Ipod drive for turntables

[Mark Kelly]

I've been asked about this a few times so I thought I'd create a thread with the appropriate information.

To make this work you need a few things:

A computer (if you are reading this I assume you have one)

An ipod or other MP3 player

Two 10k linear potentiometers, a 1/8" stereo jack, wire and access to a soldering iron.

A small amplifier (a T-amp is ideal so I'll use that in the example)

Transformer(s) - one per driven phase of your motor.

Here's a single phase example (the motor is from a Garrard 301, the amps are a small class D chip rather than the T amp but the principle's the same)



The first thing you need is to record the wave(s) for the drive as an audio track on the ipod. I used NCH's "Tonegen" (a limited time freeware download available HERE ) to do this.

There may be simpler ways but this is how I did it: I created a sinewave in the both channels at full amplitude and the frequency I wanted, starting at the zero crossing and extending for 30 minutes. The length is chosen so you'll get a full side of an LP in a single play. Just restart for each side.

For a single phase motor drive this is all you need to do. For a two phase motor drive you need to shift the second phase by 90 degrees. I did this within the wave editor by scrolling along to the first peak in one channel and deleting the bit of the wave before that peak, then shifting the whole wave back to the same start point as the wave in the other channel. You now have a sine wave in one channel and a cosine wave in the other as required.

Use any of the available wave edit programs to encode this in an appropriate form for your ipod or other player - MP3 will do. The major inconvenience here is the need for a separate track for each frequency if you want to implement variable speed drive. There seems to be no way around this.

Another inconvenience is that the digital volume control on the ipod is pretty crude so for fine control you'll need to implement a separate amplitude control. You can't rely on the input pots on the amp as they're log taper, the easiest thing to do is to put a couple of linear taper 10 k multiturn pots on a small board, wire the pots to a 1/8" stereo jack (for the ipod) and wire the wipers to the inputs of the amps.

The output of the T amps is plenty to drive a motor up to about 10 watts input power. For a bigger motor (such as the Garrard) the simplest scheme is to run two channels in parallel. From a 12V supply the T amp will max out at around 7 volts so we need to step that up for the motor.

The easiest way to do this is a small power transformer, you'll need one transformer per phase. I used 240V primary / 9V secondary toroidal transformers rated at 30VA and ran the output of the T amp to the secondary of the transformer, taking the motor drive from the primary. Due to the regulation of the transformer you'll need more voltage on the input than the simple ratio would suggest - usually about 20% higher. IIRC about 5 volts in gave me 110 volts out.

The actual type of transformer is quite important - if it has poor primary inductance or too high a magnetisation current the amp will trigger its protection circuits and shut down - or not trigger its protection circuits and let the smoke out. The toroidal types shown in the pic have been the best performers here.

Any questions post them here and I'll do my best to answer.

[GerryM5]

What a fun project and it is a very nice starter project for me, thank you. I've been wanting to start an audio project for some time, but thinking my soldering skills faded years ago. You have made the build process so doable even this Industrial Designer can confidently try.

[Jeffrey Davison]

Mark,

TD160 controller I got fromyou has worked great the past couple of years. Now I'm upgrading my front end to a Garrard 401.
VERY interested in this controller.
I was hoping that you'd continue this as I think on your website, the pages were getting a little old waiting for a follow-up.

In any case, anything you can post or send would really be appreciated.... i.e. schematics, etc....


Jeff Davison

[awedio]

Mark,

Nice intro to this heretic motor drive. This one is great for the noobs and anyone else afraid of crystal oscillator circuits.

I have a question - I have seen you mention that the exact phase lag might need to be tuned for different motors to get the best vibration-free performance. How is that determined - trial and error?

Also, is the split-phase approach a real step up from using single phase plus "caps'n'resistors" for the cosine wave, a la Rega? That's what I have running now...

Thanks much.

[Mark Kelly]

Originally posted by awedio


is the split-phase approach a real step up from using single phase plus "caps'n'resistors" for the cosine wave, a la Rega? That's what I have running now...

Yes, definitely. As explained elsewhere the phase error from the cap approach can be as much as 10 degrees.

An added advantage is that you can adjust the voltages of the two channels for minimal noise.

Another is that you can take the drive "off the grid" by running on batteries.

I have a question - I have seen you mention that the exact phase lag might need to be tuned for different motors to get the best vibration-free performance. How is that determined - trial and error?

The circuit I used to make this determination allowed the phase angle to be continuously adjusted in real time. To add this adjustment to the ipod idea is possible (it is just a pair of op-amps configured to perform vector arithmetic) but it takes away from the spirit of simplicity.

[awedio]

Thanks, Mark

I get it now, using an additional circuit to adjust the relative phase. Hmmmm... I suppose there are other approaches along with the op-amp idea. But it's reassuring to know that just taking the step of going two-phase right from the ipod should ensure smoother running. Definitely on my list to try, as I am rebuilding the Rega plinth anyway and it will be a joy to stomp out those hideous caps and resistors and throw them in the rubbish bin. Funny, more and more of the original Rega stuff is going in that direction lately!!! It's an older Planar3. Just thinking about the eccentric subplatter makes me want to puke to this day.

I knew the British were eccentric, but come on lads, you don't need to apply it to everything Groovetracer to the rescue!!!

[Mark Kelly]

One important rider:

Some ipod / mp3 types are output limited to ensure that today's Yoof deafen themselves a little more slowly.

You will need to check that the ipod / MP3 that you use is capable of driving the amp you use to full ouput.

To the best of my knowledge, standard Apple ipods are capable of about 1V output and standard Sonic Impact T amps have input sensitivity of about 0.8 V .

[honinbou]

If memory serves there is a wave generator in Adobe Audition and you could do all the neat stuff, like tweaking the phase angles, amplitudes, corner frequencies, etc.

So may be you could record a bunch of waves and by trial and error find one that works best with your turntable motor.

[smiler]

Originally posted by honinbou
If memory serves there is a wave generator in Adobe Audition and you could do all the neat stuff, like tweaking the phase angles, amplitudes, corner frequencies, etc.

So may be you could record a bunch of waves and by trial and error find one that works best with your turntable motor.

If you have a laptop, you could do all this live right? i.e. connect the laptop's output to the 1/8" jack, in place of the iPod, then you can vary the frequency and phase on the fly.

There's probably a basic function generator app that could do this, but I'd imagine it's pretty easy to knock one up in Java if not (I don't know if there are many programmers around here?). It could be a nifty way to tune the frequency & phase, then you can record that track to your MP3 player.

[smiler]

For anyone who wants to try using a computer without paying for Adobe Audition or similar: http://www.download32.com/sound-generator-d3032.html