A combination of a low pass and high pass filter, as well as a clean gain. 

The low pass filter is a 12 dB per octave high frequency filter. The LPF knob controls the cutoff frequency and ranges continuously from 20 kHz to 330 Hz.

The high pass filter is a 12 dB per octave low frequency filter. The HPF knob controls the cutoff frequency and ranges continuously from 25 Hz to 190 Hz.

The gain control ranges from unity gain to +20dB.

9v - 24v centre negative supply only. 9v should be enough for most, higher voltages may give more headroom.