[sdiy] SMT transistor for white noise?

[Mike Beauchamp]

Thanks Jay S.

I tried placing this C1 capacitor on my existing circuit (similar to Ray 
Wilsons' 
https://musicfromouterspace.com/analogsynth_new/SOUNDLABMINIMARKII/images/slmsmarkii_page_4.gif 
)

C1 going to +12 was very bad, introduced some weird high-frequency spikes.

C1 going to ground was also bad, sounded like it added a low-pass filter 
to the sound of the noise and reduced the output volume a bit.

I'm not understanding the explanation for C1 on the EDN page at all: 
"The improved output level is due mainly to the inclusion of C1, which 
provides a low-impedance AC source to the noise source while not 
disturbing the DC bias of Q1"

Also here's the original article (very cool to flip through this magazine):
https://archive.org/details/silicon-chip-2005-no-01/page/92/mode/2up

Mike





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.
> 
> On 12/13/2025 1:05 PM, Mike Beauchamp wrote:
>> On 2025-12-13 09:35, Brother Theo via Synth-diy wrote:
>>> I switched to the 2 transistor noise circuit and have had zero issues 
>>> since. The Future Retro 777 re-release uses that circuit.
>>
>> Hi Timothy, doing some duckduckgo-ing, I found a few...
>>
>> https://www.edn.com/simple-white-noise-generator/
>>
>> https://www.nutsvolts.com/magazine/article/bipolar_transistor_cookbook_part_5 (figure 20)
>>
>>
>> The EDN article is interesting. Seems like the second transistor is 
>> just the amplification stage and the increased noise level is because 
>> of the addition of C1. I don't understand the function of C1.. time to 
>> get out the breadboard.
>>
>> Mike
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