sound system issues...

[tenor_singer]

I have mentioned previously that I have just moved into a new auditeria (cafeteria/auditorium). My school spent a significant amount of their budget on our technology package... which included a very good sound system. People have been very happy with our sound (of course... they should be because our sound now as compared to our old gymnasium is like comparing a balogna sandwich to prime rib).

I have noticed a couple of issues, though.

1. At our "Orwell Idol" a fundraising show akin to the Television Show "American Idol", we stumbled into a horrible feedback. No matter what my kids tried (pull the line level back, pull the master levels back, pull the monitor levels back, turn the gain completely off, mute the channel, unplug the mic line from the board) the feedback kept screaching. Finally, in an act of desperation they **inserted thought... our system has a sequential power up** turned the system off. Finally it stopped. I contacted the company who did our sound set up and they are at a loss. The only thing that they think could have happened is that our digital compressor/affects/etc etc had a glitch. Anybody here have any ideas?

2. When they ran the 750,000 - 1,000,000 feet of cable in our building, they put it all into "trays" suspended from the ceiling. Unfortunately this means that our line cables are stacked in clusters with electric runs, technology runs, internet runs, ...etc. We get a very good hum every now and then. Sometimes we even have a channel pick up a signal even though we haven't a device running through it. Anything we can do (short of rewiring our miles of wiring) to stop this? So far we have been lucky and it only hits non-essential channels during our productions (ex... our SL hanging mic versus our keyboard during a choir concert).

Thanks!

Tenor.

[mbenonis]

Hmmm, this is a tough cookie you have here...

1. What kind of "digital effects system" do you have installed? More to the point, can you give us an overview of the system before the amplifiers (e.g., console, equalizers, compressors, crossovers, etc.) Also, do you have access to all controls on equipment that was installed, or do you have some "magic black boxes" which run everything without you knowing what they do exactly?

2. This isn't going to be an easy one to fix, but assuming everything was installed properly it shouldn't be a huge issue either. I'm assuming that all audio cables are balanced, which should offer very good noise rejection. Make sure that you don't plug unbalanced audio sources into the mic ports without a direct box (e.g., do not use an XLR to RCA or XLR to 1/4 adaptor just to plug things in). This defeats the point of having a balanced connection and will make that channel susceptible to noise.

Also, check to see that everything was grounded correctly, and that you haven't cut the grounds on any equipment in the past to remove noise. If you are getting grounding noise, try plugging the offending device into the same outlet as the rest of the system (running a long extension is often necessary) and see if that clears it up. Chances are that if it's a powered device like a keyboard, you should be using a direct box anyway, which may solve the problem.

If these suggestions don't clear things up, call the company out and make them play around with it. Any company worth their beans should be willing to work with you to make sure the system operates within parameters, which usually doesn't include random noise.

[Mayhem]

Interesting, as if you pulled the master and monitor levels back that should take any issues in the signal path before the mixer out of the equation. Is it possible that you had an amp that was oscillating? Killing the power would certainly stop that.

Is there any form of warranty on the install that would enable you to get the install guys to come back out and troubleshoot this for you?

You could try and recreate the problem and then take each FX/Processor out of the path one by one and see if it stops. I would have someone manning the amps, so they can wind down the input if need be. Alternatively, you could try running a mic directly into the board without routing through any of the FX or processors and then introducing them back in to the path one by one.

mbenosis hit the nail on the head with regards to ensuring that the grounds are present and correctly installed. However, a lot of amplifiers will have a ground lift switch on them, which isolates the signal ground from the chassis (power) ground. This will often remove the noise.

This is NOT the same as cutting the ground pin form the plug or gutting the ground wire off inside the amp, which is extremely dangerous (not to mention stupid). It simply disconnects the signal ground from the circuit.

Is it possible at all that someone has routed either a wet or dry output form one of the FX units directly to one of the amps? Highly unlikely but just something that popped into my head.

Good luck and keep us in the loop.

[AVGuyAndy]

It sounds like the FX units are actually DSP, and they are screwed up. Get the install guys in there.

As for your cabling, a good rule is that sound is in seperate conduit from other things, so you're kinda screwed on that one. But, if seperate channels hum, you may be able to lift pin 1 at the console, and that may solve it.

[mbenonis]

Lifting the ground at the console would be a very bad idea for a number of reasons, one of which being that you couldn't use phantom power on any channel with the ground lifted. A better idea would be to double-check that all equipment is grounded properly, and that there's isn't any voltage between the ground at one end of the mic run and the other end.

[AVGuyAndy]

Originally Posted by mbenonis

Lifting the ground at the console would be a very bad idea for a number of reasons,

What are the other reasons? This is standard practice. Just look at any decent split snake or DI, or even a console channel strip! If it were so bad, why would people would want to pay more for this feature?

By the way, lifting pin 1 does not always kill phantom power, the building ground could carry the return current for phantom power.

[mbenonis]

Originally Posted by AVGuyAndy

What are the other reasons? This is standard practice. Just look at any decent split snake or DI, or even a console channel strip! If it were so bad, why would people would want to pay more for this feature?

By the way, lifting pin 1 does not always kill phantom power, the building ground could carry the return current for phantom power.

My apologies for the tone of my last post; I didn't mean to attack your post but rather say that cutting pin one should be a last resort measure to reduce hum. I'm not feeling too well today so my posts may have come across as rude. I've always been taught that before lifting the ground anywhere in the system (on signal or AC connections), you should strive to make sure that all equipment is grounded correctly, and that there is nothing creating a ground loop in the first place. While cutting the ground in a signal cable (not an AC cable, of course) is a valid temporarysolution to a grounding problem, it can cause side-effects that might also be problematic. As far as this particular case goes, it would be a far better solution to call the installer back out to check all equipment and cable runs for possible grounding issues - I know I wouldn't accept a brand new system that had hum or noise issues.

Once again, my apologies on the tone of my last post.

[Mayhem]

Originally Posted by mbenonis

I've always been taught that before lifting the ground anywhere in the system (on signal or AC connections), you should strive to make sure that all equipment is grounded correctly, and that there is nothing creating a ground loop in the first place.

Very true and I agree. Any time that you cut a ground connection, that cable needs to be marked and repaired after the show, or permanently marked as a ground lift cable and only used as such.

I prefer to use short adaptor leads that are clearly marked and can be patched into a system where needed. Or as I mentioned earlier, using a ground lift switch on an amp. The particular amps that I have with these switches will reset themselves once the power has been cut to the amp.

Most of the problems that I have is when I use a room that is has signal cable already run. As you say however, most of the time you should be able to clean up the hum. If anyone complains about the hum you could always tell them it is “pre-heating” the speaker voice coils

[cutlunch]

Originally Posted by tenor_singer

I have noticed a couple of issues, though.

1. At our "Orwell Idol" a fundraising show akin to the Television Show "American Idol", we stumbled into a horrible feedback. No matter what my kids tried (pull the line level back, pull the master levels back, pull the monitor levels back, turn the gain completely off, mute the channel, unplug the mic line from the board) the feedback kept screaching. Finally, in an act of desperation they **inserted thought... our system has a sequential power up** turned the system off. Finally it stopped. I contacted the company who did our sound set up and they are at a loss. The only thing that they think could have happened is that our digital compressor/affects/etc etc had a glitch. Anybody here have any ideas?

2. When they ran the 750,000 - 1,000,000 feet of cable in our building, they put it all into "trays" suspended from the ceiling. Unfortunately this means that our line cables are stacked in clusters with electric runs, technology runs, internet runs, ...etc. We get a very good hum every now and then. Sometimes we even have a channel pick up a signal even though we haven't a device running through it. Anything we can do (short of rewiring our miles of wiring) to stop this? So far we have been lucky and it only hits non-essential channels during our productions (ex... our SL hanging mic versus our keyboard during a choir concert).

Thanks!

Tenor.

Hi Tenor
I have a few thoughts on your problems.

The feedback issue first. I don't know what equipment you have so I haven't been able to check this out in the manual. There is one thing you could check does some of your gear have test tones built in? I don't think the venice mixer has one. But what about your compressors eq sometimes the gear may have test tones to help set levels. This could be worth a look.


The hum problem could have a number of causes. Things to look for are: does the hum change with the levels of the lights or is it a constant hum.

Are the lines that are picking up the hum balanced or unbalanced? Unbalanced lines will pickup hum more easily. Are the amplifiers driven with balanced lines. Where are the amplifiers in regards to the mixing desk? ie are they near the mixing desk or down at the stage. If they are a distance apart try running a power lead from the same supply as the mixing desk and see if this effects the hum.

Are you able to get up to the cable trays? If you can, see how they have laid them out. Don't touch the power cables but see if you can move the audio leads further away from the power leads the magnetic field strength drops off with distance. The ethernet leads shouldn't have much effect.

Just some ideas from having seen this type of problem before. Hope it helps.

[tenor_singer]

He Everyone...

I have looked at our rack of equipment in our auditeria's control booth. I see two signal processors. One is a QSC DSP-30 unit. The second is covered with a plate to keep kids from fiddling around with it. I will try to find time today to remove the plate and find its name.

Here is some more information about the hum:

It is only happening on two channels so far. One of which I am using an old (purchased in 1993)... well old compared to the age of our current technology... hanging microphone made by Audio-Technica (I use them because we're about 50 miles NE of their Stow, Ohio headquarters). I am waiting for our lift to be put back on the stage so that I can go swap it out with one of our newer hanging microphones to see if it is a mic problem. I also have some direct boxes with a ground lift switches on them. I will try to lift the ground with those.

I called the technicians from RJ Martin. They told me that the random humming that I am encountering cannot be stopped because they ran our line inputs in with the other electric feeds of the auditeria. Nothing is seperated by conduit... lines with similar purposes are zip tied together and laid on these "trays" that look akin to a really long rib cage. They said simply because there is a strong field surrounding the wiring that was installed, we're going to get hums every now and then.