squeal on power down

[tempus]

Hey all;

I've got a couple of pieces of gear (including one of my own builds) that emits a squeal (sort of sounds like a laser gun sound effect) when I turn them off. Is this some type of cap discharge working its way through the audio chain? Is there any way to fix it? I was thinking perhaps a bleeder resistor from the filter caps, but I'm not sure.

Thanks

[mcknib]

Sounds like it could be a problem with something other than the pedals if it happens with two, have you tried eliminating things like cables, low battery's, distance from amp etc and have you tried it with another guitar, does your guitar have active pickups are the pedals high gain does it happen with other pedals?

Any unwanted noise problem can be a whole host of things I'd check other possibilities first before I'd suspect the circuits. Shake and wiggle things like jacks to see if it affects the noise.

[tempus]

Thanks for your reply.

No, it's definitely in the individual pieces of gear - one is an EQ (my build), one is a PA mixer amp, and the other is a tube amp from the 80s. The latter 2 make the noise with nothing plugged in.

[idy]

Neat question. More complex things now-a-days have soft power on/off circuits, a delay that leaves the output off until the circuit is on and turn the output off before turning the power off. Sometimes a relay, I hear one clicking on my stereo. I have a Marshall practice amp I use for teaching that always clicks and bumps on power up/down, probably something broken in the protection circuit. The PA maybe would have this, the tube amp less likely. Models? Schematics?

[mcknib]

It may possibly be a faulty pre amp tube in the 80's amp but I'm no expert so hopefully someone that knows will come in and advise - obviously don't mess with high voltage amps unless you know what you are doing

[R.G.]

Things squeal at power off and on when they are conditionally stable and a changing power supply voltage lets them wander off into oscillation-land. It's often a failure of either ground wiring or power supply decoupling as well as conditional stability, as these contribute to the tendency to oscillate and make it worse if it's there.

In solid state devices this is often a design problem. In tube gear, it can happen with tube aging and poor design.

Check the tubes in the tube gear. Live with it or go on an oscillation hunt for the solid state stuff. Fixing it is easier in home-built gear than bought gear because you know what's in home-built gear.

[tempus]

Thanks for all the input. Here's a schematic of my parametric EQ. Can anyone spot anything that may be causing the problem?




Thanks

[amptramp]

The 5532 has a very wide bandwidth and you only show a single 0.1µF cap as the power supply bypass.  When the voltage is up, the internal current sources may avoid current changes when the unit is in steady operation but with power off, the current starts to vary and feedback is established via the +15V power rail.  You need at least one bypass cap for each 5532 as well as the one for the regulator.  The cap for the op amps can be 0.01µF to 0.1µF.  An electrolytic may also help on this rail and you should use a reverse-biased diode from input to output of the regulator to avoid damage if the input goes down in voltage before the output if you add an output electrolytic.

[tempus]

Thanks for the replies guys. I'll look into this - although they aren't on the schematic, I think I actually put some decoupling caps on the opamps. i'll have to crack it open and check it out.

I can't believe I forgot the diode on the regulator....