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QSC amp shutting down

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David A. Parker:
had a problem last night, 8 ohm monitor rated 225 watts rms, QSC PLX 2402 amp. One monitor on one side of amp, other side of amp open. Drummer's wedge. Drummer wanted kick in his wedge, so I obliged. Drummer was happy with it, but amp would red light and then shut down, cycle, come back on, everything's fine for a while, then it would redlight and shut down again, then back on. Normally I would think it was low voltage, but the other amps were on the same circuit, and none of them had a problem. I know when a voice coil heats up, it changes the impedence, so could this have in some way triggered the shutdowns?

Ivan Beaver:
David A. Parker wrote on Sat, 26 February 2011 13:02
had a problem last night, 8 ohm monitor rated 225 watts rms, QSC PLX 2402 amp. One monitor on one side of amp, other side of amp open. Drummer's wedge. Drummer wanted kick in his wedge, so I obliged. Drummer was happy with it, but amp would red light and then shut down, cycle, come back on, everything's fine for a while, then it would redlight and shut down again, then back on. Normally I would think it was low voltage, but the other amps were on the same circuit, and none of them had a problem. I know when a voice coil heats up, it changes the impedence, so could this have in some way triggered the shutdowns?

As the voice coil heats up, the impedance rises, and this puts "less" of a strain on the amp-so that would not be your problem.

A couple of thoughts.  Maybe the speaker cable has a problem-like an intermittant short.  That could cause the amp to shut down.

Is the fan working properly? Maybe it was getting hot and is shutting down due to thermal overheating.

Or the amp could have a problem-such as a cracked connection or an intermittant in a component.

Without more testing, it could be anything.

David A. Parker:
yes, I realize it could be anything. Not an easy problem to duplicate in the shop. I don't know where to start. I have another spare amp, but if I just change it out, then I wont know if the amp in question is bad. I was hoping someone had a similar problem. Thanks for the one possibility you extinguished for me.

John Watson:
Did you try swapping channels, cables?

Scott Smith:
I still go with voltage drop.  You added kick to the monitor which pushed the channel much harder.  A dynamic peak can cause an equally dynamic dip in voltage faster than you can see, but the amp still sees it.  

I run a bunch of PLX amps, and the only thing that shuts them off for me is voltage dips.  Often though, they do not all react the same, or at the same time.  One channel can go out, while the other remains... or the entire amp can shut down.  I have had amps that were idling shut down while sharing a circuit that was seeing voltage swings from a larger amp.  Give the amps solid voltage, and they perform flawlessly (assuming you don't have a wiring problem).

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