Installs Stage monitors sound like they're underwater (at best)

[twelbes]

Hi all—long time lurker, first time poster.

I'm trying to track down the cause of a problem in the theater that I have recently inherited as manager.

Disclaimer/Warning: I am not remotely a sound engineer. Anything that sounds vague or hand-wavey is probably me hiding my ignorance. Please ask incisive questions.

My first question, is this a bad idea?

That shot is the back of the amp feeding our eight, dedicated monitor outputs. Those monitor outputs are built into the stage, four upstage and four downstage. Note in the image that there are two leads running from each amp output, one to an upstage output plug, and the other to a downstage plug (So Upstage #1 and Downstage #1 are receiving the same signal).

So is that regular practice? I remember trying similar monkey business with my home boom box when I was twelve, but does anyone sense a problem with this set up where reliability and quality is at a premium?

And I'm asking because we have a problem: those monitor outputs built into our stage are pretty much unusable. When we plug in monitors, each output has its own problem. One sounds like it's underwater, one doesn't put out anything audible, etc.

For context, here's our audio chain: Unpowered monitors -> dedicated outputs on-stage -> amplifier (wired as pictured above) -> digital signal processor (Lake LM44) -> Digico D-Rack -> sound board.

So here's what we've tested:

• The monitors themselves are fine: in any other set-up they work as expected
• The D-Racks are operational: no matter which D-Rack output we run to the built-in monitor outputs, the audio problems are the same

Possibly wild speculation:

• I think it's unlikely that the plug connections for the monitor outputs are the problem, because they would all have failed in slightly different ways at the same time.

It seems most likely to me that the problem exists between the Amplifier and the DSP. The hacky-looking wiring on the amp is the most visually obvious issue, but my ignorance regarding the DSP is contributing to the problem.

From my limited understanding, if the LM44 is set in Mesa mode, then it is passing the four signals from the board to the amplifier, after processing each of them according to its programming. I could imagine that if they were misconfigured, those four processing channels could cause the kind of problems we're hearing. My first thought when I heard the faulty monitor set-up was that someone had screwed up the EQ.

Phew.

Any thoughts or suggestions would be appreciated. And I'm happy to try and clarify anything.

 

[AudJ]

Would need more info regarding ohms and watts of amps and speakers to do a good job figuring this out. In my experience, (which is limited)the wiring could be just fine, depending what it is running.

A basic drawing of the wiring could also help if you are able to figure it out.

Could be settings, but assuming all components are functioning properly and DSP is appropriate, my first thought is that the original wiring install was intended for certain monitors to be used either together or seperately, and since then, the configuration of speaker placement or actual units might have changed using the same wiring, which changed the load on the speakers without regard to what the amp is producing - but that is just a guess.

 

[rwhealey]

The wiring at the amp is totally normal. When you parallel the monitors at the amp like that it is a bit easier for the connectors to slip out of the screw terminals than if you parallel elsewhere, however. I would pull all the connections and double check that nothing has become disconnected, then tightly reconnect them. Do the same on the stage end.

It appears, however, that the system may be designed for biamplified monitors. Notice that each speaker cable contains four conductors. How long have you had this problem? Are you using the original monitors that came with the system? If the monitor system is designed for biamped monitors and you are attempting to use monitors with a passive crossover, you will have the exact problem you describe (sounding like they're underwater). In that situation each passive monitor would only be getting one part of the signal. What monitors are you attempting to use? Do you have a one-line diagram for the original installation?

I would also be suspicious of what is going on inside the Lake. If you aren't supposed to be biamping your monitors, it's very possible that someone has screwed up the EQ settings. If you aren't comfortable looking into the lake programming you may be able to get someone to do it remotely if you have the original documentation for the system.

 

[FMEng]

What make and model of power amp?

 

[Brentgi]

I'm going to second rwhealy on this. Sounds like it's a biamp issue. A couple of questions: How many drivers does your monitor(s) have? And what sort of plug feeds them on the stage (i.e. 1/4" mono, TRS, or perhaps speakon)?

If you have more that one driver in your monitor, are you getting signal from both?

 

[FMEng]

This is one of the rare times when I suggest that the OP is venturing further than his capability, and ours to accurately advise. If this is indeed a biamp issue, there is a strong possibility of doing damage to equipment by taking the wrong turn in troubleshooting. It might be time to call in the original installer or other qualified technician.

 

[Chris15]

That amp looks like a lab gruppen C series, which would be in line with the brands in the rest of the system.

I tend to agree with the idea of getting some local assistance. That system would not have been cheap and is probably only a few years old, so it would be unwise to be messing with things that could break expensively...

 

[twelbes]

Hi all—thanks so much for the great advice and suggestions. Apologies for letting this thread languish. We were swamped with a couple of major events, plus all that *teaching* I have to do...

We are well aware of the quality of equipment that we've been graced with, and we're definitely looking to protect the investment that the school has made. We are bringing in the original installer to have a look a the system. According to a theatre tech who was here at the time of the installation, the system worked fine for about six months and then these problems suddenly arose.

I'll pop back on when we know more, and try to provide some closure

Thanks again!