[quote author="Svart"]Thanks for the info, but one more question.. If you are using Jfet input devices, DC on the input doesn't seem to impart much DC at the output usually right?[/quote]
I just used the general servo setup that Fred recommends for his opamps. :roll:
[quote author="Svart"]You mention gain switch but I see pots[/quote]
You're right, I'm sooo lazy (when drawing schematics, that is). There are pots shown there, but I am using switches.
[quote author="Svart"]pots as the level controls between the NI and INV pins of the opamps with the INV being fed also by the servo. shouldn't this already nullify the DC on the outputs if the JFET opamp didn't already signifigantly cut the DC offset on it's output? I'm just wondering how necessesary those caps really are or if they are just for added protection.[/quote]
As I said before, the DC servo puts it's corrective voltage on the inverting input, so we have a DC difference between (+) and (-) of the shelving opamp. This voltage would flow through the gain switch / pot if the caps weren't there.
Now the peaking amp: here the gain switches are connected between output and - through resistor R20 - (-) input. Since the shelving opamp has a servo we have 0VDC at the left side of R20 where the switch connects (at least in an ideal world it would be 0).
So here your assumption fits (and, in deed, there are no caps). However, we would need caps in the path labeled "feedback" where the third pole from the filters connects. Otherwise we might get switching noise when switching the filter in/out of the circuit.
I have to admit that I more or less just copied the original schematic and just replaced the opamp with something slightly more common. The Sontec opamps have these 'trim' pins, so the whole storry is completely different with the original opamps. The cap placement was chosen for the original opamps and might be less than perfect with the JFET990 amps.
Olaf
