How-To: Make a simple sound mixer
We like using a few computers at the same time -- big surprise, right? Ubuntu box here, Vista box there, Leopard box, well, somewhere; but unless the speakers have multiple audio inputs, it's a bit difficult to get audio from our everything in our stack of machines hooked up. Today's How-To involved a late night parts run, some Bawls, and some true and proper self humiliation.
See all those parts... we didn't need em. Originally we were thinking of creating a nice mixer complete with headphone/pre amplifier. For our prototyping we grabbed our CMOY amp, some pots, resistors - all the usual suspects. At 8:50PM, a mere ten minutes before Radio Shack closes, we realized that we'd grabbed the wrong potentiometers. Instead of dual potentiometers that uniformly adjust a pair of potentiometers for stereo inputs, we'd grabbed a pair with built in on/off switches.
We raced to the nearest Radio Shack where an oh so generous Robert actually kept the store open an extra ten minutes just for us. (Thank the electronic gods for 411.) We sprinted to the store and scored a single dual 100k audio potentiometer.
Our previous parts gathering trip yielded this little guy. It's RadioShack part #12-2315 - a 2-Way Audio Switch Box. It's not audiophile gear, but it's a deal at $4.99. The upper switch selects the input (computer or aux) and the lower switch selects speaker or headphone output.
Disassembly was easier than we thought. We feared the ever cheap super-glued construction but were treated to the front plate prying off easily with a screwdriver.
The two black halves separate easily with gentle pressure. It's a simple device, and perfect for our little project. (If only it had better quality cabling -- that looks like gold contacts!)
Both switches are double pole, double throw. The upper switch is our input selector. We'll be making our modifications to the board where it connects.
Remember that humiliation? Here's why. We didn't even use the potentiometers. All we need to use both inputs are four 100k resistors. We built some test versions using the pots, but in the end this is all you need. Since all of our audio sources -- computers or MP3 players -- have volume control on board, we can get away with simply mixing the signals together.
De-solder the upper switch using your favorite method. De-soldering braid is handy stuff. The solder is heated up with the iron and the braid sucks it up. Watch out for the bent pins of the switch. They tend to damage the traces when you pull them up.
Once the switch is out, start placing your resistors. Just bend one end over like the picture. If the hole is sealed in solder, heat it with your iron and slip the pin through the now soft solder.
With all four resistors in place, it should look like this. Solder the outer contacts first, then to the two center pins with the two leads.
After that, clip the leads off with a pair of cutters. Watch out for flying pins.
Reassemble it - screw the board back down and snap the cover back together.
That's it. Now we've got a simple audio mixer that'll keep the inputs from driving each other, and get the audio out from two sources to your speakers or headphones on demand. Again, it's not audiophile level stuff, but neither are the usual variety of office computer sound cards one would find in the kind of devices we're mixing.
