..Robbie MCM Response *Obsidian, and..

[petemoore]

I couldn't post under the Thread...so,
 To test if the wiring around the switch is causeing the howling, you can pull just one wire from it to physically separate input/output signals, If you make this the longest of the two wires, and it seems sensative to producing/preventing the howling...make it shielded.
 I had some noise problems, oscillation problems with either a Blackfire or Obsidian, and built another, this time with the box layout such that a very short input wire, then the length of the board away from input, was where the greatest part of unshielded signal path went, that makes the output wire back to switch being the long SP wire, [approx 1/2+ of the signal path length] was shielded, the shielding grounded to the board. That said...
 Layout may or may not be your problem. A 'narrow lane' [small switch] switch...I've read of high gain circuit oscillation problems solved using a 3pdt's inside posts for LED indicator switching, and the outside posts for signal switching with various results...the  Osci-problems may be 'from the board. That said...
 This Is a high gain circuit. High Gain circuits are prone to oscillation problems, layout, switch size, and also whether the input and output are in phase or out of phase.
 I had a 4 stage Obsidian, IIRC a 5 stager was created by Joe Davisson as an oscillation fix for the high gain circuit, after field research on other suggested oscillation elimination fixes found variable results. The problems were narrowed to the switch/Sw. wiring orientation, the extra stage made for a phase invertion from input to output...[or...Daft  :oops:  memory...can't remember enough about the Obsidian circuit proper to say which way the phase started out as..]