Randall's "Preamp From hell" debugging

[MetalGuy]

Well, not quite, because the thing is working but with few problems.

1/ The tone stack is kind of unresponsive - I can't hear big changes in sound going on especially  Bass and Treble. I checked the wiring several times.
2/ When Sustain/Boost /the pair of diodes/ is On the output level drops too much - I was expecting a decrease but not to such extent.
3/ Again when Sustain/Boost  is On and Gain is past 12 o'clock a 200-300Hz oscillation appears.

I used the R.G.'s well known 9V to 33V /up to 25V/ converter  to power up the unit.

Any comments would be appreciated.

[Marcos - Munky]

I didn`t finished my build yet, but here is the original schematic. It have some voltages in it, which may help you in your debug. Looks like the voltages are the same for both the channels.

www.geocities.com/munkydiy/randall.jpg

About the question 3, the preamp will have lots of gain, so oscillations will appears easily. Be sure you don't have crossed wires and, if possible, use shielded wires. Or maybe the power supply isn't the best one for this circuit.

[MetalGuy]

Thanks for the schematic. I'll check everything one more time. Also let me know after you finish your build.

[JimRayden]

Be sure you don't have crossed wires and, if possible, use shielded wires. Or maybe the power supply isn't the best one for this circuit.

Actually, crossed wires are good, it's the parallel wires that are evil. Make sure all the wires pass each other at 90 degrees.

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Jimbo