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VCA User Guide
Discrete Transistor Cascaded VCA
Thank you for purchasing the AJH Synth MiniMod VCA 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 Discrete Cascaded VCA exactly re-creates the VCA circuitry of the vintage Model D
synthesiser in Eurorack modular format. It features two fully discrete transistor VCA cores which
are hardwired in series, to which we have added full voltage control to both VCA cores along with
an overdrive facility. Particular attention has been paid to the selection and matching of components in the audio signal path, including the use of audiophile grade capacitors and operational
ampliers.
There are two Audio Inputs, each with their own level controls along with two CV inputs which
control the gain of VCA 1 and a Master CV Input and level control which controls the gain of VCA
2. An oset control allows a preset gain to be added to VCA 1. Internally it operates at the same
voltage and signal levels as the early Model D voltage controlled amplier and the output of VCA
1 is hardwired to the input of VCA 2.
A high gain (overdrive) mode can be selected with an onboard jumper. This overdrives the Input
of VCA 1 by a factor of 3 and forces it into soft clipping - the second VCA can be used in a similar
way to the Master volume on a guitar amplier to control levels so that variable levels of distortion (or "grit") can be added to the sound and still maintain unity gain overall.
Module width is 10 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 40mA from the +12V supply rail and 22mA 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 CV Level 1
8 CV Level 2
11 Master (VCA 2) Level
1 Audio Input 1
5 CV Input 1
6 CV Input 2
DISCRETE CASCADED VCA
mod
4
2
0
4
2
0
MASTER
4
2
0
CV 1
CV 2
AUDIO 1
4
6
2
8
0
10
AUDIO 2
4
6
2
8
0
10
OFFSET
4
6
2
8
0
10
AUDIO 2AUDIO 1
CV 2CV 1
MASTER
OUT
6
8
10
6
8
10
6
8
10
Audio Level 1 3
Audio Level 2 4
VCA 1 Oset 9
Audio Input 2 2
Audio Output 12
Master CV Input 10
1 Audio Input 1 :
2 Audio Input 2 :
3 Audio Level 1 :
4 Audio Level 2 :
5 CV Input 1 :
Audio Input 1 forms part of a 2 input audio mixer which is attached to the front end of
VCA 1, this input is mixed with Input 2 at a level set by Audio 1 Level Control (3). Signal
levels of up to +/- 5 Volts are acceptable.
Note:
Just like the VCA on the vintage Model D synthesiser on which this module is based, the
input stage is AC coupled - therefore it is not possible to process DC control voltages
through the Audio Inputs - it is suitable for processing audio signals only. However, if DC
control voltages are accidentally connected to the Audio Inputs it will not cause any
damage to the module.
Audio Input 2 forms part of a 2 Input audio mixer which is attached to the front end of
VCA 1. This input is mixed with Input 1 at a level set by Audio 2 Level Control (4). Signal
levels of up to +/- 5 Volts are acceptable. Should more Audio inputs be needed a
Eurorack compatible audio mixer can be connected to either Audio Input 1 or 2.
Sets the amount of signal from Audio Input 1 (between 0% and 100%) that is mixed with
Audio Input 2 and sent to the Input of VCA 1.
Sets the amount of signal from Audio Input 2 (between 0% and 100%) that is mixed with
Audio Input 1 and sent to the input of VCA 1.
Control voltage Input for VCA 1, this is mixed with CV Input 2 (6) and the sum of these
voltages control the gain of VCA 1. An input voltage range of +/-5 V is acceptable and +5V
gives approximately unity gain. Negative voltages will generally be ignored unless
combined with a (larger level) positive voltage from CV Level 2 control, in which case the
resulting voltage will be (CV2 - CV1). The output of both VCA 1 and VCA 2 repond in a
linear manner in relation to all CV inputs.
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6 CV Input 2 :
Works in the same fashion as CV Input 1. Any voltage applied to this Input is attenuated
by CV 2 Level control (8) and summed with CV 1 Level output voltage, the resultant
control voltage regulates the gain of VCA 1 between zero and unity.
Note that CV 1 & CV2 act on VCA 1 ONLY, see signal ow diagram below.
7 CV Level 1:
8 CV Level 2 :
9 VCA 1 Oset :
10 Master CV Input :
11 Master Level :
12 Audio Output :
Sets the amount of control voltage (between 0% and 100%) sent from CV Input 1 (5) to
regulate the gain of VCA 1
Sets the amount of control voltage (between 0% and 100%) sent from CV Input 2 (6) to
regulate the gain of VCA 1
Sets the initial gain of VCA 1 by applying a control voltage directly to the CV input node,
so that it eectively acts as a manual volume control for the VCA . See the signal ow
chart below for a visual indication of function.
A voltage applied to this input varies the gain of VCA 2. When no jack is connected the
switch contact of the jack socket is connected (normalised) to +10V and the gain of the
VCA can be manually controlled by the Master Level control (10). This emulates the
“Pedal” control and Input of the Model D, but with a greater gain range. Control voltages
up to +10V can be connected to this Input.
Controls the CV voltage level to VCA 2. If no jack is connected to Master CV Input (9) then
a setting of 5 (12 o’clock) will give unity gain for VCA 2. If VCA 1 is being over driven by
removing J1 (C) then this can be turned down so that unity gain from the VCA Output is
maintained, in a similar fashion to the Master volume control on a guitar amplier.
If a CV signal is fed into Master CV Input (9) then this controls the level of this signal fed to
the VCA gain cell between 0% (no audio output) and 100% (Unity gain with +/-5V audio
input).
Audio output. The output signal level is typically +/- 5V but can be much higher (up to
+/-11v) if the Gain Jumper (J1) is removed, so it is important that the output level is
controlled with the Master Level Control (10) to prevent overloading modules which are
connected downstream.
C Gain Jumper :
(J1 on PCB)
Audio Input 1
Audio Input 2
Removing this Jumper boosts the level of signal fed to the input of VCA 1 by almost
300%. This overdrives the VCA core and creates up to 20% of tube-like harmonic distortion. With the Gain Jumper removed care must be taken to adjust the Master Level
control to reduce the gain of VCA 2 by a corresponding amount and keep the output
level around +/-5V to prevent possible damage to other modules downstream of this
module.
Audio Level 1
Gain Jumper
VCA 2VCA 1
Audio Level 2
CV
Level
1
CV IN. 1
CV Oset
CV
Level
2
CV IN. 2
+5V
Master
Level
+10V
Master CV Input
Audio Output
Master Input Jack
Normalled to
+10V Internally
MiniMod Cascaded VCA
Audio and CV Signal Flow Chart
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Note:
This information is given for completeness, the MiniMod VCA is calibrated after manufacture
and under normal circumstances should not require any user adjustment.
A VCA 1 Oset Null
B VCA 2 Ost Null
C Gain Jumper (J1)
Adjustment and Calibration
Red Stripe aligns with -12V as shown
A VCA 1 Oset Null :
B VCA 2 Oset Null :
C Gain Jumper (J1) :
FOR MANUFACTURER ADJUSTMENT ONLY, specialist test equipment is needed to
calibrate this trimmer correctly.
FOR MANUFACTURER ADJUSTMENT ONLY, specialist test equipment is needed to
calibrate this trimmer correctly.
Removing J1 increases the gain of the signal fed to the input of VCA 1 by almost 300%.
See previous page for a fuller description and suggested use.
Power Cable
If you need any help using this module or have any technical questions please feel free to
contact us at support@ajhsynth.com
Copyright © AJHSynth 2014
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