Mixer outputs wired together?

Stevens R. Miller

Well-Known Member
I'm helping with the tech on another musical being performed in a county middle school. While trying to figure out how their systems are connected, I made a startling discovery: the two output from the school's Peavey FX mixer appear to be wired together. What I mean is that there is a female XLR plug in each of the two balanced output jacks, with an insulated wire between them, and another that runs off into the theater. Physically, it looks like this:

Code:
___
   \ <---wire
    \    _______  
     \  /       \ <---wire
      \/         \
     /  \       /  \
    /    \     /    \
    |XLR |     |XLR |
    |plug|     |plug|
    |    |     |    |
    |    |     |    |
-------------------------
    right       left
    output     output

I'm thinking someone thought this was a good way to "mix" stereo outputs into a single channel. Has anyone seen anything like this before? Is it even safe?
 

Bubby4j

Active Member
Instead of doing that all the channels should just be panned center for a mono mix. Then the left and right outputs are the same signal and there's no need to mix them together after the mixer.
 

jkowtko

Well-Known Member
I took a quick look at the Peavey FX series block diagram ... it looks like LR are fully balanced outputs. So each XLR output should have three pins -- positive signal, negative signal, and ground. Can you tell which wire(s) are connected between the two outputs, and which wire(s) are output to the theater?
 

StradivariusBone

Custom Title
Fight Leukemia
http://www.epanorama.net/circuits/linesum.html

The first output signal is going to want to send voltage into the second output and vice versa. You need resistors on each output in order to prevent the signal from going into the other output, but this may be how it's already set up if it's actually working. That site is for an unbalanced line level connection, but I would guess the only difference between that and balanced would be the addition of another resistor so you have resistance on both signal pins (2 and 3). I've only seen this once before, so if I'm not 100% accurate I'm sure someone will chime in.

What does the other end of the cable look like? Does it feed into one input?
 

FMEng

Well-Known Member
Fight Leukemia
Depending upon the design of the output stage it might work OK, but it isn't the proper way to do it. I would use four resisters to sum the two outputs. Put a resister is series with pins 2 and 3 in both connectors. Then Y the two resisters from pin2 together, and the two from pin 3 together.

The alternative is to just use one output, left or right. The result should be the same unless you have some stereo sources going into stereo input faders.

http://www.rane.com/note109.html
 

RonHebbard

Well-Known Member
Premium Member
I'm helping with the tech on another musical being performed in a county middle school. While trying to figure out how their systems are connected, I made a startling discovery: the two output from the school's Peavey FX mixer appear to be wired together. What I mean is that there is a female XLR plug in each of the two balanced output jacks, with an insulated wire between them, and another that runs off into the theater. Physically, it looks like this:

Code:
___
   \ <---wire
    \    _______ 
     \  /       \ <---wire
      \/         \
     /  \       /  \
    /    \     /    \
    |XLR |     |XLR |
    |plug|     |plug|
    |    |     |    |
    |    |     |    |
-------------------------
    right       left
    output     output

I'm thinking someone thought this was a good way to "mix" stereo outputs into a single channel. Has anyone seen anything like this before? Is it even safe?

I'm helping with the tech on another musical being performed in a county middle school. While trying to figure out how their systems are connected, I made a startling discovery: the two output from the school's Peavey FX mixer appear to be wired together. What I mean is that there is a female XLR plug in each of the two balanced output jacks, with an insulated wire between them, and another that runs off into the theater. Physically, it looks like this:

Code:
___
   \ <---wire
    \    _______ 
     \  /       \ <---wire
      \/         \
     /  \       /  \
    /    \     /    \
    |XLR |     |XLR |
    |plug|     |plug|
    |    |     |    |
    |    |     |    |
-------------------------
    right       left
    output     output

I'm thinking someone thought this was a good way to "mix" stereo outputs into a single channel. Has anyone seen anything like this before? Is it even safe?
OMG!! Two balanced outputs driving into each other? Say it isn't so.
Possibly @FMEng will chime in and explain this to you!
I'll crawl back into my hole now.
Toodleoo!
Ron Hebbard.
 

Stevens R. Miller

Well-Known Member
I took a quick look at the Peavey FX series block diagram ... it looks like LR are fully balanced outputs. So each XLR output should have three pins -- positive signal, negative signal, and ground. Can you tell which wire(s) are connected between the two outputs, and which wire(s) are output to the theater?
I could pull the wires entirely and use my multimeter to figure them out. In fact, I'd love to do that, since it would be some good, hard info about what I'm dealing with. Problem is time. We get 4 hours in the middle school auditorium each night, and the director prioritizes rehearsal over technical issues (which we techies fully understand). Also, it's not our hardware, so I'm reluctant to yank any more of it apart than I absolutely have to.

That said, I do need to understand this thing, so I may find the time to do it, no matter what the director says.

Anyone think it might be a "bridging mono" circuit?
 

EdSavoie

Well-Known Member
I hope it's not! I wouldn't want to send anywhere close to that much power down an XLR cable...

Doing a quick Google of the series, it doesn't seem like any of them are amplified, so I doubt that is the case.

I'm curious as to why it would be wired like that...

What's at the other end of that cable and how long of a run is it?
 

Stevens R. Miller

Well-Known Member
I hope it's not! I wouldn't want to send anywhere close to that much power down an XLR cable...

Doing a quick Google of the series, it doesn't seem like any of them are amplified, so I doubt that is the case.

I'm curious as to why it would be wired like that...

What's at the other end of that cable and how long of a run is it?

You are correct, it's not a powered output. The rack it's in is a rat's nest of wires, but the output appears to go into a single-channel power amplifier that drives the main house speaker, mounted on the ceiling, just over the proscenium arch. It's actually a cluster of three speakers, which I doubt are capable of working in stereo. If they are merely a monaural cluster, then it would make sense for the mixer outputs to be in bridged mono, to feed a single channel into that amplifier.

(Uh, wouldn't it :think: ?)
 

EdSavoie

Well-Known Member
Ah... So the system is entirely mono. Ok then, that makes more sense, but it seems like an odd step for the installer to have made... Is there really no second input in the back of the amplifier?
 

StradivariusBone

Custom Title
Fight Leukemia
Bridged mono is different than what's going on in the back of the mixer. If I had to guess, someone just "y-ed" the outputs and didn't bother with the resistors in order to sum both L and R to the single channel input of the amplifier. On most mixers you can run a mono output (in fact most of the mixers I work with regularly runs a mono mix). It doesn't make as much sense to run stereo separation on a sound reinforcement system like you would in a home theater or even a movie theater since a good number of public meeting spaces aren't designed such that every seat gets an equal share of audio from both sides of the system.

Most mixers can be setup to do this internally, you either put all the channels into L or R equally and you're done. I didn't look into what the equipment can do, but perhaps someone thought this was an easier fix at the time.

That all being said, bridged mono is a setup with a stereo amplifier where you effectively combine both outputs to create a single output that runs at a higher power than either output could do individually. It creates a potential between the two positive outputs of each channel and you connect your load across the two sides of the amp. That doesn't sound like what you've got. You've got a single channel amplifier, which is not super common, but may explain why someone decided to install this cable on the output of your mixer. Maybe the original amp died and they weren't sure how to configure two outputs down to one?

All that being said- is it working? Rule #1 is never mess with a working system. If it's wired without resistors it probably would have already done its damage to the output side of the mixer (if it even was going to), if you're really worried about it you can try unplugging one side and summing all your audio to the connected side.
 

Stevens R. Miller

Well-Known Member
Ah... So the system is entirely mono. Ok then, that makes more sense, but it seems like an odd step for the installer to have made... Is there really no second input in the back of the amplifier?
As far as I can tell, it really is a single-input amp. In the same rack is a dual-input amp, which powers the monitors. The continuing problem in this context, for me, is that I have no way to communicate with the installer, and no one else has any responsibility for the equipment. This means that I don't really even know if anything I find in that rack was put there by a professional, an amateur, or a student. Heck, we did load-in last Saturday, and the auditorium was used by a church for its services the following Sunday. When we came back on Monday night, all the inputs on the rack mixer were turned full up, even though most of them have no inputs. What's that say to you, eh?
 

Stevens R. Miller

Well-Known Member
Rule #1 is never mess with a working system.
Indeed. We have been having intermittent hum problems with this rig. Whenever they go away, one of us always says, "Nobody move!" We try our best to avoid changing things that work.

I could pull the cable and figure out if it's just a Y or anything more sophisticated. Do you suppose it's possible that the positive balanced line on one channel and the negative balanced line on the other channel are connected to ground, and the remaining negative and positive are being used to feed the amplifier? Would that even work? The output would no longer be balanced, but it would "mix" the two stereo channels without in-phase signals on both canceling each other out.
 

StradivariusBone

Custom Title
Fight Leukemia
Heck, we did load-in last Saturday, and the auditorium was used by a church for its services the following Sunday. When we came back on Monday night, all the inputs on the rack mixer were turned full up, even though most of them have no inputs. What's that say to you, eh?
Classic "Auditorium at a school where no one is "officially" in charge, but everyone uses everything". :(

We have been having intermittent hum problems with this rig.
That's a grounding issue. Could be related to this plug. Maybe not. Try eliminating things one at a time, when it stops you've found your ground problem. Usually this relates to having equipment plugged into different sources of power, thus different ground points within the building which can create a potential between grounds and cause hum. If it goes away, whatever you just muted is causing it.

Do you suppose it's possible that the positive balanced line on one channel and the negative balanced line on the other channel are connected to ground, and the remaining negative and positive are being used to feed the amplifier? Would that even work?
If they did lift one side of the balanced run it would make it unbalanced. In your scenario, I don't know what would happen if you connected pin 2 on one cable and pin 3 on another to pin 1. I suspect it would short and not work and possibly damage your equipment.

But, if that cable is wired as an unbalanced run, that could potentially induce hum into the system. An occasional solution is to lift pin 1 on one side of a cable run to drop signal ground. Some equipment has a button that does this for you. But the fact that you mentioned the hum goes away makes me think that it is something transient and possibly a guitar or mic receiver or some other input that's causing it instead.
 

RonHebbard

Well-Known Member
Premium Member
I could pull the wires entirely and use my multimeter to figure them out. In fact, I'd love to do that, since it would be some good, hard info about what I'm dealing with. Problem is time. We get 4 hours in the middle school auditorium each night, and the director prioritizes rehearsal over technical issues (which we techies fully understand). Also, it's not our hardware, so I'm reluctant to yank any more of it apart than I absolutely have to.

That said, I do need to understand this thing, so I may find the time to do it, no matter what the director says.

Anyone think it might be a "bridging mono" circuit?
"Mono bridging" is routinely done at the speaker level outputs of power amplifiers and to my knowledge NEVER at line level (+4 / - 10 db outputs) of various gain stages along the way.
Toodleoo!
Ron Hebbard
 
Last edited:

jkowtko

Well-Known Member
From reading the description, it could be that the ground pins on the two XLR outputs are wired together. Stevens, you did say it was a single wire that was connected between the two XLR outputs, right?

Also, can you confirm the input to the amp? Is it balanced (3-wire) or unbalanced (2-wire)?

Ultimately, if you want to get to the bottom of this, you need to pull the plugs out of the mixer, use a VOM to see which pins from each XLR output plug are connected where, and post a more detailed wiring diagram all to see.

Thanks. John
 

Stevens R. Miller

Well-Known Member
Classic "Auditorium at a school where no one is "officially" in charge, but everyone uses everything". :(
Yup. One must simply get used to it, I gather.


...different ground points within the building which can create a potential between grounds and cause hum.
Everyone tells me that coping with hum requires a bag full of tricks. In this case, we did quiet a lot of it by running a long, heavy duty power cord from the same outlet that powers the rack to the sound station in the middle of the auditorium, insuring they have a common ground. However, our sound station also has a BNC jack for a work light. The jack is loose and (surprise!) removing the light also eliminated some hum.

I am actually the lighting guy on this show, not the sound person. But, while our sound person is great at running the mixer and managing wireless mikes, she's still learning about electronics and electricity. I'm no sound engineer, but I've been a ham radio guy for over 40 years, so the electronics issues kind of fell into my lap.


If they did lift one side of the balanced run it would make it unbalanced. In your scenario, I don't know what would happen if you connected pin 2 on one cable and pin 3 on another to pin 1. I suspect it would short and not work and possibly damage your equipment.
Yeah, that was a really dumb speculation on my part. Hadn't had my coffee yet when I proposed it. Most likely, they've parallelized the hot lines and the grounds, and left the neutral balanced line out of the connection entirely, to create an unbalanced signal line that "mixes" the two channels by simple superposition, as in this set-up.
 

Stevens R. Miller

Well-Known Member
From reading the description, it could be that the ground pins on the two XLR outputs are wired together. Stevens, you did say it was a single wire that was connected between the two XLR outputs, right?
Sorry, that was imprecise. There is a single, insulated cable running between the two XLR plugs. I don't know how many wires are in it, but I suspect it is configured like the connection in the link in my previous comment here.

Also, can you confirm the input to the amp? Is it balanced (3-wire) or unbalanced (2-wire)?
Balanced 3-wire.

Ultimately, if you want to get to the bottom of this, you need to pull the plugs out of the mixer, use a VOM to see which pins from each XLR output plug are connected where, and post a more detailed wiring diagram all to see.
I'll try. Time is so tight during rehearsals that anything not absolutely on the critical path is hard to justify, but I'll see if I can do this.
 

Morte615

Active Member
It doesn't sound like this is your issue but want to bring it up just in case. I have seen installs where they run a stereo signal down a single line, the line has 4 wires plus a ground. It's remotely possible that they ran the RIght Channel into the housing for the Left and just connected it to the wire that runs to the stage to give them a stereo system on a single wire. To check just take the Left Channel out and remove the plastic shield and you should be able to see how they connected the cable.
It sounds like you only have a mono amp and this probably doesn't do anything for you, but wanted to mention it in case someone else came up with a similar setup as another option.
 

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