Schematic explanation

Hi everyone, I have question about this schematic that I have recently obtained from this forum (about some problems in the past)...

I have pretty good understanding about Op-amps but I don't understand purpose of R16, R17, R19 and C(11 or 52). :confused:
(That's just about channel 2 for guitar).
And how can it be that input impedance for channel 1 is 1 Mohm (in the manual for crate amplifier) when resistor R3 lowers it down to 10kohm?

If this question (or similar) has been mentioned before, sorry, just paste a link.
 

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It is a high-pass filter, a modified Sallen and Key? The inverting input tracks the non-inverting input, due to the feedback around the op-amp, so it can be used like a follower to make an active filter. R19 avoids oscillation due to phase shifts, as it makes the follower gain 0.985 instead of 1.0. R16 would be missing from the standard circuit, so I guess it must adjust the response a little.

Roughly speaking, at high frequencies R17 is bootstrapped so has no effect on the signal. At lower frequencies C52 introduces some attenuation and phase shift, so R17 starts to load the input and make C11 drop even more signal.