QSC USA 900 Amp hiss

[DavidDaMonkey]

So I walked into the theater today where the amps had been on all day for the theater camp I am working for to hear a nasty hiss coming from the house right speaker. I am also, btw, the house sound technician at the theater. Anyway, I tracked the problem down to the amp. The hiss stays when the board, graph EQs, and everything else upstream of the amp is off. The only upstream piece of gear still on is the DBX Driverack. I've heard before that this could result from capacitors going bad in the amp, but I don't know. Does anyone have any thoughts? A friend of mine who has been in the business much longer is hopefully going to come in and help me asses the situation before I go to the boss and tell her we need to buy a new one, but I figured I'd check here as well.

 

[rwhealey]

Give QSC a call. They're supposed to be great about supporting old products.

 

[avkid]

That's pretty old by now, so I wouldn't be surprised if some of the caps have dried out.

 

[DavidDaMonkey]

That's pretty old by now, so I wouldn't be surprised if some of the caps have dried out.

I'm decent with a soldering iron and took some basic studio electronics classes in college a few years ago, but I'm by no means an expert. Is this something that's an easy fix? Can I pull the old caps, order ones of the same specs, and then replace them and be good to go (assuming that the caps are the issue)?

 

[TimmyP1955]

If the DriveRack was still on, you have not finished troubleshooting. IME the DR is more likely to cause a loud hiss than is a power amplifier.

 

[DavidDaMonkey]

If the DriveRack was still on, you have not finished troubleshooting. IME the DR is more likely to cause a loud hiss than is a power amplifier.

Even if it hasn't been touched (although I admit this is only an assumption(but it's a very likely assumption)) and is only affecting one of it's 6 outputs?

 

[David Ashton]

The first rule of fault finding is "never assume anything"

 

[DavidDaMonkey]

Ok Ok Ok fine, I checked the Driverack, and whaddya know: I think that's where the problem is. Thanks guys!

When I plug directly out of the board into the amps the hiss goes away. Now the next question is why? Does anyone know any reason that a single output on the Driverack would be producing hiss? The only thing I knew to check so far was the EQ to see if someone had boosted a high frequency somewhere, but that's not the case.

 

[DavidDaMonkey]

Update for anyone interested:

I've narrowed it down to specifically the channel 2 output of the Driverack. When I pull the low pass all the way down or when I roll off the high end on the EQ, the hiss goes away. I'm not sure, however, what this tells me. The EQ and crossover settings are the same as they have always been in this program, so I know it's not a matter of something being incorrectly boosted. Does that mean it's an issue elsewhere within the settings, or does it mean it's something that has failed in the hardware? Or does it not tell me anything?

 

[avkid]

What does your eq curve look like on that channel??

 

[DavidDaMonkey]

Ok, so problem solved. I realized after being SURE that it was the amp, then being POSITIVE that it was the Driverack, that it was indeed the graphic EQ that's directly downstream of the board. Troubleshooting in a noisy theater full of kids is not the best way of doing things and can apparently result in error . I don't know what was wrong on it, but it's a rarely used piece of equipment that I am content (for now) to simply bypass.

Thanks for all the help guys, I really learned an important lesson about the process of troubleshooting.

 

[jkowtko]

You didn't necessarily solve the root problem, but you did identify the offending piece of equipment and were fortunate that it wasn't a critical item and could be removed from the configuration. If the EQ is not an old junker, it would still probably be nice to try to figure out what's wrong with it.

Two things that cause hiss on my DBX231s --
1) output gain up too high ... you'll introduce noise from the EQ amps(?) and in return lower your board outputs to compensate. Reducing EQ output gain and upping board outputs will reverse this effect
2) attenuating all low-end frequencies, say below 150Hz. For some reason on the DBX231 this increases hiss a bit. Zeroing out these frequencies drops the hiss back down.

If you have a chance to check out the EQ later, I'm curious to hear if you find anything.

 

[DavidDaMonkey]

You didn't necessarily solve the root problem, but you did identify the offending piece of equipment and were fortunate that it wasn't a critical item and could be removed from the configuration. If the EQ is not an old junker, it would still probably be nice to try to figure out what's wrong with it.

Aye, that's the kicker. It's an old one we pulled out of storage when we were cleaning out, no idea how long it had been back there. I mostly only use it for the monitor's anyway, as those don't go through the driverack, and that part still functions fine.