op amp buffer blasting hiss

[kvb]

I guess I'll try to answer my own question and maybe someone can tell me if I'm right or not.

I have a splitter blend (parallel effects) type thing  I've been trying to finalize.

So, just two buffers on either side of a 25K pot.

when the pot is turned all the way to the one side there is hiss blasting.

Turn the pot back toward the center, there is a point at which the hiss turns "off".
Turn the pot back toward one side; the hiss snaps back "on".

could this because there is not enough of a load at the output? (1M to ground on wiper to out)

I don't think it's a bad cap or a build error. - A cold joint at the 1M?


Thanks, if there's any answer.

[oskar]

In order for me to get the whole picture you'd better... post it!   

[R.G.]

What you describe seems to indicate that the feedback path of your second buffer might be open. The "hiss blasting" indicates a very high gain. Depending on your circuit, you may be getting cancellation of the hiss at the middle of the pot. But if it's not a really noisy part (which is always possible) then you may have a lot of gain by an inadvertent open feedback path.

[kvb]

In order for me to get the whole picture you'd better... post it!   

Yes, I'm definitely going to post a scheme. The trouble is, that I've been trying to complete the thing first - by myself - so that I can post a completed circuit, with clips, etc.

Thanks RG, that gives me something to look for.


I've been using the thing. It's 99% done, and every time I think I'm ready to put it in an enclosure another strange problem pops up.

[kvb]

Yup, feedback. - In a way that a schematic wouldn't show.


There is a push pull pot with the pot setting the volume in between two FET stages. 
The switch part has the final output caps (small/large).
So the feedback was caused by two wires just being a bit too close together. < 1/2 inch too close, > 1/2 inch OK.

I'd like to say that I took a look at the pile of wires and spotted the problem, but no. It was more trial and error. I thought the sound had gone away while I was jiggling wires around. So after a break I went back, started to follow the signal, and just touched the right wire and moved it the right way.

To me it is still a bit odd to hear the sound of the buffer taking off. Move the wires close - 'click, hiss'. Move them apart - 'click, no hiss'. There was no gradual change.

I changed the circuit of the first FET to one without a Vref on the input, a 102 instead of a 201, and set the gain low.
Now there's no problem.  I was planning on making that change anyway.

It's good to have found the problem. Now if I hear the sound of a buffer going nuts I'll know what to look for.

Thanks again.