Screwed Up Meeting Room Audio System, Need Help

September 8, 2011 8:45 AM

By inadvertently swapping amps of different power, a non-audio engineer has managed to screw up an entire sound reinforcement system and needs immediate advice on what to do next. (details inside)

My office has an audio system that starts with Shure gooseneck mics that mix down and into a Rane MA3 three channel amp which drives three sets of JBL Control ceiling cans at 70 watt TAPS. Prior to getting to the amp, the audio passes through an AMX volume control box which allows our touch panel system to control the levels.

A few weeks ago, we lost the sound coming out of one of the amp outputs. Some initial troubleshooting suggested that the amp was the problem, so we obtained a new one of the same model and connected it but the problem remained. We quickly learned that the previous amp apparently had additional transformers installed that were necessary to drive the speakers, and the new amp was having to work much harder to do the same work. So since the amp clearly was not the problem, we removed the new amp, reinstalled the old one, but now ALL THE SOUND IS GONE! Everything is powered up and the connections have been checked, so could the problem be in the AMX software/hardware? Can we simply remove the AMX volume box from the chain? Could the new amp's difference power have caused any damage?

Any help would be appreciated.

posted by Jamesonian to Technology (4 answers total)

three sets of JBL Control ceiling cans at 70 watt TAPS

I think you mean that the speakers are connected on the 70 volt taps. 70V speakers are common in commercial installations, and since the Rane amp isn't designed for such speakers you would indeed need a step-up transformer to drive them.

Can we simply remove the AMX volume box from the chain?

I don't know AMX well myself but am dubious about your assertion that the AMX box is doing volume control and nothing else (really, no other switching or control?) If you are certain this is the case, you could take it out of the chain AS LONG AS YOU REPLACE IT WITH SOMETHING ELSE TO ATTENUATE THE SIGNAL GOING INTO THE AMP. If you remove attenuation completely you will be running the amp at full power and could easily damage the speakers. You could pop in a cheesy line level controller like this one but you would then of course lose the ability to control the volume from the touchpanel.

All this is really making me wonder, though; what happened to whoever set up the system initially? If they used AMX I'm guessing this is a fairly high-grade setup, and not the sort of thing you'd want to mess around with unless you understand it pretty well. I can understand the desire to save a little money by just dropping in an exact replacement for a single component you're certain has gone bad, but at this point you've tried that and it may just be time to call in the big guns, or at least ask them for advice.
posted by contraption at 9:25 AM on September 8, 2011

I looked into it a bit and was a little surprised to discover that AMX actually does (or at least did) make stand-alone volume control boxes. Is the AMX piece the VX1? If so, I would make sure that it isn't muting. I would then try disconnecting its input and hooking that cable up to the line in of a laptop computer or boom box, just to confirm that it's receiving some signal from whatever preamp/mixer/source device is directly upstream from it. You can also hook up an iPod or some other source to the amp's inputs (but make sure the source device has its own volume control and that it's turned way down to start) and see if you can play it thorough the speakers. This will tell you whether the problem is upstream or downstream from the volume control box or (if both tests succeed) that it is within the box itself.
posted by contraption at 9:44 AM on September 8, 2011

Could there be some kind of protection circuit or mechanism somewhere?

Either a blown fuse to replace, or a cut-out circuit that needs to be reset.
posted by krilli at 1:28 PM on September 8, 2011

(As in a fuse actually inside the speakers or something ...)
posted by krilli at 1:28 PM on September 8, 2011