I picked up this little radio shack 4-way a/v switch box and I want to modify it to output mono audio with the R and L channels combined. It shouldn't be hard to do, but I've seen this done two ways. One way with just two resistors (usually a few hundred ohms each) one on the R and one on the L tied together. The other with an additional resistor (of about 10x higher value) between the combined output and ground. I'm guessing it sort of makes a voltage divider and fixes the output at a certain level. Which method works best?

My preference is using a mono-blending transformer.

One way with just two resistors (usually a few hundred ohms each) one on the R and one on the L tied together. The other with an additional resistor (of about 10x higher value) between the combined output and ground.

It all depends on the output impedance of the source(s) you're using into the "combiner". A few hundred ohms is probably really a bit too low for most things. For low impedance outputs like found on most modern solid state gear I would think more like 1000 ohms minimum to about 4700 ohms maximum would be more appropriate and there should usually be no reason to put the additonal resistor to ground in most cases.

The outputs from older tube gear are often much higher impedance and would require higher "mixing" resistors to make a proper passive mixer/combiner as their output impedances can often be up in the hundreds of thousands of ohms.

I use an adapter I got at walmart,it just combines 2 channels into 1.... Works very well!!!

I typically just short both channels together, when working with line-level signals. I have seen a stereo amplifier stereo/mono switch wired that way.

Hi to all,
Hi Adam,

here is a simple resistor combiner to convert stereo to mono (photo).
from the web site of one of our french radio forum members.

Full article :
http://www.radioman33.com/pages/mes-constructions/module-fm-et-lecteur-mp3-pour-brancher-sur-la-prise-p-u.html

This second adapter using a transistor radio small driver transformer is used for chassis isolation ("hot chassis") when feeding the phono input of a vintage tube radio. Not necessary in your case.

http://www.radioman33.com/pages/mes-constructions/brancher-un-baladeur-mp3-sur-p-u.html

Best Regards
jhalphen
Paris/France

Yes, these simple resistor channel mixer are fine for this application.