3to1 is a utility module, with three inputs and one output. The output voltage being the exact addition (or subtraction) of the three input voltages.
This module is particularly useful when you want to combine (add or subtract) accurately several Control Voltages together.
The two channels are independent and alternatively, it can be used as a simple, plain, audio mixer.
The accuracy of a Control Voltage is extremely important. In modular synthesizers, one volt represents an whole octave. And one semitone is only separated from another semitone by only 1/12th of Volt, or 83.3mV. It’s easy then to get out of tune just by having some extra milivolts added (or subtracted) due to some component’s tolerance drift.
So, for example, when you want to combine two CVs from different sources (for example: a keyboard and a sequencer), you want to get exactly 1V + 1V = 2V.
Not 2.01V, not 1.99V.
But as we live in a real world, components are not ideal and their wide tolerances sometime add a lot of errors.
Two kind of errors can be distinguished here: constant voltage offset and amplification error.
Ideally, the offset should be null and the amplification should be exactly 1.
The amplification error can be reduced by using 0.1% resistors in opamp feedback loops. Those kind of resistors are fairly cheap and can be found easily. It’s also possible to cherry pick matched sets, using your ohmmeter. If you buy a hundred, you will find good candidates.
The offset is generated by the opamp itself. And opamps are not all equal regarding offset voltage.
There are huge variations regarding offset voltage characteristics.
The higher quality opamps (“precision opamps”) are expensive and often cannot handle large voltage differences on their inputs and also they can’t be powered within Eurorack voltages.
Relatively good opamps regarding the offset problem are BJTs opamps.
And the worst ones are the FET opamps (the TL0xx series for instance).
Fortunately, single unit opamps, such as the TL071, have two extra pins specifically designed to correct that offset.
To correct the offset, you just need to connect a trimming potentiometer to the “nulling offset” pins on each opamp.