Pedal screeching (self-oscillating?) when input is unplugged

[josephfra]

Hey everyone, me again with the same circuit I keep bugging you about about.... :S


(In case it's worthy of note, I've replaced one of the diodes with a yellow LED)

This is the circuit I've got breadboarded. Thanks to a lot of help from you all before, it's sounding pretty good! The one issue I have left however, is if I leave the input unattached (either without the cable entirely, or the cable is plugged into the input of this pedal but isn't connected anywhere on the other end), it makes a viscious screeching noise. My thoughts are that maybe this just needs an input buffer, but I wanted to check in here and see if there's anything else within the circuit that could be causing the issue and if there would be any other solutions.

Many thanks again.

[Vivek]

I had designed a high gain pedal which screeched when unplugged

It got solved by

A) switching the input to ground when not plugged in

B) various caps designed to limit the high frequency response

C) shielded cable inside the box

D) repositioning the input and output inside the box

E) 0.1uf caps on rails, physically close to the opamps

F) adding 0.1uf in parallel to Vref cap

G) making sure to follow good ground principles while designing PCB


I did not need to add cap to reduce impedance at high frequency

[Vivek]

Do consider 1k resistor in series with C8, to add a bit of current limiting protection in case there is a short circuit on output (though opamps have short circuit protection)

[Rob Strand]

This is the circuit I've got breadboarded. Thanks to a lot of help from you all before, it's sounding pretty good! The one issue I have left however, is if I leave the input unattached (either without the cable entirely, or the cable is plugged into the input of this pedal but isn't connected anywhere on the other end)

For the floating cable case you can only add low-pass filtering somewhere.  Ideally the filter cut-off needs to be high enough not to affect the tone but low enough to kill the oscillation.  On some high gain pedals the oscillation won't go away until the filter cut-off is so low that it affects the tone.  In this case you are better off not worrying about the weird case of a pulled jack + cable.   Keeping the output wiring away from the input wiring can also help but it probably won't help extreme cases.

For the case where the input jack isn't inserted into the input socket, this is easy to fix.  Use jacks with terminals that switch so that when you pull-out the input jack it shorts the input line to ground.  Most amps do this.     (Don't do the same thing on the output jack.)  With many pedals when you pull the input jack it kills the power so you don't care.

[Axldeziak]

Simplest "fix" is to turn the volume on the amp down to zero (or turn it off for a minute) before unplugging the pedal. Works every time.

Nothing worse than some joker declaring a thumb war with a unplugged cable jack. Thump thump thump thumpity thump.... kek
Same for a screeching pedal, or that guy who has to plug and unplug his guitar a half dozen times.

[josephfra]

Thanks for the ideas everyone -I guess this isn't really as big a deal as I thought it was. I'll go with switching the input to ground when it's unplugged.