Hey wow, it’s a topic for noise sources. Of the electronic sort, I mean.
Prompted by an odd thing… A couple years or so ago I built a Kosmo version of the MFOS Noise Cornucopia. Now I’m designing a module that needs white noise so I thought I’d just drop in the Noise Cornucopia source.
And… it didn’t work. I mean it did generate noise, but it also generated a faint tone, up very very roughly around 1 kHz I’d say. I’d never heard that from the module I built.
Figured it had to be a mistake or bad part or something, couldn’t find it, so I built a second copy… and it had the same problem.
So I decided to try some other design and the first to come to mind was the same MFOS module in its Rev 0 incarnation. Wilson later modified the noise source design and called that Rev 1 although he said "the changes provide a negligible improvement ". The Rev 0 version is simpler:
I just now found this post. They were getting a much lower tone, 50 to 200 Hz, but otherwise it was similar to what I was hearing, and the solution seems to have been to connect the transistor base to ground rather than to -12 V. And in the Rev 0 source the base is connected to ground.
Anyway, simpler solution works better than supposedly “negligibly improv[ed]” more complicated version. I’m fine with that.
Sounds like it was picking up “noise” from the -12V rail, did you have anything else powered from that rail, a vco perhaps?
Interesting that he put a supply filter R33, C18 on the +12v rail, but not on the -12v rail.
Ive had similar problems with these type of noise circuits. Personally I prefer a digital noise source and with the availablity of the Electric Druid chips its so much easier to generate noise in this way.
What power supply were you using to test the circuit?
A while ago I changed from an internal linear power supply (which got very hot) to an external supply using 3 switch mode PSUs. Everything in the synth was happy with that except for the Noise Cornucopia, which produced a tone very much like your description, and also started triggering the random gate circuit at high speed. This was doubtless due to high(ish) frequencies on the supply rails from the switch mode supplies. I fixed it by adding 3 LC filters in the PSU cabinet. The inductors were made from about 1.5m of 0.8mm enamelled copper wire wound on ferrite rings (about 2cm OD).
The MFOS Rev. 0 circuit is very similar to the noise generator in the Turing Machine, though I don’t know if that would have been affected by the PSU - I built it after adding the filters.
This was with the bench power supply, not powering anything else. There was an oscilloscope and a module tester on the same circuit… and an LED lamp… but turning the scope and module tester off didn’t affect anything.
I don’t think I ever powered the Noise Cornucopia with that supply — I think I got that supply later — though on the bench I probably was powering it with an ATX supply that surely was dodgier. But it’s true enough PSU issues work in mysterious ways. The synth it’s now in has linear supplies.
Anyway, I regard the problem as fixed, as far as the present circuit is concerned: The Rev 0 source works fine. If the Noise Cornucopia ever starts exhibiting similar behavior I’ll start by moving the transistor base to ground and that’ll probably fix it.
I prefer not to add digital chips where I don’t need to.
Just out of curiosity I tried the Rev 1 source again this morning, on a separate breadboard. The tone was gone — but it was picking up crosstalk from the adjacent breadboard circuit I’d connected to for power. I tried adding a 47 µF cap from base to ground and the crosstalk went away.
Looking at it some more I think I do like the Rev 1 source better — with the 47 µF added.
It sometimes takes me a couple weeks to figure out the bleeding obvious but I get there. The tone I was getting was crosstalk from an oscillator on the same breadboard; it seemed to go away just because I’d lowered the frequency. Now that I know how to get the tone back I’m able to test and see if the 47 µF cap makes it go away again. It does.
In order to add my 2 cents here, the noise source from Rene Schmitz is nice.
I powered it with 12v and used TL072 as op amp, 2x 1k for replacing the 2k2 I didn’t get and it was noisy enough, maybe not perfectly white noise but I guess it is really good for my usage (making big wooooosh by filtering it and passing trough too much effects …)
Easy and cheap to build too.
I was considering building a dedicated noise module - but then I ended up with two other modules that included a noise output built in. Still… to get a few more ‘flavors’ I was considering building Moritz’s design:
The different “flavors” – chocolate (brown), raspberry (blue), yeah we’re doing this, let’s go – are just regular vanilla (white) passed through different types and orders of filters, yeah? Should be easy enough to mult out a white noise source and filter it into any flavor you want.