[sdiy] Buchla 259 integrator op amp question

[Aaron Lanterman]

Greetings,

I've puzzled over this for years:

http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2590_2_200.jpg

IC20 and C20 (0.0047 poly) form the integrator for the triangle core. 

My analysis says that the output of IC22, the comparitor, is a square wave alternating between ground and 4.39 volts (I don't get the 4.29 marked in the diagram but I doubt it's that precise anyway). These voltages correspond to voltages at the output of IC20 of -2.608 volts and 2.5453 volts, which is a nice 5.15 volt peak to peak triangle wave.

I did all this assuming the + terminal of IC20 was ground. 

But - there's a 7.5 volts put on the + terminal of IC20. By golden op amp rules, this puts a 7.5 volts on the - terminal of IC20. What is that 7.5 volts doing there? Should it change my analysis any? Does the fact that there's just a cap in the feedback loop mean that the current input to the integrator can happily sit a 7.5 volts while the output wiggles around 0 volts?

I think I may have asked this before but I've forgotten what the answer is.

It's relevant since I'm lecturing on it today, and I usually handwave when I get to that part. ;)

- Aaron