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

[Derek Holzer]

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:
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