Buchla Preset card - DIY mixer for 208 easel

One of the best things about the Buchla Music Easel is its program card.
It's really not just for presets ... esp the original card invented by Don Buchla back in the 1970s.

I'm guessing he may have wanted us users to experiment with electronic circuits.
Why else would Don have placed a mini "breadboard" with +/- 15V rails on the program card?

The Buchla SILI-CON CELLO from 1978 is a good example of his use of breadboard circuits in a fully
functioning system. It's a shame the current BEMI program cards have removed this homage to the age of DIY electronics.

This is my latest use of the card: a DIY mixer.
(The 10U system doesn't have a dedicated mixer).

The circuit is very simple.
Just one dual op-amp (TL072)

 

It's a basic unity gain inverting configuration. In the first half the 4 inputs are mixed into the inverting input of a op-amp. Since the output signal from this op-amp is inverted, the 2nd op-amp  re-inverts it. 

Power goes to pins 4 & 8. The unmarked resistor on the right is 1K

Thanks to Ken Stone for the original circuit. and Justin from MetroModular for his advice.
Ken's schematics show the possibility for using this as a CV mixer & attenuverter.

Breadboarding the circuit. Just two inputs on the left without pots.
It works on +/-12V above and +/-15V below.

Might try building a mixer using the OPA2134.
It's pinout is the same as the TL072.