It’s been a few weeks since my desktop amplifier kicked the bucket and being a bit shy on money at the moment I decided to take a slightly more creative approach to replacing it. What I had on hand was an admittedly oversized Sanyo Boombox from the 90s. Stereos and boomboxes of this vintage are fairly plentiful (and cheap) these days as people are more and more abandoning them for sleeker, smaller and newer models. And this plentiful availability makes them fantastic for experimentation. Today I’ll be adding an input for using it as an amplifier for use with a guitar or other sound devices.
The front panel of these stereos can be removed by taking out a number of screws from the back. You will usually need a fairly long screwdriver as many of the screws are very deep set. One nice thing which Sanyo does (as well as a number of other manufacturers) is mark the screws for the front panel with small arrows to let you know which must be removed.
As I began to pull this guy apart I began getting excited. Internally I found a number of complex boards built entirely of through hole circuitry (perfect for modding and circuit bending). The above image is the amplifier board, The chip in center is the main amplifier chip (A 222003 chip). Also there were a number of very interesting mechanical features. Today my stated goal is simply to add an input jack but I will be returning to this board to see what else I can force from it.
In order to add the input jack there are three wires you will need to solder onto the board. The first is a ground wire which can be connected to any ground point in the stereo. I wired mine from pin 5 (ground) of the 222003 amplifier chip. Next up you will need to locate the left and right signal inputs. I initially planned on sending these into the amplifier chip as well but after some testing I found I got a much better signal sending them into the volume control board as shown above. To locate these points I built a small 555 buzzer circuit on my breadboard. I attached the ground from the stereo and touched the output to various points on the circuit.
NOTE : I strongly recommend powering the stereo using batteries when you are feeling around for these points. If you choose to do so with the stereo plugged into the wall I take no responsibility. Mains voltage can kill. Be aware of the location of the transformer and stay far away from it.
Once I had located the speaker left and right inputs I attached lengths of wire to them. I connected these two wires to either side of a 100K (If you need to get more volume try a 50K or 10K pot) pot which will allow you to pan the signal between the two speakers. If you are not interested in having pan you could easily replace this pot with two static resistors. Then I connected the center pin to the tip of a 1/4 inch jack. In retrospect you may want to add resistors on either side of the pot to buffer the signal as it gets quite loud if you go to either end of the pot. I expect about 10K would work perfectly. If you are interested in adding a separate volume control or switch just add them in line in between the pot and jack.
From here you can drill and mount the pot and jack. Put the stereo back together and you are all set. You’ve just converted your tired old Boombox into a perfectly functional amplifier. I’ve had great results using this stereo with my guitar, circuit bent devices and my synth modules. The sound is actually quite clean. The stereo also has a 4 band equalizer and a bass booster which give you a fair bit of control of the sound.
Hope you guys enjoy this project, have a great weekend!