Windy AUTO WAH filter

[mac]

I have a modified bi-filter Dr. Quack on the breadboard, normal mode.
I'm getting a windy noise every time the transistor's resistance goes to its minimun, even with an external resistor.
First I thought it was the transistor, I disconnected the envelope detector and I put a pot, I varied it up and down quickly and I got the same noise as the pot went down.
Then I thought it could be the use of two filters. I removed one of the opamps, I used independent/changed Vbias, I removed any active device except one of the filters, etc. Same problem.
I put a roll-off cap across -IN and OUT, it helps a little but it reduces the filter's gain a lot.

I even closed the bedroom's window but the wind is still there...

Maybe it is a breadboard thing... Any advice?

mac

[R.G.]

I think you're right - it may well be a breadboard thing.

I'm guessing that it goes into RF oscillation at minimum resistance. Filters by their nature have some resonances, and it often happens that they get pushed over into oscillation in unintended ways. ultrasonic and RF oscillation often sounds like an angrier hiss than thermal noise when it affects amplifiers this way.

Try putting a 0.1uF ceramic cap from the V+ to the V- of each IC, and check the ground lines carefully to make sure they're making good contact. If that gets eliminated, we can look for other stuff.

It's a guess, but it's where I'd start.

[mac]

RG, a cap from V+ to V- makes more noise. I guess this is because V+ is tied to Vbias, which is just two 22k split resistors and a 22u to gnd. V- sees a small resistance path to gnd, 0.1 --> 22u --> gnd, increasing the gain of the filter and noise.
Adding a resistor between V+ and Vbias has the same effect.
I changed opamps, 71s instead of 741s, checked/changes gnd wires and I got less windy noise.

mac