User Manual

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doepfer

System A - 100

VCF 4 A-123

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

Level

Audio

Out

QCV

Frequency

A-123

VCF 4

FCV 2

Summary of content (8 pages)

PAGE 1
System A - 100 doepfer VCF 4 A-123 1. Introduction Level Audio In A-123 VCF 4 FCV 1 FCV 2 FCV 2 Frequency Module A-123 (VCF 4) is a voltage-controlled highpass 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.
PAGE 2
A-123 VCF 4 System A - 100 Controls: 2. VCF 4 - Overview A-123 VCF 4 24 dB High Pass Audio In Lev. 0 ➀ 10 FCV 1 Frq. 0 10 0 10 ➂ QCV QCV ➃ Audio Out Res. 2 10 Attenuator for audio input ! 2 Frq. : Cut-off frequency control 3 FCV : Attenuator for filter CV at input § 4 QCV : Attenuator for resonance CV at input $ 5 Res.: Control for setting the filter’s resonance (emphasis) In / Outputs: FCV 0 1 Lev.
PAGE 3
doepfer System A - 100 3. Controls VCF 4 A-123 CV 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. Out Out fc 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.
PAGE 4
A-123 VCF 4 System A - 100 At close to maximum resonance, the filter starts to self-oscillate, and behaves like a sine wave oscillator. Thanks to this effect, you can use the filter as an independent tone source. doepfer 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.
PAGE 5
doepfer System A - 100 $ 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. VCF 4 A-123 5.
PAGE 6
A-123 VCF 4 System A - 100 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. Audio In A-122 CV 1 CV 2 BCV f 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 frequency fM of the notch filter: fM = (fL + fH ) / 2.
PAGE 7
System A - 100 doepfer H VCF 4 A-123 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). FCV BCV fM Fig. 5: Freq. Ö Effect of FCV and BCV on the band pass filter response.
PAGE 8
A-123 VCF 4 System A - 100 doepfer 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. A-123 VCF 4 A-123 VCF 4 Audio In Audio In Audio In 0 10 0 0 P 8 • Draw in patchleads with colored pens.