6v Powered voltage controlled filter

This lo-fi low pass filter is taken from the Gakken SX-150 analogue synthesiser and has been adapted for use as a stand alone filter or a great addition to any 6v powered keyboard or toy such as the casio SA series or the Hing-Hon EK100 ETC. The circuit is quite simple and cheap to build with Resonance peak and voltage controlled frequency cut off controls. Of course any filter this simple running on 6v-dc is not going to sound like a Moog Ladder filter but for cheap lo-fi keyboards it offers an effective filter esc effect!
6V Filter schematic
Quantity Part
1
 50k trimmer pot
1
 4k7 log pot
1
 100k linear pot
1
 CA3140E op-amp
2
 2n3904 npn transistor
3
 10k resistors
2
 1k resistors
2
 1 meg resistors
1
 100k resistors
1
 2.2k resistors
1
 47k resistors
1
 100uf electrolytic cap
1
 47uf electrolytic cap
1
 4.7uf electrolytic cap
1
 10nf poly cap
1
 1nf poly cap

I have tested this filter on both a Casio SA-8 and a Hing-Hon EK100 with good and noticeable filter like results albeit very simple lo-fi style sounds. The circuit works best powered by 6vdc some Casio SA keyboards are 7.5v the circuit still works but you get a less noticeable filter sound using a 100ohm resistor from the 7.5v SA power to the +6V point on the schematic helps.

Hear is my understanding of how the circuit works. C6 couples the audio signal to VR1 (4k7 trimmer pot) which provides a ground bias to the audio signal and presents it as a varying current to the collector of Q2. VR1 should be set to provide the optimum signal level without distortion and while still obtaining a noticeable filter sound.

Q2 and Q1 are configured as a voltage controlled resistors, together with C3 and C4 they form a 2nd order low pass filter I. E low frequencies below the cut-off point pass through and higher frequencies are lowered by 12db per octave in volume. VR3 (filter cut-off) provides a voltage to the base of the two transistors, the higher the voltage the lower the effective resistance across Q1, Q2 and so the higher the frequency's cut-off point. R1 (1m) provides a constant voltage to the base of Q1, Q2 so there is always enough voltage too keep the transistors turned on, R1 also sets the lowest filter cut-off point. Increasing the value of R1 lowers the lowest cut-off point and vice versa.

IC1 is set up as a non inverting amplifier and buffers the output of the transistor filter section giving the filter a lower impedance output. R4 sets the input impedance of the buffer stage to 1meg which helps the filter stage perform more accurately. R6 and R5 set the gain of the op-amp stage to 4.7 x amplification (R6 / R5) for less gain lower R6 but no lower than the value of R5

VR2 and R7 inject some of the output signal back into the input stage of the filter and together with the fact that C3 is 10x greater in value than C4 the filter produces a resonant peak at the cut-off frequency producing a pronounced filter effect.
Web Design Matt Newlove