Chapter 6: Mix, Amplify, Distribute

This chapter describes the modules that provide the “framework” of a modular synthesizer. In contrast to filters, for example, with these modules one is often interested in signal processing that is as neutral as possible in terms of sound. However, the exception proves the rule.

Mixers combine audio signals or control voltages into a common signal and can usually adjust the level of the input signals.
Attenuators essentially do the same thing as mixers (reducing levels), but only for a single input source.
Crossfaders can blend different signals, panners distribute a signal to several (usually 2) output channels and finally polarizers can reduce the level in an inverted manner (i.e. the sign of the processed voltage is reversed).
Switches serve to route a signal to one of several outputs or, conversely, to select an input from different inputs for one output. Splitters route a signal to multiple outputs simultaneously.
Amplifiers are also used to attenuate signals, and they are often voltage-controlled. Some amplifiers can actually amplify input signals by a factor greater than 1.

Mix, attenuate and invert – the basics

We already know mixers from the audio sector and of course we can also mix several control voltages. For control voltages, mixers with linear potentiometers are more suitable. Attenuators are, so to speak, a single input of a mixer. They are useful when we want to adjust the intensity of the modulation signal, but the input module does not have a controller for this, for example. Instead of using manually controlled attenuators, you can of course also use VCAs and modulate the intensity of the modulation with a second control signal. Finally, inverters generate a negative voltage from a positive voltage and vice versa. Attenuators with a normal and inverted control range of the potentiometer are called polarizers. If a mixer with several channels is equipped accordingly, then it is a polarizing mixer.

Mixers

Admittedly, mixers are among the rather boring modules. You just plug in several sources and then you get the mixture of these sources out. In between, as little as possible should be changed, distortions or added noise are undesirable. By the way, in many systems there are far too few mixers installed – because they are boring. It’s the mixers in particular (and a handful of other unspectacular modules) that open up the most extensive possibilities in a modular system.

In principle, all mixers can be used both for audio signals and for control voltages. However, there is an important difference between audio and control voltage when it comes to the controls for the input signals. We usually want to be able to regulate the effect of control voltages completely linearly, so we will prefer mixers with linearly working potentiometers for this purpose. For audio signals, on the other hand, potentiometers with a logarithmic characteristic better fit the way we perceive volume differences.

Most mixers can be operated purely manually and have simple attenuators (potentiometers) at their inputs. In addition, there are also a few mixers that we can address via control voltages – this means that the mix can be automated. There’s even a dedicated control module to provide the appropriate control voltages for morphing four audio sources in one voltage controlled mixer – the A-144 Morphing Controller, ideally paired with an A-135-1 Voltage Controlled Mixer.

Posts on individual mixers

Attenuators, crossfaders and polarizers

The same applies here as was said for the mixers: “Boring” but indispensable! Attenuators are always required where there is no attenuator at the input of the next module in the signal chain. In principle, by the way, most VCAs (i.e. the voltage-controlled amplifiers) are actually “voltage-controlled attenuators”, because only a few VCAs amplify beyond a factor of 1. But the VCAs are not meant by “attenuators” here, but rather the manually controlled attenuators, which are often built passively without a power supply. Analogously, I have added manually controlled amplifiers (i.e. “real” amplifiers) to the category. However, there is currently only one example of this “genus”, the A-183-3 Amplifier, which can be used as a catch-up amplifier (e.g. between module systems from different manufacturers) and offers an amplification factor of up to 4.

Crossfaders can be used to blend two (sometimes even three) signals into one another – much more conveniently than would ever be possible with a mixer. Finally, polarizers are a kind of “crossbreed” between attenuators and inverters: On one side of their scale they work like conventional attenuators, but beyond the zero point the signal increases again – albeit in inverted form (i.e. positive voltages become negative and vice versa). Here, with the A-133, we find another module that can amplify beyond an amplification factor of 1 (up to 2.5) or inverted amplification.

Incidentally, the A-138e module is a very powerful and complex form of attenuator, crossfader and polarizer. The possibilities that arise with this “Swiss Army Knife” deserve a somewhat longer learning phase.

Switches and multiples

Switches are the extreme form of attenuators, so to speak. No continuum between “completely hidden” and “fully there”, but quite simply “on” and “off”. Then why not use an attenuator? Of course, the switch is much faster if you just want to switch a signal on or off – whether it’s audio or control voltage. There are also complex (partly programmable) switches that can work with multiple input and output sockets.

Finally, multiples are used to distribute a signal to several outputs.

Posts on individual switches and multiples

Voltage Controlled Amplifiers (VCAs)

As already mentioned, there are almost no “amplifiers” in the A-100 system in the sense that more level comes out than you put in.

Terrible? Not at all. VCAs are supposed to work exactly like this: You plug in audio or control voltage, modulate with e.g. an envelope or an LFO and get the signal back “shaped” in the amplitude. Namely in the range from 0 to the original amplitude.

Posts on individual VCAs

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Modulators – sounds are moved