[sdiy] Noisy white noise generator

[Michael Ruberto]

You may want to change the PCB by rerouting ground connections near those 
circuits. I had this same problem in my first modular synth with the 
multi-function module that contained 2 LFOs, a S&H, and the noise generator. 
The steps I took to fix the problem included the suggestions the others made 
for extra bypass caps, twisting the wires that ran to LEDs on the panel and 
changing the ground paths. Specifically, the ground path for the noise 
circuit was brought out and attached to the power connector for the module 
instead of sharing the ground bus on the PCB.
You may have to cut traces on the PCB in order to rerout the ground so plan 
it very carefully. Make sure you are cutting the right traces and also be 
careful not to create any ground loops.

Now that I have looked at the schematic I thought of one more thing you can 
try. Disconnect R96 from the 9V supply. Add a 220 ohm resistor between R96 
and the supply. Then connect a 10uF cap from the junction of R96 and the 220 
ohm resistor and the negative supply. This will bypass the noise transistor 
from transients on the power rails. One drawback from this is the noise 
generator will take a few seconds to reach peak output when you first turn 
the synth on.
I robbed that idea from the MiniMoog and it has served me well in my recent 
designs.

M. A. Ruberto

>From: Ian Fritz <ijfritz at comcast.net>
>To: <torpedo at demadrid.com>, <synth-diy at dropmix.xs4all.nl>
>Subject: Re: [sdiy] Noisy white noise generator
>Date: Sat, 27 Jan 2007 09:15:26 -0700
>
>At 03:04 AM 1/27/2007, Gorka Garcia wrote:
>>I've built a soundlab minisynth from http://www.musicfromouterspace.com 
>>and I am having some trouble with the white noise generator, I seem to be 
>>getting low frequency noise from the LFO that lives in the same LF444. Do 
>>you have any suggestions about what can I try to get rid of that LFO 
>>noise?
>
>
>Gorka --
>
>That's a fairly common problem with noise generator modules because of 
>their high gain.  In my present unit I decouple the power for the 
>transistor and first opamp using 3.3V Zeners and 22 uF caps.  I wouldn't 
>share the high gain stage chip with any other circuit, i.e., use a single 
>opamp.
>
>   Ian
>

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