Help me understand what the IC's are doing in my input strip? (Yamaha PM2000)

[JW]

So the PM2000 has two discrete opamps (IC1 and IC10) and a bunch of IC chips in the input strips. The IC chips are HA1457.
I've attached the schematic in two parts for better legibility.

Anyway, the chips are IC2 through IC9.

I'm considering replacement with the expensive (+/-24VDC) Opa604, so I want to maybe leave in the HA1457's that are less important.

I see that IC2 is the input opamp when the input strip is turned down to line level which makes it a good candidate for replacement.

IC3 and IC4 appear to be for two separate high pass functions. I haven't realized that switching in high passes actually runs the signal through these, so I wonder if a better approach might be to just roll off low end at 50 or 100HZ with the eq and leave the high pass functions switched out, especially if the eq is switched in for other reasons anyway.  So, these look like I could leave the original chips in since I'll likely be switching the low pass out anyway.

IC 7,8, and 9 look like booster amps for the eq, so I'll give those a shot at replacement and see how that sounds.

I guess my main question is, what are IC5 and IC6 doing?

 

[JW]

Schematic part 2

 

[JohnRoberts]

IC5 and IC6 are the EQ section.

JR

 

[JW]

Thanks John.

Good info

I have one more question that is hopefully not too annoying.  Here is a link to the PM2000 eq modification that JLM offers: http://www.jlmaudio.com/JLM%20PM2000%20EQ%20mod%20kit.htm

It makes the high mids and low mids fully inductive, replacing the gyrator eq's. I've wired these into about 20 channels on my board and they sound great. What I'm trying to figure out is if they bypass IC8 and IC9, the two chips associated with the high and low mids. That would be another area to not upgrade if possible.

???