DOEPFER A-100 Series User manual

doepfer System A - 100 VCF 4 A-123
1
1. Introduction
Module A-123 (VCF 4) is a voltage-controlled high-
pass filter, which filters out the lower parts of the
sound spectrum, and lets higher 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). Two CV inputs are available.
The cut-off slope is -24 dB/octave. The circuitry uses
a Curtis CEM 3320 chip.
Voltage controlled resonance: on the VCF 4, reso-
nance can be controlled not just manually, but by
voltages as well, right up to self-oscillation. In this
case, the filter behaves like a sine wave oscillator.
FCV 1
Resonance
FCV 2
QCV
Audio
In
Level
Audio
Out
QCV
Frequency
A-123
VCF 4
FCV 2

A-123 VCF 4 System A - 100 doepfer
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2. VCF 4 - Overview Controls:
1Lev. : Attenuator for audio input !
2Frq. : Cut-off frequency control
3FCV : Attenuator for filter CV at input §
4QCV : Attenuator for resonance CV at input
$
5Res.: Control for setting the filter’s reso-
nance (emphasis)
In / Outputs:
!Audio In : Input to the filter
"FCV 1 : Input for voltage control of the filter
cut-off frequency (1 V /octave)
§FCV 2 : ditto, level controlled by 3
$QCV : Input for voltage control of the filter’s
resonance; level controlled by 4
%Audio Out : Output from the filter
A-123
VCF 4
FCV 1
FCV 2
QCV
Frq.
FCV
QCV
Res.
Lev.
Audio In





➀
➁
➂
➃
➄
24 dB High Pass
010
010
010
010
010
Audio Out

doepfer System A - 100 VCF 4 A-123
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3. Controls
1 Lev.
Use this attenuator to control the amount of signal
entering the filter input $.
H If the filter’s output sounds distorted, turn this
control down, unless you deliberately want
the sound as a special effect.
2 Freq.
With this control you adjust the Cut-Off Frequency fc,
below which the filter attenuates all frequencies.
At zero, the filter is fully open. The more you turn the
filter up, the more the low frequencies are filtered (see
Fig. 1): the sound becomes thinner and less solid, until
at 10 the filter is completely shut, and blocks off all
frequencies below 15kHz. Frequencies above 15kHz
are still let through, but are generally outside normal
adult hearing range.
3 FCV
For voltage control or modulation of the cut-off fre-
quency using CV input §(see Fig. 1), use attenuator
3to control the level of voltage control.
Fig. 1: White noise put through a high pass filter
4 QCV
Attenuator 4gives you control over the level of vol-
tage control applied to resonance.
5 Res.
With this control you adjust the filter’s resonance (or
‘emphasis’) - the parameter which emphasises the
frequencies around the cut-off point fC(see Fig. 2). It
strengthens or emphasises the band of frequencies
around the filter’s cut-off point.
f
c
CV
OutOut
Freq.Freq.
f
c

A-123 VCF 4 System A - 100 doepfer
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At close to maximum resonance, the filter starts to
self-oscillate, and behaves like a sine wave oscilla-
tor. Thanks to this effect, you can use the filter as an
independent tone source.
Fig. 2: How resonance affects the behaviour of a
high pass filter.
4. In / Outputs
! Audio In
This is the filter’s audio input socket, where you
patch in the output from any sound source.
" FCV 1
Socket FCV 1 is a voltage control input for the filter.
It works on the 1V / octave rule, like the VCOs.
If you connect the output of a modulation source (eg
LFO, ADSR) to this input, the cut-off frequency of the
filter will be modulated by its voltage: ie, the sound
color changes according to the voltage put out by the
modulator.
P If you use this VCF as a sine wave oscillator,
connect a pitch control voltage to this input.
Do the same if you want the filter’s cut-off
frequency to track exactly with the pitch of a
note.
§ FCV 2
Socket §is also a voltage-control input for the filter.
Unlike on socket ", though, you can adjust the level of
voltage by using the attenuator 3, and thus control the
intensity of modulation effect on the filter.
f
c
Resonanz
Frequenz
0 db

doepfer System A - 100 VCF 4 A-123
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$ QCV
This socket is the voltage control input for the filter’s
resonance.
If you patch a modulation source (eg LFO, ADSR) to
this input, the resonance of the filter will be modulated
by it: increases in voltage will increase the amplitude
of the frequencies around the filter cut-off point.
% Audio Out
Filter output %sends out the filtered audio signal.
5. User examples
The filter’s cut-off frequency can be modulated in
various ways:
• VCF - LFO
Modulation of the cut-off frequency produces cycli-
cal 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. Typical uses
include filter sweeps, which slowly sweep through
the audio spectrum, picking out different harmonics
in turn.
• VCF - Keyboard CV
This modulation produces pitch-related filter ope-
ning.

A-123 VCF 4 System A - 100 doepfer
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24 dB notch filter with voltage control of
middle frequency and bandwidth
The patch in Fig. 3 shows a 24 dB notch filter with
voltage-controlled middle frequency and bandwidth.
Fig. 3: 24 dB notch filter with voltage control of
middle frequency and bandwidth.
For this patch, set both fL(Low Pass A-122) and fH
(High Pass A-123) to roughly equal cut-off points
(judging by ear).
Use a control voltage "FCV" to alter the middle fre-
quency fMof the notch filter: fM= (fL+ fH) / 2.
Use a control voltage "BCV" to alter the bandwidth,
which is determined by how far apart the two filters’
cut-off frequencies are. Using the A-175 Voltage
Inverter, these frequencies fLand fHare altered sym-
metrically around the middle frequency (see Fig. 4).
Fig. 4: Effect of FCV and BCV on the notch filter’s
response.
fH
Freq.
Ö
A-122
Freq.
Ö
A-123
Freq.
Ö
fL
A-138
Freq.
Ö
fM
A-175
Audio
In
FCV
BCV
Audio
Out
CV 1 CV 2
CV 1 CV 2
Freq.
Ö
FCV
BCV
f
M

doepfer System A - 100 VCF 4 A-123
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H To create a 24 dB bandpass filter with
voltage-controlled middle frequency and
bandwidth, put both filters in series (see the
A-122 user examples). For voltage control of
middle frequency and bandwidth, follow the
relevant notch filter instructions. (see Fig. 5).
Fig. 5: Effect of FCV and BCV on the band pass
filter response.
Freq.
Ö
FCV
BCV
f
M

A-123 VCF 4 System A - 100 doepfer
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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.
A-123
VCF 4
FCV 1
FCV 2
QCV
Frq.
FCV
QCV
Res.
Lev.
Audio In
24 dB High Pass
010
010
010
010
010
Audio Out
A-123
VCF 4
FCV 1
FCV 2
QCV
Frq.
FCV
QCV
Res.
Lev.
Audio In
24 dB High Pass
010
010
010
010
010
Audio Out
A-123
VCF 4
FCV 1
FCV 2
QCV
Frq.
FCV
QCV
Res.
Lev.
Audio In
24 dB High Pass
010
010
010
010
010
Audio Out