The MK6 uses the OPA445AP as well.
Don't worry about what the opamp is doing. I verified it long ago and I ordered my own custom versions for my experiments
This is a straight up copy of the configuration on the MK6.
My initial trials were P2P construction but I did not feel comfortable having the opamps flimsily dangling around.
The jist of it is that the opamps positive terminal is connected to ground.
The negative terminal is connected to the output.
The opamp will try to do what it can to make both inputs the same.
This means that the output of the opamp will bias the top triode of the WCF so there is ground level dc (0v) on the output of the amplifier (also negative opamp pin).
The 100k resistor/1uf cap is an RC filter so the opamp ignores frequencies above 1.6HZ.
The 470k resistors/ .22uf cap is another RC filter at 1.6HZ presumably to get rid of any noise the opamp creates and it is also a high impedance buffer from the signal.
The 1uf caps are just bypass caps to stabilize the supply voltage.
The 20k trimmer adjusts the offset error between the inputs of the opamp so when the opamp tries to keep both inputs the same they actually both measure the same. Basically you use this to tweak the DC at the output of the amp.
I've used what I've learned from the MK6 integrator to come up with much more complex and useful designs.
This is the upcoming PCB for the biasing circuits that will allow me to fully adjust the load line vertically and horizontally while in operation. It's not finished cuz these software designs are a P.I.T.A to make within the "design rules" of the manufacturer but I should have it done soon.
The pinout of the OPA445AP datasheet is correct.
Refer to my schematic for the MK6 design which now that I look at it again is kind of crappy but it works
R75, R5, and C4 in the relay schematic form a 2 second time constant to the "VCC ON mute" pin which from my short reading on "mute" seems to simply mean off/disconnected.
So effectively it seems to just delay the power to the IC for 2 seconds on startup to give the servos enough time to stabilize so you don't blow your headphones. It doesn't seem to be a timer for the protection relay in active duty and I don't see why there would be a timer.
That's actually a good point though, I didn't code my arduino to protect the headphones on startup. Glad I caught that before I used it on the real thing.
I've verified that my arduino protection circuit reacts instantly to offset above the set threshhold so If the analog circuit is really as slow as you think then my arduino solution would work, but I think such a thing is beyond the scope of this modding thread and probably a horrible idea to have people playing around with.
Keep in mind that the IC in the amp a 1990 model. Whether that's relevant or not I don't know.