Hi all, 

For years I've known that the Treble control in the ADA MP-1 has a similar effect on the tone moving the 3.1 kHz band on my external equalizer. Or maybe somewhere between the 2.5 kHz and 3.1 kHz or thereabouts. 

But the manual says that the centre frequency is 1.6 kHz, way off. Some annotations in the schematics also repeat that. 

What could be the reason? 

The naive 1.6 kHz analysis is based on just looking at the RC band-pass filter before the Treble op-amp, in in complete isolation, as if that were the circuit (just the R1, R2, C1, C2 in my circuit diagram below). 

OK, I simulated that in CircuitLab and, indeed, you get a frown curve with a nice hump right around 1.6 kHz. 

But that's not the whole circuit! I put in more of the circuit: the op-amp after that, with the feedback resistors. 

And ... BLAM! ... that circuit has a peak at 2.5 kHz, based on my best attempt at reading the blurry values in the schematic: 

1.6 kHz, ... my MIDI footswitch! 

My ears tell me that my actual unit has the peak higher. When I compare the "flavor" of the Treble boost to the 2.5 and 3.1 kHz sliders, it's somewhere between.  There is some additional nuance in how this sub-circuit fits into the overall EQ framework, and the component values could be off. 

Cheers ...