Hello.
I did little modification on the board, change R7 and R8 to 10kohm resistors, so obtain unity gain in first stage.
Paralell to R7, following Hookup Guide, I use a capacitor in C4, something like 2.2nF expecting to obtain a low-pas filter with cut-off frequency of aproximatelly 7kHz.
The unity gain was observed in the whole bandwidth, but not action of a low-pass filter.
From 1 to 10kHz I observed a reduction of a senoidal input as expected, howerver in frequencies up to 20kHz to 1MHz the reduction of amplitude was almost half from 10kHz and remain constant, when I expected to reduce to something near 0.