Convert an old cassette player, boom box into an audio input jack for your MP3 player

My new phone has enough storage to keep all my music and it can also play music from a variety of online sources. It has a 3.5 mm or 1/8" output jack to connect to headphones or an external amplifier.

We also have a couple of old boom boxes around the house, in varying states of decay. Unfortunately, these don't have audio input jacks, but they do all have cassette players. We haven't listened to cassette tapes in years.

There are inexpensive devices available that look like an audio cassette with a short pigtail that you can plug into an mp3 player. They work, but the audio quality is generally miserable. I wanted to have a direct electrical connection that bypasses the original tape head. I also wanted an input jack where the cassette player was. This post describes my first conversion. Some soldering skills required.

The tape head is the pickup element of a tape recorder. It's a set of small coils that, in the presence of a changing magnetic field, such as that of a moving magnetic tape, will produce a small changing electrical signal that can be amplified to be audible. Here is a close up of the tape head from a cassette recorder:



I had a boombox with dual cassette players. By converting one of them to an audio input jack, I still have the option to play cassettes and I could even record the audio signal from my phone to cassette. I don't think I'll ever want to do that though.

I started out by removing the cassette door and ripping out as much of the tape players robotic guts as possible. Ideally, I would have cut the wire to the motor that drives the spindles to move the tape, but both dual cassette players share a single motor (a cost cutting measure) and disabling it would have disabled the other tape player as well. It might have been possible to disable the switch that turns on the motor when the Play is pressed, but I wasn't able to easily check this. Sony, the manufacturer of this boom box seems to have specially designed the boom box so that an unusually long screwdriver is required to take the thing apart. Not very friendly of them, but I suppose they make more money if their products ends up in a landfill when they break rather then being repaired. This is called planned obsolescence and it is one of the reasons I will never buy a Sony product again.

I removed the tape head and desoldered the wires to it. Then I painted the old cassette door black, drilled a hole in it and glued a 3.5mm jack in it.

Two 10kΩ trimmers are soldered to the input jack and the old wires that went to the tape head now go to the trimmers, as in this photo:



As you can see, the slider connection of the trimmer is connected to the cassette player wire. One of the other leads of each trimmer is connected to ground, the other to the signal connection on the jack.

The type of this standard audio jack is called "tip ring sleeve". The sleeve is ground, the tip is the left channel and the ring is the right channel. On the wires to the cassette player, red is right, white is left and black is ground.

By turning the trimmers, the strength of the audio input signal can be adjusted so it's at the right level and the amp doesn't clip. The tape head preamp is very sensitive. It would be ideal to use trimmer with a so-called "audio taper", but regular trimmers will work too.

Once the audio levels were adjusted, I glued the jack into the cassette door with some two part epoxy glue and then put the tape door back in its original place with some silicone glue and double sided tape.

You can see the finished result at the start of this post. The audio quality isn't great, but it works well.

For my next conversion, I might use pot meters instead of trimmers and mount them on the cassette door, so the signal and stereo balance can be adjusted later. Also, I intend to disable the tape motor, since it produces some noise.