The effect circuit works this way: A high input impedance buffer amp (T1, T2) receives the signal, which is then split in two ways: it is fed directly to one of the inputs of the mixer amp (IC1-A), and is fed into a high-pass Sallen-Key filter (T3), which provides high-frequency signal to the distortion circuit (IC1-B, D1, D2). The filter is tunable between approximately 800Hz-8kHz with a dual-gang potentiometer (R21/1, R21/2), and two more passive filters (C4-R7 and C5-R8) ensure that all the frequencies below 500Hz are being firmly cut. The output of the distortion circuit is further filtered with another tunable high-pass filter (T4, R22/1, R22/2) in order to eliminate the lower base frequencies; the generated upper harmonics are finally mixed with the original signal by the mixer amp (IC1-A).
The circuit is based on the ideas from the Harmonic Sweetener, but it uses only a single opamp package, and furthermore all the filters are tunable.
The effect is built inside a 1590BB enclosure. The controls from left to right are: 1st filter (R21), Drive (R23), 2nd filter (R22) and Mix (R24).