Stereo > Mono Line Level Mixer

Mixing line level stereophonic signals into full dimensional monophonic ;)

Here's a DIY project for mono aficionados that's so basic, I should have uploaded it many years ago. But other things got in the way.

Just like an MC step-up, these are entirely passive devices. 

The simplest way to convert stereo to mono is to connect left and right channels via a Y connector. But there's a more elegant way than just shorting two channels.

Resistors can blend two signals into one better by...

...using Allen-Bradley carbon composition for warmer sound ;)

We can end the mixing quest here but...

...the most elegant way of mixing two channels as was done in the studios by audio pros of yesteryears was through transformers.

This single UTC A-20 transformer was in a box lot of tubes and parts I picked up at a radio show. It's a high quality transformer designed for mic, mixer or line matching applications. Instead of trying to find a mate for MC step-up duty, I repurposed it for a greater calling in my mono rig.

With a claimed bandwidth of 10-50,000 hz and dual windings in the primary and secondary, it is perfect for mixing line level stereo signals into mono!

Left and right channels from a stereo source are fed into each primary winding. The summed mono output is taken from the series-connected secondary winding, which goes into the line level input of a mono preamp.

I also had a pair of 1:1 Tamura line level matching transformers. With the secondaries connected in series, it functioned like the UTC A-20. I tried an Altec 15095A but the step up ratio provided too much gain in my set up. It will work better in systems with lower gain.

Aside from mixing duties, transformers also serve as great digital signal processors. Who needs a DAC? ;)


  1. Hi,
    If I want to convert to the monaural two-channel recording , I can use this way?

    1. Yes this blog is about mixing stereo signals into mono.


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