Yusynth Minimoog Filter
Yves Usson has kindly allowed me to produce a 4U board for his minimoog filter.
This is the classic Moog 4 pole (-24db) low pass filter. Serge never made a 4 pole low pass filter and this is a great addition to my synth.
My circuit is the same as Yves design with the exception of my using 2 TL071 op amps where he uses a single TL072.
Ladder Filter build notes
Please note that the circuit requires 4 pairs of matched NPN transitors. Many people also match the 4 47n capacitors in the ladder.
Current build is Version 1.0
Download documentation here: Ladder Filter Docs
Yve's documentation for his original design can be found here
TRIMMING NOTES FROM YVES WEB SITE (edited to match my schematic)
Trimming V/Octave tracking :
Apply 0.000V to the V/Oct input
Turn the frequency knob fully counter-clockwise in order to measure 0mV at the base of Q7 (BC557) (node between R14 to R21)
Apply 1.000V to the V/Oct input
Adjust TR1 (500R) in order to measure 18.2mV at the base of Q1
Apply 5.000V to the V/Oct input Check that you have 91.0mVat the base of Q1, if not adjust TR1
Apply 0.000V to the V/Oct input Set the filter to auto-oscillation (EMPHASIS turned fully clockwise)
Connect a keyboard (CV/GATE) to the V/Oct input Play a tune and check the goodness of the tracking Slightly adjust TR1 to achieve a good chromatic tracking.
Frequency range : Apply a sinewave with frequency 32Hz to the audio input
Emphasis potentiometer to minimum resonance (fully counter-clockwise)
Turn the frequency knob fully counter-clockwise
Adjust TR3 (10K), in order to mute the 32Hz signal.
Emphasis : Adjust TR2 (1K) in order to reach auto-oscillation near 95% of the full range of the EMPHASIS pot.