Making an audio mixing box

A few months ago, I got a new desktop and handed down my previous desktop to my wife as a gaming rig to complement her day-to-day laptop.
Ending up with two computers, but with the same single set of speakers we had before raised the question of how to connect the audio from both computers to the speakers.

One’s immediate instinct is to just connect all the wires and let ‘er rip. However, that’s a very bad idea.

Your science is bad and you should feel bad!
Your science is bad and you should feel bad!

This post explains exactly why this is a bad idea and describes a simple mixer circuit that sums together two inputs into one output. However, it doesn’t allow any mixing control.

Simple and useful
Simple, useful, yet lacking.

In case you’re not familiar with circuits and whatnots, check out sparkfun.com for some tutorial goodness.

This doesn’t deal with a very common situation where the volumes on the two inputs might not be set to comparatively reasonable levels, causing the output to effectively feel one sided. with no means to control the mixing ratio, I felt it wasn’t good enough. Additionally, if one of the inputs is connected to a computer that’s shut down at the moment, some static noises might be annoying, so an option to disconnect one of the inputs temporarily might be nice.

Complete with switches and double pots.
Complete with switches and double pots.

My design required a few specific components, for example the two pots are actually a double potentiometer (two pots on a shared axis) affecting both left and right channels in the same intensity. the switches are also double switches (123circuits only had triple switches) so both U1_S1 and U1_S2 should connect and disconnect together, without actually touching one another electrically (crossing the streams, sword-fighting if you will).

It took me a while to collect everything I needed, including buying solder for the first time in like a decade. but last week I was finally ready to build.

here’s the final result:

A very "sound" design
A very “sound” design

Using it is as intuitive as pointing the knob in the direction of the input you want to be more dominant. I skipped on the switches as the ones I had from dx.com were a bit flimsy and messed the audio a bit. I will probably post an update once those are added.

Plugs go here.
Plugs go here.

The actual inputs are aligned with the labels and directionality of the pot, so it should all just “make sense” to the user. Output is in the middle.

a quick lookie loo
A quick look inside

This was my first soldering work in over a decade so I won’t be posting any close ups of that poor poor PCB.

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