Summing Stereo into Mono

[4pyros]

Will this Sum stereo inputs into mono?
https://www.rolls.com/pdf/M_DB24.pdf

 

[Nigel Goodwin]

Of course (it's got a mono/stereo switch), but it's a bit of overkill just for doing that

As always, it depends exactly what you're trying to do - in the vast majority of circumstances you simply need to join the two channels together.

 

[4pyros]

Yes but do the left and right resister networks provide true summing before the input to the transformers is shorted out?

 

[Nigel Goodwin]

Yes but do the left and right resister networks provide true summing before the input to the transformers is shorted out?

Basically - but it's not really required - you could just join the inputs together in almost ALL cases.

 

[4pyros]

Basically - but it's not really required - you could just join the inputs together in almost ALL cases.

But would not out of phase stuff cancel each other out?

 

[Nigel Goodwin]

But would not out of phase stuff cancel each other out?

It would regardless of how you mono it, or even if you kept it stereo it would cancel out in the air.

 

[tomizett]

This reminds me of a conversation I had with a sound engineer earlier this year. I wanted to use a passive DI box of my own design, but similar to the one we're discussing here, to mono up a DJ mixer to feed our PA. He insisted that that a simple passive network wasn't the way to do it, and that we'd end up with cancellation and phase problems (I can't quite remember how he phrased it). If it was anyone else I'd have dismissed this suggestion out of hand, but the engineer in question was an old hand and highly experienced, so I have to at least give his suggestion some serious consideration.
Now I agree with Nigel - this seems like as good a way as any other to sum stereo to mono... But am I missing something more subtle about the way stereo signals are produced?
The only difference I can see is that a passive voltage divider allows cross-talk between the two inputs, whereas with a summing stage based around a virtual earth the impedance seen at each input is entirely independent of the signal on the other. But I can't see why that'd make any difference...

(edit for typos)

 

[Nigel Goodwin]

This reminds me of a conversation I had with a sound engineer earlier this year. I wanted to use a passive DI box of my own design, but similar to the one we're discussing here, to mono up a DJ mixer to feed our PA. He insisted that that a simple passive network wasn't the way to do it, and that we'd end up with cancellation and phase problems (I can't quite remember how he phrased it). If it was anyone else I'd have dismissed this suggestion out of hand, but the engineer in question was an old hand and highly experienced, so I have to at least give his suggestion some serious consideration.

It could well just be an 'old wifes tale' he'd heard

Look in any amplifier with a mono/stereo switch - they simply join the two channels together, very often by simply joining the tops of the volume control.

 

[audioguru]

"Out-of-phase stuff" is the ambience of the hall or studio the stereo sounds were recorded in. That is what makes stereo "alive" and makes mono "dull and dead".
A choir of singers do not all sing exactly the same frequency and phase. Some are a little flat and others are a little sharp. A group of musical instruments do not all play exactly the same frequency and phase. Some are a little flat and others are a little sharp.
So their different phases combine as they sing and play which creates ambience in stereo but causes serious level fluctuations in mono.

 

[4pyros]

So basically you will lose some information no matter what?

 

[Nigel Goodwin]

So basically you will lose some information no matter what?

It's only mono - so obviously you lose all the stereo information

But the way of 'converting' it has no effect on the process.

 

[tomizett]

We all seem to be in agreement then. I always understood that it was part of the job of the mastering engineer to make sure that a recording would merge to mono elegantly, without loosing anything important or sounding too odd (ie, a limit on level panning and phase/delay differences between left and right).

 

[audioguru]

Stereo is normal and natural. Our hearing is in stereo. Nothing needs to be done to make mono sound the same because mono sounds different.

Stereo sound is a big improvement over mono sound. Surround sound is a big improvement over stereo sound. Many yeas ago I made a Motorola PRO-SQ surround sound decoder and bought records (remember them?) with SQ surround sound encoded on them.
I don't listen to surround sound anymore, mostly stereo except my clock radio is mono.

Sound engineer? One time I heard some odd phase errors from my FM stereo radio. When I switched it to mono all sounds completely disappeared. I phoned the radio station and the sound engineer apologized that the recording had one channel out-of-phase! He didn't have or couldn't find a phase reversal switch. So the channels cancelled when in mono.

Have you ever heard out-of-phase mono music on headphones? It seems to pull your ears away from your head. You can do it easily by feeding the live terminals of both earphones but not connecting the common ground terminal. Then the earphones will be in series and out-of-phase.