Turntable Motor Speed Control

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vinylnvalves
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#1 Turntable Motor Speed Control

Post by vinylnvalves »

I know that my turntable could benefit from an improved AC supply to the turntable motor, I was loaned a wave mechanic some time back and the skeptic in me heard something, but it wasn’t £600 better, so didn’t buy one. I missed out on the group buy for one of these https://www.diyaudio.com/forums/swap-me ... u-set.html. One is now for sale on DIYaudio, so was thinking of buying it. The principle I am happy with playing a sine wave into an amp that then drives turntable’s synchronous motor. Lots of TT’s use something similar with 16v motors. Both of my turntables with synchronous motors have 240v ones. The suggested route is to use a mains transformer in reverse to step up the voltage to 240v. What pitfalls does anyone see with this approach?
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rowuk
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#2 Re: Turntable Motor Speed Control

Post by rowuk »

So you are suggesting a 12V 50Hz or 60Hz tone generator that you step up to 240V using a transformer backwards? Essentially a UPS without batteries.

I would suggest that a transformer is lossier in that direction and that a 50Hz sinewave may look very much different depending on the core winding geometry...

I breadboarded a valve amp and used a transformer backwards. 230V-> 12V -> 12V->230V actually resulted in 212VAC due to losses. As I rectified the result, it was OK for that use.
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izzy wizzy
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#3 Re: Turntable Motor Speed Control

Post by izzy wizzy »

This might be helpful for ideas. This is the Alphason Sonata PSU. Many of the AC motors were run at much lower voltage to reduce vibration. I think a very similar thing was used in the Linn Valhalla PSU and the Rock TT. All this from memory. I have the PCB layout drawn by hand too but it might take some "interpreting". Could be a bit old fashioned these days, maybe.

alphasonPSU.jpg
vinylnvalves
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#4 Re: Turntable Motor Speed Control

Post by vinylnvalves »

Thanks for the comments, there is no free lunch then, my hunch that the step up transformer hysteresis and losses will affect the lovely sine wave coming out of the tone generator. The comment about UPS’s is interesting they are dirt cheap as a mass produced commodity.. however I would image they have a chopped square wave, if the VFD attached to my cnc router is anything to go by.
Linn’s solution had a Wein bridge oscillator arrangement from memory, saw that one and liked the idea, those chip amps can output 60W’s more than enough to drive any of my turntable motors. I have the same motor arrangement as the hyperspace on my rock, as Tom Fletcher convinced me the minimal motor to make up for losses was the way to go, would imagine if I dropped the voltage it wouldn’t even turn.

Maybe a mechanical solution adding a ring to the outside of the platters in something heavy to improve the platters moment of inertia would offer a benefit. I remember hearing the Annalogue with and without the wave mechanic.

Before Mark suggests the rim drive.... I haven’t come across one I cannot hear with a stethoscope.. The number of Garrards I hack up in the 80’s trying different plinths to get a rumble free one, I don’t want to think about as I would cry as I could have had an Annalogue now if I hadn’t butcher them.
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#5 Re: Turntable Motor Speed Control

Post by IslandPink »

How did you know I was building up to a post about rim-drives ? :)
My test is can you hear anything on the music, not a stethoscope. The Verus is an expensive product though, granted. I have no hands-on experience with Vic's DC pod though. In any case I would never go back to the weedy N.A. AC motor, the bass was transformed. I would have thought that if you're an old rocker prog-rocker, it would be a no-brainer as the Americans say .
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rowuk
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#6 Re: Turntable Motor Speed Control

Post by rowuk »

I think that the assumption that a sine wave is necessary for a synchronous motor is not backed up by anything. A 50/60Hz generator that puts out 6V. 12V, 15V should not be a big deal. There are enough cheap class D amplifier modules that should work fine. With a stepup transformer to get the voltage in the ballpark would be a relatively cheap experiment. An oscilloscope could provide confirmation what the waveform looks like. You could try out a toroid or a simple EI. Maybe it is enough?
A valve amp with the right output transformer would probably be the most efficient way to do this...
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JamesD
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#7 Re: Turntable Motor Speed Control

Post by JamesD »

I'm going to go out on a limb and suggest a hybrid stepper motor run in micro stepping mode - these have the most precise and smooth control of variable rotory speed and precision and with modern motors they do not suffer from clogging at all. Have these driving a mylar belt for the most accurate and precise control either open or closed loop of a belt drive turntable.

Effectively this would be an up-to-date direct drive motor system driving a turntable through a belt - best of both worlds?

Oh and it was used on 1" broadcast video tape machines for its low noise and precision so its a 40 year old concept.

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izzy wizzy
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#8 Re: Turntable Motor Speed Control

Post by izzy wizzy »

The Versus rim drive gizmo I understand has a torque setting and changing that can increase drive but also introduce motor artifacts. It's a trade off ... well from what I heard when listening to one. Is that the case Mark?

I use a 1cm mylar streamer belt but it's not easy on a parallel pulley as there's no give and I don't use a crowned pulley for centering. Just BTW if that was up for consideration. The splice ain't easy either.

A bit more info for the ideas pot https://galibierdesign.com/eeny-meeny/
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Nick
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#9 Re: Turntable Motor Speed Control

Post by Nick »

I'm going to go out on a limb and suggest a hybrid stepper motor run in micro stepping mode
That does though start to look very much like a synchronous motor :-)
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pre65
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#10 Re: Turntable Motor Speed Control

Post by pre65 »

Nick wrote: Sat Aug 08, 2020 12:43 pm
I'm going to go out on a limb and suggest a hybrid stepper motor run in micro stepping mode
That does though start to look very much like a synchronous motor :-)
Or a hybrid synchronous stepper ?
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Nick
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#11 Re: Turntable Motor Speed Control

Post by Nick »

pre65 wrote: Sat Aug 08, 2020 1:17 pm
Nick wrote: Sat Aug 08, 2020 12:43 pm
I'm going to go out on a limb and suggest a hybrid stepper motor run in micro stepping mode
That does though start to look very much like a synchronous motor :-)
Or a hybrid synchronous stepper ?
Don't know, its been some years since I was directly involved in machine control.
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vinylnvalves
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#12 Re: Turntable Motor Speed Control

Post by vinylnvalves »

This thread started as advise on the idea of using the transformer to set back up to 240v, on a ready built unit I can get for £50. The transformers seemed to be the weak link, so maybe I need a different lower voltage motor, you can pick up a Pro-Ject one new for £40, so that’s one approach.

Mark has thrown his hat into the ring with the rim drive option... I have re equated myself with this, maybe it has legs for the Hyperspace as the top platter is decoupled from the main flywheel part of the platter, but it needs to be a vibration free drive.

Others have suggested reducing the voltage reduces the vibration. From my limited work with electric machinery (only upto 75hp) the more poles and 3 phase definitely help reduce vibration, so logic would suggest more poles and more phases.
Stepper motors and servos I have on my cnc equipment, had a play this morning with a spare motor and driver the 400 steps per rev motor with a micro stepper driver isn’t vibration free, shames are stepper and servo drivers are cheap for the technology you get.

I had seen this https://www.diyaudio.com/forums/analogu ... drive.html.
Which is a slightly more sophisticated approach to the first solution I shared, it basically gives you a 3 phase supply. With the right ac synchronous motor this would give a low vibration solution, which could be used with either a belt or rim drive. Going back to Marks rim drive, on the Transfi website, Vic started with a simple rim drive using DC motor before he ended up with the Teres one which Mark has; which according to the website utilises a 3 phase supply. So it looks like if I get the right low voltage AC motor and drive it with a synthesised 3 phase AC wave form, that’s as good as it gets. Now getting the right motor is the hard bit....
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#13 Re: Turntable Motor Speed Control

Post by IslandPink »

izzy wizzy wrote: Sat Aug 08, 2020 12:32 pm The Versus rim drive gizmo I understand has a torque setting and changing that can increase drive but also introduce motor artifacts. It's a trade off ... well from what I heard when listening to one. Is that the case Mark?
I struggled to hear any downside, in terms of noise, to having the higher torque settings, I must admit. Perhaps if my system had been more hum-free I might have heard something ?
I'm at either full torque or one or two steps down. I have a friend Mike ( Carrick ) in Essex who uses one with a Teres, and I think he uses about 75% of full torque.
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vinylnvalves
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#14 Re: Turntable Motor Speed Control

Post by vinylnvalves »

On the teres website some interesting stuff, when you filter out the sales bull*it. https://www.teresaudio.com/manuals/teres_speed_tech.doc. The motor looks like a permanent magnet synchronous motor, which are similar to permanent magnet stepper motor in construction. A stepper motor is a synchronous motor, if driven so. So maybe James suggestion has merit, I think my trials using the microstepper driver which is a pulsed signal which is great for positioning on the CNC, isn’t representative of driving a stepper motor at a constant speed.
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#15 Re: Turntable Motor Speed Control

Post by vinylnvalves »

I know I had seen something similar. ..http://www.altmann.haan.de/turntable/

I think I may disagree with him over using a 7.5 deg stepper... just use a higher frequency supply with a 1.8 deg one. What would be the challenge if your synthesising the supply to use a supply running at a few kHz, instead of 50Hz?
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