Mixer-to-Mixer Connections

Stevens R. Miller

Well-Known Member
My community theater company has its own Behringer UB2442FX that we use for our shows. We perform in local middle schools. The schools all have rack-mounted Peavey FX mixers and amplifiers that drive their built-in loudspeakers. The racks are wired in such a way that it isn't always feasible for us to connect the output of our mixer to the inputs of their amplifiers. In those cases, we must connect the output of our mixer to two inputs of the schools' mixers.

We have been connecting XLR outputs from our mixer to XLR inputs on their mixers. By reading the manuals, I have become aware of something called "microphone level," and something else called "line level." Thus, if I understand what I am reading, I have discovered we are connecting a line level output to inputs intended to receive microphone levels. Mostly, I think we do this because it seems as though the "main" output jacks on our mixer are the XLRs ("main" because they're bigger than the quarter-inch TRSs, I guess), and it just seems natural to connect one XLR to another. But now I am wondering if we wouldn't be better off connecting the output of our mixer to the line level inputs on their mixers.

Specifically, here are my questions, and my own guesses at the answers:

1. Should we connect our line level output to the schools' line level input? (Guess: Yes, because line level is higher than mic level, so the input will have no pre-amp, which helps keep noise down.)

2. Is it harmful to connect line level outputs to mic level inputs? (Guess: No, so long as you don't overdrive the input, but there's no up-side to it, either, and gawdnose what would happen to our mixer if someone turned on the phantom power at the schools' mixers.)

3. Is there any reason not to connect an XLR line level output to a TRS quarter-inch line level input (as opposed to using TRSs at each end)? (Guess: No, but it's easier to find TRS-to-TRS cable than it is to find XLR-to-TRS cable.)

As ever, thanks to an CB'ers who can help sort me out.
 

RonHebbard

Well-Known Member
Premium Member
My community theater company has its own Behringer UB2442FX that we use for our shows. We perform in local middle schools. The schools all have rack-mounted Peavey FX mixers and amplifiers that drive their built-in loudspeakers. The racks are wired in such a way that it isn't always feasible for us to connect the output of our mixer to the inputs of their amplifiers. In those cases, we must connect the output of our mixer to two inputs of the schools' mixers.

We have been connecting XLR outputs from our mixer to XLR inputs on their mixers. By reading the manuals, I have become aware of something called "microphone level," and something else called "line level." Thus, if I understand what I am reading, I have discovered we are connecting a line level output to inputs intended to receive microphone levels. Mostly, I think we do this because it seems as though the "main" output jacks on our mixer are the XLRs ("main" because they're bigger than the quarter-inch TRSs, I guess), and it just seems natural to connect one XLR to another. But now I am wondering if we wouldn't be better off connecting the output of our mixer to the line level inputs on their mixers.

Specifically, here are my questions, and my own guesses at the answers:

1. Should we connect our line level output to the schools' line level input? (Guess: Yes, because line level is higher than mic level, so the input will have no pre-amp, which helps keep noise down.)

2. Is it harmful to connect line level outputs to mic level inputs? (Guess: No, so long as you don't overdrive the input, but there's no up-side to it, either, and gawdnose what would happen to our mixer if someone turned on the phantom power at the schools' mixers.)

3. Is there any reason not to connect an XLR line level output to a TRS quarter-inch line level input (as opposed to using TRSs at each end)? (Guess: No, but it's easier to find TRS-to-TRS cable than it is to find XLR-to-TRS cable.)

As ever, thanks to an CB'ers who can help sort me out.
Perhaps not so much an answer as a collection of related thoughts for you to further cogitate upon:
S/N ratio, Signal to Noise ratio is / should always be a concern.
Every cable will have some amount of residual noise imposed upon it.
This residual / background noise will be there regardless.
When you route your desired signal via a given cable, your desired signal will always be in addition to whatever level of residual noise is present. If you're routing mic' level, you will be a given amount above the residual noise floor.
If you're routing at line level, either -10 or +4, your desired signal will be that much higher than the residual noise.
When your interconnect cable reaches an input stage you'll be saddled with whatever the signal to noise ratio is. You can amplify / increase (or decrease) the overall level but you can never again improve upon the signal to noise ratio of your path of choice.
From my personal perspective, I run balanced line level interconnects whenever they're an available option.
Not all devices will be equipped to supply balanced line level outputs. Similarly not all devices will be equipped to receive balanced line level inputs.
As with so many things, we are forced into compromises.
As I wrote initially, perhaps not so much an answer as a few more thoughts to cogitate upon.
Take care Stevens, the pleasure is always on my end.
Toodleoo!
Ron Hebbaard.
 

Stevens R. Miller

Well-Known Member
S/N ratio, Signal to Noise ratio is / should always be a concern.
...
If you're routing at line level, either -10 or +4, your desired signal will be that much higher than the residual noise.
That makes perfect sense. Once combined, signal and noise will be amplified together.

Could you tell me more about what "either -10 or +4" means? My manuals suggest that the mixers' microphone pre-amps add 20dB, but I've seen the -10 and +4 numbers here and there as well and don't know what they mean.
 

RonHebbard

Well-Known Member
Premium Member
That makes perfect sense. Once combined, signal and noise will be amplified together.

Could you tell me more about what "either -10 or +4" means? My manuals suggest that the mixers' microphone pre-amps add 20dB, but I've seen the -10 and +4 numbers here and there as well and don't know what they mean.
Again, I can offer a few thoughts to tie you over until one of our more eloquent posters (@FMEng ) comes along.
O.K., don't misconstrue any of the following as verbatim but merely personal thoughts and illustrations of concepts.
Put your computer guru hat on and think of the similarities / differences between RS232 vs. RS422 vs. RS485.
Bear in mind I'm totally computer illiterate.
In my naively warped perspective, I think of RS232 as an unbalanced bi-directional serial comms protocol designed for short distance / consumer level / non mission critical / economy applications. (In my mind, unbalanced single ended -10 db is the audio interfacing equivalent but don't tell Fostex that.)
Again, I think of RS422 as a more robust / reliable / bullet proof / industrial strength bi-directional / point to point comms protocol aimed at a higher priced / less cost conscious segment of the market.
RS485 ratchets it up one more notch being able to have one source simultaneously communicate with multiple receivers.
This gets you into the world of balanced +4db level audio interfacing.
You don't usually find the more expensive interfaces available on devices aimed at the more cost sensitive segments of the market.
Optimistically @FMEng, or a similar poster, will be along soon to poke holes in everything I've told you and get you sorted.
Take care Stevens,
Always my pleasure.
Toodleoo!
Ron
 

BobHealey

Active Member
My community theater company has its own Behringer UB2442FX that we use for our shows. We perform in local middle schools. The schools all have rack-mounted Peavey FX mixers and amplifiers that drive their built-in loudspeakers. The racks are wired in such a way that it isn't always feasible for us to connect the output of our mixer to the inputs of their amplifiers. In those cases, we must connect the output of our mixer to two inputs of the schools' mixers.

We have been connecting XLR outputs from our mixer to XLR inputs on their mixers. By reading the manuals, I have become aware of something called "microphone level," and something else called "line level." Thus, if I understand what I am reading, I have discovered we are connecting a line level output to inputs intended to receive microphone levels. Mostly, I think we do this because it seems as though the "main" output jacks on our mixer are the XLRs ("main" because they're bigger than the quarter-inch TRSs, I guess), and it just seems natural to connect one XLR to another. But now I am wondering if we wouldn't be better off connecting the output of our mixer to the line level inputs on their mixers.

Specifically, here are my questions, and my own guesses at the answers:

1. Should we connect our line level output to the schools' line level input? (Guess: Yes, because line level is higher than mic level, so the input will have no pre-amp, which helps keep noise down.)

2. Is it harmful to connect line level outputs to mic level inputs? (Guess: No, so long as you don't overdrive the input, but there's no up-side to it, either, and gawdnose what would happen to our mixer if someone turned on the phantom power at the schools' mixers.)

3. Is there any reason not to connect an XLR line level output to a TRS quarter-inch line level input (as opposed to using TRSs at each end)? (Guess: No, but it's easier to find TRS-to-TRS cable than it is to find XLR-to-TRS cable.)

As ever, thanks to an CB'ers who can help sort me out.

1.) Depends if the line level input is balanced or unbalanced. If its balanced, shouldn't be an issue. If the input is unbalanced, unless the cable is short, you'll probably have issues with noise and interference.

2.) Depends. Some mixers can't handle +48V phantom being fed into an output. DDA consoles were really good at having their outputs friied by being connectd to a phantom power source.

3.) Assuming the TRS is balanced, no issues.

4.) Option 4: Run the output of the mixer into a suitable DI to drop it back down to mic level, and just treat your mixer like a generic unbalanced line level source.
 

Jay Ashworth

Well-Known Member
-10 and +4 are 0vu points for non-professional and professional line-level audio signals, respectively.

They're not in the same unit of measure, though, so they're more than 14db apart.

Indeed: many boards have enough range on the channel trim control that you can run a -10 signal into them and set the channel faders at a reasonable level, and run that way, and as long as the rest of the channel is flat, you should be ok.

If someone turns something up unexpectedly, you might blow things, either components in the channel strip (unlikely) or speaker drivers (more likely).

But no, it's not an illegal thing to do or anything; just requires more caution.

If you've done it and it works, no reason not to do it. I would take *some* of your gain drop on the masters on your own board, and the rest on the input channel trims, or find a set of XLR-M to XLR-F attenuators; -20db loss is a common number, and would let you open the trims back up on the second board and worry less about popped cones.
 

FMEng

Well-Known Member
Fight Leukemia
The Behringer has main buss outputs on both XLR and TRS. On that model of mixer, they are electrically identical, so either connector is fine. The Peavey has balanced, line inputs on TRS. The best way to interfaces the two is to connect the Behringer main outs to the Peavey line inputs on mono faders. The Peavey stereo line inputs are unbalanced, which makes them more prone to noise, so I would avoid them.

As for levels, it isn't an easy thing to explain, but I will take a stab at it. (Thanks for dragging me into trouble, Ron.) Decibels (dB) of anything is a logarithmic ratio of two numbers. We can use dB as just a ratio, or we can use it as a ratio compared to a reference value, which is what dBu is.

The decibel was named after Alexander Graham Bell. Audio technology has its roots in Ma Bell (Bell Telephone, later AT&T) and Bell Laboratories, and they invented many standards that are used today. Consequently, line level refers to the level used on telephone lines. The phone company was concerned about audio levels and optimum audio power transfer between sources and loads. (That was a big deal with long lines, transformer coupling, and vacuum tube electronics.) Bell Labs came up with a standard reference level for audio of 0 dBm, which was equal to a power level of 1 milliWatt across an impedance of 600 ohms.

The "m" in dBm = 1 milliWatt. With modern, solid state electronics, we don't worry about matching impedance very often, and inputs and outputs are no longer 600 ohms, so we don't reference to 1 milliWatt anymore. Instead of power, today we care about voltage. As it turns out, at 0 dBm, the voltage across 600 ohms is 0.775 v, so we use that as our reference standard. The "u" in dBu simply means referenced to 0.775 volts. The math formula to convert from voltage to dBu is:

dBu = 20 Log ( v / 0.775 )

A few things to note about this formula: Any value that is less than the reference is a negative number. Any value above the reference is positive. Twice as much = 6 dB and half as much is -6 dB.

Let's say we run steady tone through a mixer and measure the AC voltage on the output as 1.23 volts. Let's plug that in to our formula...

dBu = 20 Log ( 1.23 / 0.775 ) = 4.01 dBu

On many audio devices with balanced outputs, that's the nominal level, and the level meter would read 0 VU. The balanced outputs of well designed equipment clip at +24 dBu, or higher, so a nominal level of +4 dBu gives us 20 dB of headroom which is considered ample for most untamed audio signals. Less professional devices might have outputs capable of only +18 dBu, so their nominal level could be lowered to 0 dBu.

Some typical, nominal audio voltage levels:
Professional, balanced, line level +4 dBu = 1.23 v
Un-balanced line level -2 dBu = 0.615 v (also for pseudo or impedance balanced)
Vintage broadcast and recording equipment line level +8 dBu = 1.95 v (from the days of tubes)
Consumer, unbalanced line level -10 dBu = 0.245 v (typical for RCA jacks)
Microphone level -50 dBu = 0.00245 v (a dynamic mic reproducing a speaking voice)

Why do we use dB? Because it makes the math of gains and losses easy when we translate between voltages, power, and acoustics, and because it relates well to how people hear. Most people will say that just noticeably louder or softer is about 3 dB. Subjectively, ten times louder is 10 dB. If we need to amplify mic level up to line level, we know we need 54 dB of gain.

Now to further complicate things. We can use dB to describe a ratio of powers, but the formula becomes 10 Log ( P1 / P2 ). That's fodder for another day.
 

Jay Ashworth

Well-Known Member
Very close, but all my sources tell me that the zero reference points are +4 dBu, and -10 dBv. That moves the consumer zero-point up to about .34 volts.
 

FMEng

Well-Known Member
Fight Leukemia
Well, yeah, I'm starting to see dBV used more often for consumer equipment, lately. dBV uses a 1 v reference. The 0 dBV = 2.2 dBu. Happily, that amount of difference is small enough to ignore most of the time. The pro audio equipment world is sticking to dBu. I suspect the Audio Engineering Society would raise a fuss if they did otherwise.
 

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