Hi Arfa,

Arfa Daily Inscribed thus:

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

I'll bet that your first suspicion is right and the amps have or are
being used in bridge configuration in some other gear.

--
Best Regards:
Baron.