A-120 - Audio module Doepfer - Free user manual and instructions

USER MANUAL A-120 Doepfer

A-120 VCF 1 CV 1 CV 2 CV 2 CV 3 CV 3 Frequency Level Audio In Audio Out Resonance

1. Introduction

Module A-120 (VCF 1) is a voltage controlled low-pass 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.

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.

2. VCF 1 - Overview

A-120 VCF 1 LOW PASS FILTER ① CV 1 Freq. ② CV 2 CV 2 ③ CV 3 CV 3 ④ Audio In Lev. ⑤ Audio Out Res.

Controls:

① Freq.: Cut-off frequency control
② CV 2: Attenuator for CV at input
③ CV 3: Attenuator for CV at input
④ Lev.: Attenuator for audio input ④
⑤ Res.: Control for setting the filter's resonance (emphasis)

In / Outputs:

① CV 1: Voltage control input
② CV 2: ditto, level controlled by ②
③ CV 3: ditto, level controlled by ③
④ Audio In: Input to the filter
⑤ Audio Out: Output from the filter

3. Controls

① Freq.

With this control you adjust the Cut-Off Frequency f_c , 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. f_C

Fig. 1: White noise put through a low pass filter.

② CV 2 • ③ CV 3

For voltage control or modulation of the cut-off frequency, use these CV inputs ② and/or ③ (see Fig. 1). 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 ④.

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_c (see Fig. 2 on p.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 oscillator.

Fig. 2: How resonance affects the response of a low pass filter

4. In / Outputs

① 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

modulated by its voltage: ie., the sound color changes according to the voltage put out by the modulator.

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.

② CV 2 · ③ CV 3

Sockets CV 2 and CV 3 are also voltage control 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.

④ Audio In

This is the filter's audio input socket. Patch in the output from any sound source (eg. VCO, noise generator, mixer).

⑤ Audio Out

Filter output ⑤ sends out the filtered 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 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. Typical uses 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.

Producing wind FX with A-120 and A-118

☐ Patch coloured noise into the audio input, and 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 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.

graph LR
    subgraph_A-118["NOISE / RANDOM"]
        A1["Blue"] --> A2["Red"] --> A3["Color"]
        A4["Rate"] --> A5["Level"] --> A6["Random Output"]
        A7["Random Control"] --> A8["+"]
    end

    subgraph_A-120["VCF 1"]
        B1["CV 2"] --> B2["CV 2"] --> B3["Level"] --> B4["Audio In"] --> B5["Audio Out"] --> B6["Resonance"]
        B7["Frequency"] --> B8["+"]
    end

Fig. 3: Creating wind effects

A-120 VCF 1

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.

Make multiple copies of your composite diagram, and use them for remembering good patches and set-ups.

• Draw in patchleads with colored pens.
• Draw or write control settings in the little white circles.

A-120 VCF 1 LOW PASS FILTER CV 1 Freq. CV 2 CV 2 CV 3 CV 3 Audio In Lev. Audio Out Res.

A-120 VCF 1 LOW PASS FILTER CV 1 Freq. 0 10 CV 2 CV 2 0 10 CV 3 CV 3 0 10 Audio In Lev. 0 10 Audio Out Res. 0 10

A-120 VCF 1 LOW PASS FILTER CV 1 Freq. 0 10 CV 2 CV 2 0 10 CV 3 CV 3 0 10 Audio In Lev. 0 10 Audio Out Res. 0 10

A-120 VCF 1

System A - 100

DOEPFER