Making sounds with analogue electronics - Part 4: Voltage-controlled amplifiers

[Part 1 briefly reviews the differences between analogue and digital synthesis, and discusses "one of the major innovations in the development of the synthesizer" - voltage control. Part 2 begins a look at subtractive synthesis with a discussion of VCOs, waveforms, harmonic content, and filters. Part 3 discusses envelopes - the overall 'shape' of the volume of a sound, plotted against time.]

3.3.7 Amplifiers
Most analogue synthesizers have a VCA as the final stage of the modifier section. The CV is used to change the gain of an amplifier.

The VCA controls the volume of the audio signal and is sometimes connected directly to the output of an EG. An offset voltage can also be used to provide a volume control; so even the output volume of a synthesizer can be voltage controlled.

The following are the two types of input to VCAs:

• 1. Linear inputs are used for tremolo and AM (amplitude modulation). They are also used with exponential curve envelopes.


• 2. Exponential inputs are used for volume changes and linear curve envelopes.

Tremolo is a cyclic variation in the volume of a sound. It is produced by using an LFO CV to alter the gain of a VCA. Tremolo normally uses a sine or triangle waveform at frequencies between 5 and 20 Hz. Higher frequencies from an LFO or a VCO produce AM, where the output of the VCA is a combination of the audio signal and the LFO or VCO frequency. See Section 3.3.1 for more details on AM.

Apart from their normal use as volume-controlling devices, VCAs can also be used to provide 'filtering' effects. By connecting the keyboard pitch voltage to the CV input of a VCA, the gain of the VCA is then dependent on the pitch CV from the keyboard. Since the keyboard pitch voltage normally rises as the keyboard note position rises, the VCA will act much as in a high-pass filter, since low notes will be at a lower volume than higher notes. By inverting the keyboard pitch voltage, a low-pass 'filter' effect can be produced. This coupling of the VCA to the keyboard pitch voltage is called 'scaling', since the output of the VCA is scaled according to the pitch (Figure 3.3.31).

FIGURE 3.3.31 A VCA can be used to produce control of volume which follows the keyboard by routing the keyboard CV to the VCA gain control. This is similar to the tracking of a filter, and produces a coarse high-pass filtering effect, where higher notes are attenuated less than lower notes.