Controfuzz: Crossing over to the frizzy side

[Mark Hammer]

breadboarded it and something weird happens

oscillation,

triple checked everything, any ideas?

I made two of them this past weekend, and no problems of this sort at all.

Sure you have all your inverting and noninverting pins connected properly?

Incidentally, while the first one was made with a TL072, as per Dano's drawing, the second one used an LM748 and 27pf compensation cap for the gain/fuzz stage (no feedback cap, though), and an NE5534 with added feedback cap for the second/difference-mixing stage.  Taking all component tolerances into account, the two sound very similar.  I take from this that the added feedback cap I proposed in the TL072 version was a reasonable idea.

Another incidental is that whatever noise is generated by the gain stage will not show up in the output, as long as it amounts to less than the forward voltage of the Ge diodes.  This was pleasing to me since I made my prototype units without any shielding whatsoever (cables, box, ground plane, etc.) and tested them in a room with fluorescent lightings.  I wouldn't say dead quiet, but VERY quiet considering how much potential gain is in the circuit.  Nice.

[Arn C.]

Mark, would this work or do the values have to be changed with this effect?
Thanks,
Arn C.

[Mark Hammer]

All the values I showed work just fine.  The only change from the drawing I posted is a pop-immunity resistor (pick your value) to ground on the input.

[Arn C.]

Thanks Mark, I am going to give this a try!

Peace!
Arn C.