Another Theremin Circuit

It's not a normal OP Amp, it's a transconductance amplifier. That means that the amplification factor is not set by feedback as with a "normal" opamp, but it is controlled by a control current on a separate input. Another extra input allows biasing the main inputs which will affect distortion and also the gain. And there is no output voltage but an output current which has to be transformed into a voltage by the means of an external resistor and (at least) an emitter follower stage.

The LM13600 is outdated but may be replaced by the pin-compatible LM13700. The only obvious difference between both is that the internal emitter followers are doubled in a darlington configuration on the newer variant.

I was looking at this circuit myself, Just LOOKing (before i get jumped on)

When it comes to IC, they come and go, but there are normaly always suggestions of replacment IC you can use, just keep an eye on what it wants in and gives out tho.

Well i couldnt find the LOST R22 ever, i was intrested in the Inductors, never really worked with them much myself, but always in my maths classes was two ends, but the circuit shows another connection  going to the middle (side) of the inductor!
Or is it Two connections on one side

With all the really good circuits available, why are people drawn to the rubbish ?

The EM Theremin by Bob Moog is far better than any of this rubbish, is fully documented, and is updated by reference to the EW Manual

The idea that the rubbish is simpler than the good stuff. as an illusion - Sure - A single schematic without pages of construction notes and testing details may look simpler - But thats only because YOU need to create everything by yourself from the "simple" schematic.. The missing detail is a disadvantage not an advantage!

Ok, parts cost for the EM will be a bit higher - Add about £20 for the inductors etc.. But you will have a higher chance of a playable theremin than you have with a high frequency instability generator you are likely to be building with this rubbish "another theremin circuit" -

Simplest ? Buy a Jaycat kit!