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VCF User Guide

Thank you for purchasing the AJH Synth MiniMod VCF module, which like all AJH Synth Modules,

has been designed and handbuilt in the UK from the very highest quality components. We hope

that it will help and inspire you towards creating some great music and soundscapes!

The MiniMod VCF is a very faithful replica of the four pole 24db/octave low pass lter used in the

early Model D synthesiser. By using fully matched discrete ladder transistors along with audio-

phile grade capacitors and operational ampliers in the audio signal path, it captures the vintage

sound and vibe of this epic lter and combines it with the versatility of the Eurorack modular

format.

The front panel provides a three channel input mixer and a large lter cuto control, along with

1/3 and 2/3 CV inputs to emulate the action of the original VCF keyboard switches. There is also a

variable Exponential CV input which is about 30% more sensitive than the 1v/oct input, which

allows some nice over modulation eects.

Added features include Voltage Controlled Emphasis (Resonance) which uses a Vactrol as the

control element. This approach was chosen because it is a purely resistive element and impor-

tantly has no impact on the vintage sound of the lter. The fast acting Vactrol chosen allows

modulation speeds up into the audio range. The lter will of course self-resonate and act as a sine

wave oscillator too.

An overdrive facility can be selected using the onboard jumper which allows the input to the lter

core to be substantially overdriven - great for adding a bit of extra dirt to the sound!

Module width is 14 HP of Eurorack space and it is compatible with standard Eurorack cases. The

height of the panel is 128.5mm, and depth is 26mm . There are four mounting holes at the

corners of the module and we provide 4 of M3 rack xing screws, along with a Eurorack compat-

ible power cable. Current consumption is 45mA from the +12V supply rail and 30mA from the

-12V supply rail.

All AJHSynth modules are covered by a one year guarantee against manufacturing defects.

Note:

It is very important that the power supply ribbon cable is connected correctly, see the “adjustment and

calibration” section for an illustration of the correct orientation.

Discrete Transistor Ladder Filter

mod

www.ajhsynth.com

Summary of content (4 pages)

PAGE 1
mod VCF User Guide Discrete Transistor Ladder Filter Thank you for purchasing the AJH Synth MiniMod VCF module, which like all AJH Synth Modules, has been designed and handbuilt in the UK from the very highest quality components. We hope that it will help and inspire you towards creating some great music and soundscapes! The MiniMod VCF is a very faithful replica of the four pole 24db/octave low pass filter used in the early Model D synthesiser.
PAGE 2
Controls, Inputs and Outputs mod TRANSISTOR LADDER FILTER FREQ 7 Cutoff Freq. 9 Freq. CV Level -2 -4 4 FREQ CV 4 Input 1 Level 4 8 0 INPUT 1 4 11 1/3 CV Input 8 0 1/3 CV 8 0 10 INPUT 2 INPUT 3 6 6 2 10 6 2 10 4 6 2 1V/Oct 8 0 10 4 6 2 Emphasis Level 13 EMPHASIS EMP CV 6 2 4 Emph. CV Level 15 2 4 8 0 2 FREQ-CV Freq. CV Input 1 Audio Input 1 2 Audio Input 2 IN 1 IN 2 Input 3 Level 6 10 2/3 CV Input 12 Emp.
PAGE 3
8 Freq CV Input : Connect an external control voltage to this Input for voltage control of the filter cutoff frequency. The amount of signal passed to the filter core control circuitry can be adjusted with the Freq. CV Level control (9) 9 Freq. CV Level : Sets the amount of control voltage (between 0% and 130%) which is sent from the Freq. CV Input (8) to the filter cutoff circuitry. It has a greater range than the 1 V/Oct Input so that overmodulation effects are possible. 1V / Octave from the Freq.
PAGE 4
Adjustment and Calibration Note: This information is given for completeness, the MiniMod VCF is calibrated after manufacture and under normal circumstances should not require any user adjustment. A Scale Adjust B Regen Adjust C Offset Adjust D Range Adjust E Gain Jumper Power Cable Red Stripe aligns with -12V as shown A Scale Adjust : Sets the 1V/Oct voltage scaling. To adjust, first remove all inputs, then turn Emphasis Level (13) to 10 (full on) and adjust Frequency Control (7) for 440hz output.