I tried to write something short about this system and problem but saw it getting to be a term paper. I'll try to be short.

There's a music/paging system using 8 TOA amps. The same input is daisy-chained to all 8 amps. Some of the amps have U11S input modules -- inputs are labeled 1, 2, and 3. Other amps have U13s, with inputs labeled H, C, and E.

There's buzz coming from somewhere and we're going to find it, but meanwhile something puzzling is slowing me down.

The TOA tech guy tells me that the U-11S input modules are for unbalanced inputs. He says the third connection, Pin 1, is not for anything and isn't used. While trying different things today (we also were getting about two watts out of a 100 watt amplifier), I connected the input to pins 2 (hot) and 3 (Neutral). I got sound. Then, on a whim. I connected Pin 1 to Pin 3. The sound instantly stopped.

Now.... if Pin 1 isn't for anything and isn't used, why would connecting it to something else have any effect?

I can't find an actual schematic of the U-11S on line, just a representation of it. Does anyone have a schematic of it? Know anything about the actual connections of Pin 1?

I haven't gotten into the U-13 yet, but the TOA tech guy said Pin 1 doesn't do anything on that, either. Yet there's a wire from each of the three pins to the output connector of the board. So I'll have to see what's going on there, too.

The amps are all Series A-900 type.

Thanks for any help.

While you wait around trying to fix the buzz you are just screwing around with in/out modules? Put the gun down, slowly back away from the amps. We will get you some help. Seriously, you make no mention of a reason to investigate the modules. So don't bother.

You mention 2 different modules.

1 of them is balanced (U13s) the other is unbalanced (U11s).

You are getting buzz from a grounding issue somewhere along the line.

Buzz on one amplifier? All amplifiers? I'd break the chain apart, then start adding amplifiers until the buzz begins. I'd also be curious which pins, if any, are tied to the chassis and if 2 & 3 float or not.

You'd actually want not to do anything, since the audio source is a dozen feet from the amp stack, and the entire thing is wired with unlabeled undocumented CAT5. It's pretty impossible to tell what the hell is wired to what without bringing the whole company to a halt. They're a LARGE Toyota dealer nearby who just expanded one of the zones: it's the Toyota waiting area, about 3,000 square feet, with a Starbucks and a Subway. (The AAA and DMV offices are at the other end, in the Lexus part of the property.)

I'm figuring the buzz is coming from the cabling that goes out to at least three other racks on the property. There's a transformer isolating the XM radio from the audio system, but then these different buildings are all connected together. With freakin' CAT5. It's a mess we'll conquer.

EDIT: there's more going on here. See [Link: remotecentral.com].