[sdiy] SMT transistor for white noise?

[Mike Beauchamp]

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