Gareth Magennis wrote in message
...


"N_Cook" wrote in message
...
N_Cook wrote in message
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repair m on e-S and there is an inverter stuck in before the R ch o/p,
not
for the L





Is it just for evening up the ps?. If a momentary high draw from one PA
on
+/ve rail dropping reservoir by 1V then also a matching 1V drop on
the -ve
rail from the other ch, wheras usually that would be a 2V drop on + rail
only






Yes there is a specific reason/technique involved in running the outputs
"out of phase" and inverting the signal to one channel.
Yamaha also do this with the Stagepas range, Lab Gruppen do this with
their
far more pro series of amps. There are others. You can get more out of
the power supply by doing this.


I personally consider the technique fundamentally extremely dodgy, since
the
"ground" connection on one of the amplifiers is actually live, which is
not
at all obvious to anyone not in the know. In the case of the Stagepas,
you
have 2 speaker jack outputs next to each other. Plug in 2 metal jack
plugs
to your speakers and you have the almost unimaginable situation of 2 jack
plug bodies a few mm apart, with a power amplifiers output between them.
If you use right angled metal jack plugs, a dead short and a blown amp is
pretty much a certainty at some point, as is accidentally touching
anything
grounded against the one live speaker output jack plug body "ground"



Gareth.




First looking at output board, knowing L ch was flakey, saw that there was
no continuity to R ground of a 1/4 jack plugged in, eventually deducing that
continuity was to the tip.

Anyway all PbF "usual suspects" dealt with on output board,ps and relevant
shannels of mixer panel. PA section not touched, inputs swapped to isolate
if the problem emerged (owner having to swap L and R speaker leads), would
isolate to mixer or PAs problem. Unlikely pa problem as L ch of meter
dropped , nothing on schema indicating meter feed from PAs .
I was just wondering, as the feed for the ps is unipolar rectified mains and
there was an obvious PbF failing on that. Especially as problem noticed at
switch on, no great load, whether the raw DC feed reducing due to
intermittant crimp contact could have led to a mior supply dropping and
thence registering as 1 ch problem on the bargraph meter.

Run for 6 hours since, by owner, with problem not emerging.