Whine on high gain buffered pedal - not when double buffered

[Ksander]

A word of warning, In case anyone tries the schematic: it appears to fry the lm386! The ac voltages created by the op amp exceed the maximum input ratings of the lm (1.5v vs 0.4v). Smaller gains and/or a protection stage may be necessary...

[Ksander]

A word of warning, In case anyone tries the schematic: it appears to fry the lm386! The ac voltages created by the op amp exceed the maximum input ratings of the lm (1.5v vs 0.4v). Smaller gains and/or a protection stage may be necessary...

I thought the power amp was fried because the circuit started 'gurgling', and an audio probe gave me a signal until the input of that IC. However, I have now tested it separately and it still works. Something else must be wrong. Also, using an oscilloscope, I found that the guitar puts out a much higher voltage than I previously thought: 0.2v instead of 0.04v, and even without a buffer/amplifier in front of it, the LM386 gives a 4v output even with standard gain of 20, which should be plenty for the transformer to put out a signal strong enough for the neon bulbs to discharge... it's back to the breadboard for now.

[Ksander]

I've found a nice solution: it is basically the Ruby amp, with instead of a speaker the transformer + neon bulb and output circuit. I used a 230-6v transformer. If there's any interest I can post a sound sample...

[Elektrojänis]

The solution seems to be found already, but I thought to mention something I found out years ago. With high gains and high input impedances, shielding teh input and output wires doesn't always help to stop feedback from output to input. The signal "wires" between input and output are closest together inside the bypass switch! Hardwire the circuit to input and output and it stops feeding back, but that's not very usefull...

One thing that can help is changing the design so it inverts the signal polarity (some call it phase) if it previously didn't. If it did it might help to make it not invert.

On a 3PDT switch it might help to use the mid contacts for the led indicator and the sides for the signal. That way the input and output signals stay as far from eachother as possible and the led circuit is relatively low impedance to ground so it might actually help as a shield in between too.