Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two completely separate and identical power amps, one of which was running very hot very quickly. No schematics, but problem was quickly traced to an open circuit resistor, by comparison ohms checks between the two amps. In order to replace the resistor, it was necessary to remove the offending ("B") power amp to get to the board underside. The power rails and ribbon from the mixer desk section were on removable connectors, but the 4 pole speakon connector was bolted to the rear panel and the wires were soldered directly to the board. When I came to unsolder these wires from the socket, I noticed that the "B" channel one was wired the opposite way round to the "A" channel one. I noted this down to check on later.
When the amp was tested ok, I bolted it back in, and reconnected the speakon socket the same as it was when I unsoldered it. The wires lay exactly where they originally were, and looked absolutely factory-original, even down to the one which went to the uppermost connector tag, having a rubber sleeve on it, exactly the same as on the "A" channel. Except the two channels were definitely wired arse-about-face to one another. On both channels' speakons, the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel "A" the ground wire was connected to the "+" pair, and on the "B" channel, the ground was on the "-" pair.
I fired it up again, and put a sine wave in, then hooked up two channels of my 'scope, one to the "A" output and one to the "B", both with the same polarity, and was surprised to find that the two signals were completely anti-phase, which meant that with the reversed connections to the speakon sockets, they would be back in phase again to the outside world.
I then went back to the power amps' front ends near where the ribbon connector was, and found the same thing. At the same point on each amp, the signals were in opposite phase to one another. I then spotted three little 3 pin header blocks, with one pin pair on each designated "B", and the other pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to the power of' position. On the B amp, these three jumpers were set to "B", and on the A amp they were set to "A", so clearly, these jumpers reverse the phase of the signal coming in from the desk section. The two amps then process the signals in antiphase to one another right to the final output, where the 'correct' phase relationship is again restored, by wiring the speakon connector 'wrongly'.
I have puzzled over this, and can think of no good reason for doing it that way. I did wonder at one point if it was anything to do with being able to make the amps bridgeable, but I downloaded a copy of the user manual, and there is a dire warning paragraph towards the end, specifically saying that under no circumstances should any attempt be made to operate the amps in bridge. It also warns against making any connections between the two amps or any speakers connected to them, although in actual fact, the 'ground' side of each output is truly ground, and is common to both amps, the whole (dual) power supply, and all metalwork.
What am I missing here ? Not of any real consequence, as the unit is mended and back in its original condition, but for academic interest, it would be good to understand the designer's thinking.
Arfa