by Doug Coulter » Sun Aug 15, 2010 1:29 pm
I will second some of John's remarks here. When in bridge mode, there is no longer a path straight to ground from either terminal in case of a back-arc, and this can cause troubles indeed, as the protection circuitry cannot handle drives of above the power amp's own rails coming in backwards unless the amp design is really robust. In the case of the Beheringer amp I got cheap, it's no problem -- that thing is as idiot proof as they come - clipping anticipation and limiting, and fat diodes to the rails inside. But I only do that after working out the other inevitable bugs in the rest of things. Remember, this stuff, if bought right, was designed for some fairly inept users (stoned roadies) and will really take the punishment. We were running it last weekend at full power into an XRT in the presence of some arcing -- no problems. When we tried for a lower frequency drive (for other reasons) its fans did speed up -- it was dissipating some power into the too low inductance of the transformer primary, but that's it -- it will take a one ohm load at full power! Less than that, and it shuts itself down gracefully. The new stuff at say, Musician's Friend tends to be good indeed, and cheap enough to not bother with questionable older designs in surplus gear. Like with some other things, the good stuff doesn't hit surplus, just the stuff someone is sick of.
Lots of times having one wire from the main supply going straight to ground will help the amp live longer as there is now a lower impedance path for peak currents than the amp output pin, so most of the stuff goes there instead of back into the amplifier. Or so it seems from observations here. A way over-specced amp doesn't hurt in any case. The only troubles I've had with the big guys is turn-off time in their output devices (haven't tested the Beheringer yet on this) so they don't like full power at high frequencies -- shoot through currents cook them. Probably less trouble with the newer FET output designs. A disaster with the old bipolar ones. Haven't tried a Class-D switcher for this yet, but there they HF response is probably limited in other ways for other reasons.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.