Synth-diy-test

On 03/22/2026 8:48 PM PDT Mike Beauchamp <list@mikebeauchamp.com> wrote:

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Just a 3-month-later follow up to this... info below..

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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.

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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.

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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.

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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.

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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.

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Hope this helps,

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Mike

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