Optimally driving a (VPI) synchronous turntable motor - diyAudio
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Old 10th August 2016, 10:29 PM   #1
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Default Optimally driving a (VPI) synchronous turntable motor

Driving a synchronous turntable motor needs two signals 90 degrees apart, generated with a stable 50Hz/110V or 60Hz/230V frequency depending on the country where you live.
The simplest way is to use the local mains frequency and a capacitor to do the phase shifting for the second phase.
The mains frequency however is most of the time not being a perfect sine shaped wave and may be further polluted by many high frequency peaks caused by domestic equipment, apart from the fact that the mains frequency is jittering around the nominal frequency caused by varying loads in your local area.

Stable phase shifting by means of a capacitor works only when 1) the input signal is a perfect sine shaped signal and 2) with a non-varying load. None of both is true when feeding the motor directly from the mains supply.

To have a better situation to start with, VPI offers the SDS, a device that is producing a stable sine shaped signal to directly drive the motor, thereby isolating all problems that comes with the mains supply.
However, phase shifting needed for the second motor drive voltage is still done by the same capacitor as before. Since the load that the motor presents to this capacitor is not stable, the second phase will be a jittering signal leading to torque changes and hence to rumble.


Linn with its well praised LP12 turntable, uses the Lingo power supply to provide the two motor phases electronically, thereby ensuring a perfect constant torque motor drive, preventing the need to use a super heavy platter to damp the motor vibrations.
That a capacitor is not the ideal component for making the second phase can be shown with a so called Lissajou figure, by offering one phase to the X input and the other phase to the Y input of a scope.
Two sine shaped signals, each 90 degrees apart, will result in this way in a perfect circle.
Zero degrees phase shift will result in a straight line, tilted 45degrees and all phases in between will result in an oval shaped, but only if the phase shift between the two phases is constant all the time.
If the phase shift is not constant, it will result in a distorted oval or circle.

So lets have a look at the Lissajou figure of the phases directly on the motor driving a VPI Scoutmaster.

220V-Condens.jpg

So what you see is a distorted oval, telling two things. Because it is not a circle but an oval means that the phase shift is not 90 degrees, causing the motor to have less torque.
And because of the distortion of the oval it is obvious that the phase shift jitters, and so does the motor thereby causing rumble.
The solution is of course the way Linn did it by supplying the two phases instead of just one, and to get rid of the dreaded capacitor used for this function.

Block Diagram.jpg

The 50/60 Hz generator can be made in many ways, to give a few examples:
- precision sine wave generator
- digitally, starting with a Xtal osc, dividing to 50 or 60Hz, then bandpass filtering to get a sinus shaped signal
- using a uProcessor and let the internal D/A convertor generate a sinus
In my case I used a Project Speed box that is using the third example and generated a signal of 50Hz and 67.5 Hz for 33 1/3 and 45 RPM, but it will be pretty easy to use some low cost uP and do the programming yourself.
Depending on the quality of the signal you may have to add some filtering.

Then comes the phase splitter.

Phase Splitter.jpg

The frequency where the phase shift is 90 degrees is 1/(2 x Pi x R3 x C1).
In the example above this is 50Hz.
By changing the value of R3, you can alter the 90 degree phase shift point to your need.
You could even add a second resistor plus a switch parallel to R3 to be able to switch between 33 1/3 and 45 RPM.

Gain of this splitter is 1X, so both phases have the same amplitude. It is therefore wise to have a signal coming from the generator that can be varied in amplitude, this will affect both phases.

Next in line are the 2 amplifiers steering the turntable motor.
You could make your own design for a 110V or 230V amplifier, which can be done, but it is not the easiest way.

What I did was to use a cheap (second hand) stereo amplifier and feed it with the signals from the phase splitter to get an output signal of 15 Volt, by varying the amplitude of the generator output.
The output of the stereo amp is fed to two small 15V/230V toroid trafos that are directly feeding the turntable motor.

Again displayed as a Lissajou figure, this is how it looks.

two-phases.jpg

Hans
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Old 11th August 2016, 12:06 AM   #2
TomWh is offline TomWh  United States
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Great post Hans

I will get educated on the options and build one of these. It will be fun to see what it does with the 4 different Hurst motors I have.

Thanks Tom
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Old 11th August 2016, 12:34 AM   #3
TomWh is offline TomWh  United States
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Hi Hans

Was looking at the Project speed boxes looks like they want low watt motors. Yours has a amp before the step up tramformers so how many watts can it put out?

The reason I ask is the one Hurst motor is a 14 watt motor. Also could I come off my phoenix eagle which is good for 15 watts and split there?

Thanks Tom
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Old 11th August 2016, 09:10 AM   #4
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Hi Tom,

I used a Quad 303 Amp for my project, and used 2 toroids each 15W 15V/2x115V.
This is beyond in power for what I needed, but I never cared to scale it down.

Hans
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Old 11th August 2016, 11:45 AM   #5
martyh is offline martyh  United States
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Hello Hans,
what do you think about using a commercial amplifier with 100V outputs to directly drive the motor?
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Old 11th August 2016, 12:35 PM   #6
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Originally Posted by martyh View Post
Hello Hans,
what do you think about using a commercial amplifier with 100V outputs to directly drive the motor?
Any amplifier capable of generating a high enough voltage and supplying at least 100mA will do.
A motor normally running at 110v rms will probably work at 100v rms.
Give it a try, and if it works you won'tneed the trafo's.

Hans
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Old 11th August 2016, 12:51 PM   #7
Pyramid is offline Pyramid  United States
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You would need a very large amp to directly drive a 115VAC motor. 115V RMS is 325VPP so you would have to have 163VDC rails minimum. This would equate to a 1.6kW amp into 8 Ohms. You could drive the motor at a lower voltage (75VAC), but it will most likely have trouble starting a heavy platter, especially at 45 RPM. 75V RMS is ~700W into 8 Ohms.

I've completely given up on Hurst motors for turntables, I think they are ill-suited for the task. All of the Hurst motors I have vibrate terribly, and some of them would not run from a dual phase supply without modification. The Premotec motors behave better in all aspects, but are very low power (torque). I've been using a BLDC motor with 3 phase drive and it is head and shoulders above the Hursts:

BLDC Turntable motor
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Old 11th August 2016, 02:00 PM   #8
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Originally Posted by Pyramid View Post
You would need a very large amp to directly drive a 115VAC motor. 115V RMS is 325VPP so you would have to have 163VDC rails minimum. This would equate to a 1.6kW amp into 8 Ohms. You could drive the motor at a lower voltage (75VAC), but it will most likely have trouble starting a heavy platter, especially at 45 RPM. 75V RMS is ~700W into 8 Ohms.

BLDC Turntable motor
There are many PA amplifiers being able to supply these high voltages. However they are meant to be driving the speakers through local step down transformers, their power output is just average.
The advantage is that you can attach many small speakers and the line from the amplifier to speaker can be relatively thin.
Such amplifier could without a doubt drive a turntable motor.

Hans
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Old 11th August 2016, 02:21 PM   #9
martyh is offline martyh  United States
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Thanks Hans,
I have verified that the hurst motor that came with my VPI HW-19 will start up on 100 VAC and the standard phasing cap. It is an early platter with lead filling so pretty heavy compared to the latest stuff. When I run it at a lower voltage it seems to help with the vibration. I ended up building a separate motor pod though as the vibration never got to the level where I could not detect it in the plinth when the motor was attached to the base.
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Old 11th August 2016, 04:04 PM   #10
Pyramid is offline Pyramid  United States
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I hadn't considered PA amps which are available in 70VRMS and 100VRMS. The 70V version would most likely be too low to drive a motor, but 100V might work. Both are rated at peak output, not continuous levels that would be needed for a motor, although the motor power is considerably less than most of these amps are rated for. Most of these amps use internal step up transformers, so I'm not sure what you are gaining by doing this? There are directly coupled 100V PA amps, but they are considerably mroe expensive and usually high power (250W or more). To each his own I guess.
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