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Stereo to mono the easy way, by Mike Sokol - guest post from 1996

Searching for a way to combine stereo channels for my FM mono transmitter, I found this article back in 1996. I had a link to this information in my favorites for years until it finally came up "404." Thanks to the Internet Archive I was able to recover this seminal missive and present it here for you.

We’ve all been in a fix from time to time when we needed to sum two audio signals into one. Most of the time, it’s taking a stereo signal and turning it into mono."

I designed Bus’tr simply as an aluminum project box with 1/4” circuit jacks and 680 ohm build-out resistors. To use it, you just hook 2 outputs of anything you want to mix together into 2 of the jacks and the signal comes out the 3rd jack, simple as that.

Notice that all 3 jacks have a build-out resistor, allowing any two jacks to act as an input. You can substitute RCA jack for the 1/4” jacks if desired. For this configuration, be sure that the equipment you’re hooking together is rated at 600 ohm or less output impedance. Higher or lower impedances can be made to work by substituting different resistor values

I have also used lower resistor values when the source output voltage was higher than a line-out connection.

See the complete article by Mike Sokol


Anonymous said...

Don't just short the two channels together because this causes reduced output and noticeable distortion. Virtually all audio circuits are low impedance (8 - 100 ohms), intended to drive dedicated or higher impedance loads. If your transmitter has L and R inputs, use separate L and R audio cables. If your transmitter has only one mono input, be sure the L and R signals are combined with a proper mixer circuit somewhere in the audio chain before connecting to the transmitter.

Many modern consumer audio and computer outputs have modes to enhance stereo and simulate surround sound. These modes can wreak havoc on the combined mono signal so turns enhancement modes off.


Anonymous said...

Try cnnecting a 47 k resistor in series with the center conductor of the L channel, and another 47 k resistor in series with the center conductor of the R channel. Connect the other ends of the resistors together and connect this junction to the center conductor of the load (your transmitter audio input). Connect all the grounds (outer shells of the plugs) together.


Anonymous said...

Increase the value to more than 50K, even 100K if the audio level is too low. Try to match your transmitter's audio gain to make up for the loss. It's ridiculously cheap to test.