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Turntable Speed Control

Printed From: Graham Slee Hifi System Components
Category: Turntable Audio
Forum Name: High Fidelity Turntable User
Forum Description: Technical Q&A, hints and tips
URL: https://www.hifisystemcomponents.com/forum/forum_posts.asp?TID=4359
Printed Date: 28 Mar 2024 at 4:21pm
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com


Topic: Turntable Speed Control
Posted By: Graham Slee
Subject: Turntable Speed Control
Date Posted: 15 Nov 2017 at 9:43am
As my new posts in the Rega Rim Drive topic are attracting very little interest, but turntable speed controllers are a hot topic all over the internet, I thought it best to start a new topic.

The most popular turntable motor is the synchronous alternating current Premotec 9904 111 31813.

It is a 110V motor and it is probably used in the majority of turntables and is definitely used in my 1979 vintage Rega Planar 3.

With 230V dropper circuit removed and a 0.22uF mains rated capacitor between red and blue wires it will take 110V ac from a suitable speed controller.

Most turntable manufacturers who use this motor drop the voltage further to around 90V to reduce motor vibration.

Because it is synchronous with the mains frequency reducing the voltage like this has no effect on the speed (unless the load drags it down).

It runs at a constant speed because it gets the constant mains frequency. Most users are satisfied with this but some purists realise the mains frequency can fluctuate (slightly) and want a non-fluctuating supply. This is where a speed controller or "speed box" comes in.

Most speed controllers simply provide 50Hz or 60Hz to suit the mains (line) frequency of the geographical location.

With these turntables speed change from 33.1/3 to 45 is by pulley swapping, but it would be nice just to click a switch.

To get the Premotec 9904 111 31813 to do the revs required for 45 rpm it needs 67.5 Hz instead of 50Hz, and 81Hz instead of 60Hz.

It is easy to generate these frequencies and amplify them and step them up using a transformer to supply the voltage required. You simply need to set the frequencies using an oscillator.

However, it isn't so easy to keep the speed constant because the frequency stability might not be as good as the mains!

The oscillators need to be dead stable, and in my experiments I've found that simple resistor capacitor (RC) oscillators drift too much with temperature.

I've tried fixing temperature using an "oven" but still get drift, so there needs to be a better way.

That better way is to use a quartz crystal, and it is easy to divide down to 50Hz or even 60Hz, but to do 67.5Hz or 81 Hz as required for 45 rpm is a whole lot more difficult.

This topic picks up at the point where I am investigating the crystals which will enable me to do all the required frequencies, and what I do to create a speed controller suitable for my ageing Rega Planar 3, which will also be suitable for several other turntables.




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Replies:
Posted By: Ernie
Date Posted: 15 Nov 2017 at 1:29pm
If I’m reading you correctly you’re going to amplify a crystal oscillator to drive a transformer to drive the motor. A sort of Armageddon where you can switch frequency to give 33 1/3 and 45 rpm.

A different approach from a straight inverter. I like the approach.

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There are 10 kinds of people. Those who understand binary and those who don't.


Posted By: Graham Slee
Date Posted: 15 Nov 2017 at 8:05pm
Originally posted by Ernie Ernie wrote:

...Armageddon...


Excellent name Wink and by the time I get it finished it may well be...

The big difficulty is finding the right value of stock crystals to suit all the frequencies required to do 33.1/3 and 45 rpm on both 50Hz and 60Hz turntables.

50Hz is easy, just look it up on Wikipedia: a 3.2768 MHz crystal divided by 2^16. Available from most UK suppliers.

Now try 60Hz, again on Wikipedia: a 3.93216MHz crystal divided by 2^16 again, but not available from the regular UK suppliers. This one can be imported from the States where 60Hz is the mains frequency.

The above covers 33.1/3 rpm in both instances, but 45 rpm?

That needs 67.5Hz for a 50Hz turntable and 81Hz for a 60Hz turntable. Now try finding crystals for them!

I could have them made I suppose, but using a combination of binary and decade counters, stock values can be divided down to give 67.5Hz, and 81.002Hz. It may be possible to "pull" the 81.002Hz to exactly 81 through the use of a trimmer capacitor.

As there will be no adjustment possible and the crystal oscillators give the exact frequency required - any deviation from a strobe disc will be purely mechanical - there is no need for a digital readout of frequency. Perhaps there will be a need for a true 50Hz/60Hz strobe lamp? And if so the 50 and 60 Hz oscillators will provide the signal.

I had originally decided to use RC oscillators because they can be tuned to the exact frequency, whereas crystals slow down with age, but I discovered that the ageing takes a seriously long time. The RC oscillators were found to be too temperature dependent, and a real fiddle to set up. I will still publish the frequency counter circuit I made as promised on the Rega Rim Drive topic.



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Posted By: Ernie
Date Posted: 15 Nov 2017 at 10:34pm
Graham, can’t you get programmable oscillators, set the frequency output via an input voltage or digital value?

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There are 10 kinds of people. Those who understand binary and those who don't.


Posted By: Graham Slee
Date Posted: 16 Nov 2017 at 6:24am
You mean dial-a-frequency? Wink

Being a hardware guy means I don't understand how to get a microprocessor to do it, so I'm stuck with doing it in "the real world". But even a microprocessor must get it from somewhere, and that is its clock, which is a crystal.

The crystal if it oscillates fast enough can be divided down to any frequency but maybe not the exact frequency wanted but very close.

Given the correct instructions I'm sure the microprocessor will take the complexity of all the logic gates required as a software representation, which will be quite cumbersome, but it's software and so isn't seen as a board full of components, which is what it will end up as in hardware.

But the source is still a crystal - a lump of rock.

Setting it by an input voltage means the use of a VCO or voltage controlled oscillator (or even a VCXO - a voltage controlled crystal oscillator -- but the frequency range it outputs will still need to be divided down to the range we want), but this depends on the stability of the control voltage, and as I have seen, the stability of anything analogue is all over the place. What is needed is the smallest parts per million per degree Kelvin, but finding it is the problem.

With a crystal it's in the order of 30ppm/K. A metal film resistor is about the same, but add more to make dividers and it becomes cumulative, although the stability of voltage references at 5ppm/K is excellent.

But a crystal will probably be the most stable because that's all there is to it. OK, there are the load capacitors which will be circa 200ppm/K but they do not have a direct relationship with the tuned frequency.

The divide down logic will not have any effect on accuracy, it just divides as logic does.

Obtaining 50Hz and 60Hz is easy, requiring something like a CD4060 and some spare data flip-flops (or even another CD4060, or CD4020 or CD4040... here I'm using cmos because of the final voltage).

The crystals are 3.2768MHz and 3.93216MHz respectively. They cost pence. The logic chips also cost pence. It is so easy to design a fixed speed controller that they're easily available from the usual turntable manufacturers. Some cost peanuts - some are premium badged products.

Buy a PJ turntable and the microprocessor speed box will cost you 129 Euro and it does 45 rpm too, so why am I bothering?

Well, I use a Rega Planar 3 fitted with a 110V motor set to work on 230V. I could buy Rega's high performance 24 volt motor upgrade kit for £129 and then I could buy the Rega Neo speed controller for £225 making £354. How a badged-up 24 volt Premotec 9904 111 318 is high performance compared to the 110V Premotec 9904 111 318 I have no idea. Perhaps it is marketing?

As in my introduction above, these speed boxes are microprocessor controlled and obviously a microprocessor uses a crystal for its reference frequency, or "is built upon a high stability crystal" as Rega marketing likes to say.

Rega show what it calls the speed control step size (0.01 rpm) which is what I meant by "maybe not the exact frequency wanted but very close".

With hardware it will be exact for 50Hz, 60Hz and 67.5Hz, and the "step size" (which in hardware isn't a step) at 81Hz will be 0.002.

I must add, within the tolerance of the crystals (plural in the hardware solution) and their stability which is 30ppm/K, but the same applies to all crystal driven things including the proprietary products above. OK, maybe their crystals are 20ppm/K?

I would say the only good thing about a hardware solution is it will be easily repaired anywhere in the world while ever they keep making c-mos glue logic chips.

The only thing that is really crazy about doing a hardware solution is the 81Hz requirement for 45rpm on a motor fitted with a 60Hz pulley (as in all 110 - 125 V countries). It really takes the biscuit, and I could have done with Carol Vorderman's help here!

I ought to set a quiz: what crystal frequency (readily available crystals that is) is required to obtain 81Hz, and how do you arrive at that?

I'll tell you how I arrived at it: divide by 5; divide by 3034; and then divide by 2 using a data flip-flop to obtain an equal mark-space ratio 81.002Hz squarewave... you name the crystal!



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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Dave Friday
Date Posted: 16 Nov 2017 at 10:32am
A few thoughts...
Speed control,use an eddy current brake?,drive each motor winding separately and alter the phase a bit on one of the drive amps?
Ps,when do you sleep?
Kr.

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lp12,oc9mk3,ca610p,krimson40watt pa,kef105.4


Posted By: Ernie
Date Posted: 16 Nov 2017 at 4:29pm
2.4576MHz UART clock crystal? A 12.96MHz would be better.

Only winding you up Graham, I love Google.

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There are 10 kinds of people. Those who understand binary and those who don't.


Posted By: Graham Slee
Date Posted: 17 Nov 2017 at 9:02am
Simplicity is good for reliability. It's neater. I like tidy circuits.

The 81 Hz simply to do 45 rpm on 60Hz mechanisms - I mean, who plays 45's? (OK, a DJ perhaps) - does not fit the words simplicity, or neat, or tidy.

It will take 6 16-pin and 14-pin integrated circuits to divide the crystal clock frequency by the required 30340.

The 67.5 Hz required to do 45 rpm on a 50Hz mechanism is only one IC fewer. Whereas to obtain 50Hz and 60Hz for 33.1/3 rpm on either mechanism requires just 3 each.

But just think of the accuracy! That's what I keep telling myself.

Now, the accuracy also depends on everything being glitch free. And what seems easy isn't easy when you start to consider glitches. Logic circuits can seriously misbehave because of propagation delays, and some ICs are far from perfect. Take the CD4017 decade counter for example: a great solution for dividing by 10, except when you need to divide by 10,000 where 4 are required. Inside it is packed with decoding circuitry to turn binary counts into decimal, and these can glitch because of their complexity. Cascade 4 of them and think of the result just one glitch will bring?

Therefore it is better to count in binary coded decimal which removes all that decoding circuitry, but that's much harder to get ones head around. Worth the effort though.



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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Ernie
Date Posted: 17 Nov 2017 at 1:12pm
Exciting stuff Graham. LP12 owners (me for one) will be interested too as well as Rega.

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There are 10 kinds of people. Those who understand binary and those who don't.


Posted By: BackinBlack
Date Posted: 17 Nov 2017 at 1:20pm
There are quite a few albums recorded at 45 rpm these days, so it would seem necessary to include it. Surely 50/60Hz would also be an either/or choice, could it be as simple as just changing between pairs of crystals?


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Just listen, if it sounds good to you, enjoy it.


Posted By: Graham Slee
Date Posted: 17 Nov 2017 at 6:31pm
Originally posted by BackinBlack BackinBlack wrote:

There are quite a few albums recorded at 45 rpm these days, so it would seem necessary to include it. Surely 50/60Hz would also be an either/or choice, could it be as simple as just changing between pairs of crystals?


1. It's worth doing a good job on 45s then!

2. I wish it were simple. It would save me testing each section like this...



(81Hz section awaiting delivery of crystal - from the USA)

Although 50Hz and 60Hz would use the same divider circuitry (but not on 45 rpm sections) the square/sine converter has to be different and rather than complicated switching it may as well have a section each.

Also, with the number of "globetrotter" customers we have, it might be best to put everything in the one box so they have the choice (the external PSU also adjusts to the local voltage).

The generation of 67.5 and 81 Hz presents challenges, very mathematical challenges, but it's not impossible.

I suppose there will be some who say "I'd be buying things I wouldn't use..." Wink


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Posted By: Graham Slee
Date Posted: 21 Nov 2017 at 6:28am
30340/2 = 15170/2 = 7585/5 = 1517/37 = 41/41 = 1

Neat isn't it?

And 2458000 divided by 30340 = 80.015 Hz.

Bingo!

So all we need is a 2.458 MHz crystal oscillator followed with divide by 41; divide by 37; divide by 5; divide by 2; and another divide by 2, and we get 80.015 Hz?

Well maybe... it depends on the counters, or more precisely the width of their reset pulses, which may not allow sufficient dwell on their last count outputs - i.e. 41; 37; and 5. Yes, they count the complete integers, but don't keep their last output high for long enough - that's what is meant by dwell - like how long do you dwell on a thought?

Therefore although the logic is doing 41 or 37 or 5, there is error in that the count falls short of the required time by a tiny fraction.

So now we try a count of say 38 instead of 37 to try and lengthen the time a little, but find it isn't quite sufficient.

The result, which is quite logical, provided the above logic is understood, is 81.49 Hz. It is 0.6% out. The electrical mains fares better at 0.4%. However, unlike the mains where the frequency can increase or decrease by 0.4%, the 0.6% error is rock-solid.

However, playing a 45 rpm record at a constant 45.3 rpm might not be what people want. The pitch will obviously be wrong. I need better logic!

For the technical among us, I used CD4518BC binary coded decimal counters for the 41, 37 and 5 counts. Now looking for other suitable counters.



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Posted By: Graham Slee
Date Posted: 21 Nov 2017 at 9:26am
A better idea would be a programmable divide by n counter/frequency divider with n being 30340.

There is such a device! It's called a CD4522BE which runs at a suitable voltage for this design. It can take up to a 4MHz clock - ours is 2.458 MHz - so it should work...

We need one per digit making 5 ICs which is the same number of ICs used in the previous failed example. Actually it's 6 because we need a gate from somewhere to make the crystal clock!

So that's six chips to make 81.015 Hz... great! LOL

One may well laugh, but a very well respected turntable manufacturer instead used a CD4060 plus a couple of CD4013's to divide down to 50Hz, but a CD4060 is a ripple counter which suffers glitches. Therefore on some cycles the speed won't be as 'advertised' and therefore speed stability is not stable...

And if software speed controllers were to be based on this...

So, a lot of hardware chips of the right sort might just give the best stability??




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Posted By: Dave Friday
Date Posted: 21 Nov 2017 at 9:38am
How are you going to adjust the motor/turntable speed ( for bearing oil drag?)
Kr.

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lp12,oc9mk3,ca610p,krimson40watt pa,kef105.4


Posted By: BackinBlack
Date Posted: 21 Nov 2017 at 10:07am
A synchronous motor (Premotec, Airpax or similar) operating within its design parameters, ie torque will maintain it's synchronous speed of the frequency applied to it. If bearing drag slows it below locked synchronous speed then it's overloaded and will be unstable. These motors are quite different in the way they are designed to operate to say a Garrard 301/401 "synchronous" motor which is in fact a shaded pole, a type of squirrel cage motor. The Garrard principle is to provide a constant load to the motor via the eddy brake and bearing to stabilise its speed. Also the Garrard motors run at about 25 to 30 watts compared to the Premotec's 1.5watts. This is why a very free running bearing is necessary on Premotec driven turntables.


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Just listen, if it sounds good to you, enjoy it.


Posted By: Dave Friday
Date Posted: 21 Nov 2017 at 12:21pm
Ta,but...Lin have mechanical speed adjustment ( on the motor pulley)..

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lp12,oc9mk3,ca610p,krimson40watt pa,kef105.4


Posted By: BackinBlack
Date Posted: 21 Nov 2017 at 2:21pm
I'm not directly familiar with the L*nn arrangement, but it appears that the standard pulley is probably slightly tapered. Adjusting the belt position on the pulley effectively gives a small amount of speed adjustment for fine tuning. The adjustment also ensures that the belt does not scuff on the pulley, ie it runs true. Whether this is for overcoming manufacturing inaccuracies, personal preferences etc I cannot say. Nevertheless a synchronous motor should normally run at a speed locked to the frequency of the ac voltage applied to it.
When correctly set up there should be no need for further adjustment as long as the supply frequency is correct.


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Just listen, if it sounds good to you, enjoy it.


Posted By: Graham Slee
Date Posted: 22 Nov 2017 at 8:49am
Originally posted by Dave Friday Dave Friday wrote:

How are you going to adjust the motor/turntable speed ( for bearing oil drag?)
Kr.


Crystal clocked motor speed controllers can only output a fixed frequency. The L*nn Valhalla output was a fixed 50Hz (60Hz using a different crystal) suitable for 33.1/3 rpm. BackinBlack has answered the mechanical issues much better than I can so I won't add anything here.

If the gearing (pulleys) is correct and the motor has sufficient torque given the available power - which I guess will be around 12W although I have not measured it yet - it should rotate at the specified rpm (this is the power available from the circuit, not the motor power it is designed for).

The controller's job is to output the required voltage at the correct frequency required for the motor to turn the platter at the required revolutions per minute. These are 50Hz for 33.1/3 when the belt is set on the 33.1/3 pulley, and 67.5 Hz for 45 rpm without moving the belt. On turntables manufactured for 60Hz mains/line it is 60 Hz for 33.1/3 and 81 Hz for 45 rpm.

Crystals can only be adjusted by a fraction of a percent. It would be nice if they could be "pulled" by more than this, but in my experience they either oscillate or don't depending on the circuit conditions. Varying load capacitance too far stops oscillation and as the correct values are approached the crystal "snaps" to the specified frequency plus or minus the tolerance.

If they were adjustable then the stability would depend on the passive components used, and these do not have the same temperature coefficients as does the crystal and as such there would be drift.



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Posted By: Graham Slee
Date Posted: 23 Nov 2017 at 8:38pm
The stumbling block has always been obtaining the correct frequency to play 45 rpm records on a turntable meant for 60Hz operation. The pulley ratio for the Pemotec 110V motor is based on the motor doing 300 rpm, because that's what they do at 60Hz. So to switch to 45 rpm when using the 33.1/3rd pulley a frequency of 81Hz is required.

All the other frequencies will comparatively easily divide into a suitable standard crystal frequency.

81Hz isn't easy!

Taking a suitable crystal often used to generate 50Hz: a 3.2764 MHz crystal; 81Hz won't go easy. In fact, after 2 weeks of maths, and with the help of online number crunchers, I wasn't anywhere close.

I tried dividing a 2.4576MHz crystal by 30340 which should have given 81.002 Hz, but propagation delays meant it ended up being 81.49 Hz.

The problem with that number is it won't divide down much further (save for one divide by 4) before it hits prime numbers which are too big for an accurate divider to handle (0-15 count).

Cascading CD4522B's isn't all that reliable. Also any zero makes it impossible. Try dividing any number by zero, so 30340 isn't going to work (n/0 = infinity). The CD4522B works best in isolation. It looks like a cascade but it is only the output of one driving the input of another.

This way a very close approach to 81Hz is possible, if you're prepared to do lots of math and search out number pairs, or ones which look close.

The nearest I have got is 80.8997 Hz (measured), and if there was a hour long 45, it would run over 4.464 seconds. I don't think 0.124% will be noticeable at all.

Here's the work it took to get there.







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Posted By: Graham Slee
Date Posted: 28 Nov 2017 at 6:41am
Showing my ignorance, or maybe it's my forgetfulness? I can't remember, but I had not taken into account the propagation delays which is the price you pay for using CD4000 logic. It is not high speed like the 74HC series, but that would go BANG on the supply voltage I want to use.

OK, yes, the CD4522B data sheet says it will do 4MHz but with propagation delays the next stage faced with the original clocking rate (from the previous IC) cannot "lock in" properly so a cascaded arrangement fails. Separating them into individual dividers where a longer output pulse drives the next one works, as the previous post clearly shows.

I'm sure all of this is dead boring in these days of Raspberry Pie, but there is more chance of discrete logic staying the course being repairable by somebody who doesn't have programming skills.

And this is probably why very few readers here are taking any notice, which is great really because it isn't using up bandwidth and so is a quickly available resource for the tiny minority who are interested.

The CD4522B is not recommended for new designs which means that any day the plug will be pulled on its manufacture. It is therefore a good idea to use something else.

What's left is the popular stuff of the science class, because it would seem that the main use of cmos logic is in academia. Bloody progress!!!

So, we can still get hold of through hole PDIP devices like cmos logic gates, a limited range of counters/dividers and shift registers, and amongst counters it would seem that the only presettable and cascadable divide by n counter considered worth keeping is the CD40193B.

It's getting a bit like valves! Therefore this topic should really be attracting swathes... Cool

So, using the CD40193B we can easily see what it can do? But logic data sheets never do that, no, they're reserved for those who already know! A trifle stupid when you think about it - the student who could make the most of these things migrates to something he can understand like programming Pies! It's a self destructing mentality which has ensured the dearth of these things, set in place so the then consumers (other manufacturers) could keep their market position. And now? And that's why such devices are generally only consumed by academia (obvious?).

So back to the CD40193B. It's all I have left to make a hardware frequency divider with, and I am not falling into the "speed trap" as before. It will work in a cascade only if the clock is divided down by something else first, to make the clock it receives slow enough such that the last stage sees a real clock signal. This dictates what we can get from the speed controller.

Rated at 4MHz which most CD4000 ICs are, each cascade will slow down between a factor of 3 or 4, so the third in a cascade will be doing something like 250 to 440 kHz. Clearly a 3.27MHz clock is too fast. A CD4017 used simply as a divide by 10 with nothing fancy going on, should reliably output 327kHz, so we have the chance that a cascaded 3 section divide by n frequency divider will work.

And if so we can divide by up to 4095 (decimal number). So back to the stumbling block frequency of 81Hz which our American 45 rpm users will be most interested in. How accurate can we make it?

Divided by 10 already (the CD4017 stage) we have 327680Hz left to play with. The cascade won't output a nice 50:50 squarewave so we will need to flip-flop the result using a data flip flop (DFF), and as it divides by 2 this further reduces the frequency we can divide into. It is now 163840Hz.

The nearest integer number it will divide by to get close to 81Hz is 2022, and 163840/2022 is 81.02868447 which is within 0.035% of 45 rpm. I think that's 1.275 seconds per hour. Do you think the strobe mat will appear stationary at that?

I think we can all agree it will.

So off to make it, and hopefully it will work and I can show you the result later.



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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 28 Nov 2017 at 8:00am
Talking about datasheets. This taken from a TI datasheet of the 74HC193 - the 74HC version of the CD40193 - has "carry" and "borrow" swapped over. The logic is that terminal up count (TCU) "carries" and terminal down count (TCD) "borrows". Obviously the compiler of this sheet was somewhere else.






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Posted By: Graham Slee
Date Posted: 28 Nov 2017 at 12:32pm
So this is the result from the one before last post:



You can see that 81.0198Hz is an improvement upon 80.8997Hz, and is only 0.0243% high which is excellent! So do we have a circuit?

Well, actually no. The circuit didn't work as the maths said it would. One of the jam inputs which I thought should have been low resulted in the wrong output, but when set high, resulted in 81.0198Hz which is very close to the predicted 81.02868447Hz ...

... but I want to know why my maths was wrong!




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Posted By: Graham Slee
Date Posted: 28 Nov 2017 at 8:35pm
Still trying to find the mathematical proof but here are the results...

50.0194 Hz using a 2.7MHz crystal
67.5012 Hz using a 2.7MHz crystal
60.0078 Hz using a 3.2768MHz crystal
81.0197 Hz using a 3.2768MHz crystal

50.0194 Hz being the worst result, out by 0.0388% which I think is something like 0.01 steps in Rega language. A 20 minute LP will be over in 19 minutes 59.5344 seconds (0.4656 seconds short). Good enough?


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 29 Nov 2017 at 4:49pm
Originally posted by Graham Slee Graham Slee wrote:

Still trying to find the mathematical proof but here are the results...

50.0194 Hz using a 2.7MHz crystal
67.5012 Hz using a 2.7MHz crystal
60.0078 Hz using a 3.2768MHz crystal
81.0197 Hz using a 3.2768MHz crystal

50.0194 Hz being the worst result, out by 0.0388% which I think is something like 0.01 steps in Rega language. A 20 minute LP will be over in 19 minutes 59.5344 seconds (0.4656 seconds short). Good enough?


Not good enough because the mathematics don't make sense, therefore although I got all the right answers I cannot show how I arrived at them, and as such this is a worthless circuit. Others may wish to dabble, but this speed control has to be properly engineered.

The reason it doesn't work was alluded to earlier. Due to propagation delays the third CD40193 was unable to latch on to the clock, which by the time it reached it, looked like a shark swimming backwards.

The poor old dear is rather slow. It was after all an early 70s device. However, this old CMOS being slow, is good for EMC! What with relatively slow crystals and slow logic there is little chance of spurious emissions. After all it will be used in the vicinity of the turntable which is wide open to interference from uP clocks.

The good thing about the CD40193 is that I was able to get it counting in two stages. This means with another two stages I can obtain the correct frequency division I'm after.

In this case I need 1/1517, and the factors of 1517 (apart from 1 and itself) are prime numbers. The only way to divide using prime numbers is by a cascade of presettable divide by n counters, such as these. OK, the jam inputs are prone to glitches if you count too far, but so far none detected.

If this works the accuracy on 81Hz should be 0.0033% Smile




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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Dave Friday
Date Posted: 30 Nov 2017 at 12:39am
I find your work on the " motor speed control unit" very interesting,I did try to find out how Quartz cristals are made( not much information!) it's a pity that a bit can't be ground off the cristal (to tune it up a bit!)

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lp12,oc9mk3,ca610p,krimson40watt pa,kef105.4


Posted By: Graham Slee
Date Posted: 30 Nov 2017 at 7:35am
Originally posted by Dave Friday Dave Friday wrote:

I find your work on the " motor speed control unit" very interesting,I did try to find out how Quartz cristals are made( not much information!) it's a pity that a bit can't be ground off the cristal (to tune it up a bit!)


I think crystals are made by cutting a slot in the quartz and then metalising either side of the slot and connecting the wires to the metalization. The low frequency 32kHz ones used for digital clocks look much like tuning forks, and yes, by grinding a bit off the crystal it will change its frequency. Radio Hams reportedly do such things for fine tuning.

However, I find it preferable to use standard crystals and not tweak them so that if this project reaches production it will be a standard assembly job.

81.0026Hz (for 60Hz TT users playing 45) is I think good enough and within 0.00321%, which will give a speed of 45.00144 rpm. It is never going to be exactly 45.000000 rpm due to component tolerances and temperature coefficients, but this is much better than the speed obtained from the line frequency will ever be.

As for stability, it never changed over a 12 hour period and I don't expect it will change given a reasonably constant temperature. Temperature coefficient drift should be in the region of 35 parts per million per degree C, which is 0.0035%. When added to our 0.00321% that makes 0.00671%, and if subtracted, -0.00029%.
 




The build below pre-scales the clock by 10 bringing its frequency low enough for reliable CMOS operation.

Two 8-bit binary dividers divide this frequency to twice that required, and then a D-type flip-flop (a bistable) divides by two to give an equal mark/space squarewave, which is what we need to produce a low distortion sine wave which can be manipulated to a suitable voltage (and current) to drive the turntable (synchronous) motor.





Reliability is of the utmost importance as we don't want the motor spinning out of control, which is what would happen if a division was skipped.

The signal shown below is the load pulse applied to the 8 bit binary dividers so they load the divide by numbers set on the presettable inputs. You can see it is a very short pulse. It has to be or the division would last longer and the output frequency would not be as accurate.

However, it isn't a very nice shape by which I mean it gets narrower just where the trigger to count again occurs. In theory this should have straight sides and a squared off base, but propagation delays - the response time due to our old enemy capacitance (capacitance is sometimes an ally though) - cause it to be rounded off. The pulse width is only around 50nS (0.00000005 sec) and in frequency terms (if repeated) would be 10MHz. Remember that CMOS is only reliable to around 5MHz and you can see why this pulse isn't squared off.





The next stage is to try a different binary counter (CD40161) where the clock is synchronous rather than rippled through, which might help ensure better reliability. In fact it would be the end of the road but is prefered by digital electronis engineers, so hopefully it will be the "be all and end all" of frequency dividers.

As long as I'm happy with this method of frequency division - which is less glitchy than some of the competing motor speed control designs - then I am sure with 2 x 8-bit binary numbers to play with, and with a choice of four crystal frequencies (selectable by data switches - yet to be developed), that all four required frequencies can be accurately obtained.

If so, then the task will be to develop a way of setting the counters using simple to use switches: one for 50/60Hz; the other for 33.1/3 and 45 rpm (assuming the use of the 33.1/3 pulley).

After that I will concentrate on the output stage; quality of sine wave; and ways to achieve different voltages to suit as many production synchronous motor turntables as possible (including my own 230V Rega Planar 3).



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Posted By: Graham Slee
Date Posted: 02 Dec 2017 at 8:55pm
Now, anybody can do a frequency generator, so why is this topic lasting so long?

It's because I want this to be accurate, reliable, and easy to use.

I also want it to tick all the boxes, and that is bound to include:

33.1/3 / 45 selection
50 / 60 Hz turntable selection
Stability - rock steady constant speed - quartz locked
Vibration reduction - full start-up voltage to get the platter up to speed followed by variable ramp-down for quietest operation (variable so it can be owner tweaked to suit the turntable)

Then there is the voltage it outputs - the variants required. But that has to be investigated once we have the frequency generation part done.

And we're not far off now. The latest circuit does all 4 frequencies very accurately and stably. Here are the results...

50Hz frequency generator
50Hz (33.1/3 rpm on a 50Hz turntable)

60Hz frequency generator60Hz (33.1/3 rpm on a 60Hz turntable)

67.5Hz frequency generator
67.5Hz (45 rpm on a 50Hz turntable)

81Hz frequency generator
81Hz (45 rpm on a 60Hz turntable)

Breadboard

And the bread-boarded circuit which does it.

Unlike the 40193 circuitry this one is fully synchronous to the clock but is a two stager, so the second stage takes the first stage output as its clock.

What I want to do now is accomplish the whole division in one stage cascading three modulo-n counters all synchronous with the crystal. This might not be successful because of propagation delays, but if not I can always revert to this one.

OK, a microprocessor solution might be more elegant and less "busy" but think about this: do you really want spurious interference as can be seen https://www.hifisystemcomponents.com/forum/understanding-record-repro_topic4323_post53549.html#53549" rel="nofollow - here ? The tone arm has better chance of shielding out much lower frequencies. After all, this will be used with and near the turntable!



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Posted By: Graham Slee
Date Posted: 03 Dec 2017 at 7:00pm
Originally posted by Graham Slee Graham Slee wrote:


What I want to do now is accomplish the whole division in one stage cascading three modulo-n counters all synchronous with the crystal. This might not be successful because of propagation delays, but if not I can always revert to this one.


Well? It works. So we're now on a 12 bit counter (one chip fewer), after a divide by 10 pre-scaler, and before a data flip-flop which divides by 2. The outputs (50, 60, 67.5 and 81 Hz) all exhibit a 50:50 mark/space ratio, and now we need it shaping up to be a perfect flat top/bottom squarewave, which the AP scope monitor doesn't show but the real oscilloscope does.

And notice the 4uS load pulse. Far better than the 40193. This is because it is synchronised to the clock (1/Fcrystal / 10 (prescaler)) which shows that asynchronous at these speeds isn't reliable.

Load Pulse

Another reliability factor is increasing the supply voltage, as CD4000 can go to 16V, and its speed increases with it.


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Posted By: BackinBlack
Date Posted: 03 Dec 2017 at 7:45pm
Just to reassure you at least one of us is following this project.
I find this quite fascinating, the use of "old" IC technology to overcome the potential interference from todays superfast ICs. I'm assuming that you will be using a bespoke amplifier output stage to drive the turntable motor, but what output power will be available? I have both a Garrard 401 and a  24 volt motor twin belt driven turntable. The Garrard can be configured to run at a nominal 120 volts by running the motor field coils in parrallel rather than series but consumes significantly more power than the usual premotec/airpax motors, will this be within the scope of the output.
Might you consider making this turntable drive available in kit form to cover a range of output powers?

Ian


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Just listen, if it sounds good to you, enjoy it.


Posted By: Ernie
Date Posted: 03 Dec 2017 at 10:14pm
If Graham, as he originally suggested, is using a transformer to up the voltage he could get it wound with some standard tap offs to give 24v, 90v, 110v etc.

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There are 10 kinds of people. Those who understand binary and those who don't.


Posted By: Graham Slee
Date Posted: 04 Dec 2017 at 2:56am
The output stage isn't simple. Just as bad as the frequency generator but in different ways. A power amplifier to do 230V rms AC would require a power supply of more than 650 DC volts. For a 24 volt motor the power supply would have to be nearly 70 DC volts.

We can half the supply voltage by using a bridge amplifier: that's two amplifiers running 180 degrees out of phase and using their "pos" terminals to drive the load (motor).

Any discrete amplifier will be costly to make. Do we really need to do a bespoke design? There are a number of chip amps which could be utilised instead. And in-fact that is the way I tested the basic design for this project, using a cheap car radio chip amplifier.

I also used an off-the-shelf transformer reversed to step up the voltage. Taking transformer regulation into consideration the step-up ratio is less than its step down ratio, so if it's a 12V transformer I would have to input 15V to its secondary to get 230V out.

For my initial experiment I chose a 12VA transformer. Its secondary inductance was measured and by calculation the secondary presents a 3.2 Ohm load to the amplifier at 50Hz (our lowest sinewave frequency). The amplifier (rated to do about 30 watts into 3 Ohms) however, would not do 15 volts but it would do around 9.5 volts into this load, and the resulting voltage across the primary was about 170V rms, suggesting the transformer has better regulation than estimated.

By attenuating the amplifier input an output of 110V rms was obtained, suitable for the premotec motor which is rated at 110V.

How far can we go? Perhaps a 20VA 24 volt transformer would have a similar secondary impedance such that a chip amp were able to drive it. It would need to swing about 25 - 26 volts, and if a bridge amp could run on say 40 volts DC. But on calculating the wattage we find it to be around 200 watts (into a 20VA transformer??). A bit of a tall order.

It becomes an empirical matter of balancing impedances, ratios, and power ratings. There may be something to be gained by the fact that a 12VA transformer actually consumes around 20 watts, so when reversed it might supply a 20 watt motor?

We are in the territory of power inverters here, and learning that subject could be a steep curve.



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Posted By: Graham Slee
Date Posted: 04 Dec 2017 at 1:16pm
Perhaps if the winding ratio were greater, necessitating a lower VA rating because otherwise the secondaries impedance would become too low for the amplifier, it could then do 230V?

But then you have the problem of insufficient current to drive a motor such as a 30 watt Lenco.

The secondary on the 12V transformer seemed "happy" enough with 9 to 9.5 volts into it's approx. 3.4 Ohms impedance (corrected from 3.2 Ohms which I stated earlier). That means the secondary winding can take nearly 25 watts rms! Although rated at 12 VA (watts by any other name) it was being driven backwards at twice that, and it didn't even break sweat! But that was with the light primary load of a 1.8 watt motor.

We understand because of magnetising current that the primary can do 20 watts (VA really), but in this "normal" mode it will get hot (try it). Therefore, although the secondary runs cool, the primary, when loaded, will run hot.

So how about paralleling output stages? A reminder: an output stage consists of an amplifier and a transformer. If we parallel each primary, and if we parallel each amplifier input, then the amplifiers and transformers share the load. To parallel inputs all that is needed is a impedance matching circuit - an emitter follower should do.

We should be mindful of ensuring one transformer isn't shorted by the inverted phase of another, just in case one production batch is wound opposite to the other.

To do 30 watts however, using 6VA transformers, would require 5 output stages. Whereas with the premotec/airpax motors only one is required as they are 1.8 watt rated.



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Posted By: BackinBlack
Date Posted: 04 Dec 2017 at 7:33pm
When I referred to bespoke I was really thinking more toward optimised to provide a clean 50 to 81Hz output. Chip amps seem an obvious choice. Might a bridged pair drive a 24v motor directly with only a 30v supply?
As for the higher power demand of motors such as the Garrard and Lenco is it just a case that a much larger transformer will be necessary.


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Just listen, if it sounds good to you, enjoy it.


Posted By: Graham Slee
Date Posted: 04 Dec 2017 at 9:28pm
Originally posted by BackinBlack BackinBlack wrote:

Chip amps seem an obvious choice. Might a bridged pair drive a 24v motor directly with only a 30v supply?


I'd agree that is the best way should a suitable chip amplifier be available. A bridged amplifier would require a power supply DC voltage at least 1.5 times the AC output voltage (36V) because of the difference between peak output and the rms output we require.

Originally posted by BackinBlack BackinBlack wrote:

As for the higher power demand of motors such as the Garrard and Lenco is it just a case that a much larger transformer will be necessary.


A more powerful transformer would have a very low impedance secondary which the chip amplifiers might be unable to drive. I think it will be a case of trial and error as far as calculations go.


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Posted By: Graham Slee
Date Posted: 04 Dec 2017 at 9:38pm
Final part of the frequency generator logic, and this is the minimum component count I could achieve to obtain a reliable circuit. Sorry I cannot give you a detailed view which could be easily copied.



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Posted By: Ernie
Date Posted: 04 Dec 2017 at 10:46pm
Really interesting stuff Graham.

No nasty inverter chopping up mains level voltages. Not seen anything else on the market with this concept.

Lots of L**n owners swear by the N**m Armage*don power supply which is basically a big transformer but fixed at the incoming mains frequency.

Are you going to shift the phase angle on the second motor winding with a simple RC network or have you something else up your sleeve?

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There are 10 kinds of people. Those who understand binary and those who don't.


Posted By: Graham Slee
Date Posted: 05 Dec 2017 at 12:00am
Originally posted by Ernie Ernie wrote:

Really interesting stuff Graham.

No nasty inverter chopping up mains level voltages. Not seen anything else on the market with this concept.


It just seemed obvious to me to do it this way. I don't think pulse width modulation into a big inductor (the transformer) is a good idea. I once drove a couple of BMW electric window motors that way, but that was to open and close some posh wooden doors that hid a rear projection screen, and there wasn't a turntable within miles. I've done plenty of complicated logic control circuits to do some crazy things for exhibits, but they've always been hidden behind the scenes in plastic project boxes.

Originally posted by Ernie Ernie wrote:

Are you going to shift the phase angle on the second motor winding with a simple RC network or have you something else up your sleeve?


The simpler the better for me, as long as it turns in the required direction. I'll be happy enough if I can get a 230V output with sufficient current that I can hook it up to the Rega mains cable and let the existing circuit board do the rest. As for direct connection to the standard 110V premotec, I simply wired the 220nF mains rated capacitor to the other phase. It vibrates a bit at full voltage so I'll put in a pot to attenuate the output a little - basically what the 230V supply 10k series resistor does - the data sheet states 6k8 for full power.


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Posted By: Graham Slee
Date Posted: 05 Dec 2017 at 11:07pm
Previously each of the frequencies which dictate the RPM were selected using DIP switches to set their binary codes (12 bit words). That's a bit too awkward for the user.

A simple switch is what is required to set the four 12 bit words, and that needs encoding onto the counter. The simplest solution being open collector transistors whose collectors may be paralleled without causing shorts, from which a suitable switching matrix can be formed.

As I didn't have a switch handy I bridged the four connections with a piece of wire (illustrated on the left of the image). These connections can be subdivided to allow for two switches: 1 for 50 or 60 Hz operation; the other for 33.1/3 or 45 RPM selection.
 
Rather than discrete transistors I chose a monolithic solution: several transistors on a chip complete with base resistors, and had 4 channels spare to drive some LEDs which confirm to the user that he/she has selected the appropriate RPM (illustrated on the right of the image).

I need further switching to sync the appropriate sinewave filter to each frequency, but not wanting complicated switch wiring or multi-pole switches, I realised that the LED drive outputs can also be used to gate (steer) the output to the appropriate filter. I can then sum all four (only one active at a time) into a simple op-amp stage where the start/run "ramp" can also be accommodated.

Speed Selector Logic



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Posted By: Graham Slee
Date Posted: 07 Dec 2017 at 3:38pm
And now we move onto the transformer end.

I've been asked about it being suitable for use with Garrard/Lenco motors. I don't think this is going to happen, and this is why...

According to http://www.lencoheaven.net/forum/index.php?topic=68.0" rel="nofollow - Lenco Heaven "Two main versions of the motor exist, a 110/220 volt 50 Hertz (Euro) version, and a 110 volt 60 Hz (US) version. For both versions, power consumption is 15 to 19 watts. (Actual currents: 85 ma @ 220V/50 Hz, 140 ma @ 110V/60 Hz)"

An amplifier will be required to drive a transformer to provide the voltage (110 or 220V). An amplifier alone can be made to do this, but we're talking about one which will deliver 220V rms (in the UK) and greater than 19 watts. You might find mention over at Lenco Heaven of the use of a Crown 300 watt amplifier being used to drive a 110V motor? So what will we need for 220V?

Honestly, we are approaching the realms of little return.

To transform it up using a smaller amplifier we need the amplifier to deliver some serious current, because the secondary impedance (the end of the trasformer we need to drive to provide the voltage step-up) is so dreadfully low.

But we can drive such as the Premotec motors at 110V and 220V using a chip amp and transformer.

Most chip amps will drive as little as 3 Ohms, especially those designed for automotive use. They will deliver near 30 watts and the voltage swing is around 10 or 11 volts.

They will however go into shut down the nearer the load gets to a short circuit, so 3 Ohms should be treated as a minimum.

Now we ask, what transformer has a secondary which at 50Hz, and up to 81Hz has that impedance?

You find that out by measuring the inductance and then finding the reactance for a particular frequency: in this case 50Hz.

A 12VA 6,0,6 V transformer offers us around 3.4 Ohms (series wired across the 6V terminals), and if we put into it near on 12 volts we get 220V out??

Wrong!

You might get 150 volts. I can't remember what I actually measured but it wasn't far off with the amp at clipping. Not that the amp was overloaded, far from it. It was just that it reached its limit of output swing.

We could go for a lower secondary voltage but that reduces the secondary impedance below what is good for the amplifier.

The only thing we can do is reduce our expectations by looking at 6VA instead of 12VA. Here we can go to a lower secondary voltage because it is lower current which means it is higher impedance. It is now a case of scoring a match.

9V isn't quite there, it can only reach 190V. A 6V secondary 6VA transformer might do it (I hope so, I just ordered one).

OK so far? Not really. The amplifier develops 30 watts or thereabouts into 3.4 Ohms, and the transformer is 6VA (AC watts). To be honest I've been driving a 12VA transformer at around 24 watts (around 9V AC into 3.4 Ohms, check my maths if you will), and it didn't get hot, but at half the power the 6VA could have its life shortened?

We could borrow a trick from the PA world and put two transformers in series...

But that's a job for tomorrow.



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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: morris_minor
Date Posted: 09 Dec 2017 at 12:29pm
Thanks for the information, Graham. Looks like my Solid12 will have to make do with the motor as-is Ermm.

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Bob

Majestic DAC/pre-amp
Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V
Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50
2 x Proprius + Spatia/Spatia Links


Posted By: Graham Slee
Date Posted: 10 Dec 2017 at 2:27pm
Originally posted by morris_minor morris_minor wrote:

Thanks for the information, Graham. Looks like my Solid12 will have to make do with the motor as-is Ermm.


Not so fast!

Now, one thing everybody seems not to have considered is the effect of temperature on frequency stability.

RC oscillators were found to be no good because temperature changes affected stability too greatly.

So, everybody else uses quartz crystals, and so am I, but the specs say something in the order of 35ppm/degree K. K having the same delta as C, so we will use C.

So, if the temperature was to differ by say 25C the accuracy of the crystal would change 25 x 35 ppm or 875ppm, which is 0.0875%.

Check my maths: 875/1,000,000 = 0.000875; multiply by 100 to get percent: 0.000875 x 100 = 0.0875, yes?

That's almost 0.1%, so what use is it going for accuracies of 0.0025%?

Because by mounting a transformer and amplifier, or just an amplifier with its heat sinking - which drives the motor - heat will be given off proportional to the energy required...

And so far I have measured that very temperature rise above ambient (25C). That just being for the 1.8W premotec/airpax, never mind a 14; 19 or 30 watt motor!

So what do we have to do? Well, you could put off listening until the device warms up, and by rights, according to standards such as IEC 60950 that takes 72 hours (3 days).

Or the power side of things can be isolated from the frequency generation side. That requires 2 boxes instead of 1.

If the power side is housed separately then you can have any power you want! It just needs a big enough box. But would you want a behemoth for a premotec/airpax motor? So would it require a range of power boxes?

Think of it like a preamp and power amp. Here we have the control circuit and power circuit separated.

Too many boxes for you? Well, it looks like sticking with the mains.



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Posted By: Ernie
Date Posted: 10 Dec 2017 at 8:53pm
Graham, can’t you use a temperature stabilised crystal?

This is 30 year memory kicking in, think it’s called an oven.

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There are 10 kinds of people. Those who understand binary and those who don't.


Posted By: Graham Slee
Date Posted: 11 Dec 2017 at 5:36am
Originally posted by Ernie Ernie wrote:

Graham, can’t you use a temperature stabilised crystal?

This is 30 year memory kicking in, think it’s called an oven.


Yes Chris, correct answer, and you remembered it but I had to look it up (a few pages ago).

Thermal considerations seem to have plagued at least one speed controller (on the LP12 I believe) and I don't want this one to fall into a similar trap.

As for a crystal oven, I really don't want to go there. I used to make temperature controllers for the PCB processing industry (with a little help from my friends) and they had problems of their own; and so did everybody else's it would seem, judging by the number of artwork mods I had to do for a West Midlands company.

There is only one kind of really stable temperature sensor in my experience, and that is the thermistor. Everything else is too sensitive to noise.

For the crystal oven to maintain a constant temperature (+/-5% should be adequate) it would need to be hotter than the highest expected temperature in the product, and not to give off it's own heat into the product which could ultimately give rise to thermal runaway.

The highly stable Hewlett Packard frequency meters had something on the lines of a thermos flask in the back of a 3U 19 inch rack case for its crystal oven.

Maybe I need to settle on the fact that this project will heat up within a range of temperatures depending on its use regardless. And from measurements taken so far, the temperatures of the amplifier and transformer averages out around 20C above, but can be as high as 30C above ambient depending on motor load.

Assuming that the temperature inside the product falls short of the heat coming from its contents - using its aluminium case as amplifier chip heatsink - should average its 20 - 30 C rise with the ambient (let's say it gets 10-15 C hotter than ambient), then the transformer becomes the main point of interest. It will obviously need holes in the case for convection cooling.

Provided the user is happy about speed stability only being fully achieved when the unit has reached its temperature equilibrium, it may be acceptable?

It should be pointed out that this post is only considering the use of the 1.8W motor (as used in several modern turntables) running on 110V ac or 230V ac, a voltage selector switch therefore also being necessary.

It has not considered the heat contribution from an inbuilt power supply, which sould take it even further "over the top", and the use of an external power supply therefore seems logical (I would have thought?).

As for higher powered motors, I definitely believe the power side should be dealt with in a separate box.




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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 11 Dec 2017 at 4:55pm
Taking stock, the idea was for the controller to be simple to connect to the Rega (my Rega) and as such needed to output 230V.

However, it isn't going to be simple to connect. The controller cannot have a 13A socket for a start. 1. it wouldn't fit, and 2. anything could be plugged into it by family members (not good).

Therefore it would need to have a different connector, and that requires DIY skills, which I have but not all customers do. And therefore its appeal becomes limited.

And if so, I may as well save myself a lot of unnecessary design work and wasted power, and just go for an 110V output. This cuts the power required by half, it being wasted by the series resistor used in the turntable to drop 230V to 110V. May as well rewire and use a more suitable plug from the turntable.

Getting rid of half of the power also reduces the temperature rise in the unit, improving the speed stability between switching on and so many sides in.

Solved.


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: BackinBlack
Date Posted: 11 Dec 2017 at 7:17pm
I'm beginning to think that the DC motor and drive might be an easier and more economic option, even with the cost of the dc motor.
I'd also like to correct an earlier comment I made regarding the Garrard 401 motor power requirement; I've measured it running at just under 12 watts, not the 30 watts I mistakenly quoted.
Any thoughts on brushless motor drives?
Sorry to spoil the fun.
Ian


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Just listen, if it sounds good to you, enjoy it.


Posted By: Graham Slee
Date Posted: 11 Dec 2017 at 9:02pm
I'm not a motor expert although I designed and manufactured an industrial motor control many years ago (around 1/8th HP). That was relatively easy because it used an appliance motor control chip developed by Plessey. It could be used open-loop but some loading conditions made it oscillate, and so I developed an optical tacho for it, which having a CNC drill/router to hand it was easy to make the rotor. It was also AC.

The only other motor work I've done is last year's DC motor experiments and the AC synchronous motor control being developed here.

DC motors run far too fast in my opinion, and require considerable "gearing down". A simple belt and pulley drive soon fails as the belt joint peels itself apart on the small motor pulley diameter. Gearboxes are too noisy (remember the Philips VCR?). If the voltage is reduced sufficiently to get a DC motor turning slow enough it has too little power/torque to overcome resistance and therefore wows and flutters.

Asynchronous motors (induction types) do not lock onto mains frequency and rotate because of a combination of volts and frequency, and so are not easily controlled.

Brushless motor drives are good, my Technics uses one... However, the circuitry is quite complicated and the motor is obviously bespoke.

Much as I've tried to be clever in doing the turntable motor any which way than the premotec/airpax synchronous way, I have to say that in my opinion it is the most logical choice (and uses one heck of a lot of it in the process - logic ICs that is).



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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 18 Dec 2017 at 11:02am
List all the turntables using the Premotec/Airpax/Allied Motion/Philips 110V synchronous motors up to 1.8 watts, and this should be able to supply them all. Not only suitable for 50Hz geared turntables but 60Hz too, and at the flick of a switch. 33.1/3 and 45 rpm on the 33.1/3 pulley.

...provided it works.

I managed to squeeze it all down to a eurocard size board and it will fit nicely into our standard eurocard case.

At 17 holes per square inch it's close to the limit of single sided boards.

Now waiting for delivery of the boards.






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Posted By: Ernie
Date Posted: 18 Dec 2017 at 8:11pm
You certainly don’t hang around do you Graham!

Wonder how it’ll compare to a L##go?

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There are 10 kinds of people. Those who understand binary and those who don't.


Posted By: Graham Slee
Date Posted: 18 Dec 2017 at 9:07pm
I noticed the above mentioned uses two crystals and the 45 rpm one isn't exactly the 1.35:1 ratio to the other, which it should by rights be.

Because of the increased clock speed (I'm guessing it's because of that) it uses 74 series 'multiple divide by two's' instead of the slower 4060 used in the V*****a.

OK, it takes some understanding to work out a synchronous divide by n counter, especially with 12 bits, and the input frequency has to be prescaled (divided by 10) for the last 4 bits the old CMOS can just about keep up with, but we're all supposedly electronics engineers...

My excuse is I'm over 60.

And for 60Hz versions???


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Posted By: Graham Slee
Date Posted: 18 Dec 2017 at 9:43pm
If L##go accuracy is acceptable then I could have just used one crystal to get all four frequencies, but I wanted to be more accurate than 0.02% on 81Hz (45rpm on 60Hz gearing - the worst frequency to get right), and so I switch in another crystal just for that to obtain 0.00244% (Americans matter too).


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Posted By: Graham Slee
Date Posted: 20 Dec 2017 at 9:28pm
Built, tested and working fine.





Runs up to 45rpm in 15 sec at 87.6V. No "hand assistance" required.


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Ernie
Date Posted: 20 Dec 2017 at 10:24pm
Well done Graham!


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There are 10 kinds of people. Those who understand binary and those who don't.


Posted By: Graham Slee
Date Posted: 21 Dec 2017 at 7:03am
On the subject of hand assistance (sounds so rude) I need to express (ruder!) my opinion.

If a motor does not have the torque to start the platter turning at all, then the motor has absolutely zero control over the platter! And if that is the case, no speed control can have any effect whatsoever.

I hope those who have such things are listening/reading because I recently realised (I must be very reclusive) such enmities of engineering exist. Look up the word enmity because I think it describes what those who make such things must have in their minds. If that is not the case then another word springs to mind: naivety. If neither of those two words fit then try stupidity.

This speed controller will not be able to operate such turntables.

Remembering back to the days of my youth and Bardwells of Sheffield, the motors so "highly praised" for use in turntables today (and since around the late 70s) were seen in abundance in the surplus washing machine programmer basket.

They drove cams via high ratio gearboxes which switched microswitches timed appropriately to switch the different functions of automatic washers.

I remember the thought when examining the Rega Planar 3 I had just purchased in 1979 when I realised that what was driving the platter bore a strong ressemblance to a "cam motor". I think the word was audacity.

But at least it starts the platter moving and it reaches speed.



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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 21 Dec 2017 at 8:22pm
And another danger with non-self starters is motor overheating, especially if using some form of voltage boost in an attempt to start it. These synchronous AC motors have a permanent magnet rotor and its magnetism can be reduced by overheating, compounding the problem until speed regulation simply will not happen. The only solution: a new motor - and keep the voltage low if it needs hand assistance.


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 27 Dec 2017 at 9:18am
Today I will be measuring the stall current of a Premotec 9904-111-32311 motor, because that is exactly what this motor will be doing for long periods of forgetfulness when used in turntables requiring hand assistance.

When stalled because the motor is unable to run because of the load its rotor is locked.

The Premotec 9904-111-32311 motor is pathetic in my opinion as it will not even turn the load of an approx. 2 inch diameter plastic pulley!

It is priced at £42 which IMO is a total rip-off. It used to be a third of that price a few years ago. The price increase could be due to reduced demand, and I am pretty sure that is the case. Some may desire a turntable which refuses to turn without hand assistance, but I am sure the vast majority see it as poor engineering.

Locked rotor stall current is the equivalent of a transformer shorted turn, and the increase in current suggests that the stator coils are overheating.

I will report back with my findings.

(I'm actually thinking of asking Mclennan Servo Supplies what applications apply to a motor which will not start???)








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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 27 Dec 2017 at 1:31pm
And here are the results:

220V supply. Run: 6.3mA; Stall: 6.7mA (wattage 1.474 rms)
174V supply. Run: 4.9mA; Stall: 5.1mA (wattage 0.887 rms)

So we find that the Premotec 9904-111-32311 motor must have been designed to exist in a stalled state for considerable periods of time. It would be nice if Mclennan Servo Supplies were to put that on their data sheet, but they do not. Is it supposed to be some form of trade secret? Or perhaps they don't know?

The problem I found with a proprietary turntable design is that by running this motor at an elevated voltage (140V wired for 110V) via its dropper resistor meant for the Premotec 9904-111-31813 motor, that the motor's performance had suffered, hence why I'd bought a replacement.

If you look at my results above you will see that the difference between run and stall currents increased with an increase in supply voltage. It doubled. If it continued in that direction linearly, it would resut in 8.1mA run, and 8.9mA stall. Wired in parallel each winding would be required to disipate 1.246 watts and 2.492 watts in total.

However, wired for 110V this motor produces only half the torque stated for 220V (allegedly), and in use the proprietary turntable wows and flutters. Not so with a new motor wired for 220V.

In my experience most things disipating a watt or less don't get ridiculously hot, so if this motor were wired for 220V and then run at around 170 - 180V it could have better longevity - and functions well as a hand assisted turntable.

And in use the proprietary turntable wired 220V and run at 174V does not suffer wow and flutter, and sounds very good indeed. Motor noise is not noticeable and I would say it runs silently.

The only problem I can see is its use on 110V supplies, limiting its sales potential to 220-250V countries.

The controller can circumvent this problem because it can be manufactured to output 220V, and wound down to around 170 volts by its potentiometer to assist motor longevity. And therefore would need to be sold with such a controller in that case.




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Posted By: morris_minor
Date Posted: 27 Dec 2017 at 2:41pm
Originally posted by Graham Slee Graham Slee wrote:

Some may desire a turntable which refuses to turn without hand assistance, but I am sure the vast majority see it as poor engineering.
Don't the "hand-assisted" turntable makers tout a lack of torque as a beneficial feature? A massive platter, once spinning, only needing minimal assistance to keep going . . . That has a certain philosophical attraction, but I'm technically challenged, and prefer turntables without belts anyway Wink


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Bob

Majestic DAC/pre-amp
Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V
Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50
2 x Proprius + Spatia/Spatia Links


Posted By: Graham Slee
Date Posted: 27 Dec 2017 at 3:30pm
Having gotten round the motor problems we now have a red herring!

I've been very fussy about getting the 45 rpm speed bang on, especially for American customers where obtaining the correct frequency for 60Hz sync motor turntables isn't that easy.

I now realise the significance of the 45.11 rpm markings on 50Hz strobe discs, which led me to research things further.

For a 50Hz sync motor turntable 45 rpm is so easy to obtain from a quartz crystal. It just requires a 67.5Hz sine wave. And if you understand how to work with divide by n counters, obtaining 67.5Hz isn't rocket science.

For a 60Hz sync motor turntable 45 rpm is not easy at all. It requires 81Hz and it is difficult to get exactly 81Hz from all the quartz crystals which run at sensible rates, and with logic which again runs at sensible rates, by which I mean frequencies which aren't going to generate EMC problems.

So for 45 rpm, if 67.5 Hz is achieved for a 50Hz sync motor turntable, and 81Hz is achieved for a 60Hz sync motor turntable, then you have 45 rpm bang on - end of story...

Not!

Because 45s made in 50Hz countries are actually 45.11 rpm, and 45s made in 60Hz countries are 45 rpm.

The most difficult to get right, in the 60Hz USA (and other 60Hz countries), is 45 rpm, which really needs to be 45 rpm.

The easiest to get right, in 50Hz countries, just happens to be of no use, because 45s in these countries were mastered at 45.112 rpm...

Why? Because the lathes were synchronised to the mains, not a crystal.

60Hz is 3600 cycles a minute and if divided by 80 gives 45 rpm.

50Hz is 3000 cycles a minute and would have to be divided by 66.66666 recurring to get 45 rpm. For some reason they could divide it by 66.5, but not 66.6 or 66.66.

So, in the UK (and Europe etc) 45 rpm is 45.112 rpm, and in the US (and Canada etc) 45 rpm is 45 rpm.

Is it still 45.112 rpm on new pressings? Nobody knows. Probably they don't give a damn.

But the point is this. If a controller for a 50Hz sync motor turntable set to 45 rpm is used with a 50Hz strobe disc, it will not look locked... because 50Hz strobe discs are made for 45.11 rpm.



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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 28 Dec 2017 at 7:22am
I have come to my own conclusion regarding the 45.11 rpm thing, and being unable to find any official confirmation I can only put it down to the strobe disc itself.

In otherwords I don't believe the arguments for 45s being cut at 45.11 in 50Hz mains countries.

However, I can see why strobe discs for 45 rpm in 50Hz countries are only accurate when run at 45.11 rpm, and that is because of the non-integer number of bars/dots required for 45 rpm.

It would need 133.33333 (recurring) bars/dots - try doing 1/3rd of a bar/dot - can't be done, you need whole ones.

And therefore the strobe disc for 50Hz use will always show 45s running slow (the bars stepping backwards very slowly).

As for the cutting lathe sync motor requiring the 3000 cycles per minute dividing down by integers then 3000/45 is 66.66666 (recurring) and 3000/45.11 is 66.5, and neither are integers.

It could be 66 or 67 which would give 45.4545 (recurring) or 44.7761194 which is nowhere near.

If the 45.11 thing were true then they divided down by 66.5, and if they could do fractions then they could do 66.6 and 66.66, and so on.

Therefore I am leaving the frequency at 67.5Hz and so a 50Hz sync motor turntable will turn at 45 rpm, period.

And if it is proven to me that European 45s are actually 45.11 rpm, then we're only 0.2444 (recurring) percent out, but imported 45s from the US will be spot on.




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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: BackinBlack
Date Posted: 28 Dec 2017 at 9:57am
I would  agree with your logic regarding the bars on the 45rpm strobe disc. It can be done, just needs 133 bars at 2.7 degree intervals and a .9 degree space. Alternatively 400 bars at equal spacing would give the correct indication as every 3rd bar would be picked up as stationary.
Surely, the assumption that the "error" is due to the inability to divide the clock frequencies to exact integers would only apply in this digital age?
The classic Neumann vinyl cutting lathes use/d Lyrec low speed (331/3, 45 and 78 rpm) synchronous motors with 3 different rotors on a common shaft and 3 stators with windings in the same casing. They drive the very heavy turntable via a fluid coupling to reduce motor noise and cogging. Power supply to the motor was direct from the mains.
For me this does raise the question as to how accurate the speed of the orignal master is.




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Just listen, if it sounds good to you, enjoy it.


Posted By: Graham Slee
Date Posted: 28 Dec 2017 at 12:46pm
Originally posted by BackinBlack BackinBlack wrote:

Alternatively 400 bars at equal spacing would give the correct indication as every 3rd bar would be picked up as stationary.


Brilliant Clap


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: BackinBlack
Date Posted: 28 Dec 2017 at 1:30pm
Thanks, it's just numbers!
No doubt someone who is able with CAD can produce a template.


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Just listen, if it sounds good to you, enjoy it.


Posted By: Graham Slee
Date Posted: 30 Dec 2017 at 7:25pm
Tried it with my PCB CAD footprint wizard but it will only go to 99.

Instead, I bought a Dynavox strobe disc for around £14 from Amazon, which has 50Hz and 60Hz patterns. I will have a go at making a hand held high brightness LED strobe with both 50Hz and 60Hz flash rates.


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 01 Jan 2018 at 9:23pm
Earlier in this topic (or one linking to it) I promised to publish how a discrete logic frequency counter can be built using cmos logic integrated circuits.

Since developing my own I found a problem, and that was capacitance drift due to ambient temperature change.

The window in which the frequency is measured varied to the degree that the counter did not accurately display the input frequency.

All RC time based frequency counters will be the same. And as I am not able to publish a circuit that I consider accuarate, I'd rather not publish it at all.

But if such inaccuracy is tolerable, which it would not be in this application, a good design for students of electronics engineering can be found at:

http://www.electronicdesign.com/analog/period-rpm-converter-measures-very-low-frequencies" rel="nofollow - http://www.electronicdesign.com/analog/period-rpm-converter-measures-very-low-frequencies

The 74HC4040 can be replaced by the CD4040 making the circuit cmos voltage range tolerant.




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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 05 Jan 2018 at 10:17am
Last week I got hold of a 3.5W 220V Airpax motor capable of being used in a Rega turntable, so I could test the controller at what I envisage to be its maximum load.

Normally a Rega is fitted with a 1.8W 110V motor and for 230V use a dropper resistor is placed in series with the motor. Usually this is 10k and results in a current of 14mA which reduces the motor's input power to roughly 1.3W.

It is however wasteful of power as the 10k resistor consumes power too. The load across 230V is approx. 16,800 Ohms and so the total power is 3.15W with only 1.3W reaching the motor.

It would be far better to rewire the motor such that the 10k resistor is removed and power is supplied by the controller at 110V. The controller would run much cooler, and the motor's "sweet spot" for minimum vibration can be adjusted by the controller's output adjustment pot.

However, there will be some who do not want to, but they will need to fit the controller's plug instead of the mains plug, and the controller's power will be wasted as above, but it will still work.

The resulting load will be less than the 3.5W 220V motor, but I needed to try it anyway to establish the controller's capabilities.

First thing I noted was the run up speed on the Rega was halved, and the motor was much quieter than the stock one fitted.

The controller runs hot however. And this is due to the transformer saturating at 50Hz. At 60Hz it runs noticeably cooler. I've been using an off the shelf transformer and I hope to replace it with one designed specifically for the job. If that runs cooler then I will consider it job done, and we can start making them.


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 08 Jan 2018 at 10:09pm
Naming time.

Series: GramSpeed (a follow-on from the GramAmp)
Type: Synchro-Quartz
model: 3.5

Did a search for "gram speed". Should get lots of drug related search results... Wink


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: BackinBlack
Date Posted: 09 Jan 2018 at 8:51am
GramDrive ?


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Just listen, if it sounds good to you, enjoy it.


Posted By: Graham Slee
Date Posted: 09 Jan 2018 at 9:41am
Originally posted by BackinBlack BackinBlack wrote:

GramDrive ?


That's better!Smile


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: ICL1P
Date Posted: 09 Jan 2018 at 1:54pm
Originally posted by Graham Slee Graham Slee wrote:

Naming time.

Series: GramSpeed (a follow-on from the GramAmp)
Type: Synchro-Quartz
model: 3.5

Did a search for "gram speed". Should get lots of drug related search results... Wink

Why 3.5?

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Ifor
=====
Reflex M & ACCESSION M, CuSat50, Majestic DAC, a Proprius pair.


Posted By: Graham Slee
Date Posted: 09 Jan 2018 at 3:47pm
3.5 for its watts handling. But as decimal points are awkward I since decided on 35, and also Ian's suggestion. Also adding a H because it is the high voltage version. The 24V version will be the L.
So it becomes the GramDrive Synchro-Quartz 35H.

The whole point of it being a sync motor driver under quartz crystal control is for speed accuracy and consistency. I am now being asked for speed adjustment...

A crystal is not adjustable. An RC oscillator is but it will drift. A PLL would help but can still drift  slightly. The only way of adjustment is by changing the number of divider bits. This would require a rethink of the divider circuit. And anyway, what's the excuse for sloppy engineering? A good machine shop should be able to turn out accurate pulleys and hubs/platters.




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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 23 Jan 2018 at 7:33am
The next stage in development would be to go back to Ian's question regarding driving the motor phase winding.

There are two identical windings on the reversible synchronous motors and one is supplied via a capacitor and depending on which one, the motor rotates clockwise or anti-clockwise.

The capacitor produces a 90 degree phase difference and if the alternating current voltage supplied is in the form of the (expected) sine wave, then the voltage to the phase winding must be the cosine - the same but stepped a quarter further on.

The logic can be stepped to produce this difference, and so the controller will require a second output, and therefore a semi-parallel copy of the output stage. This will require a larger box and will therefore be more expensive to produce.

A number of commercial controllers do not bother with this second output, but purists would argue of its necessity, in which motor vibration would be under better control.

With just one output phase is dependent on the motor capacitor but at different frequencies the phase angle will be different to the expected 90 degrees, and it is argued this leads to vibration.

Calculations show that if an average of the two frequencies being used results in a half-way capacitor that the phase will be 16% out at both frequencies. Is this bad?

Mains capacitor tolerances are usually +/-10% and so might vary a total of 20% if many are sampled.

A motor I tried using phasing components (a resistor and capacitor) recommended for 60Hz produced noise at 60Hz and none at 50Hz. What will it do with the phase generated by a second output? We wait and see.



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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: BackinBlack
Date Posted: 23 Jan 2018 at 11:49am
Originally posted by Graham Slee Graham Slee wrote:

The next stage in development would be to go back to Ian's question regarding driving the motor phase winding.

There are two identical windings on the reversible synchronous motors and one is supplied via a capacitor and depending on which one, the motor rotates clockwise or anti-clockwise.

The capacitor produces a 90 degree phase difference and if the alternating current voltage supplied is in the form of the (expected) sine wave, then the voltage to the phase winding must be the cosine - the same but stepped a quarter further on.

The logic can be stepped to produce this difference, and so the controller will require a second output, and therefore a semi-parallel copy of the output stage. This will require a larger box and will therefore be more expensive to produce.

A number of commercial controllers do not bother with this second output, but purists would argue of its necessity, in which motor vibration would be under better control.

With just one output phase is dependent on the motor capacitor but at different frequencies the phase angle will be different to the expected 90 degrees, and it is argued this leads to vibration.

Calculations show that if an average of the two frequencies being used results in a half-way capacitor that the phase will be 16% out at both frequencies. Is this bad?

Mains capacitor tolerances are usually +/-10% and so might vary a total of 20% if many are sampled.

A motor I tried using phasing components (a resistor and capacitor) recommended for 60Hz produced noise at 60Hz and none at 50Hz. What will it do with the phase generated by a second output? We wait and see.



I think that Ernie asked the question before I got round to it, about driving each phase winding.
I note that one manufacturer uses a relay to switch between 2 different phase circuits for 33 and 45. Each phase circuit has a potentiometer trimmer to optimise the phase/voltage applied to the 2nd winding.

Ian




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Just listen, if it sounds good to you, enjoy it.


Posted By: Graham Slee
Date Posted: 29 Jan 2018 at 9:57pm
One bit of useful information this electronics guy of 40 plus years experience didn't know - because I do the electronics to make things work, usually to a brief - unless it's audio which I understand...

And I don't do electrical stuff like motors...

Sync motors are not ALL self starters. Some are - some not - don't ask me why.

It would seem that the motors like the one Rega use are made to be self starters, and a few others, but mainly they don't!

Therefore I've been doing a lot of messing about for nothing.

Shame the data sheets don't tell you!!! Shame on those who write them!!!

The internet is quite useful in digging things like this up if you search day and night. Books are bloody useless!

So now you know. And the motors which do self start have some additional "items" which make them start, like the sync motors used in clocks. Those in washing machine controllers use a clock spring...


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Ernie
Date Posted: 29 Jan 2018 at 10:50pm
A Shaded pole which is a copper ring around one of the poles, creates a weak rotating field (current is induced into the ring slightly out of phase to the main winding, which is fine if the required starting torque is low. You’re basically using inductance rather that capacitance to shift the phase angle.

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There are 10 kinds of people. Those who understand binary and those who don't.


Posted By: Graham Slee
Date Posted: 30 Jan 2018 at 8:29am
Originally posted by Ernie Ernie wrote:

A Shaded pole which is a copper ring around one of the poles, creates a weak rotating field (current is induced into the ring slightly out of phase to the main winding, which is fine if the required starting torque is low. You’re basically using inductance rather that capacitance to shift the phase angle.


Understood, but don't you think the data sheets should mention the inclusion?

It has become rather an expensive job buying all these motors to find some self-start whilst other types just sit there vibrating (correct rotation capacitor fitted).

And here's another observation: the higher the power rating the greater the hand assistance to get them to spin the platter.

Apart from the few turntables which are produced in quantities of a few a week, what else could use these motors - the ones which don't start that is?

And what are the techniques used to start them. This seems to be an official secret!

So far it has been ascertained that the Airpax 9904-111-31104 and the Pemotec 9904 111 31813 self start, but all the others tried do not start, including some Hurst motors BAK helped me obtain.

There is nothing on any of the data sheets to suggest they will self start or not. This seems to be a bit of a rip off!

The capacitor is there to cause the sine wave to lead or lag (depending on the way you look at it) one coil before/after the other, which sets the direction of rotation. In an induction motor this also starts the motor, but not with synchronous motors, but some do. So does that suggest some have one of these poles (the copper ring) and some don't?

I would have thought it was important for the data to state this? Or perhaps the distributors don't have a clue?



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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Ernie
Date Posted: 30 Jan 2018 at 9:47am
Hi Graham,
Have to agree with you. Just looked at the Airpax data sheet Clear as mud and looking at the drawings I wouldn’t have thought it was self starting. Maybe there’s enough reluctance to get it to spin off line but is there enough torque to start a turntable platter?



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There are 10 kinds of people. Those who understand binary and those who don't.


Posted By: ICL1P
Date Posted: 30 Jan 2018 at 3:39pm
I wonder what motors Analogue Works (Divine Audio) use in their turntables. They are always on with platter start and stop done by hand.

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Ifor
=====
Reflex M & ACCESSION M, CuSat50, Majestic DAC, a Proprius pair.


Posted By: Graham Slee
Date Posted: 30 Jan 2018 at 5:34pm
@Chris: Amazingly the two motors will self start one of those replacement acrylic platters on a Rega Planar 3. The smaller standard motor does it within 15 seconds and the larger 3.5W motor in 7 seconds. And by increasing the load slightly by applying a finger to the edge of the platter they still start but it obviously takes longer. They should easily accomodate a heavier platter. There must be something about these motors which assists self starting but it would be a rather expensive do buying one to destructively take it apart to find what?

@Ifor: Using a proprietary hand start turntable loaned to me for R&D purposes the standard motor is a Philips 9904 111 32311. I had to fit an identical replacement because it was originally using a dropper resistor intended for the Premotec 9904 111 31813 which gave it 138V instead of 110V. It could have been that the voltage was too much for it, especially with no breeze to cool it... (no names, no pack drill).

@both (and all): Fitting a 2W Hurst 110V sync motor to the proprietary turntable the result is far worse, taking a considerable spinning up effort. Otherwise the motor sits there vibrating, and changing phase capacitors between 4uF down to 0.8uF (Mfrs value 0.25uF) results in no improvement apart from vibration being much less the lower in value the capacitor goes.

Conclusions:

1. the capacitor has little (or no) effect on starting except for reducing vibration.

2. the capacitor fulfils the requirement for rotational direction and little else.

3. given the above, a separate (cosine) output to drive the phase coil would seem to be wasted effort.

4. it is suggested that a sync motor might be started by ramping up from a very low frequency and in such case transformer coupling can be ruled out. It would have to be a 315V p-p amplifier for a 110V motor. Also there would need to be feedback to detect if the motor was actually ramping up in revs. This precludes quartz control which is fixed frequency and the oscillator's final frequency being RC defined would be subject to large tempco fluctuations; unless it switched over to quartz control after confirmation of being up to speed. Comment: very complicated to achieve and therefore very costly.

5. spinning up by hand using a sync motor which is "happy" to remain in a stalled condition without resulting in damage or danger seems to be the best option. Here again this is something the data sheets do not delve into.

6. or use one of the motors other manufacturers have found to self start from empirical investigation (because the data sheets never say).

7. In either of the cases (5 or 6) ensure controller power isn't wasted as heat; which means: a, rewiring the
Premotec 9904 111 31813 to operate on 110V instead of 230V (UK, EU etc) where the 10k (6k8 on data sheet) resistor wastes power; b, run hand spun motors within their voltage rating and not at elevated voltages which investigation suggests causes motor damage.



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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 31 Jan 2018 at 1:16pm
Give an engineer ALL the information and he can do his job.

Tell the engineer that these motors are el-cheapo 24-pole can-stack construction permanent magnet stepper motors, and he can design something suitable that cuts through the great hifi misinformation society (to the tune of "the self preservation society" please... something seems a bit criminal here like £77 for an el-cheapo stepper motor).

Like I have said before, the data sheets tell you nothing.

What told me exactly what they are was taking two motors apart. What a pity that the customer/user has to do this??

What was found is pictured by this drawing at http://www.ibiblio.org/kuphaldt/electricCircuits/AC/AC_13.html


image reproduced here for educational purposes (I will take it down if objections are raised)

The reason why the larger diameter motors self start (using a capacitor to step 90 degrees'ish) is probably down to the finger spacings alone.

Photo to be uploaded when my camera battery charges.

Steps back 27 years and I hear a manager saying "I thought you being an electronics engineer you would know that?", to which I replied "how many branches are there to electronics engineering?". What I would have liked to have said was: "you are BSc in electronics engineering so why can't you design all these broadcast audio circuits you get me to do!?"

The above being mentioned in case anybody reading this thinks I'm a bit thick.


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 31 Jan 2018 at 1:43pm
Now, having said the above, can we really gain anything by driving these stepper motors as stepper motors?

At 50, 60, 67.5 or even 81 Hz they are "join the dots" and I can tell you now, having used CNC XYZ positioning using precision stepper motors, there will be vibration - yes, even by applying the correct phase intervals. The maximum torque being applied when the motor is stood! And for good reason in locking the position for a process, usually in the X dimesion, to complete. Think of a CNC printed circuit drilling machine using a 0.4mm tungsten carbide drill?!

Put simply, they cog, and the belt plus platter inertia smooths this out.

So all the things hi-fi sync motor "control" does is really a job of cheating the application. For example: nudging the phase difference to lower vibration; making one phase voltage different to the other; reducing phase voltages altogether.

An original case of "it'll do" back when it would do, now being touted as a great solution because it gives manufacturers things to do to make money.

All hail Technics...



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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 31 Jan 2018 at 7:15pm
Here are the views of the insides of the Premotec 9904 111 31813 (larger) and Philips 9904 111 32311 (smaller) stepper motors.

inside stepper motors


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps


Posted By: Graham Slee
Date Posted: 07 Feb 2018 at 9:29pm
Quite incredibly the 3.5W 220V Airpax motor I bought and fitted to my old Rega Planar 3 turntable is now running at 89V!

And even more miraculous it goes from stop to synchronous speed in 3.5 seconds!

Why bother running it at 220V when it behaves perfectly at 89V?

Or did they badge up a different motor wrongly?

It can be obtained at less than half the price of the Rega motor... Wink


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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps



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