[sdiy] SMT transistor for white noise?

[Mike Beauchamp]

Beating a dead horse here, for the sake of future generations of 
internet searchers looking for white noise circuits.

Jay S.'s recommended mods seems correct. With the original circuit of C1 
going to V+, I experienced a lot of microphonics in my power supply - 
tapping on my psu board was audible through the noise circuit. Moving C1 
to ground as Jay S. suggests, the sound is identical and the 
microphonics went away.

Also increasing the value of C1 extends the noise to lower frequencies.

Mike



On 2026-03-23 21:24, Jay Schwichtenberg wrote:
> Thanks for the feedback Mike.
> Jay S
>> On 03/22/2026 8:48 PM PDT Mike Beauchamp <list at mikebeauchamp.com> wrote:
>> Just a 3-month-later follow up to this... info below..
>> On 2025-12-13 19:59, Jay Schwichtenberg via Synth-diy wrote:
>>> Looking at the EDN schematic again I think there is an error. Think C1
>>> should go from Q1's emitter and ground, not emitter and +12. I built
>>> mine with C1 going to ground.
>>> Jay S.
>> I just received a pcba that I had fabbed with a few different transistor
>> noise circuits, including the one from the EDN article as published and
>> with your suggested modification.
>> Both outputs look exactly the same on the scope and on the scope's
>> spectrum analyzer, with flat noise from 0 to 20kHz. Granted, the FFT on
>> the DS1054 isn't great, but enough to see.
>> So not sure what C1 is doing, why it's different on several different
>> schematics, or why it seems to work in both positions just fine.
>> With the latest batch of transistors I have, where the noise is
>> super-quiet.. this circuit outputs a very nice 1Vpp noise with no
>> asymmetry or clipping that I can see. It is also resilient to mechanical
>> noise, unlike 1 transistor circuits using several stages of high-gain
>> opamps, where I think the AC coupling caps become microphonic and
>> problematic when the PCB is being shared with mechanical switches and 
>> jacks.
>> Hope this helps,
>> Mike
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