Two Into One

[TBird]

I am occasionally faced with the problem of directing the stereo output of two desks into one set of FoH speakers. The program does not run concurrently; first one desk, then the other (then repeat). For operational reasons I cannnot just run the stereo output of one desk into spare channels on the second desk.

However it is a pain to turn the speakers off, unplug one set of output leads, replug with different leads and then turn the speakers on again (and repeat). I suspect a more elegant solution would be to acquire two passive line combiners (two XLR inputs into one XLR output) and take it from there.

I had been looking at a Studiospares RED503 but they don't seem to make them any more. Can anyone recommend any other make/model that would do the job?

[Hugh Robjohns]

Passive combiners can literally destroy the output drivers of electronic outputs, because the parallel connection results in one output trying to shove current up the other output... and they don't like it up 'em!

At best, you'll lose signal level and suffer transient distortion, and at worst the magic smoke will escape and you'll never hear anything from one or both device(s) again!

So my advice is to NEVER use parallel-wired 2-into-1 passive combiners. Transformer-coupled combiners are okay, as are resistively padded versions (although the latter will result in some signal level reduction), but only because they prevent one output trying to directly force-feed another output.

The much better solution if you need signal mixing is an active mixer... and there are plenty of low-cost small mixers on the market to do that.

In your case, though, you don't need signal combining at all, just independent source selection, so a two input / one destination stereo balanced selector switch box would be a much more practical solution. And it would have the added bonus that it won't affect signal quality or level at all, and doesn't need power, either.

There are lots of suitable devices around, such as this one on Amazon (UK):

https://www.amazon.co.uk/Nobsound-Audio ... KPR2V?th=1

Or this expensive but robust one from RADIAL: https://www.radialeng.com/product/backtrack

Or the LS3 from Coleman: http://www.colemanaudio.com/swtchr.htm

... and there are many more if you search...

[AlecSp]

For operational reasons I cannnot just run the stereo output of one desk into spare channels on the second desk.

That would have been one of my suggestions.

The other would be to get a small mixer and use that. More kit, but much tidier - and you can mute the desk not in use, which would help avoid problems.

Searching for the Red 303 gave the PLC21 Passive Line Combiner by Lambden Audio (in stock) as the very next item.

[Hugh Robjohns]

Searching for the Red 303 gave the PLC21 Passive Line Combiner by Lambden Audio (in stock) as the very next item.

Its designed to combine dynamic mics via a small transformer, but there's a good chance that the transformer will saturate if fed with line level signals from mixer outputs. So probably not a good solution in this application.

[TBird]

Thanks Hugh! I knew there was something I was missing.

Slightly off-topic - Would the Lambden Audio PLC21 be ok for combining the left and right outputs from a keyboard into a single feed or would it still fail for the reasons you have already given?

[Hugh Robjohns]

Frustratingly, I can find no specs for the unit, and while it's called a 'line' combiner the blurb only mentions combining mics... so I have to assume the transformer is optimised for mic levels.

Keyboards typically operate at around -20 to -10dBu, which might be within the capabilities of the transformer.... but I can't guarantee it. If the transformer is struggling you'll get some distortion and possibly less bass than normal.

Many keyboards have a mono mode which often sounds better than summing left and right – especially for sampled pianos and synths with reverbs and chorus effects.

Some Radial DIs have a 'merge' facility which passively mixes stereo inputs to a mono output.

[TBird]

Ok, thanks for looking.

[Wonks]

I had the same thoughts about that merge unit so tried looking up Lambden Audio as a manufacturer in the hope of finding specs or a manual with more details on a product page, but just got hits on the Studiospares site.

So you would probably have to ask the question directly to Studiospares.

[Dave Rowles]

Great things said, but I have to say that a 3rd sub mixer to run them through is the best option. That's what I've always done on larger festivals so that we can switch desks without cutting playback or compare mics.

[James Perrett]

I had the same thoughts about that merge unit so tried looking up Lambden Audio as a manufacturer in the hope of finding specs or a manual with more details on a product page, but just got hits on the Studiospares site.

I guess that Lambden Audio is named after Studiospares' founder, Barry Lambden so it is probably an own brand product.

[MarkOne]

Just wondering how the big festivals do this, I’ve seen BTS tour vids of big acts where the mix position has 4 or 5 big digital consoles set up for the various acts.

[Hugh Robjohns]

Breakout consoles usually route into a main console, or – with the increasing use of Dante and similar AoIP – routing into the speaker management system is selected from the console Dante outputs.