QSC CMX500Va Amp in Bridge mode problem

[Alan Hamilton]

I'm trying to see if I am missing something before deciding the fault is in the amp. The QSC CMX500 recommends bridged mode to drive 70V speakers.

I was asked to troubleshoot the system because the amp was cutting out. I assume it was going into protect because it would go silent and signal meters go dark... then after a bit audio would return. When it did the RIGHT side of the amp was showing heavy clip. Not the same as the right side at all. Right side (ch 2) input attenuator was all the way down (fully counter clockwise)

Input is balanced line feeding the barrier strip channel 1 input.

Speakers are tapped at 480W IIRC (16 x 30W ea).

When speakers were working I heard no audible distortion, but I'm listening to 70V horns so I wasn't expecting hi-fi... but they sounded fine. Just cutting out.

Dip switches set exactly as shown in manual page 12 (FYI, unshown switches are set to the LEFT side):http://qsc.com/files/8713/8911/9180/CMXa_Series_User_Manual_5-Language.pdf

The speaker wires were connected to the bridge output section of the rear terminals.

I disconnected one leg of the speakers and there was no change at the amp as far as metering goes (still heavy clip showing on the RIGHT side).

If I switch the amp out of bridge mode, there is no clipping on the left input (just normal signal meter). And if I connect the speaker line to that side all is still fine. If I put it in parallel mode, channel 2 metering behaves identically with channel 1 (I did not put a load on channel 2 to test past this). This tends to indicate to me there is no problem in the speaker line.

As far as input level goes, what was being sent to amp when I took the amp out of bridge mode was the same as what was sent when it was in bridge mode. So the signal was not hotter being fed to it... Yet no clipping in parallel mode. Big clipping on RIGHT side in bridged mode.

Bringing down the the channel 1 input knob on the amp didn't take the right side out of clip either when it was bridged. It either had no or little effect.

I was working in combat audio conditions just to get them back working so I patched around the problem and didn't get to test every last detail. Between the amp and the speakers there is an Altec 70V level control (similiar to these: http://www.mcmelectronics.com/product/ATLAS-SOUND-AT100-/555-7342)
I'm not sure if that somehow throws a monkey wrench into the equation or not... but it doesn't when the speakers are connected to the amp in normal mode (versus bridged). But I would've liked to have tested with this level control out of the circuit in bridged mode. But I did disconnect a leg of the speakers at one point directly from the back of the amp with no change so I'd think that would eliminate this line of thinking anyway.

Sooooooo Is there a fault in the amp? Did I miss something? Does the level control throw a monkey wrench into the equation in bridge mode? Something I should try before I pull this? Does this point to an external fault I'm not thinking of?

The gist of it is, is there anything that would cause the RIGHT meter to be in hard clip and not registering the same as the LEFT meter in bridged mode?

-Alan Hamilton

 

[Jason Lavoie]

Re: QSC CMX500Va Amp in Bridge mode problem

Speakers are tapped at 480W IIRC (16 x 30W ea).

but what did the load actually measure as? (without a measurement you're guessing and assuming there are no wiring issues)

I disconnected one leg of the speakers and there was no change at the amp as far as metering goes (still heavy clip showing on the RIGHT side).

Which leg did you disconnect? why didn't you disconnect both if the problem didn't go away when you did the first leg?

The gist of it is, is there anything that would cause the RIGHT meter to be in hard clip and not registering the same as the LEFT meter in bridged mode?

-Alan Hamilton

Yes. in bridge mode you're asking two amplifiers to work together to make a combined signal of twice the voltage, so if either leg is shorted to ground somewhere it can cause that channel's amp to shut down.
I'd suspect that one side of your speaker line is shorted to ground. does the problem go away when you completely remove the speaker wires?
measure the load of your line, then measure impedance to ground of each wire.

you can get away with one wire shorted to ground with a normal amp but not with a bridged amp.

Jason

 

[Alan Hamilton]

Re: QSC CMX500Va Amp in Bridge mode problem

but what did the load actually measure as? (without a measurement you're guessing and assuming there are no wiring issues)


Which leg did you disconnect? why didn't you disconnect both if the problem didn't go away when you did the first leg?



Yes. in bridge mode you're asking two amplifiers to work together to make a combined signal of twice the voltage, so if either leg is shorted to ground somewhere it can cause that channel's amp to shut down.
I'd suspect that one side of your speaker line is shorted to ground. does the problem go away when you completely remove the speaker wires?
measure the load of your line, then measure impedance to ground of each wire.

you can get away with one wire shorted to ground with a normal amp but not with a bridged amp.

Jason

I did disconnect both legs but that was the point I shut down and switched to parallel operation. Things were already rolling so I didn't have time methodically go thru the checklist more than just get things working one way or another. So it was more about a workaround and then figuring out the rest when the building is dark and unused this week if there wasn't an immediate fix right in front of me.

So does that 70V level control throw a monkey wrench into the equation that the amp would see as a short to ground in bridged mode that wouldn't normally be an issue?

 

[Jason Lavoie]

Re: QSC CMX500Va Amp in Bridge mode problem

It's not about the level control, it's that when you bridge the amp you're using the + output of the left and the right amps at the same time, so if either wire is shorted to ground it will trip up the amp.
In stereo operation the - of each amp is ground, so theoretically one leg of the speaker line could be grounded and not have much of an effect.

This is all just speculation though until you get a chance to diagnose.

ps: if the amp is shutting down periodically due to the fault, I'm sure you could take it offline for a minute to diagnose?

Jason

 

[Alan Hamilton]

Re: QSC CMX500Va Amp in Bridge mode problem

It's not about the level control, it's that when you bridge the amp you're using the + output of the left and the right amps at the same time, so if either wire is shorted to ground it will trip up the amp.
In stereo operation the - of each amp is ground, so theoretically one leg of the speaker line could be grounded and not have much of an effect.

This is all just speculation though until you get a chance to diagnose.

ps: if the amp is shutting down periodically due to the fault, I'm sure you could take it offline for a minute to diagnose?

Jason

I can take it offline now... But at the time, since it worked fine in stereo operation, that was easier/quicker than anything else.

 

[Alan Hamilton]

Re: QSC CMX500Va Amp in Bridge mode problem

It's not about the level control, it's that when you bridge the amp you're using the + output of the left and the right amps at the same time, so if either wire is shorted to ground it will trip up the amp.
In stereo operation the - of each amp is ground, so theoretically one leg of the speaker line could be grounded and not have much of an effect.

This is all just speculation though until you get a chance to diagnose.

ps: if the amp is shutting down periodically due to the fault, I'm sure you could take it offline for a minute to diagnose?

Jason

I'm not sure exactly how those level controls are designed. So the question is, since it is between the amp and the speakers, is its design such that it would essentially be a short in bridged mode as far as the amp is concerned?

 

[Jason Lavoie]

Re: QSC CMX500Va Amp in Bridge mode problem

I'm not sure exactly how those level controls are designed. So the question is, since it is between the amp and the speakers, is its design such that it would essentially be a short in bridged mode as far as the amp is concerned?

70V volume controls are just multi-tap transformers with several different turns ratios. if properly wired it should just appear to the amp as if you're lessening the load as you turn them down.
Even if it was wired so that a short was possible, your problem would go away when you lifted just one leg of the speaker line. which is why I'm saying it seems like one leg of your speaker line may be shorted not to the toher, but to building ground somewhere (pinched wire on a sharp edge, miswiring somewhere, faulty device)
You really can't tell without a proper meter.

Jason