User Manual

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doepfer

System A - 100

Vactrol Multitype Filter

A-101-1

1. Introduction

Module A-101-1 is a special multitype filter based on

an idea by Nyle A. Steiner from the year 1974. Injecting

an audio signal into different points of a standard low

pass filter circuit leads to the three filter types low pass

(LP), band pass (BP) amd high pass (HP). For each

filter type a separate input

is available and the sum of

the three filtered signals appears at the common out-

put. Even a notch filter can be realized.

In contrast to the original circuit so-called

vactrols

are

used instead of diodes as variable resistors for fre-

quency and resonance control. A vactrol is a combi-

nation of a light depending resistor (LDR) and a light

emitting diode (LED) both put into a small light-proof

case. Vactrol based circuits are known for their soft

low-distortion sound. For details about vactrols and

the A-101-1 circuit principles please use the correspon-

ding links in the A-101-1 info page on our web site

www.doepfer.com.

The three filter audio inputs are normalled via swit-

ching sockets, i.e. the BP input is connected to the LP

jack socket provided that no cable is plugged into the BP

socket (same applies for HP/BP inputs). Each audio

input is equipped with an attenuator. The audio inputs

are very sensitive and enable distorted sounds too.

For both frequency and resonance manual control and

several control voltage inputs are available.

Two LEDs display the current frequency and reso-

nance settings.

Summary of content (6 pages)

PAGE 1
doepfer 1. Introduction System A - 100 Vactrol Multitype Filter A-101-1 Module A-101-1 is a special multitype filter based on an idea by Nyle A. Steiner from the year 1974. Injecting an audio signal into different points of a standard low pass filter circuit leads to the three filter types low pass (LP), band pass (BP) amd high pass (HP). For each filter type a separate input is available and the sum of the three filtered signals appears at the common output. Even a notch filter can be realized.
PAGE 2
A-101-1 Vactrol Multitype Filter System A - 100 Controls: 2.
PAGE 3
doepfer System A - 100 Vactrol Multitype Filter A-101-1 3. Controls / Inputs / Outputs 1 Frequency (control) / ! CVF In 1 (socket) 2 CVF2 (control) / " CVF In 2 (socket) 3 CVF3 (control) / § CVF In 3 (socket) This group of elements is responsible for the filter frequency. Control 1 Frequency is used to adjust the frequency manually. The control voltage inputs ! CVF In 1, " CVF In 2 and § CVF In 3 enable voltage control of the filter frequency by means of external voltages (e.g.
PAGE 4
A-101-1 Vactrol Multitype Filter System A - 100 Resonance can be increased up to self-oscillation. But due to the vactrol tolerances and tracking errors mentioned above resonance resp. self-oscillation deviations over the frequency range may occur. 6 LP (control) / & LP In (socket) 7 BP (control) / / BP In (socket) 8 HP (control) / ( HP In (socket) These are the audio inputs of the module with the corresponding level controls (attenuators) for low pass (LP), band pass (BP) and high pass (HP) function.
PAGE 5
doepfer System A - 100 Vactrol Multitype Filter A-101-1 Pay attention that the notch filter does not arise from a combination of low pass and high pass only - as one would expect. Even the band pass control has to be turned up. The reason for this behaviour are the phase conditions inside the filter circuit. Band pass with offset means that a share of all frequencies passes the circuit. High pass gate means that the filter frequency setting also affects loudness but not only the frequency.
PAGE 6
A-101-1 Vactrol Multitype Filter System A - 100 4. User Examples not yet ready • • • 6 examples with different audio signals (e.g.