Could you explain something to me here?

[KarenColumbo]

I breadboarded the in- and the output stage of this here and threw an active baxandall (with opamps) inbetween.

Result: HUGE gain, extremely clean. Very nice circuitry there.

My question: Is this 3906 contraption a constant current source? What does it do?

[Rob Strand]

There's a reasonable whack of gain in the first stage.

My question: Is this 3906 contraption a constant current source? What does it do?

It is working as a voltage amplifier. 

The JFET and 2N3906 form a discrete opamp.    Both are contributing to the *open-loop *gain.  The resistors from the output to the JFET source and then to ground form the feedback network of a non-inverting opamp configuration.

Here's a very common all BJT version:
https://www.electronics-notes.com/images/transistor-two-device-amplifier-circuit-01.svg
http://darrenyates.com.au/electronics/wp-content/uploads/2011/03/ELEC00041-300x250.jpg


C4, R6 and R7 look a little odd to me.   They look like left-overs from some other amplifier configuration.

[KarenColumbo]

Thanks - that clears it up!

The "double Rs" there were interpreted as a pot by me. Which worked, sorta "Gain" knob.

[Rob Strand]

The "double Rs" there were interpreted as a pot by me. Which worked, sorta "Gain" knob.

OK, that makes sense.  A 10k gain pot would work  (and the 10uF cap C4 would prevent the DC level at the output changing with the gain pot setting). 

R7 still looks odd.  It might be trying to get exactly x100 gain but it doesn't work as R4 and R5 need to be taken into account.   Anyway no need to get bogged down in the fine print!

[mac]

A Heathkit TA-28 with a gain pot

http://www.experimentalistsanonymous.com/diy/Schematics/Fuzz%20and%20Fuzzy%20Noisemakers/Heathkit%20TA-28.gif

mac

[Rob Strand]

A Heathkit TA-28 with a gain pot

And a heap of gain as the 100uF has no series resistor.