The Official Turntable Drive System/Motor Thread! - diyAudio
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Old 26th October 2010, 06:33 PM   #1
gtyler is offline gtyler  Canada
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Default The Official Turntable Drive System/Motor Thread!

Hi All,

I have been working on a TT design for some time now and I quickly realized that the most difficult step was designing a good drive system. As I have searched I have noticed that there are many people who are also looking for a good DIY turntable drive system, some to upgrade existing tables, some for new designs. There are some great ideas out there, but they are all over in different threads and sites, so I thought I'd start this thread that is specifically dedicated to TT drive systems.

Please, if you have ideas for DIY drive systems post them here. Maybe you have a schematic for a nice variable frequency drive (VFD) that could drive synchronous motors, or maybe you have a feedback system that could be used on encoded DC motors, or maybe you have some way of using old VCR drive motors. Whatever your plans, post your links, and share your idea here so everyone can benefit.

Also, if you are looking for a specific kind of drive system and need some help, post here and hopefully the collective knowledge of the followers of this thread can help.

I hope that this thread will become the "one stop shop" for those looking for TT drive systems.

Thanks for your contributions.
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Old 26th October 2010, 10:45 PM   #2
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I have something I posted in another thread earlier - it derived from me making some initial plans on how to go about driving a turntable controller I'm planning to build easily and cheaply and I needed to figure out some ratios, resistor values for feedback circuits etc...

This would be used in conjunction with a Buck converter in my design, but it's fairly multi-purpose if you want to go about using another regulation method.

http://liammartin.com/Misc/TTCalc/Tu...river_Calc.xls

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Old 26th October 2010, 11:16 PM   #3
gtyler is offline gtyler  Canada
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Thanks Psychobiker for the input.

Because I am the creator of this thread, I though it only makes sense that I also share some of my ideas ideas. After all the searching that I have done I have decided to with a drive system inspired by the Altmann DIY turntable but just a little more sophisticated. The Altmann DIY Turntable

I am going to use a Hurst 7.5 deg stepper motor like this:
http://cgi.ebay.com/HURST-300-RPM-SY...#ht_1151wt_958

To drive this motor I will use a stereo sound file (created using a program like Audacity) of the right frequency to drive my table at 33 1/3 rmp with one channel of the file shifted 90 degrees. This will then be played though an amp, and the speaker outputs will be hooked up through a step-up transformer to the motor to get the voltage that I need for this motor. Once this is all done, all you have to do is simply press play on a sound file that you have created and the motor begins to spin. I also plan to tweak my sound file so that it will begin at a higher voltage but a slower frequency and quickly speed up to the required frequency for my TT. This will allow me to accelerate the platter while remaining in synchronization the whole time. It should make for a very quick and quiet start up.

Once I have this all worked out I am planning to build the whole unit including an MP3 player into it’s own housing so that all I will have to do to start my TT is press the built in play button, and to stop, just press stop. If I want to play a 45, or a 78 I will just have to switch to the correct track and press play. There will be no belt switching or any of that mess. When all is said and done it should be sleek and reliable.

This is a very quick description of this system and if there is anyone that is in need of some more details, let me know and I will be happy to help. I will also continue to post my progress on this project as I go.
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Old 29th October 2010, 12:33 AM   #4
Nanook is offline Nanook  Canada
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Default possible TT drive schemes...

For a DC motor, one must take into consideration any possible temperature dependant drift that may occur. Some sort of compensation may be needed for this to work correctly, as temperature rises, resistance decreases and voltage will increase (usually).

Using an AC motor, one needs to find something that can create a rock solid frequency. This can be pretty easy. I've considered using mp3 players for some time. Not elegant, but easy enough to change so that a solid speed is maintained. I use a digital tachometer for rotational speeds. It works quite well and is accurate +/- 0.05 RPM. The only thing that I think you may have trouble with is the amplifier of choice. Pure Class A amps (as in non-switching, non-digital amplifiers) have been suggested to be the best. Certainly a MOS-FET type biased heavily towards class A would take you a long way there.
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Old 29th October 2010, 01:22 AM   #5
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Quote:
Originally Posted by gtyler View Post
To drive this motor I will use a stereo sound file (created using a program like Audacity) of the right frequency to drive my table at 33 1/3 rmp with one channel of the file shifted 90 degrees. This will then be played though an amp
I was thinking about this a while ago and it occurred to me that I have an unused car CD radio here. I could write the sound file to a CD and use the player to drive a motor. Of course the file would run to an end after a while which would probably end up being irritating.

After that I thought about some Direct Digital Synthesis. You can get chips for this which run off a crystal clock with fractional Hz resolution, or you could use an FPGA or CPLD, you could configure some of it as ROM and read out the sinewave values to a DAC and thence to an amplifier. Then I thought about dsPICs, and in fact you can get a PIC with a 16-bit stereo audio DAC for £3 UK or about $5 US. You can program it with a Pickit2, which I've got. In fact there are so many options for creating a stable sinewave it's more a question of which to pick.

w

Last edited by wakibaki; 29th October 2010 at 01:32 AM.
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Old 29th October 2010, 01:28 AM   #6
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Quote:
Originally Posted by Nanook View Post
For a DC motor, one must take into consideration any possible temperature dependant drift that may occur. Some sort of compensation may be needed for this to work correctly, as temperature rises, resistance decreases and voltage will increase (usually).

Using an AC motor, one needs to find something that can create a rock solid frequency. This can be pretty easy. I've considered using mp3 players for some time. Not elegant, but easy enough to change so that a solid speed is maintained. I use a digital tachometer for rotational speeds. It works quite well and is accurate +/- 0.05 RPM. The only thing that I think you may have trouble with is the amplifier of choice. Pure Class A amps (as in non-switching, non-digital amplifiers) have been suggested to be the best. Certainly a MOS-FET type biased heavily towards class A would take you a long way there.
Nanook - perhaps a series trimmer pot on the feedback loop will sort it - I shall build it and let you know!
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Old 2nd November 2010, 01:11 AM   #7
gtyler is offline gtyler  Canada
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Here's another interesting TT drive system idea. I bet it would work pretty well for a low mass platter. I wonder if it would be possible to link several of these motors for a heavier platter.

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Old 2nd November 2010, 02:06 PM   #8
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I own two turntables - a VPI HW-19 Mark III. I ended up using the (overly expensive) SDS power supply to have some control over the speed. Honestly, this is a silly way to do it.

My second turntable is an 80s vintage Dual CS5000 that has a quartz locked DC motor with a belt drive.

As far as speed stability, the both seem to perform about the same - just different ways of reaching the same goal.

Whatever is done for a DIY table has to be simple to use - I do like the DC motor with the quartz lock. It shouldn't be that hard to make a circuit/motor combination that can replicate this.
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Old 2nd November 2010, 06:24 PM   #9
gtyler is offline gtyler  Canada
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Originally Posted by kstagger View Post
Whatever is done for a DIY table has to be simple to use - I do like the DC motor with the quartz lock. It shouldn't be that hard to make a circuit/motor combination that can replicate this.
This would have to have some sort of a feedback loop to adjust for temperature fluctuations, drag, etc. That would of course mean that you would need a DC motor with an encoder or some sort of speed measurement device. I think it may be trickier that it initially sounds, but I would love to hear more ideas regarding this.

Has anyone managed to make something like this work?
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Old 2nd November 2010, 10:53 PM   #10
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I continue to be perplexed by the ongoing fascination with using PMBDC motors to drive a turntable. They all suffer from significant ripple torque relative to average torque, and cogging (detent) torque can also be an issue, especially with the rare earth magnet motors. True enough, they are better in that regard than the commonly available synchronous stepper motors, but that's not the only synchronous solution. Not to mention the issue of maintaining accurate speed.

A specially designed 3 phase PMBDC, perhaps more accurately called PMBAC, with sinusoidal torque/angle characteristic will, when excited with clean sinusoidal current in each phase, generate a ripple free output torque, depending upon how well the magnetics and controls were designed.

My personal favorite for turntable drive, however, is the hysteresis synchronous type that used to be used for capstan drives. They are incredibly smooth in operation, far superior to PMBCD types. They don't seem to be manufactured any more but they do turn up as surplus regularly. Perhaps not a good candidate for new production, but just fine for DIY. For example, I picked up two 450 RPM units off ebay last week for $10 each.

You can run them off the AC line with a phase splitter cap on one phase with excellent speed control, or use a low distortion two phase AC power supply for even better (smoother) performance. One could easily be put together with a small microcontroller using lookup tables to drive a couple of chipamps, for example. Or an mp3 player. Or...
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