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.