🚀 go-pugleaf

    I'd like to build a walkman jamming device for
    use on the train.

I would never have believed that a walkman could produce enough sound
to even be heard, much less be objectionable, over the background noise
on a train if I hadn't seen exactly this thing myself.  Imagine how loud
this must be to the person with the earphones on!

If I wanted to build such a device, I would try transmitting a signal
on about 50KHz or so.  At this frequency you have a reasonable chance
of getting a significant amount of energy coupled into the play head
on the walkman.  The theory is that this would intermodulate with the
bias signal on the tape and produce audible noise.  For maximum effect
the signal should be approx. 1 KHz or so off from the bias frequency,
and frequency modulated with the noise source of your choosing.

I would start with an audio amp chip, the kind used in car stereo
applications.  Get one with external equalization so you can run it
above normal audio frequencies.  You should be able to get a couple
of watts out of it.  For best results you'll need a honking big
coil to act as an antenna (either that or a couple hundred feet of
wire, hard to hide on a crowded train).

I have no idea whether you could couple enough energy into the play
head at reasonable range, or whether there is even enough residual
bias voltage on the play head to get the desired intermod products.
The play head and associated electronics will be designed to reduce
the residual bias, so it will be an uphill battle.  Maybe Larry Lippman
could grace us with some of his elegant calculations, or suggest a
better method?