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.
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)
An externally hosted image should be here but it was not working when we last tested it.
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.
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
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
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.
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.
awedio said:
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.
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.
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!!!
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!!!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 .
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 .
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.honinbou said:
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.
For anyone who wants to try using a computer without paying for Adobe Audition or similar: http://www.download32.com/sound-generator-d3032.html
Three Phase?
Obviously the players involved (Ipods, etc.) are two channel devices.
If I were to lag the initial sine wave by 120 degrees to record in the
second channel, is there a sum/difference trick to synthesize the third
signal. I just get confused thinking about this one. I suppose you
could get a cheap home theater receiver that would have more than
enough channels, but creating a functional four-six channel signal that
really only used three is a stumbling block.
thanks,
Skip
Obviously the players involved (Ipods, etc.) are two channel devices.
If I were to lag the initial sine wave by 120 degrees to record in the
second channel, is there a sum/difference trick to synthesize the third
signal. I just get confused thinking about this one. I suppose you
could get a cheap home theater receiver that would have more than
enough channels, but creating a functional four-six channel signal that
really only used three is a stumbling block.
thanks,
Skip
Skip
There are two easy ways to do this. One is to have the two channels at 120 degrees as you suggested then to combine them by adding. The simplest adder also inverts. In phase terms:
(1 x 0) + (1 x 120) = 60 and (-1 x 60 ) = 240
so the required answer pops out.
What I do is slightly more complex because I create quadrature in the digital domain using an 8 step direct synthesis architecture. I invert the 90 phase so I have 0, 90 and 270 then use a pair of inverting multiplying adders:
(0.5 x 0) + (1/2 x sqrt3 x 90) = 60 which inverts to 240
(0.5 x 0) + (1/2 x sqrt3 x 270) = 300 which inverts to 120
These sorts of arithmetical operations are the raison d'etre of op amps, hence the name. To get good results use instrument grade amps with low input offsets.
There are two easy ways to do this. One is to have the two channels at 120 degrees as you suggested then to combine them by adding. The simplest adder also inverts. In phase terms:
(1 x 0) + (1 x 120) = 60 and (-1 x 60 ) = 240
so the required answer pops out.
What I do is slightly more complex because I create quadrature in the digital domain using an 8 step direct synthesis architecture. I invert the 90 phase so I have 0, 90 and 270 then use a pair of inverting multiplying adders:
(0.5 x 0) + (1/2 x sqrt3 x 90) = 60 which inverts to 240
(0.5 x 0) + (1/2 x sqrt3 x 270) = 300 which inverts to 120
These sorts of arithmetical operations are the raison d'etre of op amps, hence the name. To get good results use instrument grade amps with low input offsets.
Thanks for the explanation -- that seems very approachable with
IC opamps. Use a dual chip driven in parallel from each channel's
input, send one side of each to the 0. and 120. power amps, and
sum the other two into an inverting input for the 240. channel
with a pot to that power amp input to balance the amplitudes.
This would stretch me a bit but seems like a good stretch.
Skip
IC opamps. Use a dual chip driven in parallel from each channel's
input, send one side of each to the 0. and 120. power amps, and
sum the other two into an inverting input for the 240. channel
with a pot to that power amp input to balance the amplitudes.
This would stretch me a bit but seems like a good stretch.
Skip
There is no need to balance the amplitudes if they are correct in the first place. It's a vector sum, so 1 + 1 = 1.
An easy way to visualise this is to place the vectors in a unit circle. Draw the vector for zero (from the centre to 12 o'clock) and the vector for 120 (from the centre to 4 o'clock). To add them together, draw a line the same length and angle as the 120 vector starting at the end of the zero vector. You will see that this is a line from 12 o'clock to 2 o'clock. Invert it by rotating the whole thing 180 degrees. It now goes to 8 o'clock.
If you change the length of any vector the sums I gave don't work any longer.
An easy way to visualise this is to place the vectors in a unit circle. Draw the vector for zero (from the centre to 12 o'clock) and the vector for 120 (from the centre to 4 o'clock). To add them together, draw a line the same length and angle as the 120 vector starting at the end of the zero vector. You will see that this is a line from 12 o'clock to 2 o'clock. Invert it by rotating the whole thing 180 degrees. It now goes to 8 o'clock.
If you change the length of any vector the sums I gave don't work any longer.
a suggestion already made....
by Mark (and even myself, previously)
The Altmann uses essentially the same idea, but uses DC stepper motors . The end result is something that can be driven by an AC wave..and the implementation of the frequency generator/iPod, etc is very similar to Mark's suggestion. I even had suggested in the past recording 60Hz (or 50 HZ depending where one might live in the world) on a cd and playing it though a cd player to a large stereo amp---thus creating your own clean AC signal, without filters...as per "the Voyd" turntable.
I've been laughed at for this suggestion, but it does make sense.
Thanks Mark for the clear cut instructions for myself and others. Simpler is most often really better.
Here's a thought too... you could even create some +/- "X" % files so that you could have some speed adjustment built in (if already mentioned, sorry. I didn't read Mark's complete post).
by Mark (and even myself, previously)
The Altmann uses essentially the same idea, but uses DC stepper motors . The end result is something that can be driven by an AC wave..and the implementation of the frequency generator/iPod, etc is very similar to Mark's suggestion. I even had suggested in the past recording 60Hz (or 50 HZ depending where one might live in the world) on a cd and playing it though a cd player to a large stereo amp---thus creating your own clean AC signal, without filters...as per "the Voyd" turntable.
I've been laughed at for this suggestion, but it does make sense.
Thanks Mark for the clear cut instructions for myself and others. Simpler is most often really better.
Here's a thought too... you could even create some +/- "X" % files so that you could have some speed adjustment built in (if already mentioned, sorry. I didn't read Mark's complete post).
Prompted by this thread, I rummaged through my growing supply of unused gear (stuff I thought I definitely was going to need at one point or another in my insane DIY adventures) and I found an extra t-amp, an extra trafo, and an extra iPod. Now my question is, I have a Pro-Ject 1xpression mkII, but I am not sure what kind of motor I have. Is it a single phase, or is it biphasic with the 90 degree offset? I know it's an AC motor, but I don't know which kind. Would I ruin it by trying to run it single phase? If anyone knows, I would be grateful (and so would my wife because I could build something useful without having to spend any money)
-T
*Edit: I got impatient and took it apart. It is biphasic with a passive delay network.
-T
*Edit: I got impatient and took it apart. It is biphasic with a passive delay network.
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