The SP-1 is a parametric equalizer with one frequency band only. The spectrum knob sets the frequency range in the interval 500Hz to 5kHz. The balance knob sets the peak within the range. Today it is one of the most sought after collectors items and a mint condition SP-1 doesn't come cheap so build your own for a fraction of the price!
The original unit used dual potentiometers and the Balance one was a 10KA/10KC. We modified the circuit board to use a standard 10KA/10KA dual potentiometer. The circuit functions exactly the same. Just no need for a special/hard to get potentiometer.
Controls: Balance, Spectrum
Current Draw: 23mA
Input Impedance: 1 Mohm
Output Impedance: 1 kohm
PCB Dimensions: 1.95 x 1.79 inches (50 x 45mm)
3/7/2016 - Added trimmer VR1 adjustment instructions.