Play Along CDs are great for learning how to read sheet music. Some require that one of the speaker be muted -- that's the part you play; the other speaker is the band playing without your part. Some plugs let you cut one of the tracks off but it is not consistent and it's a real hassle. This gadget makes playing along fun again.
Step 1: Sound Source to Switch Box to Guitar Amp
To start with, someone suggested this on Instructables years ago. I just finally got around to building it. S/he deserves the credit for the idea.
The CD player at the top left corner of the picture sends the sound from both tracks through the earphone jack to the box in the middle of the picture. The 2 switches at the front of the box let one or the other (or both or neither) of the tracks go on to the guitar amplifier at the right of the picture. I play along using the amp on the left.
The sound signal splitting box takes the input from the cable shown at the front of the box, splits it depending on which switch is on, and transfers it to its respective speaker (inside), which then induces that signal in the guitar pickup (inside).
So what's inside? The two switches, the left and right speakers, and the guitar pickup. That's it. The first version had the speakers lying down. The idea was to have them vibrate a guitar string (or some other metal wire) so that the guitar pickup would think it was on a guitar. It turns out that the magnetic fields were being picked up (no string needed) -- as a previous commenter had posted before.
The cones and the magnets on the speakers may not be needed either, just the coils from the speakers (resting directly on the pickup). That configuration may even signal transfer with less energy loss.
Here's the inside of the lid. The input cable splits off into the 2 switches connected to their respective speakers. These shown are SPST (Single Post, Single Throw). The guitar pickup cable exits out the back of the box and on to the guitar amp. The grounds from the input cable close the loops with both speakers (at all times, they don't go through the switches). BTW, on a stereo cable, the tip is the left speaker/track, the middle sleeve is the right speaker/track, and the sleeve closest to the cable is the (common) ground.