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CV 2
CV 3
CV 1
CV 2
CV 3
A-120
VCF 1
Frequency
System A - 100
1. Introduction
Module A-120 (VCF 1) is a voltage controlled lowpass filter, which filters out the higher parts of the
sound spectrum, and lets lower frequencies pass
through.
The Cut-Off Frequency determines the point at which
filtering takes effect. You can control this manually, or
by voltage control (filter modulation, for instance by
an LFO). Three CV inputs are available, and the sum
of the voltages from these affects the filter cut-off.
VCF 1 A-120
Lev el
A udio In
Audio
Out
Resonance
VCF1 is a co-called "transistor ladder" design, with a
cut-off slope of -24 dB/octave, as in various Moog
synthesizers. That’s what gives it its classic, legendary
Moog sound.
Resonance (or Emphasis) is adjustable all the way up
to self-oscillation - in which case the filter behaves like
a sine wave oscillator.
1
A-120
VCF 1
System A - 100
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2. VCF 1 - Overview
A-120
LOW PASS FILTER
CV 1
0
CV 2
0
CV 3
0
Audio In
0
Audio Out
0
VCF 1
Freq.
10
CV 2
10
CV 3
10
Lev.
10
Res.
10
➀
➁
➂
➃
➄
Controls:
1 Freq.: Cut-off frequency control
: Attenuator for CV at input
CV 2
2
CV 3: Attenuator for CV at input
3
4 Lev.: Attenuator for audio input $
: Control for setting the filter’s reso-
Res.
5
nance (emphasis)
In / Outputs:
! CV 1: Voltage control input
: ditto, level controlled by
CV 2
"
CV 3: ditto, level controlled by
§
Audio In
$
Audio Out: Output from the filter
%
: Input to the filter
2
3
"
§
2
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System A - 100
VCF 1 A-120
3. Controls
1 Freq.
With this control you adjust the
above which the filter attenuates all frequencies. At 10,
the filter is fully open. The more you turn down this
control, the more the high frequencies are filtered.
The sound becomes mellower and less bright (see
Fig. 1) until at 0 the filter is completely shut, and there
will be no output signal at all.
CV
Out Out
f
c
Freq. Freq.
Fig. 1: White noise put through a low pass filter.
Cut-Off Frequency f
f
c
2 CV 2 • 3 CV 3
For voltage control or modulation of the cut-off frequency, use these CV inputs " and/or § (see Fig. 1).
,
c
Use attenuators 2 and/or 3 to adjust the control
voltage level.
4 Lev.
Use this attenuator to control the amount of signal
entering the filter input $.
H If the filter’s output distorts, turn this control
down, unless you deliberately want the
sound as a special effect.
5 Res.
With this control you adjust the filter’s resonance
(emphasis) - the parameter which emphasises the
frequencies around the cut-off point f
p.4).
Close to its maximum setting, the filter becomes so
resonant that it goes into self-oscillation, and starts
behaving like a
of this effect, and use the VCF as an additional oscillator.
sine wave
. You can take advantage
(see Fig. 2 on
C
3
A-120
➨➨➨➨
VCF 1
System A - 100
modulated by its voltage: ie., the sound color changes
according to the voltage put out by the modulator.
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Resonance
0 db
Frequency
f
c
Fig. 2: How resonance affects the response of a low
pass filter
4. In / Outputs
! CV 1
Socket CV 1 is the filter’s standard voltage control
input, and works on the 1V / octave rule, like the
VCOs.
If you patch a modulation source (eg LFO, ADSR) to
this input, the cut-off frequency of the filter will be
If you use the VCF as a sine wave oscillator, connect
the pitch CV into this socket. Do the same if you want
the filter’s cut-off frequency to track exactly with the
pitch of a note.
" CV 2 • § CV 3
Sockets CV 2 and CV 3 are also
inputs for the filter. Unlike CV 1, you can control the
level of voltage - the intensity of modulation effect on
the filter - with attenuators 2 and 3.
voltage control
$ Audio In
This is the filter’s audio input socket. Patch in the
output from any sound source (eg. VCO, noise generator, mixer).
% Audio Out
Filter output % sends out the filtered signal.
4
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System A - 100
VCF 1 A-120
5. User examples
The filter’s cut-off frequency can be modulated in
various ways:
• VCF - LFO
Modulation of the cut-off frequency produces cyclical changes of the sound spectrum. At low
frequencies (c. 1 - 5 Hz), you get a "Wah-Wah"-
effect. Modulation in the audio range produces
interesting sounds; the same principles apply here
as with frequency modulation of the A-110 VCO
(see chapter 6).
• VCF - ADSR
Modulation by an envelope results in gradual
change of the
sound spectrum
would be the synthesis of electric bass or drum
sounds, and filter sweeps, which slowly sweep
through the audio spectrum, emphasising different
harmonics.
• VCF - Keyboard CV
This modulation produces pitch-related filter opening: the higher the pitch, the more the filter opens,
and the brighter the sound becomes.
. Typical uses
Producing wind FX with A-120 and A-118
Patch coloured noise into the audio input, and
D
random voltage CV into CV2. Set the filter CV to 0,
cut-off to a middle value, and the resonance just
below self-oscillation.
Now turn CV 2 up. A whistling wind sound will
D
occur. You can control the rate and amount of its
irregularity with controls 3 and 4 on the A-118, and
the tone colour with the A-118’s controls 1 and 2.
CV 2
Audio In
Audio
Out
A-120
VCF 1
Frequency
Resonance
A-118
NOISE / RANDOM
Blue
Red
Colored
Rate
Random
Level
Output
Random Control
- +
: Creating wind effects
Fig. 3
CV 2
Level
5
A-120
VCF 1
System A - 100
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6
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6. Patch-Sheet
System A - 100
VCF 1 A-120
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.
A-120
LOW PASS FILTER
CV 1
CV 2
CV 3
Audio In
Audio Out
0
0
0
0
0
VCF 1
Freq.
10
CV 2
10
CV 3
10
Lev.
10
Res.
10
A-120
LOW PASS FILTER
CV 1
CV 2
CV 3
Audio In
Audio Out
0
0
0
0
0
VCF 1
Freq.
10
CV 2
10
CV 3
10
Lev.
10
Res.
10
A-120
LOW PASS FILTER
CV 1
CV 2
CV 3
Audio In
Audio Out
0
0
0
0
0
VCF 1
Freq.
10
CV 2
10
CV 3
10
Lev.
10
Res.
10
7
A-120
VCF 1
System A - 100
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