Anyone a clue about this circuit ? What is it used for ? Make an educated guess! 
Lots of coils, capacitors and analog stuff inside, YES, it's a Theremin, a musical instrumet that was popular in the late 70th. Basically there are two antennas´attached to resonat LC tanks, player's hand changes this circuit so resonance frequency changes which can be used to dirve filter, VCA and VCF stages to control sound and volume. Very sensitive to noise injection, not very stable regarding maintaining anything from frequency and sound but Mr Moog did a great job with the invention, appereance and sond ist very outstanding. I already dig into this using LTCs famous switching caps AmP LTC 1043 (there are really good application notes you can start with) and Cypress PSoC devices but didn't get too far. Still more analog then digital and at least a new circuit for an old principle. About to start a new approach based on optical sensors being cheap as often used in tablets and s smart phones (Silicon Labs sensors are looking very promising) I learned about the LDC1000 so the idea grow to stick to the basic principle but switch to the digital domain as early as possible in the signal chain. And WHAT about MIDI ...
