LP12 a.c. motor voltage questions

Hi folks,

I'd like to get a PSU for my LP12 so I can play 45s. I've looked around a lot at what's available and am half toying with the idea of designing one. I'm an electronics engineer by trade and making a board that would produce a couple of sine waves from a DAC would be simple for me.

What I'm not clear about is exactly how the motor voltages need to work. It looks like you need two a.c. voltages out of phase going into the red and blue leads and grey is 0V. The a.c. voltages, I think, are 85V rms,

However, looking at the Valhalla schematics it looks to me like it's not a splt rail supply, but simply rectified mains. (The schematics I've found are not great quality though.)

I'm guessing it's producing a sine wave of around 0-240 pk-pk and then just putting it through the cap to produce the phase difference bwtween red and blue. Is that right?

If so, I guess there's no advantage in a split rail of roughly +/-120Vdc and I should just create two sines 0-240V ish?

Am I mad thinking a better idea would be to produce two sines and have the micro set the phase between the two?

Thanks!
 
I should probably reconfirm that for me, creating the sine wave is simple (having done software to control three phase motors in the past). Amplifying the sine wave is fairly straightforward too, it's the exact waveforms needed I am unsure of in terms of whether I can simply produce two sines of 0-Vx that are 90 degrees out of phase, where (I think) Vx is root 2 times roughly 85 times 2 which amounts to about 240V. I'd use 230 * root 2 (ie rectified mains) as the rail for the push/pull outputs driving the motor and connect greys to 0V.

Tweaking the size and phase in software is a sinch and I would use an STM32 with a USB interface so I could tweak over a vitual com port. I wouldn't used an Arduino, I'd knock up a board with the analogue circuitry AND the micro.

The trickiest part once I am happy my above assumptions are correct is getting the push pull output drives right, but even that isn't massively difficult.
 
Fair 'nuff, only a suggestion.
Sometimes it's not worth making something where a ready made solution is available.
With the ability to ramp speed, set speed, set current limit (to avoid blowing the chip) and if required set full power for starting & reduced power for (possibly quieter) running, it's a pretty neat chip / ready made board.

What is an LP!2? Google doesn't find it for me.
 
Motor control is traditionally done with PWM, for a multiphase AC motor multiphase PWM is used. Analog waveforms are avoided due to all the expensive parts needed to handle them like amps and heatsinks.

Motors are highly inductive so PWM voltage can give very smooth AC current waveforms.

However you have to worry about the switched waveforms causing interference - choosing an ultrasonic PWM frequency is a first defence, and shielding the motor cabling is wise.

Stepper motors are not the solution, the mechnical vibration and noise from them is large and smack bang across the audio band - the degree of mechanical isolation required would be tricky to achieve, whereas synchronous AC motors are very quiet.
 
Fair 'nuff, only a suggestion.
Sometimes it's not worth making something where a ready made solution is available.
With the ability to ramp speed, set speed, set current limit (to avoid blowing the chip) and if required set full power for starting & reduced power for (possibly quieter) running, it's a pretty neat chip / ready made board.

Yeah, you're not wrong, it's just now I've started looking into it, the engineer in me wants to know how it works!

What is an LP!2? Google doesn't find it for me.

:p

It's made by L!nn, have you never come across it before?

It seems I can't edit thread titles. :rolleyes:
 
Last edited:
The stepper driver outputs 2 PWM sine waves at 90 deg.
It is used for smoother running of stepper motors.
However, it should be equally suited to providing 2 motor phases for a normal induction motor.
I am a duffer at programming but even I managed to program 2 speeds with around 2 second ramp up and down.

Program speed to give 45 RPM by trial & error checked with a strobe.
Program 33.33 RPM to be (45 RPM speed / 0.74).
Then you have only one speed value to set.

If you're a musician you could even incorporate pitch control.
 
The stepper driver outputs 2 PWM sine waves at 90 deg.
They use constant current control with feedback to control the switching,
and step through a table of current drive levels. Steppers are current controlled, not voltage. Steppers have separate windings so multi-phase PWM wouldn't be needed anyway.

It is used for smoother running of stepper motors.
Microstepping indeed makes it smoother, but its still very noisy and all over the audio band.
However, it should be equally suited to providing 2 motor phases for a normal induction motor.
That's an interesting idea, but you presumably mean a synchronous motor, not induction motor? They are designed for mains voltage, not low voltage current control however, stepper drivers go up to 60V or so using individual chips, beyond that requires an industrial stepper driver, expensive. So you'd need to rewind the motor for a lower voltage / higher current.
I am a duffer at programming but even I managed to program 2 speeds with around 2 second ramp up and down.

Program speed to give 45 RPM by trial & error checked with a strobe.
Program 33.33 RPM to be (45 RPM speed / 0.74).
Then you have only one speed value to set.

If you're a musician you could even incorporate pitch control.
Some stepper driver chips use frequencies in the audio band, note, some have variable frequency output (hysteresis switching aka bang-bang control).
 
Microstepping indeed makes it smoother, but its still very noisy and all over the audio band.

I think you would be surprised at what passes for acceptable designs, even in the commercial space. The VPI HW40 Direct Drive uses a BLDC motor driven with block commutation using PWM at 10kHz and no filtering. I measured ~180mVPP into a 47KΩ load directly below the table and almost 3mVPP into a MM cartridge over the groove area but not lowered (resting on the queuing arm).
 
Just bumping this as I am still unclear on:

1. Whether producing two 90 out of phase signals is better than one that is put through a cap. (Or indeed even possible.) I am guessing it is because Pyramid's little board seems to produce multiple outputs.

2. Am I right about the voltages needed yo drive the motor? Can I supply 85Vrms into red and another 85Vrms into blue that is 90 degrees out of phase with red and have the greys grounded?

Pyramid, I did look at your design, but those power amps boards are not available any more. I have found an excellent little dual power amp from Apex that looks like it should do the trick that I could run from around +/-130Vdc and have worked out a good, stable dc supply from LM317/337.

I want to have a go at this for the self satisfaction. Although I could save myself some time getting other bits and cobbling them together I fancy doing a single board solution. If I really wanted to save time I'd nuy a Hercules! ;)
 
Just bumping this as I am still unclear on:

1. Whether producing two 90 out of phase signals is better than one that is put through a cap. (Or indeed even possible.) I am guessing it is because Pyramid's little board seems to produce multiple outputs.

Synthesizing 2 signals is preferable; you can never get to 90° with a cap and sometimes the optimum operating point is greater (or less than) 90°.

2. Am I right about the voltages needed yo drive the motor? Can I supply 85Vrms into red and another 85Vrms into blue that is 90 degrees out of phase with red and have the greys grounded?

Yes this will work, although the motor will consume more power than if driven by a single phase and a cap. It will also generally have more torque than a single phase drive.