The Tube EQ Analog Parametric/Shelving Equalizer from Electro-Harmonix is a vacuum tube filter equalizer. The Tube EQ consists of two filter sections that are summed together for a wide range of options. A passive 'shelving' 'type', 'tone stack' with Bass and Treble controls is combined with an active 'parametric' bandpass filter with its own gain, frequency and 'Q' controls. Turning the Bass, Treble or Bandpass Gain controls clockwise, increases the level of each respective range. Keeping each control at maximum counter-clockwise, or OFF, attenuates the input signal by about 20 dB. In addition to the input and output jacks at the front of the unit, there is also an insert for an expression pedal. When this is used, the pedal will control the frequency center of the bandpass filter only. This produces nice 'wah' like effects and more. The Frequency control must be set to the maximum clockwise setting to get the full range of the pedal. The Frequency control can then be used as a range trim with the expression pedal to make a fine adjustment to the range of the pedal sweep. Turning the Frequency knob counter-clockwise with the pedal plugged in has the effect of both reducing the range and lowering the frequency center of the bandpass filter. In addition to an expression pedal, a DC control voltage can be inserted to allow remote sweeping of the frequency control. The range of this input is 0 - 5.0 VDC.