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Turntable Speed Control

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dave Friday Quote  Post ReplyReply Direct Link To This Post Posted: 21 Nov 2017 at 12:21pm
Ta,but...Lin have mechanical speed adjustment ( on the motor pulley)..
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post 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.



Edited by Graham Slee - 22 Nov 2017 at 8:50am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post 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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