Variable frequency drive as a replacement for VPI SDS? - diyAudio
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Old 16th September 2014, 10:42 PM   #1
hopat is offline hopat  United States
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Default Variable frequency drive as a replacement for VPI SDS?

Hello,

I got my hands on a used VPI Super Scoutmaster, a few years back, but it came without the SDS, which in short means that there is no 45 rpm option on this table. I have become increasingly interested in being able to play 45 rpm records, but simply can't afford the SDS option. In an attempt to find an alternative, I came across the term variable frequency drive(VFD), and it seems that these industrial components are very similar to the SDS in function (though probably less user friendly to set up, and less visually integrated with the table). That being said a VFD can be had for a small fraction of the SDS's price. So here are my two questions...

1) Could a VFD be used to add 45 rpm functionality to my table without compromising sound quality?

2) Are there commercially available versions that could also perform the SDS functions of dropping voltage after the table gets up to speed, or producing a sine wave that is accurate to .01 Hz?

My budget for the switch would be in the $150-$250 range, and I would really like to keep things as cheap as possible. In order to gain SDS like features, I would be willing to consider waiting and saving up for a $400 alternative. Here is a link to an example of a drive that I am considering <http://lsdrives.com/ig5a/>. I would obviously be interested in the lowest powered 0.5 horsepower option as I understand it to be overkill already.

Thanks for the help!

Last edited by hopat; 16th September 2014 at 10:45 PM.
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Old 16th September 2014, 11:46 PM   #2
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those are 3 phase drives..
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Old Yesterday, 12:06 AM   #3
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Yes they're three phase but most will quite happily cope with a single phase load as long as you tell the control electronics that the imbalance is acceptable.

The downside is that they put out a terrible waveform and are electrically noisy.

The further downside is that the phase shift capacitor works for one frequency only so if you use the correct value for say 60Hz / 33 RPM it will be wrong for 81 Hz / 45 RPM.
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Old Yesterday, 12:33 AM   #4
hopat is offline hopat  United States
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There are one phase versions by other manufacturers, and those are actually cheaper. Would that solve the problem, or would they still put out an unacceptable waveform?
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Old Yesterday, 12:57 AM   #5
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The ones I've used put out a stepped wave and use the inductance of the motor to provide rudimentary filtering.

I believe there are quasi sinewave VLTs but they are more expensive.
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Old Yesterday, 04:05 AM   #6
hopat is offline hopat  United States
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So, I guess I have two questions then. First, do you think that feeding the turntable motor a stepped wave would cause it to function poorly or wear prematurely? It was my understanding that the precursor to the SDS, the VPI PLC, would simply output a square wave, so I'm not sure that their motors require a good waveform. That being said, I'm a real novice on this stuff, so I could be really off on that. Second, is there any commonly available device that would allow me to vary frequency relatively cheaply? I was hoping to find something that would act as a switch, but if I can't find anything at or under that $400 price point then I might have to go back to my old table for a few years, when I'll hopefully be able to afford the SDS. I'll try and look into quasi sine wave VLTs in the meantime.
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Old Yesterday, 06:10 AM   #7
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1. Yes, the waveform is important. The SDS and its ilk filter the waveforms to make a sinewave.

2. There are plenty of cheap frequency generators around. I once built one for about $50 using an ipod playing a pair of sinewaves, a pair of tiny chip amps and a couple of step-up transformers.
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Old Yesterday, 07:01 AM   #8
hopat is offline hopat  United States
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I was starting to move in the same direction myself, although I hadn't figured out the transformer solution yet! If you have any additional information on the aforementioned project, I would love to get my hands on it. In the meantime I'm going to start looking at amps chips and small signal generators. I think I might barely be able to pull this off...
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