"Mark Zacharias" wrote in message
.com...
"Arfa Daily" wrote in message
...
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

Carver has also done this on some models. Not so good when someone tries
to demonstrate through a common grounded speaker switcher...

Mark Z.

No, I guess it wouldn't be ... :-\

Arfa