Doepfer A-108 User Manual

doepfer
System A - 100
1. Introduction
6/12/24/48 dB Low Pass
A-108
Audio Level
CV 2
CV 3
Audio
In
A-108
VCF 8
CV 1
CV 2
CV 3
Feedb.
6dBLP12dBLP24dBLP48dB
BP
Freq.
Res.
LP
Module A-108 is a completely new, unique voltage­controlled low pass/band pass filter based on the well-known transistor ladder (Moog ladder).
The module has internally an 8 stage low pass filter with different slopes available: 6, 12, 18, 24, 30, 36, 42 and 48 dB per octave. In the factory the outputs with 6,
12, 24 and 48 dB are connected to the four low pass sockets at the front panel band pass output (i.e. band pass with transistor lad-
der).
The module has manual controls for frequency and
resonance
right up to self-oscillation, in which case the filter will behave like a sine wave oscillator. Three CV inputs for frequency control are available. Two of them are eqipped with
The A-108 features an enables the insertion of additional modules into the feedback path.
The audio input is very sensitive so that distortion is possible even with normal A-100 levels.
available. Resonance can be adjusted
attenuators
. In addition it features an
.
external feedback input
that
1
A-108
6/12/24/48 dB Low Pass
System A - 100
doepfer
2. VCF 8 - Overview
CV 1
CV 2
CV 3
Audio
In
A-108
6/12/24/48 dB Lowpass
Ouputs
BP
6db LP
12db LP
24db LP
48db LP
Feedb.
VCF 8
0
0
0
0
0
Freq.
10
CV 2
10
CV 3
10
Audio Level
10
Emph.
10
Controls:
Freq. : Manual frequency control
1
2 CV 2 : Attenuator for frequency control
voltage at input § / CV2
CV 3 : Attenuator for frequency control
3
voltage at input § / CV3
Audio Level : Attenuator for audio input
4
!
5 Emph. : Resonance control
In / Outputs:
Audio In : Audio input to the filter
!
" BP, 6 db LP ... 48 db LP : Filter outputs
: Control voltage input for frequency
CV 1
§
control, approx. 1V/Oct.
§ CV 2: Control voltage input for frequency
control, level controlled by
CV 3: Control voltage input for frequency
§
control, level controlled by
Feedb. : external feedback input for reso-
$
nance (audio input)
2
3
2
doepfer
System A - 100
6/12/24/48 dB Low Pass
A-108
3. Controls
1 Freq.
Control 1 is used to adjust the filter frequency manu­ally, i.e. the
middle frequency f
resp. the cut-off frequency f fig. 1).
The actual filter frequency results from the sum
H
of the manual control 1 and the external control inputs CV1 ... CV3.
2 CV 2 • 3 CV 3
For voltage control or modulation of the cut-off fre­quency the CV inputs § can be used. The controls 2 and 3 act as attenuators for the inputs CV2 and CV3 to adjust the control voltage level.
4 Audio Level
This attenuator is used to control the amount of signal entering the filter audio input !. If the filter’s output distorts, turn this control down, unless you deliberately want the distorted sound as a special effect. The filter audio input is very sensitive so that distortion is possi­ble even with normal A-100 levels. Distortion appears about from position 5 with normal A-100 audio levels.
for the
M
for the low pass (see
C
band pass
Out
Freq.
f
M
Out
Emph.
Freq.
Freq.
f
C
Emph.
Freq.
Fig. 1: Frequency response of band and low pass
5 Emph.
Control 3 is used to adjust the filter’s emphasis) - the parameter which emphasises the fre­quencies around the cut-off point f this control changes the bandwidth (see Fig.1-4). Close to its maximum setting, the filter becomes so resonant that it goes into self-oscillation, and starts behaving like a
sine wave
. You can take advantage of this effect, and use the VCF as an additional sine oscillator. Self oscillation will break off at high distortion levels as the internal feedback signal is drown out by the distorted audio signal. This feature may intentio­nally be used to create new sounds. If you want to use the filter as a sine wave oscillator no audio signal should be fed in (or control 4 to zero).
resonance
. For the band pass
C
(or
3
A-108
6/12/24/48 dB Low Pass
System A - 100
doepfer
Fig. 2
Fig. 3
Fig. 4
Fig. 2: Filter response curves (minimal resonance)
Fig. 3: Filter response curves (middle resonance)
Fig. 4: Filter response curves (high resonance)
For all figures:
a : Band pass b : 48 dB Low pass c : 24 dB Low pass d : 12 dB Low pass e : 6 dB Low pass
4
doepfer
System A - 100
6/12/24/48 dB Low Pass
A-108
4. In / Outputs
! Audio In
This is the filter’s output from any sound source (eg. VCO, noise gene­rator, subharmonic oscillator, sampler, mixer output).
audio input
" BP • 6db LP • ... • 48db LP
The sockets " are the filter outputs with the filter types band pass (BP) as well as 6, 12, 24 and 48 dB low pass (LP).
H You may change the factory settings of the
cut-off slopes (6, 12, 24, 48 db) for the four low pass outputs (see chapter 6 for details).
§ CV 1 • CV 2 • CV 3
The sockets CV1, CV2 and CV3 are control voltage inputs no attenuator and works approximately to the 1V/ octave standard (not as exactly as the VCOs). Inputs
CV2 and CV3 are equipped with the attenuators 2 and
3
sted. With attenuators set to 10 (fully clockwise) the sensitivity for CV2 and CV3 is about 0.5V/octave.
to control the
so that the sensitivity of these inputs can be adju-
filter frequency
socket. Patch in the
. Input
CV1
has
If you patch one or more modulation sources (e.g. LFO, ADSR, Random, S&H, Theremin, Ribbon, MIDI­to-CV) to these inputs, the cut-off frequency of the filter will be modulated by the sum of the control voltages: i.e., the sound color changes according to the sum of the control voltages put out by the modula­tors.
P If you use the VCF as a sine wave oscillator, it
is recommended to connect the pitch CV to socket CV1 as this input works approximately to the 1V/octave standard. Do the same if you want the filter’s cut-off frequency to track ex­actly with the pitch of a note.
$ Feedb.
Module A-108 has available an external audio input for resonance. This socket is normalled, i.e it is
connected to the 48 dB low pass output unless another signal is patched into this socket. Different modules can be inserted into the feedback loop (see chapter 5 for examples). E.g. inserting a VCA enables voltage controlled resonance. It is also possible to feed back other filter outputs than the 48 dB low pass to obtain another resonance behaviour.
5
A-108
6/12/24/48 dB Low Pass
System A - 100
doepfer
5. User Examples
The filter’s cut-off frequency can be modulated in various ways: e.g. ADSR (A-140, A-141, A-142), LFO (A-145, A-146, A-147), Sample & Hold (A-148), Ran­dom (A-118), Joy-Stick (A-174), Theremin (A-178), Ribbon (A-198) or via MIDI (A-190/A-191). Many of these examples can be found in the manuals of the other filters of the A-100 system.
One special feature of the A-108 is the external feedback input. This enables the insertion of any audio processing module into the feedback path. In­serting a VCA leads to voltage controlled resonance. In this case the control voltage of the VCA works as voltage control input for the filter’s resonance. Other modules that are suitable for insertion are e.g. phaser (A-125), frequency shifter (A-126), waveform proces­sor/distortion (A-116, A-136) or even other filters.
Another example is a filter with voltage controlled
cut-off slope
led mixer A-135 and the morphing controller A-144. Fig. 5 shows the corresponding patch.
The four low pass outputs (6/12/24/48 dB) of the A-108 are connected to the four audio inputs of the voltage controlled mixer A-135.
in combination with the voltage control-
The levels of the four mixer paths are controlled by the control voltages coming from the morphing controller A-144. The audio output signal of the filter with voltage controlled cut-off slope is available at the output of the voltage controlled mixer A-135.
The control voltage that is used to control the cut-off slope is connected to the control voltage input of the morphing controller A-144. That way it is possible to sweep the cut-off slope from 6 ... 48dB (fig. 5, the control voltage applied to the CV input of the A-144. In the example the voltage is generated by a joy stick (A-174) and called “character modulation”. Of course any other control voltage (e.g. LFO, ADSR, MIDI-to-CV, Theremin, Random) could be used to control the “character”, i.e. the cut-off slope. For more examples please refer to the A-144 user’s manual.
The patch in fig. 5 has these control inputs available:
Char. Mod.
Freq. Mod. 1 filter frequency 1 (e.g. ADSR)
Freq. Mod. 2 filter frequency 2 (e.g. LFO)
If the 48dB low pass output of the A-108 is connected to the feedback input of the A-108 through a VCA even the resonance is voltage controlled (= control voltage of the VCA).
filter cut-off slope
2) with
6
doepfer
System A - 100
6/12/24/48 dB Low Pass
A-108
A-144
MC
Char.
Mod.
Joystick
CV
VC
ext. CV
CV Out
1
2
3
4
1
t
: Low pass filter with voltage controlled cut-off slope
Fig. 5
VC-Mixer
ext. CV 1
ext. CV 2
ext. CV 3
ext. CV 4
A-135
Audio In
Audio Out
Audio
Out
A-108
VCF 8
Audio Level
1
2
3
4
6dB
LP
12dB
LP
24dB
LP
48dB
LP
CV 2
CV 2
CV 3
CV 3
Feedb.
Audio In
Freq. Mod. 1
Freq. Mod. 2
2
100 Hz 1 kHz 10 kHz
7
A-108
6/12/24/48 dB Low Pass
System A - 100
doepfer
6. Changing the assignment of the four low pass outputs
Only 4 output stages are available as this seems to be sufficient from our experience. In the factory the 4 outputs are connected to the filter stages with 6, 12, 24 and 48 dB cut-off slope. This is a well-considered compromise as these outputs generate audible diffe­rent sounds. E.g. the audible difference between the 48dB and 42dB or between 42dB and 36dB is very little. So it would make not much sense to have 30, 36, 42 and 48 dB available instead of 6, 12, 24 and 48 dB.
But if desired any of the 8 filter stages can be connec­ted to one of the 4 outputs. By changing the internal connections (jumpers or wires) each filter stage is available - but only four at a time. Even a multi-way switch could be used but from our results the factory setting (6/12/24/48dB) is the best combination for mu­sical applications.
The factory settings can be changed if the correspon­ding jumpers on the pc board are removed and re­arranged for the desired new output combination.
On the pc board 4 double row pin headers are avai­lable (see picture on next page). The positions of the jumpers on these pin headers determine the assign-
ment of the filter stages to the outputs. One pair of jumpers is responsible for each of the filter stages. The factory setting of the jumpers is marked
These are the possible jumper settings (the factory settings are printed bold):
Output 1: only 6 dB possible
Output 2: 12 dB or 18 dB
Output 3: 24 dB or 30 dB or 36 dB
Output 4: 42 dB or 48 dB
In principle each filter stage can be connected to one of the four output stages but this would require additio­nal wiring and cannot be carried out by changing the jumper settings only (e.g. 18 dB to output 3). All combinations not listed above have to be made with wires two by two instead of the jumpers. The upper row of the pin headers are the 8 filter stage outputs in pairs. The lower row of the pin headers lead to the inputs of the 4 output stages in pairs whereby the pins below 6 db belong to out 1, the pins below 12 and 18 to out 2, the pins below 24, 30 and 36 to out 3 and the pins below 42 and 48 to out 4. For details please refer to the A-100 service manual (additional charge).
black
.
8
doepfer
System A - 100
6 | 12 dB 18 | 24 dB 30 | 36 dB 42 | 48 dB
6/12/24/48 dB Low Pass
A-108
output 1 output 2
output 3 output 4
9
A-108
6/12/24/48 dB Low Pass
6. Patch-Sheet
System A - 100
doepfer
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-108
6/12/24/48 dB Lowpass
Ouputs
CV 1
BP
6db LP
CV 2
CV 3
12db LP
Audio
24db LP
48db LP
In
Feedb.
VCF 8
10
0
10
0
10
0
10
0
10
0
Freq.
CV 2
CV 3
Audio Level
Emph.
A-108
6/12/24/48 dB Lowpass
Ouputs
CV 1
BP
6db LP
CV 2
CV 3
12db LP
Audio
24db LP
48db LP
In
Feedb.
VCF 8
0
0
0
0
0
Freq.
10
CV 2
10
CV 3
10
Audio Level
10
Emph.
10
10
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