Hi All,

I manage a studio theatre with 2 patch bays installed. The channels from the patch bays are reaching the sound board for sure as when an input is plugged in there is a pop. And, when something like a microphone is plugged in (with phantom power on) and the channel is turned up fully there is lots of hissing but no actual signal.

This happens with all inputs in both patch bays. Same symptoms.

I'm a technical stage manager who majors in lights and minors in sound. This particular problem is beyond my expertise so I'd be so happy if someone can offer an insight!

Thanks in advance.

Hi,
Best advice would be to get a cheap cable tester (although you will probably want one with a seperate input/output units (i.e. 2 boxes one that plugs into the patchbay and another unit that connects to the cable going to the mixer). This will show up any cable problems (jut beacuse it goes pop (although really all connections should be done with mixer power OFF especially when phantom power is involved) doesn't mean the cable works.

Also, I presume, that you have tried running a mic DIRECTLY into the mixer just to make sure that is definatly not the problem.

What happens when you plug a dynamic mic in?

And are these XLR patchbays or jack patchbays? (Jack patchbays aren't ideal for microphones)

James.

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JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net

Quote pppmmunuay:

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The channels from the patch bays are reaching the sound board for sure as when an input is plugged in there is a pop.

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...but presumably no wanted audio signal?

You have to approach this kind of problem solving very methodically.

1. Check that there really is an audio signal ont he cable you're trying to plug in.
2. CHeck that the mixer channel reall is working by plugging a known good audio signal directly into it (bypassing the patch panel)
3. using a cable tester or multimeter, check the wiring from the patch panel to the console.

Was this patch bay working previously? If so, what has changed recetnly to cause it to stop working?

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And, when something like a microphone is plugged in (with phantom power on) and the channel is turned up fully there is lots of hissing but no actual signal.

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Plugging mics on a patch panel with phantom power switched on (if we're talking about a jackfield rather than an XLR panel) is quite a good way to destroy the input stages of most budget mixing consoles!

As James says, start off proving that the console works (or not) by plugging a known good dynamic mic directly into the desk and see if you can get that working.... then work back along the chain to the patch bays.

THi kind of fault finding is very hard to do remotely. Where abouts are you located?

Hugh

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Technical Editor, Sound On Sound

Quote pppmmunuay:

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Hi All,

I manage a studio theatre with 2 patch bays installed. The channels from the patch bays are reaching the sound board for sure as when an input is plugged in there is a pop. And, when something like a microphone is plugged in (with phantom power on) and the channel is turned up fully there is lots of hissing but no actual signal.

This happens with all inputs in both patch bays. Same symptoms.

I'm a technical stage manager who majors in lights and minors in sound. This particular problem is beyond my expertise so I'd be so happy if someone can offer an insight!

Thanks in advance.

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"Patch bay" may be a misleading term. Is this simply a wall box with a set of sockets leading directly to mixer inputs?

Are they XLR sockets, leading directly to XLR inputs on the mixer?
Does the same microphone with the same cable plugged directly to the same mixer input work?

Wow, lots of answers and help - thanks guys!

To answer the questions:

Yes I did try all the channels in the mixer to check they work. They all had a mic directly plugged in and worked perfectly.

I was using a dynamic mic and also an iPod going through a DI box to create 1 channel of XLR. Both methods of testing the line had the same results.

Definitely audio signal in the cable as each test was replicated directly into the mixer (bypassing the patch bay) with satisfactory results.

I don't know if the patch bay was working previously. The equipment is 4 years old and I've only recently started the job. The equipment has been rarely used and poorly maintained.

Today I have got some signal from the iPod/DI box method but need to crank it up too high in order to get enough volume. This distorts the signal and causes either a hissing sound or a noise similar to that of an idling jet engine. Even at full volume on the iPod, gain, fader and padding the level of audio is really too low.

Now, there are 2 patch bays in the room (both XLR to XLR), the one further away from the mixer has less signal/more hissing than the one closer. There is consistent behavior within all the lines in the two different patch bays.
( "patch bay" / "patch panel". Which is correct? I wasn't sure which to use. )

I do have a signal tester. How would I use it here? Which points should I test across?

Thanks everyone for your time and assistance.

As Hugh says, you have got to be sternly methodical about this.

My first approach would be to knock up a two part tester. A battery and a pair of resistors and tother end a red and green LED. This would save you time in the long run since it would test all three conductors at once and give a phase check.

There must be a "silly" miswire somewhere in the bays since you could not afford enough mic cable to make any audible attenuation of the signal!

Dave.

Quote pppmmunuay:

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Yes I did try all the channels in the mixer to check they work. They all had a mic directly plugged in and worked perfectly.

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Okay, it's obviously very comforting to know that the only complicated part of the signal path is working properly! The rest of it is hopefully just wiring which is probably difficult to get at, but very simple to fix!

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Today I have got some signal from the iPod/DI box method but need to crank it up too high in order to get enough volume. This distorts the signal and causes either a hissing sound or a noise similar to that of an idling jet engine. Even at full volume on the iPod, gain, fader and padding the level of audio is really too low.

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A length of cable obviously shouldn't affect the signal level. So if the iPod/DI box and/or the dynamic mic work when plugged straight into the mixer, but not when plugged into the patch bay, clearly something is amiss with the wiring.

There are really only two possibilities:

1) The wiring has been mislabelled or misplugged and you're not making the connections you think you are, or

2) the wiring has been damaged (or there are major wiring errors) and the connections are actually broken

In an ideal world the cables will have been identified in some long-lasting way at both ends where they terminate in the XLR panels and mixer cables so you can easily check option (1) above... but I suspect that wont be the case here if it was an amateur installation. In which case, you'll just have to work through all connections to find out what is connected to what , either by tracing the cables by hand, or using a cable tester. It's a tedious and time consuming process, but there's no real alternative. The same largely applies to option (2), in trying to identify what is faulty.

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Now, there are 2 patch bays in the room (both XLR to XLR), the one further away from the mixer has less signal/more hissing than the one closer.

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That's probably logical... I don't suppose there are some active amplifiers connected in the signal paths before, between or after these patchbays are there? Some distribution amnps or external preamps? Something that needs power and isn't getting it?

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( "patch bay" / "patch panel". Which is correct? I wasn't sure which to use. )

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Either or both... and in broadcasting terms it's often called a jackfield, too! (although that strictly applies to panels that accept only jack plugs, rather than XLRs)

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I do have a signal tester. How would I use it here? Which points should I test across?

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If it is an XLR cable tester of some sort then you'll need two good XLR cables, one shortish one to go from the tester to the patch bay, and one long one to go from the cable ends at the mixer back to the tester.

Start by testing both of these cables to make sure they are wired correctly!

Next, plug the long cable into the mixer cable end for channel 1, and then using the short cable, plug into the XLR socket on the patch panel for channel 1. If it identifies a partial fault, (eg, pin 3 disconnected) great -- you've found the problem. If not, work along the patch bay testing each socket in turn until you find one connected directly to the mixer cable. This will obviously identify any mis-plugging or mislabelling.

Next, move the mixer end cable into channel 2, and repeat ad nauseum until you've resolved all the problems. ...and then repeat the process for the second patch panel.

However, It seems very odd to me that all patch channels should go down in the same way and at the same time -- that's not typical of normal wiring faults. I would suspect either:

1) the patch panels aren't actually connected to the desk at all -- it's not wired up the way you think it is!

2) there are some powered buffers or distribution amps in the signal path somewhere that aren't getting power

3) there has been some misplugging or mislabelling so that you're not plugging into what you think you are (or you're reading the labels incorrectly -- often easy on a jackfield bay, but less so on an XLR panel)

4) there has been a catestrophic disaster and someone has managed to saw through the cable looms!

Hugh

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Technical Editor, Sound On Sound