[sdiy] PWM noise in analog ground in Optigan-like system

[Derek Holzer]

Thanks for the quick reply.... as I mentioned, schemos and layouts are 
here: http://www.umatic.nl/tonewheels_technical.html

Except that in place of the PWM circuit listed there, I'm using an 
Arduino with PWM output to a TIP122. Layout of analog side remains the same.

I found some tips on shielding here: 
http://www.edn.com/article/CA6288042.html

which seemed to work, namely the following:

Keep motor cables as short as possible.
Use a motor cable with a high-quality shield.
At the motor end of the cable, make a low-impedance connection between 
the shield and the motor frame.
At the amplifier end of the cable, make a low-impedance
connection between the shield and the chassis ground.

Best!
Derek


On 3/29/10 2:30 PM, cheater cheater wrote:
> Hi Derek,
> It might be easier to say what's up if we can get some schematics
> and/or board layouts. Here are some ideas that might or might not help
> you:
>
> - more decoupling
> - maybe you're not decoupling the right thing
> - EMI issues? Try shielding the bugger away.
> - use separate ground for the PWM; make sure it doesn't touch anywhere
> - if all else fails, two-wire signaling in the analog path: normally
> synths use single-wire signaling, which means that the audio is an AC
> signal, the value of which is referenced to ground. But the question
> is, 'what is ground'. In difficult situations the ground will be
> different from point to point, or contain noise, which - when
> referencing the audio to the ground - makes it appear as if there's
> noise in the audio signal. Instead of referencing your audio to random
> places in the ground plane, you can carry the ground your previous
> 'part' (op amp, transistor, ...) was referenced to, from the place
> where it gains the reference, parallel to the signal trace; then you
> reference the next 'part' from that ground trace, and go on like that.
> That's one practice used in big mixers where there's a lot of noise
> attacking your audio no matter what you do.
> - kill it with fire
>
> HTH
> D.
>
> On Mon, Mar 29, 2010 at 13:27, Derek Holzer<derek at umatic.nl>  wrote:
>> Hi folks,
>>
>> the system I am building uses a PWM motor controller (Arduino+TIP120) as
>> well as an op-amp buffer for a phototransistor. Think of it a bit like an
>> Optigan... technical details here:
>> http://www.umatic.nl/tonewheels_technical.html
>>
>> I've been running the PWM controller and the op-amp buffer from separate
>> wall-warts for some time now, but this week I wanted to combine everything
>> into one box with one power supply. And, of course, I can hear the PWM
>> coming through the + and ground lines to the op-amp.
>>
>> I tried using a separate LM7809 voltage regulator with decoupling caps (100
>> uF between supply and ground, 100nF between regulated and ground) for the
>> audio portion, but enough noise still comes through the ground alone to be
>> heard (noise can be heard with + disconnected). And it still fails to remove
>> the noise from the + side as well!
>>
>> I also tried small ferrite beads on the + and ground lines. No noticeable
>> difference.
>>
>> Short of plugging two different wall warts into the same box, one for the
>> PWM and one for the audio, what can I do to remove this noise?
>>
>> How far back in the power path do I need to separate the powers and grounds
>> of the two systems? For example, at the AC/DC conversion stage?
>>
>> Thx+best!
>> Derek
>> --
>> ::: derek holzer ::: http://macumbista.net :::
>> ---Oblique Strategy # 8:
>> "Accretion"
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>

-- 
::: derek holzer ::: http://macumbista.net :::
---Oblique Strategy # 90:
"In total darkness, or in a very large room, very quietly"