Utilities

CV Mix

Three-channel CV mixer with attenuverters - sum, invert, and offset modulation sources.

What is a CV Mix?

A CV mixer is a summing utility for control voltages. It takes two or more CV inputs, scales each one by an attenuverter (an attenuator that can also invert), and adds the results together at a single output. Patch an LFO and an envelope into a CV mixer and the output is LFO + envelope - the modulation source has both shapes layered.

The math is the most elementary in modular synthesis: OUT = a*X + b*Y + c*Z + ..., where a, b, c are the attenuverter knob positions and X, Y, Z are the input voltages. If a knob sits at +1, that input is added at full level. At -1, it's inverted and added (i.e. subtracted). At 0, that input is muted. At +0.5, it contributes half its swing.

What makes CV mixers indispensable is the unpatched normalization. On most modules, an unpatched input means a constant 10V reference internally connected to that jack. Combined with the attenuverter, this turns each unused channel into a manual offset voltage source: knob at +0.3 produces a steady +3V, at -0.5 produces -5V, at 0 produces 0V. This is the cheapest, most reliable way to generate fixed control voltages anywhere in a patch - so a CV mixer is a CV mixer and a programmable voltage source rolled into one.

Combining modulation sources is one of the most powerful gestures in modular: a slow envelope opens a filter, while a fast LFO adds vibrato to the same parameter, while a knob on a CV mixer adds a constant offset to set the resting cutoff. Three knobs, one cable to the destination, and the same parameter is doing three things at once. Without a CV mixer you'd need three separate destinations or three modulation slots - the mixer collapses all that into one stream.

Our CV Mix

Our CV Mix is three channels, each with a single attenuverter from -1 to +1, summed to a unity-gain MIX output. Compact (4 HP), no extra outputs, no DC blocking - it stays out of the way and does the one job. The three knobs scale and sign their inputs, the output sums them, and that's it.

Unpatched inputs are normalled to +10V, so any unused channel becomes a manual offset: knob at +0.3 outputs a steady +3V, knob at -0.5 outputs -5V. We chose 10V (rather than the more common 5V) so the full attenuverter swing covers the entire Eurorack modulation range from -10V to +10V cleanly. Two cables in, one knob held back as a fixed bias - that pattern shows up everywhere once you start looking for it.

If you need more channels, chain CV mixers. Patch the MIX of one into a channel of the next; the second mixer's attenuverter becomes a master level for the first three sources, and the second mixer's remaining channels add new ones. Six channels, then nine, then twelve - the topology scales without any dedicated master panel.

In a patch

The most common patch: LFO and envelope summed and sent to a filter cutoff. Set the LFO attenuverter low (a touch of vibrato), set the envelope attenuverter high (the main motion), set the third channel to a fixed offset to tune the resting cutoff. The filter modulates with all three signals from a single jack.

CV mixers are also where inversion happens. Patch an envelope into a CV mixer with the attenuverter at -1 and you've made an inverted envelope - useful for ducking a parameter down while the envelope rises, or for making a VCA close as another opens. No dedicated inverter needed; the negative side of every attenuverter is one.

When you need a manual offset somewhere in the patch - a +2V constant added to a sequencer line to transpose it up two octaves, a -1V bias to push a filter into self-oscillation - the unpatched-channel-as-offset trick is the standard solution. One CV mixer can supply offsets for several places in a patch from its unused inputs.

Inputs

CV 1 (cv) — First CV input. Receives 10V when unpatched, making the Level 1 knob a voltage source.
CV 2 (cv) — Second CV input. Receives 10V when unpatched, making the Level 2 knob a voltage source.
CV 3 (cv) — Third CV input. Receives 10V when unpatched, making the Level 3 knob a voltage source.

Outputs

MIX (cv) — Summed output of all three scaled inputs. The result can exceed +/-10V if multiple signals add up.

Controls

Level 1 — Attenuverter for CV 1. Scales from -1 (inverted) to +1 (full). At 0 this channel contributes nothing to the mix.
Level 2 — Attenuverter for CV 2. Scales from -1 (inverted) to +1 (full).
Level 3 — Attenuverter for CV 3. Scales from -1 (inverted) to +1 (full).

Inspired by

A small CV summer in the lineage of every modular mixer ever made. Three attenuverters, one output, unpatched inputs normalled to 10V so each knob can act as a constant offset source.

Doepfer A-138
Make Noise Maths summing channels
classic CV summing mixer

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