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.
Reader Comments (Page 1 of 1)
Dario @ Sep 20th 2006 2:44AM
kool.. i was wondering what happened to the "HOW TO:" topics.
well this looks like something i can do for my vacations
AndyB @ Sep 20th 2006 2:51AM
This really isn't worth it if you value your time.
Especially when you can buy a good quality name-brand audio mixer for thirty dollars. Check out the low end Behringer mixers - unbelievably cheap!
http://www.samedaymusic.com/product--BEHUB502
sparrow @ Sep 20th 2006 9:17AM
For about $40, the Rolls MX42 will mix four stereo sources. See: http://www.amazon.com/Rolls-MX42-Stereo-Mini-Mixer/dp/B0002KX74Y
Will O'Brien @ Sep 20th 2006 3:03AM
After loosing a pair of nice seinheisers a few years ago to workplace theft, I don't keep high end gear on my desk. Even a cheap Bheringer is overkill if you just need to combine some signals.
Mike @ Sep 20th 2006 9:36AM
Why not just use the RadioShack 2-Way Audio Switch Box to switch between the comps? Whats the benefit of this mod?
Hardy @ Sep 20th 2006 10:06AM
You can have audio from more than one source __at the same time__ useful for music from one machine while still hearing error beeps on the other or perhaps just not having to switch between either at all.
J. R. @ Sep 20th 2006 10:03AM
I was thinking about doing something like this for getting two audio sources to play into one pair of headphones. Basically, I want to plug my aviation headset into the comm radio as well as my cell phone. I realize you can buy these things on the internet, but they are really expensive. Anyone know of a do it yourself solution?
Hardy @ Sep 20th 2006 10:09AM
The problem with aviation headets is that they are really high impedance so you'll probably need some form of amplifier to let the cell phone actually drive the speakers.
Hardy @ Sep 20th 2006 10:09AM
However you could probably power this from the line voltage used to power the microphone. (Just a thought)
Amos @ Jul 6th 2007 6:08AM
Yeah, as if _drivers_ talking on cell-phones weren't bad enough... :P
Chad @ Sep 20th 2006 11:09AM
I built something similar to this for mixing 5.1 outputs from two different computers - http://www.flickr.com/photos/iamthechad/226629264/in/set-827767/
Michael Geary @ Sep 20th 2006 1:18PM
"Watch for flying pins [when you clip the leads]."
No, don't watch for flying pins. Keep them from flying!
Don't just clip the leads, *hold* each lead when you clip it so it doesn't fly. Much safer that way.
aperson @ Sep 20th 2006 6:15PM
Pardon if this is obvious... but does the output switch function correctly after this mod is done?
Nik Mani @ Sep 21st 2006 9:07AM
1001 uses for an altoids box.
thomas.rowse @ Dec 11th 2006 5:41PM
Will this work for mixing the audio from ...
http://www2.oregonscientific.com/shop/product.asp?cid=3&scid=9&pid=595 and my iPod Shuffle (1st Gen)? ... if so , why do you pick 100K resistors? ( I saw this http://www.tkk.fi/Misc/Electronics/circuits/linemixer.html where they used 10K ...does it matter?
Will O'Brien @ Dec 11th 2006 5:51PM
It depends on your setup. The 100k's work great with my home machines and altec lansing speakers. However, with a crappier soundcard and amp at my office, they're too much. I highly suggest trying out a few different levels of resistance before finishing the job.
Jon H @ Jul 2nd 2007 7:14PM
I'm sad. I did this, but it no workee. (Probably bad soldering)
The audio only comes out through the pink mic-in socket, and the audio only goes in via the pink mic-out wire.
I'll have to open it up and check the connections this weekend.
One thing that came to mind: using it as designed, you need a male/male 1/8" headphone cord to go from the second device to the switchbox. It would be better to use the pink 'mic' cable that is built-in as a second input.
AdamNZ @ Jul 6th 2007 4:58AM
I built a fairly similar mixer - http://tinyurl.com/26zkaz
Randomstuff.co.nz