Audio problem-undesired attenuation - AVS Forum
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Old 08-27-2012, 03:13 PM - Thread Starter
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I have a pair of systems that are behaving oddly. It is a rather simple system of a QSC amp, 35W wall attenuator, 4 EViD 8.2 speakers with a Bose Subwoofer (all 70V), iPod dock and wireless microphone. The issue is that at the start of the class (it is located at a gym) the volume on the iPod music is fine and loud, but over the course of the 1 hour class the music volume decreases but the microphone volume stays the same. Any ideas?
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Old 08-27-2012, 03:35 PM
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The Amp you refer to must also have a built in Mixer of some sort or you are using an external mixer to drive the Amp.
I would try switching the mic and music inputs to see if the problem is related to the specific input that the iPod is the problem.
The other thing to verify is that the music level of the iPod output is actually remaining constant.

Try running a few tests to closer identify where the problem might be.

"So many tweeks....So little time!"
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Old 08-27-2012, 05:19 PM - Thread Starter
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The amp is a QSC 302V so no mixing on the unit. I haven't had a chance to visit the system since a client called about the problem. I think it might be the wall attenuator that is failing. It might be the client is driving the system so hard the pot is overheating causing increased resistance over time.
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Old 08-27-2012, 06:19 PM
 
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I think it might be the wall attenuator that is failing.

Wouldn't the microphone volume decrease as well?
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Old 08-28-2012, 03:55 AM
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Got to be something else in the system. The QSC won't deal with a mic at all, barely would handle an iPod very well. There's a mixer in there somewhere, and that's where the problem is.
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Old 08-28-2012, 06:28 AM
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Originally Posted by has7738 View Post

Got to be something else in the system. The QSC won't deal with a mic at all, barely would handle an iPod very well. There's a mixer in there somewhere, and that's where the problem is.

Wireless mics generally put out a line level signal that could possibly drive a power amp fairly well.
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Old 08-28-2012, 06:28 AM
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Originally Posted by SAM64 View Post

Wouldn't the microphone volume decrease as well?

+1
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Old 08-28-2012, 09:21 AM
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Originally Posted by arnyk View Post

Wireless mics generally put out a line level signal that could possibly drive a power amp fairly well.
Ah, missed the wireless part...quite right, but what a weird way to build a PA system. Mixers these days are cheap. But that brings to mind another couple of possibilities. The QSC has an internal "clip limiter" that can be disabled with a DIP switch in the back. That implies some sort of variable gain element inside, which may be failing. If the system were wired so that music went to one channel and the mic to the other (hey, there's no mixer! Anything's possible), perhaps one clip limiter failing would cause this. I've also found the gain control pots on QSC amps can get dirty causing a gain drop. The mixing could also be happening at one of the balanced inputs, where say the mic feeds the + input and the iPod feeds the -. No CMRR that way, of course, but it does mix with isolation to the inputs. But my point is, given the type of installation, check for bad connections everywhere. A bad connection in the mic circuit would be the most likely.

Might want to be there when this happens to verify the complaint. Many user complaints are...ah...somewhat unreliable. This is sort of like getting a doctor to diagnose a disease over the phone.
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Old 08-28-2012, 10:05 AM - Thread Starter
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I would agree that some clients are...unreliable...this is a particularly unreliable client as well. The system IS pretty straight forward. There are two rooms in question with the same problem but slightly different configurations. One is a temporary measure until a new cable from a remote rack location with the amp in it can be replaced and the other is a standard configuration for the site. In the first (temporary set up) the amp is in the room with the iPod dock being routed into the "mix in" of the wireless receiver. The receiver is using the line level output to the amp. In the other room, the amp is in the remote location (another room in the gym) and the iPod dock and wireless are separate runs to a TOA mixer and then to the amp. They are both having, according to the client, the same issue. I am scheduled to be there next week to do some other work there as well as fix this issue. I can only think it is the amp (thank you Has 7738, I will look at the dip switchers) as it is the only common thread in all of them.
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Old 08-28-2012, 10:32 AM
 
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The mixing could also be happening at one of the balanced inputs, where say the mic feeds the + input and the iPod feeds the -.

the resulting signal would be the difference between the ipod and the mic...it would be very noticable.
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but it does mix with isolation to the inputs

No, it doesn't.
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A bad connection in the mic circuit would be the most likely.

Why? that's the part that works.
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I can only think it is the amp

2 amps, different configurations, exhibiting the same problem? Unlikely.
the common thing here is the clueless client. I'd start by having them demonstrate the problem to you in person.
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Old 08-28-2012, 08:38 PM
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Originally Posted by SAM64 View Post

the resulting signal would be the difference between the ipod and the mic...it would be very noticable.
And what would that difference be? The two signals have nothing in common, so the difference would be the sum of the mic and the inverted ipod (or vice versa), which is, effectively, a usable sum. No, not noticeable. I've done it, it works just fine, thanks. And yes, the two signals remain isolated from each other by virtue of how a diff amp works. They literally can't talk back to each other or the thing wouldn't work as a diff amp. Remember, this isn't a transformer input, it's an active balanced input.

Yes, I mis-spoke again, I meant iPod not mic. Very little sleep lately, sorry. Would you agree that the fading iPod could be the result of a connection issue, or not?
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Originally Posted by SAM64 View Post

2 amps, different configurations, exhibiting the same problem? Unlikely.
Sorry, where are the two amps exactly? Seems the OP mentioned only one QSC, albeit stereo. If you're referring to my comment that I've found QSC amps with bad level controls, the reality is that level controls of all kinds are prone to dirt, wear and failure. When I noticed low gain on the amp in question, the very first thing I went for was the level control (knowing it's a common issue), exercised it, and cleared the problem at least temporarily.
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the common thing here is the clueless client. I'd start by having them demonstrate the problem to you in person.

I hate to call a client I've never met clueless. Their observations may be flawed, but they aren't usually complaining because the haven't heard something wrong. I'd respect their complaint and even if the problem is not demonstrable, sit with them for a while to see if it comes up. Let's not blame the client until he's been proven clueless. In actual fact, all of us here are in essence clueless until somebody goes there and collects more clues!
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Old 08-29-2012, 06:17 AM
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Quote:
Originally Posted by InlandJohn View Post

I have a pair of systems that are behaving oddly. It is a rather simple system of a QSC amp, 35W wall attenuator, 4 EViD 8.2 speakers with a Bose Subwoofer (all 70V), iPod dock and wireless microphone. The issue is that at the start of the class (it is located at a gym) the volume on the iPod music is fine and loud, but over the course of the 1 hour class the music volume decreases but the microphone volume stays the same. Any ideas?

My best guess is that the level of the music on the iPod recording is itself decreasing.

If the problem was in the amp or the attenuator, it would affect both the mic and the recording. Since the level of the mic input is remaining the same, everything from the amplifier input onward must be maintaining consistent performance.

It is possible that an iPod would develop a fault that would cause its analog output to decrease with use, but that two of them would have the same fault seems highly unlikely. To futher test the system, simply plug in an iPod that is known to be working reliably.

IME people are used to working with systems that have considerable reserve gain, but this particular system probably has smaller gain reserves than most.
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Old 08-29-2012, 07:24 AM
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Originally Posted by arnyk View Post

My best guess is that the level of the music on the iPod recording is itself decreasing.
It's possible, that they are running into music that's mastered differently. As we know, there are huge differences in recording average levels. However, if they use SoundCheck in an iPod, and iTunes, those differences should be reduced to the point that most people wouldn't notice. Something for the OP to check. There isn't much in the iPod that would artificially change levels over time outside of the volume control itself. If they use the dock connector, the OP didn't specify, the volume control is out of the circuit.

Still doing the Dr. on the phone thing...

I hope the OP figures this out and tells us so we don't all go crazy with theoretical worry!
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Old 09-01-2012, 09:28 AM
 
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Still doing the Dr. on the phone thing...

Lol! You think you're a doctor?
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Old 09-01-2012, 11:18 AM
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Lol! You think you're a doctor?
It's an analogy. Get it?
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Old 09-02-2012, 07:35 AM
 
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It's an analogy. Get it?

Not a good one.
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Old 11-26-2012, 10:13 AM - Thread Starter
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It's been a while since I have posted here but the problem was resolved. It turned out that the amp itself had a built in limiter. The client was driving the system so hard for so long that it would pull itself back to prevent overheating and failure, thus the fluctuating audio level. There is a way to turn it off, but since the client ran it so hard all the time I didn't want to let the smoke out. Thank you everyone.
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Old 11-26-2012, 10:46 PM
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Using an underpowered amp for the desired SPL and/or mismatched loads on the amp.
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