Plug a stereo output to a mono input, the "mono emulator"

I want to plug my 8 bit computers tape port (input) to my new stereo cassette player another stereo set as my smartphone, a mp3 player, my raspberry pi in order to play games.

The idea is simple, plug all ground/mass conection together and stereo right and left signal to mono one.

To connect the stereo source to a computer with a homemade port, i.e. ZX Spectrum +2A or Commodore 64's tape unit, I make a cable with two male connectors, one stéreo and another mono, like this:

But If I want to make an adapter to an Amstrad CPC6128 or MSX tape cable the connections will be the same but mono connector will be female.

If I want to record in tape I need to this cable two times.

Also I can make a specific adapter for new smartphones (CTIA connection) or for my raspberry. At this cases we change the male stereo connector for an 4 contacts jack where most external connection will not be connecter (MIC or Video signal), ground will be the next connection.

If you use a very old mobile (OMTP connection), ground and MIC are swapped.