and now I'm using software that emulates the sound of these old machines

Commodore SID chip emulation is used allot in music these days.

, Ian2 wrote

Not sure if we have done this before but i found a nice pic:

 

More here:

http://www.informationtechnologyschools.org/blog/2010/30-old-pc-ads-that-will-blow-your-processor/

I still have the 16k RAM pack but gave the ZX81 away a few years ago to a colleague who was mourning the loss of his own.