As appeared in Radio Electronics back in 1972-73, a so called zero distortion preamp with a unique
tonecontrol and amplifier cascodes built like discrete operational amps.
In a way it's pretty cool and probably totally outdated but I got a few crazy ideas and decided to give the amplifier a try.
The ideas are to use relays for switching of the tone controls instead of the "unique" switching
arrangement in the original design, as well as input switching, mono/stereo-switching and loudness.
This is the original amplifier for the preamp. Transistors aren't that easy available, but can easily be replaced by almost anything.
Alternative schematic, found at Elliot Sound Projects.
One idea was to use the original PCBs (cloned), remade to house the relays, but I think it's as easy to mount the relays and components on small experiment boards. Also I have to figure out how to activate the relays. Using an Arduino of any kind would solve that problem as well as give an opportunity to use a remote control - I must use a motor potentiometer for the volume in that case.
Got a bunch of relays - one pole 5V NO reed relays from China