About
While probably one of the least fascinating thing a modular synthesizer system can have, a voltage summator of this sort is also the most important core element of them all. Mixing and matching is one of the basic techniques of sound synthesis, and it is important to have facilities to do so in the system. As time went on, summation stages for CV and inputs became integral parts of modules and are often taken for granted, reducing the need to have a dedicated module sprinkled around in multiple instances, however, arbitrary summation still remains one of the core operations to run a patch.
This unit has three identical summation units. Each has three inputs, an offset knob and the output, indicated by a bicolour LED. Each knob acts as an attenuverter (attenuator + inverter), meaning that the amplitude of the signal is zero when the knob is at noon, and as it gets turned clockwise, the amplitude grows. However, turning it counter-clockwise from noon will also kick the signal in, however, it will then also get inverted.
The module is handy for instantly mixing different modulation signals (inverting or not inverting envelopes and LFOs at will without a need to repatch anything) or searching for artistically useful phase cancellation in sounds by, say, mixing a sound and an inverted version of itself passed through some filter. Usual operation as a sound mixer is also possible, but it may be tricky to find the exact spot of the knob where the sound input is completely inaudible at the output. If this is a concern, it is best to build a dedicated audio mixer.
The offset knob operates in -6...+6 volts range, with 0 at noon, like the input amplitude control knobs. It is handy to move the resulting modulation mixture around, center it around zero or bring it entirely to the positive or negative voltage range. Becasue of this module being in a system that uses banana cables, that usually stack, it's easy to configure it as a 3x3 matrix mixer, too. This workhorse module is very useful, easy-to-build, and a crucial part of the system.
Schematic
The design is an better version of the SFP28 Dual Utility Summator one, which, in its turn, is derived from the Doepfer DIY Matrix Mixer II. The main improvement is the precise LED driver circuit, without which the output signal suffers a noticeable amplitude loss and the LEDs do a crappy job in actually showing what's going on, as well as a dedicated offset knob. The switch selecting between unipolar and bipolar operation modes was omitted, and the unit hard-wired to bipolar (attenuverting) mode, but you can look at the SFP28 design and bring it back yourself, if needed. The circuit isn't anything spectacular, really, but it is very important to have a few of these in your system.