The 904A Low Pass Filter is a Modcan re-creation of the 4 pole 24db low-pass filter from the Moog Modular synthesizer. The 44B honors the Moog spec. with precision matched 2N3392 transistor pairs throughout.

904A Demo sequence electropop.mp3
All sounds were created using the 904A

Another demo with bass line from the 904A

Range Switch

Retained from the original design, sets the frequency band over which the Fixed Volt knob operates in two octave steps.
Position 1: 1Hz to 5kHz
Position 2: 4Hz to 20kHz
Position 3: 16Hz to 60kHz

Input Level
The filter input level is adjusted using the Level control attenuator.

1V/Oct Input
Exponential converters track linear voltage input at 1 volt per octave. This input is primarily used for keyboard tracking and pitch control of filter in self-oscillation mode.

CV Input 1
Control Voltage input with attenuator to control the depth of frequency modulation. When the knob is centered no modulation occurs. Positioning the knob to the left of zero inverts the incoming voltage (negative) and to the right the voltage is non-inverted (positive).

CV Input 2
Secondary Control Voltage input with attenuator. CV inputs are used to control the filters cutoff point. The most common input signal would be from an envelope generator or LFO.

Regen Control
Knob varies the amplitude of the resonant peak at the cutoff frequency of the filter.

Fixed Voltage
Sets the cutoff frequency of the filter

Specs: Wide-range lowpass filter with variable-height
resonant peak at cutoff frequency.
4 pole response
24db/oct cutoff slope
Range:Continuously variable from 10Hz-18kHz
Input impedance: 100K ohms
Output impedance: 1k ohms
Control Sens: 1 volt per octave