Many modules can generate and mix CV in your modular system. What's particular about a Precision Adder is that it's calibrated in such a way that it can combine signals and make very exact voltage changes. These known voltage changes enable us to transpose sequences accurately, offset modulators and create intervals for musical interactions. It's the sort of utility that you won't feel you need until you start using one.
To design a precision adder, you must use extremely high-quality components to ensure the voltage is measurably accurate. You can mix CV signals together to generate interesting modulation shapes with much less accuracy using simpler modules like mixers and VCAs. But if we're dealing with melody, intervals and octaves, then they need to be correct, and that's why we have these specially designed modules.
Precision Adders revolve around the 1-volt-per-octave standard, which you'll find on all oscillators to drive the pitch. At any given pitch, if you add one volt the pitch will jump up by one octave. It follows then that each semitone is defined as one twelfth of a volt. By precisely adding voltage, you can play the pitch in a regular scale. But don't we have sequencers and quantizers to do that? Yes, we do, but a Precision Adder can come into play when we want to combine or vary sequences while keeping the tuning correct and the results interactable.
Let's look at some examples using the classic Doepfer A-185-2 Precision CV Adder.
Keyboard Transposition
You have an 8-step sequence running a VCO, and you'd like to transpose that sequence using a keyboard controller with a CV output. Here's what you do. Plug the output of the sequencer into the second input on the A-185. The top input involves the attenuator knob, which we'll come to in a minute, but for now, we want the simplicity of the non-attenuated inputs. Set the switch to "+", which adds the voltage from the sequencer to the outputs at the bottom. Patch one of the outputs to your VCO, and you should hear your original sequence. Now, plug the pitch CV output of your keyboard into the third input. Switch it to "+", and any voltage generated by the keyboard will be added to the sequencer voltage.
It does depend a little on what the keyboard is kicking out, but we can probably assume that at the lowest note, the bottom C, is set to 0V. So, pressing this key will have no effect on our sequence. Now press the C key an octave up and it will transpose the sequence by an octave. Try the F and the G for fourth and fifth transpositions. It's beautifully simple and effective.
Sequencer-on-Sequencer Action
Instead of using a keyboard to shift your sequence, you could program another sequencer to do that for you. So, you could use a slow-moving sequence to transpose the original sequence by plugging it into where the keyboard was. Using a clock divider, you could slow one sequencer down to an eighth of the speed of the original so that on every loop, the sequence is transposed.
It doesn't have to be that simple or regimented. You could combine two sequencers that have different lengths or unrelated clocks to create wild variations on the sequence.
I regularly use a Precision Adder to combine the output of a Music Thing Turing Machine that's generating random melodies with a slow-moving arpeggiator. It creates some very pleasing and far-reaching variations.
Adding Vibrato
The first channel on the A-185 has a nice big attenuation knob. This allows you to scale back modulators, like LFOs or envelopes, for more subtle use or to invert the modulation. One useful application is to add vibrato to a sequence.
So, we have our original sequence still going into channel 2. In the top channel, you can patch a relatively fast sine wave LFO and switch it to "+". Immediately it will be throwing your sequence all over the place, but as you dial down the knob, you can reduce it to a mere wobble of some classic vibrato.
Sequencer Octave Shift
Here's another scenario. You have two VCOs and one sequence driving them in unison. Patch the sequence going to one of the VCOs through the Precision Adder. With nothing patched into the other inputs of the A-185, the switches on the empty channels can add a flat 1V to the sequence. While one VCO stays on the original melody, the other VCO can be transposed up to three octaves on the flick of a switch or two, giving a really nice tonal variation.
Sequencer Interval Shift
You could use a keyboard or another sequence to do an interval shift in the double VCO scenario, or we could use a different Precision Adder that has that function built in. The Voltio from Befaco is a very musically orientated Precision Adder that can generate voltages in semitones. Patch your sequence into the Sum In, and you can shift it in octaves, semitones or whatever interval you choose.
The Voltio can add up to 10 volts, which makes it perfect for offsetting other voltages, like turning a -5 to +5 bipolar LFO into a 0-10v unipolar one.
Obsolete or Essential?
You may find that many of the functions a Precision Adder can bring to your modular are already taken care of elsewhere. Many VCOs have FM inputs with attenuators to use with vibrato and other modulations. Others have built-in octave selection switches. Some sequencers have the ability to transpose themselves. However, the presence of a Precision Adder gives you so many more modular possibilities. It's interactive, useful in experimentation and is easily bent into doing unexpected things.
The usefulness of the Precision Adder is perfectly demonstrated by Mylar Melodies in this video describing his current performance modular system.