Doepfer A-130/A-131 VCA (linear / logarithmic) User Manual download pdf (Page 6)

A-130 / A-131 VCA System A - 100 doepfer

Amplitude modulation

In Fig. 5, an LFO is modulating an A-130 linear VCA

(with Gain > 0), so that the amplification changes

cyclically with the LFO’s voltage. (Amplitude modu-

lation / AM.)

With an LFO frequency in the sub-audio range (1 Hz to

around 15 Hz) the result is Tremolo (see Fig. 5).

With a modulation frequency in the audio range, side-

bands occur like those produced by FM (Frequency

Modulation), and interesting timbres emerge.

Fig. 5: VCA amplitude modulation with an LFO

Modulation depth is adjusted with control 2.

Fig. 6 shows a way of voltage-controlling this modula-

tion depth using another VCA. In this example, the

VCAs have the following functions:

• VCA 1 (A-130): AM control

• VCA 2 (A-131): total volume control

• VCA 3 (A-130): modulation depth control

The voltage control input A to the modulation depth VCA

can come from an ADSR, MIDI controller, etc..

Fig. 6: AM with voltage-controlled modulation depth

LFO

VCO

Au dio

In 1

C V 2

Ga in

A-130

VCA-LIN.

In 1

Aud io Out

Out

Gain > 0

VCO

LFO

VCA 1

VCA 3

VCA 2

1 2 3 4 5 6 7 8

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Doepfer A-130/A-131 VCA (linear / logarithmic) User Manual Page 6