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

[cheater cheater]

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"
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