doepfer System A - 100 VCA A-130 / A-131
1
1. Introduction
Modules A-130 (Linear VCA) and A-131 (Exp. VCA)
provide voltage-controlled amplification.
H This section of the manual applies equally to
the A-130 and A-131, because apart from the
one difference of their response curves, they
are otherwise identical.
For audio signals, you would normally use the exponen-
tial VCA (A-131), and for control voltages, the linear
VCA (A-130). It doesn’t always have to be that way,
though.
The amount of amplification the VCAs provide is deter-
mined by the voltage at the CV input, and the position
of the gain control, which sets the overall gain in the
system.
The VCA has two audio inputs, each with an attenua-
tor. They are amplified by an amount determined by the
combination of the gain and the two CV controls.
CV 1
Audio
In 1
CV 2
Audio
In 2
In 2
CV 2
Gain
A-130
VCA-LIN.
In 1
Audio Out
Out
A-130 / A-131 VCA System A - 100 doepfer
2
2. VCA - overview
Controls:
1 Gain: Overall gain control
2 CV 2: CV attenuator for input "
3 IN 1: Attenuator for audio input §
4 IN 2: Attenuator for audio input $
5 Out: Attenuator for the output signal
In / Outputs:
! CV 1: Control voltage input
" CV 2: ditto, with attenuator
§ Audio In 1: Audio input
$ Audio In 2: ditto
% Audio Out: Audio output for the combined audio
signals input at § and $ and then
amplified by the VCA.
CV 2
Audio In 1
Audio In 2
Audio Out
Gain
CV 1
Ê
Ë
Ì
Í
Î
À
Á
Â
Ã
Ä
A-130
VCA-LIN.
0
10
0 10
0
10
0
10
0 10
CV 2
In 1
In 2
Out
doepfer System A - 100 VCA A-130 / A-131
3
3. Controls
The A-130 has a linear response: the amplification is in
direct linear proportion to the control voltage input (see
Fig. 1).
The A-131 has an exponential (or logarithmic) re-
sponse. That means that control voltage changes have
less effect at low levels of amplification than at high
levels, and makes the A-131 more sensitive and sui-
table for subtle changes at low levels (see Fig. 1).
Fig. 1: Response curves for the A-130 and A-131
1 Gain
This controls the overall gain of the VCA.
At "0", with no control voltage present, there is no
amplification at all: no signal is present at the VCA’s
output. Once the gain control is turned up, amplification
occurs, even if there is no CV input present. The gain
control shifts the whole VCA response upwards (see
Fig. 2).
Fig. 2: Effect of gain control on VCA response
Control Voltage
èè
10
5
0
5 10
lin.
exp.
Gain = 3
Control Voltage
èè
10
5
0
5 10
lin.
exp.
A-130 / A-131 VCA System A - 100 doepfer
4
H Note that if a control voltage is partially nega-
tive (for instance an LFO modulating the VCA)
you need to set the VCA gain above zero,
since otherwise the input signal is only ampli-
fied when the modulation CV is positive (see
Fig. 3).
Fig. 3: VCA output level at different gain settings.
2 CV 2
This attenuator affects the level of voltage control at
socket ". It controls the amount of effect the CV has
on amplification.
3 IN 1 • 4 IN 2
Attenuators 3 and 4 control the level of the signals
input into the VCA.
H If the output signal distorts in an unwanted
way, turn down the input level, using control
3 and/or 4.
5 Out
This attenuator controls the total volume of the VCA’s
output.
Input
CV
Output
Gain > 0
Gain = 0
doepfer System A - 100 VCA A-130 / A-131
5
4. In / Outputs
! CV 1 • " CV 2
Sockets ! and " are control voltage inputs, whose
voltages are combined. The effective range of the VCA
goes from 0 V (no amplification) to +5 V (maximum
amplification).
§ Audio In 1 • $ Audio In 2
The signals you wish to amplify are input through audio
inputs § and/or $.
% Audio Out
The output signal here is the audio inputs amplified by
the VCA..
5. User examples
Typical voltage controlled amplification
A standard VCA patch is shown in Fig. 4. An ADSR
envelope produces a time-dependent amplification
curve, which can affect any sound source you choose.
The curve can be very quick (with a fast envelope) or it
can produce long, slow changes in the volume of a
sound.
Fig. 4: Time dependent amplification using an ADSR
NOISE
ADSR
Au dio
In 1
CV 2
CV 2
Ga in
A-131
V C A-EX P.
In 1
Aud io Out
Out
A-130 / A-131 VCA System A - 100 doepfer
6
Amplitude modulation
In Fig. 5, an LFO is modulating an A-130 linear VCA
(with Gain > 0), so that the amplification changes
cyclically with the LFO’s voltage. (Amplitude modu-
lation / AM.)
With an LFO frequency in the sub-audio range (1 Hz to
around 15 Hz) the result is Tremolo (see Fig. 5).
With a modulation frequency in the audio range, side-
bands occur like those produced by FM (Frequency
Modulation), and interesting timbres emerge.
Fig. 5: VCA amplitude modulation with an LFO
Modulation depth is adjusted with control 2.
Fig. 6 shows a way of voltage-controlling this modula-
tion depth using another VCA. In this example, the
VCAs have the following functions:
• VCA 1 (A-130): AM control
• VCA 2 (A-131): total volume control
• VCA 3 (A-130): modulation depth control
The voltage control input A to the modulation depth VCA
can come from an ADSR, MIDI controller, etc..
Fig. 6: AM with voltage-controlled modulation depth
LFO
VCO
Au dio
In 1
CV 2
CV 2
Ga in
A-130
VCA-LIN.
In 1
Aud io Out
Out
Gain > 0
VCO
LFO
A
CV
CV
VCA 1
VCA 3
VCA 2
doepfer System A - 100 VCA A-130 / A-131
7
Keyboard control of VCA (tracking)
You can use the CV output from a keyboard to modu-
late the VCA, and so have level of amplification de-
termined by the pitch of a note - what’s usually cal-
led keyboard tracking.
In the example in Fig. 7, high frequency notes are
amplified more than low frequency notes. Use control 1
on VCA 2 to vary the degree of keyboard tracking.
Fig. 7: Keyboard tracking - the higher the pitch, the
louder the output
To produce the opposite effect (that is, inverse keyboard
tracking, where lower sounds are more amplified than
higher ones) patch a Voltage-Inverter A-175 in before
VCA 2 (see Fig. 8).
Set the gain control 1 at maximum, and use CV2
control 2 to determine the intensity of the effect
Fig. 8: Inverted keyboard tracking: the higher the
pitch, the less amplification.
VCO
mfp f
CV
Gate
VCA 1
CV1
VCA 2
ADSR
VCO
CV
Gate
VCA 1
CV2
VCA 2
ADSR
mf pf
A-175
Gain = 10
A-130 / A-131 VCA System A - 100 doepfer
8
6. Patch-Sheet
The following diagrams of the module can help you
recall your own Patches. They’re designed so that a
complete 19” rack of modules will fit onto an A4 sheet
of paper.
Photocopy this page, and cut out the pictures of this
and your other modules. You can then stick them onto
another piece of paper, and create a diagram of your
own system.
Make multiple copies of your composite diagram, and
use them for remembering good patches and set-ups.
P • Draw in patchleads with colored pens.
• Draw or write control settings in the little
white circles.
CV 2
Audio In 1
Audio In 2
Audio Out
Gain
CV 1
A-130
VCA-LIN.
0 1 0
0
1 0
0
1 0
0
1 0
0 1 0
CV 2
In 1
In 2
Out
CV 2
Audio In 1
Audio In 2
Audio Out
Gain
CV 1
A-131
VCA-EXP.
0 10
0
10
0
10
0
10
0 10
CV 2
In 1
In 2
Out
-
1
-
2
-
3
-
4
-
5
-
6
-
7
-
8
DOEPFER A-130/A-131 VCA Owner's manual
- Category
- Musical Equipment
- Type
- Owner's manual
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