Fairly Low Frequency Random Voltage Generator

[jhaible]

Hi Matthew,

as you intended to post this to DIY, I'm forwarding the whole text,
so others can comment as well.

There are 3 things that come to mind, 2 of them I've tried, the
3 being the most promising (and I plan to try it as soon as I find the
time):

1) Just add several simple triangle LFOs with slightly different
frequencies.
I have a bank of 5 such LFOs in my JH-3 (now that's only 2.5 TL074's,
and 0.5 TL074 for adding the LFOs, so 3 ICs in total), and while it's not
strictly random, in practice it is "random enough".

2) Low pass filtered noise. I also have this, copied from the elektor
formant, but I like it not very much. Too noisy, not LFO-like enough.

3) Buchla's Source Of Uncertainty. Most promising. I'm currently working
on a Vactrol-less version of this. Done simulations already, like it a lot.
I'll publish the circhit when I've built it. It goes like this: Noise
amplified
and low pass filtered, amplified until it's clipped, this drives a 1st order
PLL, the PLL output is sampled by the pulse of a simple VCO. The CV
for the VCO also controls a 2pole LPF, which is used to smooth the
S&H output.

JH.

> I'm pursuing an idea for which I need advice, and, as the synth-diy
> server seems to be unresponsive tonight, I thought I'd try you first! :)
>
> I want to develop a circuit that produces a low frequency (of the order
> of 0.1Hz - 10Hz) 'wandering' random voltage.
>
> The best way I can describe what I'm after is to envisage the signal
> strength meter on an AM radio or somesuch. The meter will wander over
> the scale in a somewhat unpredictable fashion, sometimes quickly,
> sometimes slowly, in response to atmospheric effects, people moving in
> the room and so on. I want to emulate this effect.
>
> I'm not sure what is the best approach to use. Maybe an AM radio's
> signal strength output will be the easiest way! I thought of trying
> high-Q noise generators, but the bandwidth of the output isn't wide
> enough for my requirements.
>
> Any advice or pointers you can provide will be greatly appreciated. I'm
> after a simple cct if at all possible. A soon as I can reach synth-diy
> I'll post this there too.
>
> Thanks again; bye for now
>
> Matthew S. Padden
> Glasgow, UK
> mattp at mindless.com
>
>