[sdiy] Circuit noise / oscillation - sorted.

[Justin Owen]

Thought I'd mention this for the archives and because I was pretty pleased with the solution.

I set up a passive lag/slew circuit - the classic two reversed diodes, each into a resistor, both resistors into an anti-parallel capacitor with one leg to ground - signal out of the junction at the other leg.

Set it fast enough to let audio frequencies through unaffected but not so fast that the HF oscillations get through. 

This wiped out the problem entirely - but I still kept all the nice, rich, complex overtones I was getting from the feedback circuit.

Cheers,

J



-----Original Message-----
From: Harry Bissell [harrybissell at wowway.com]
Received: 27.01.2011 18:46:31
To: Justin Owen
Cc: SDIY List
Subject: Re: [sdiy] Circuit noise / oscillation

should the oscillation get to a power amplifier, or to your tweeters, it might indeed damage them. You won't hear it but
it will still produce heat in the load...

H^) harry



----- Original Message -----
From: Justin Owen <juzowen at googlemail.com>
To: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Thu, 27 Jan 2011 13:24:53 -0500 (EST)
Subject: [sdiy] Circuit noise / oscillation

Hey again,

I've been playing with feedback quite a bit lately - literally feeding part of an output back into an input.

I've got a curious little circuit here that, under certain circumstances, will begin to oscillate within audio frequencies - which is cool - but under another circumstance it starts oscillating right up at 130-135 KHz.

While the oscillations do appear at the outputs I only noticed it when I had the scope on the outs. There's no signal registering at my soundcard at all - even with *loads* of gain on the soundcard input and I certainly can't hear it!

It's reasonably well behaved, symmetrically shaped and it's centred on exactly 0V. It doesn't suddenly thump in from supply levels or jump around and freak out - so it's basically just self-oscillation out of audio range.

This circuit isn't meant to be linear or well-behaved - it's meant to scream and shout.

My question - can oscillations of approx. 7V PP at this high frequency range potentially damage other audio gear / modular Ins/Outs - or am I just chasing worrying about *stuff*?

Thanks.

J  

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Harry Bissell & Nora Abdullah 4eva