"The Smartax from Empirical Labs is an equalizer, compressor, and saturation effect designed specifically for acoustic instruments, packed into a 500-series module. In addition, it can process line level signals. Circuitry is all analog, but controls are digital. The EQ section consists of a 70 or 100 Hz 18 dB/octave high pass filter followed by three bands of parametric (though not continuously variable) EQ with frequencies selected to be particularly useful for acoustic instruments. The compressor and saturation circuit follow the EQ. The degree of gain reduction and percentage of saturation distortion is determined by the Input gain control (that’s input to the compressor). A Mix control adjusts the amount of compressed/saturated signal is added to the clean signal. The compressor can be turned off to allow the saturation circuit to work independently. An internal jumper can engage a 150 Hz high pass filter in the compressor detector signal path (this is the factory setting) to prevent excessive compression from the boomy low end of a guitar. Two units can be ganged together to equally compress a stereo source, though this requires installing a jumper between pin 6 of the card slots containing the Smartaxes. When those pins are connected together (this involves soldering in most if not all 500-series racks) a front panel switch can engage or disengage stereo linking."