[sdiy] SMT transistor for white noise?

[Tom Wiltshire]

They're not "usually" pseudo-random, they *are* pseudo-random. They produce a fixed sequence the same every time. However, that sequence is statistically indistinguishable from a random signal. It certainly "sounds random". There is only any audible repetition if the LFSR is too short. The MM5837 that gave digital noise sources a bad reputation back in the day (Prophet 5, Korg Monopoly, etc) used a 17-bit register, giving a repetition rate of only a few seconds at its typical output rate of 10's of KHz. No-one complains about the 31-bit register LFSR noise in the TR-909, for example. Since the length of the sequence doubles for each extra bit you add, going a few bits beyond the 17-bit register of the MM5837 gives massive improvements. Moving up to a 24-bit register gives you an output that doesn't repeat for several minutes. By the time you're to the TR-909's 31-bit register, you're into hours, even with its much better output rate (around 250KHz).

While you _can_ screw up the implementation, LFSRs are a solid technique for good quality audio noise. There's no way you can tell the difference. There's really no need for "truly random" noise unless you're running a lottery.

Tom

> On 14 Dec 2025, at 00:48, Jay Schwichtenberg via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> I've used the EDN noise circuit at least 3 times and it has worked just fine. I always socket the noise transistor and select it. I bought a bag of 500 2N3904 transistors (for 1 cent each) about 15 years ago.
> 
> Thing about LFSRs is they are usually pseudo random. They typically contain small sections of noise that repetitive. In most case that is good enough but not truly random. In order to get truly random noise digitally it can be somewhat complicated.
> 
> 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
>> ________________________________________________________
>> This is the Synth-diy mailing list
>> Submit email to: Synth-diy at synth-diy.org
>> View archive at: https://synth-diy.org/pipermail/synth-diy/
>> Check your settings at: https://synth-diy.org/mailman/listinfo/synth-diy
>> Selling or trading? Use marketplace at synth-diy.org
> ________________________________________________________
> This is the Synth-diy mailing list
> Submit email to: Synth-diy at synth-diy.org
> View archive at: https://synth-diy.org/pipermail/synth-diy/
> Check your settings at: https://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org