[sdiy] WNoise Gen Isolation

[Tim Daugard]

From: "Michael Ruberto" <frankentron at hotmail.com>
> On my old modular I used the circuit from the Minimoog but
noise bleed was
> very bad...

The first noise module I built had the same problem - noise
bleeding into everything.

> I'm worried about the noise bleeding onto the power and ground
rails. What
> is an effective way to isolate this noise circuit from the rest
of the
> system?

I leave the ground rails alone. I use a (semi-) star ground
system and figure that the ground is robust enough to handle the
noise. But then as I tried discussing earlier, I use current
paths instead of voltage paths for signals.

For the positive rail (or both if you use a three wire supply), I
put a 100 ohm resistor in the module power supply path. This
increases the impediance of any signals trying to bleed through.
There are capacitors to ground at both ends of the 100 ohm
resistor. I use my normal bypass cap at the power supply end of
the resistor. At the circuit end of this resistor is lots of
capacitance. I use a large electrolytic as a second power supply
filter cap (and theorectily the actual voltage supply for the
module). I also use a medium and small ceramic for further noise
isolation (10 - 100 nF for the medium, 50 pF for the small). This
reduces the noise signficantly.

The next noise module I build will also have a mucher lower
buffer gain. I've found that with other signals in the 10V ptp
range, 1V of noise is enough. I though about adding inductors in
to the power supply line, but have't tried that yet - inductors
are to hard to find to use this way.

Remember to use the right size resistor. The first resistor I
used was 1/8 watt. It got hot! I bought some 100 ohm 1W resistors
to use in these types of power supply filters.

The same filter design is used in my low noise - instrument
preamp module. The other thing to remember is that the modules
with these filters will be charged and operating long after the
other modules are powered down. These filters also require time
to charge. I give the noise module a 3 to 4 minute warm up before
making any adjustments.

Tim Daugard
AG4GZ 30.4078N 86.6227W Alt: 12 feet above MSL
http://home.sprintmail.com/~daugard/synth.htm

not spell checked or proff read. Don't bother telling me wruds
are mizspeled.