I promised a new thread to cover the buzzing problem, and here it is. I don't have any pictures handy at the moment, so I'll have to just explain all the connections and the debugging I've done so far.

My hypothesis at the moment is some kind of grounding issue, and the problem only appears when the Kookaburra is hooked up to the RevC. The buzz is present on either RevC channel, when isolated from the other (but still hooked up to the Kookaburra). Now, if I remove the Kookaburra's output from the RevC's input, the buzz disappears completely.

On the scope, the buzz looks like a flat line, with small triangular peaks - not a proper triangle wave, or a sine wave, or anything else that'd seem obvious. If I were to try and compare it with anything else, it almost looked like (a very attenuated version of) the output of the capacitors after the rectifier bridge, but with narrower peaks.

If I hook the scope up to just the output of the Kookaburra, I see no such waveform. This is why I'm suspecting a grounding issue.

Signal-wise, everything's hooked up just as you'd expect. 2 RCA inputs with their grounds tied together and signal lines to their respective input pins on the Kookaburra's input. The Kookaburra output's ground splits to 2 wires, and the two output pairs go to the two RevC boards. Each RevC has its output going to a pair of speaker terminals on the chassis.

Power-wise, I have the trafo going to a screw-down distribution block. It attaches to the RevC boards with the same lines going to AC1 and AC2 (i.e. I didn't reverse the phase of AC1 and AC2 between the two boards) and Kookaburra board isn't labeled for AC1/2 so I can only assume it's the same. Switching the Kookaburra's AC phase didn't change anything.

Ground-wise, I do *not* have the mains ground hooked into the chassis. I'm a little nervous to do it since I still have one LM3886T chip that I'm afraid might short to the case. I don't imagine it'd make any difference, but I'm not the experienced amp builder here. If it's absolutely required, I'll have to swap out the other board's 3886T for a 3886TF for safety's sake.

Based on the above, what do you recommend I try checking out next? Also, is there anything in particular that you'd like to see photographed or diagrammed?

Thanks,

Chris