# Summing stereo to mono safely



## Starliner (Jan 9, 2010)

Hi everyone,

I would like to build a small switch box for my home recording setup which would allow me to combine the left and right outputs from my headphone amp or preamp to produce a mono signal (L + R). I would then be listening through either my headphones or a set of powered studio monitors. 

This is for monitoring the mixing process, for audio comparison of both the combined L+R signal to the separate left and right channel signals.

I need to be able to do this* safely*, so as not to damage the output stages of these devices, on which some of these devices there was a warning against connecting the left and right channels together. 

Your suggestions and ideas for the electrical design are appreciated. Thank you.

Starliner


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## Wayne A. Pflughaupt (Apr 13, 2006)

Back when I was doing pro audio installations we used to use this little gizmo for summing. Works like a charm. If you want an active device, all you need is a little project mixer or a two-channel USB interface.

Regards, 
Wayne


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## Starliner (Jan 9, 2010)

Hi Wayne,

Thank you very much for your quick reply. I will look into both of these possibilities.

Does the RDL passive device reduce the overall output level to a noticeable degree?

What I am trying to do is compare the stereo signal to a summed audio signal in an A-B test, to see if there are issues with either of the channels when combined, such as masking of certain frequencies or instruments.

The concern I have about an active device is whether it will add coloration and / or noise to my signals.

My problem is made a bit worse due to a hearing problem, mostly in one ear. I plan to build a box to swap left and right channels for comparison, and also to allow me to sum the channels to mono, as mentioned earlier. I have an old Heathkit AC-13 Headphone Control Box, which I might modify for the task. 

Thanks again!

Al


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## Wayne A. Pflughaupt (Apr 13, 2006)

I honestly never did a “before and after” test with the RDL, so I don’t know if it reduced the signal level or not. It’s basically nothing but a couple of10k resisters in line with the signal (+) before the two are tied together, you could easily make the summing network yourself if you can solder. We had the subject come up here recently, I’ll post a question about the levels there and we’ll see what Mr. Myers comes back with.

Regards, 
Wayne


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## AudiocRaver (Jun 6, 2012)

Thank you, Wayne P, for bringing this thread to my attention, and pardon my slowness in getting to it, Starliner.

The summing network that Wayne P suggests is the simplest way to do it. It will cut the signal level by 6 dB, which equals cutting it in half. It will not seem like that much because sounds that are panned to center will be combining electronically instead of acoustically and will get about 3 dB of perceived gain (less loss) as a result.

The network completely protects the outputs feeding it from any damage and is perfectly safe. It can feed the input of an amplifier, line level mixer, headphone amplifier, or powered speaker. The loss can be made up by bumping the gain either before or after the mixing network by 3 dB. Do not worry about extra noise, the signal level stays way above the point where that could become a problem.

The network cannot drive headphones directly, the signal level will drop down to inaudible or close to it with heaphones following it. Again there is no possibility of damaging anything, it simply will not work that way. A headphone amp will be needed to drive the 'phones and can make up the loss mentioned above.


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## Starliner (Jan 9, 2010)

Wow! It's been a LONG time for me to get back here and thank you guys for your technical assistance.
So... THANKS!!!

Maybe next time it won't be so long before a return visit!

Starliner


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