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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 lter used in the
early Model D synthesiser. By using fully matched discrete ladder transistors along with audiophile grade capacitors and operational ampliers 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 eects.
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 importantly 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 compatible 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.
www.ajhsynth.com
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Controls, Inputs and Outputs
7 Cuto Freq.
9 Freq. CV Level
4 Input 1 Level
11 1/3 CV Input
10 1V/Oct Input
8 Freq. CV Input
1 Audio Input 1
2 Audio Input 2
TRANSISTOR LADDER FILTER
FREQ CV EMP CV
4
2
0
INPUT 1
4
2
0
FREQ-CV OUT
FREQ
-2
-4 4
4
6
8
10
6
8
10
6
2
0
10
INPUT 2 INPUT 3
4
6
2
0
10
2/3 CV1V/Oct
IN 2
8
8
EMP-CV1/3 CV
IN 3IN 1
2
EMPHASIS
4
2
0
4
2
0
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Emph. CV Level 15
Emphasis Level 13
6
8
10
6
8
10
Input 2 Level 5
Input 3 Level 6
2/3 CV Input 12
Emp. CV Input 14
Audio Output 16
Audio Input 3 3
1 Audio Input 1 :
2 Audio Input 2 :
3 Audio Input 3 :
4 Input 1 Level :
(Audio)
5 Input 2 Level :
(Audio)
Audio Input 1 forms part of a 3 input audio mixer which is attached to the front end of the
lter. This Input is mixed with Input 2 and Input 3 at a level set by Audio 1 Level control
(4). Typically the output from a VCO would be connected to this Input, but any AC signal
with a level of up to +/- 5 Volts is acceptable. .
All three Audio Inputs are for audio signals only, the lter signal path is AC coupled so DC
control voltages are not useful here but will not cause any problem if accidentally
connected.
Audio Input 2 forms part of a 3 input audio mixer which is attached to the front end of the
lter. This input is mixed with Input 1 and Input 3 at a level set by Audio 2 Level control
(5). Signal levels of up to +/- 5 Volts are acceptable.
Audio Input 3 forms part of a 3 input audio mixer which is attached to the front end of the
lter. This input is mixed with Input 1 and Input 2 at a level set by Audio 3 Level control
(6). Signal levels of up to +/- 5 Volts are acceptable.
Sets the amount of signal from Audio Input 1 (between 0% and 100%) that is mixed with
Audio Inputs 2 & 3 and sent to the lter core.
Sets the amount of signal from Audio Input 2 (between 0% and 100%) that is mixed with
Audio Inputs 1 & 3 and sent to the lter core.
6 Input 3 Level :
(Audio)
7 Cuto Freq. :
Sets the amount of signal from Audio Input 3 (between 0% and 100%) that is mixed with
Audio Inputs 1 & 2 and sent to the lter core.
The Cuto Frequency control manually varies the cut o frequency of the low pass
lter. At the minimum setting (fully counter clockwise) the lter will cut o all frequencies
with no audio output and at the maximum setting the lter will pass all frequencies. It is
still active when using CV control, in which case it acts as an oset control.