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Turntable Speed Control

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Topic: Turntable Speed Control
    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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Ernie View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ernie Quote  Post ReplyReply Direct Link To This Post 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.

Edited by Ernie - 15 Nov 2017 at 2:21pm
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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post 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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Ernie View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ernie Quote  Post ReplyReply Direct Link To This Post 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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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post 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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Dave Friday View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dave Friday Quote  Post ReplyReply Direct Link To This Post 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.
lp12,oc9mk3,ca610p,krimson40watt pa,kef105.4
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Ernie View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ernie Quote  Post ReplyReply Direct Link To This Post 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.
There are 10 kinds of people. Those who understand binary and those who don't.
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