No line in on your amp? No problem! (If you're ok with headphones)

[joecool85]

The K-20X doesn't have a line in.  What this means is that to play to a backing track I need to have a CD player or radio etc playing over a speaker while I play my guitar over the amp's speaker.  This works but is clunky and can also get loud.  Rather than purchase a Vox Amplug or similar ($40 or so and have great reviews, I still might do it at some point), I decided to go the cheap and DIY solution and raid my parts drawers.  What I found:

1 x small wooden box
1 x 1/8" stereo jack
1 x 6' stereo 1/8" cable cut in half
4 x 200 ohm fire-resistant 1w resistors (1/8w would have worked and they don't need to be fire-resistant, but it's what I had)

With this I made a passive mixer designed for headphone level signals (the same design can work with line level signals but use 10k resistors). Now I plug this into my amp (with stereo 1/8" -> 1/4" adapter), and cell phone, then plug my headphones into the passive mixer box and BAM! I'm rocking to tunes and not bothering anyone in the house. If only somehow my singing could be dampened...

I don't have a schematic, but the gist is this: For each channel coming from each device you put a resistor in line with the positive.

So Device One's left positive goes into a 200 ohm resistor.  Device Two (amp) has it's left positive go into another 200 ohm resistor.  Both of these resistors terminate at the left positive of the headphone out jack.  Do the same with the two right channels, tie the grounds all together and you're done!  Easy peasy and works a treat.