Lehle Parallel, schhematic [schematic]

[apollomusicservice]

Lehle Parallel, a very well built device. Schematic is only for reference and entertainment.

[ilcaccillo]

Thank you,
Lehle has really nice useful boxes

[Chugs]

Why is the Vbias so crazy asymmetrical?

Could they have not just reference the Bias resistors to ground as they have a + and - power supply for the pampas?

[Intripped]

I think that Vbias is for the op-amp offset

[Pruttelherrie]

Weird, you get a liiiitle bit more headroom this way on the negative side, but I'm not sure if it is really needed. All opamp stages are regerenced to GND and powered by +9 (or +Vbatt!!!) and -8.42, from the ICL7660.

To be honest, I didn't know that the ICL deliveres a negative voltage that is slightly below the supply voltage

[Satch12879]

The send and return inverting amplifier sections don't appear to be correct. The specification states they provide +/-15dB with unity at the center of the control's travel. R7, 3k9 and R45, 15k0 should probably be the same value. That being said, I'm having trouble working out how this configuration gives +15dB or x5.62 with the 22k log pot turned fully clockwise. By my calculation, the parallel combination for the 22k pot and R7 plus R46 680R in series for the send amplifier gives x1.02, while the parallel combination of the 22k pot and R45 15k0 plus R44 680R in series gives x2.46.

I also think that unity gain at mid-band doesn't work, either.

Am I missing something?