# Summing cable XLR (x2) Female to (x1) Male



## Fab (Feb 4, 2020)

One for the cable experts out there!

The idea is to connect 2 audio interfaces to the same monitoring setup using a pair of summing cables that have this configuration: 

XLR (x2) Female to XLR (x1) Male.


The setup is:


OUTPUT 1 of both Audio interfaces go to the left speaker
OUTPUT 2 of both Audio interfaces go to the right speaker


Would there be any potential problems or loss of signal strength/quality you might run into?


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## pderbidge (Feb 16, 2020)

I think this could be bad. Two separate outputs with different impedence loads to one input. Possibly some type of overload issue? This is where some type of mixer comes in to play to manage that load. I'd have to think it over but my first thought is you will have problems.


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## wst3 (Feb 17, 2020)

Nope- you WILL have problems, for the exact reason given above. Outputs are (generally) designed with a very low source impedance, thus they will drag down another low source impedance, making a mess of things (at best) and possibly even damaging one or both outputs stages. (ah for the days of output transformers<G>!)

You do not need a mixer, although that is probably the easiest solution (if you have one.)

Otherwise you can sum the outputs passively or actively. If you want to build this into a cable then passive it is.

There are two, maybe three common configurations...

If you want to handle ANYTHING that is thrown at you put a 10K resistor in series on each signal conductor. This is as close to guaranteed as you can get.

If you know that you are working with outputs that were designed to "pro" standards you can reduce the resistors to 1K (some folks say 470 ohms, that's a little low for my tastes).

And if you end up with an exaggerated low frequency bump you can add a 20K resistor from the junction of the two series resistors to ground, but that is seldom necessary.

Does that answer the question??? Let me know if you need more info...


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## Fab (Mar 7, 2020)

Hey, thanks for the information @wst3 and @pderbidge . I actually forgot about this thread as I didn't get a reply immediately!

What has happened since is:

I got the summing leads and currently I'm using them for just one audio interface connecting to powered monitors. The extra connectors (+1 female XLR for each cable) are just dangling unused. This seems to work fine.

Next: I was going to connect another audio interface in a few weeks, but from what you are saying it's sounds like it's not possible without overloading the speaker inputs. Is that right?

I'm going to read a bit more though because I can only really get the jist of what you guys are saying with my (extremely) limited electrical circuitry knowledge.

Thanks,

Fab

EDIT:


It's a little annoying though because the cable shop guy told me it should work fine with 2 interfaces connected, using the set up I described in the first post.


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## wst3 (Mar 8, 2020)

It might work as you described in the original post, but only if all the stars align, which is to say that the source impedance of both audio interfaces is sufficiently high to minimize the loading effect, and that also means they have to be equal, or at least very close. This also requires that the inputs on the loudspeakers should be very high.

From my experience, limited though it is, the odds of finding two audio interfaces with similar output configurations is actually pretty good - far too many designers these days just copy reference designs. However, the odds of that source impedance being equal, and sufficiently high is much less.

Perhaps if you listed the specific equipment we can figure that part out? Maybe?!?!?

Your best bet remains to use a passive summing network (or a mixer?) in front of each monitor.

On the other hand, if you try it and it doesn't sound awful then you save yourself some aggravation.

FWIW, I would not wing it, but I'm a little odd that way.


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## Fab (Mar 8, 2020)

Thanks @wst3 I did some light reading on the subject last night, very daunting subject. Ohms, resistors and capacitors, algebra...

To be honest, I might just go the mixer route when I have the money, as suggested.

What do I do with these cables then? Can I just continue to use them as single output in the time being? It's not going to change the signal or anything is it, by having an unused connector?

They weren't cheap for xlrs!

EDIT:
I was looking at the following materials:






Understanding Impedance


No home studio is immune from issues of impedance, yet the subject can seem very confusing. In this workshop we explain what the recording musician needs to know about impedance, and show you how to avoid lifeless guitar sounds, digital glitches, and fried amps!




www.soundonsound.com








Fab


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## wst3 (Mar 8, 2020)

Hi Fab - not sure why I didn't think of this sooner! 

Please read this - https://www.prosoundweb.com/rane-note-why-not-wye-or-y-for-that-matter/ (Why Not Wye)

Dennis is one of the best technical writers to ever grace the professional audio field. This paper, in particular, is considered to be the yardstick. There is one minor flaw, but the paper was written years ago, before some standards existed. If you are dealing with balanced inputs and outputs you can use his examples as is, or you can omit the resistors that connect signal to ground and increase the series resistors. If that makes no sense then just use his suggestions.


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