I’m looking at the schematic for the Links module and am wondering what about the 2:2 section makes it a precision adder/mixer. The schematic for the 3:1 section looks almost identical except for the op-amp feedback resistor - 2:2 is 100k and 3:1 is 33k. My current understanding is that the minor differences in resistors with the same values make it difficult to precisely add and mix DC signals.
It is precise in the sense that the gain of this section is equal to 1 with a reasonably high accuracy. If you look at the BOM, you will see that the 100k resistors have a 0.1% tolerance. There are four 0.1% resistors involved in the circuit, one stage with a gain of -1, another with a gain of -1, so the overall gain will be somewhere between 0.996 and 1.004.
This means that you can use the circuit to transpose and combine note CVs from sequencers, MIDI interfaces, quantizers… without much tracking error (a few cents per octave at worst).
The 3:1 section has a gain of 0.33, so obviously you will lose tracking! It does add and mix DC signals, precisely (in the sense that there is no distortion) but with such a gain you can’t use it for note CVs.
Ah, okay. So the trick is to use resistors with a 0.1% tolerance. This will allow the circuit to be precise. I hadn’t checked the BOM.
This is awesome. Thank you.
And a gain of 1 of course!