The inputs are AC-coupled in the current design, but that’s something I’ve gone back and forth on. Making them DC-coupled wouldn’t be the only change necessary to turn the module into a good CV mixer, though.
For CV, I think it would be best to use constant-voltage panning, with the inputs spaced evenly between the two outputs:
For audio, constant-power panning is arguably better, and is what I’m using:
I’m not yet sure about the best placement of the inputs, but I know that spacing them evenly doesn’t work well.
So, even with DC-coupled inputs, it would be a pretty weird CV mixer. Plus, for audio, the mix would be more likely to clip and distort when incoming signals have DC bias. (I know of at least one popular Eurorack sound source module that has an inherent DC bias: the Bastl Grandpa.)
Of course, all of this applies only to the “standard” build of the module. If you DIY one, you can change the panning to whatever you like, and also make the inputs DC-coupled (by replacing the capacitor at each input with wire).
Thinking about this made me realize that even “normal” Eurorack audio signals (10Vpp without DC bias) might cause the mix to clip, if they are strongly correlated, e.g. from oscillators tracking the same voltage source. To fix that, I’ve added jumpers to the second revision of the board, that let you attenuate all signals by 6dB, much like on the Intellijel Unity Mixer.
Expander pins, then. At first glance, they seem like a great idea, but for a module like this one, they turn out to be a lot of trouble. (This is the kind of design issue I enjoy reading about, so I’m gonna brain dump a bit here.)
First off, where in the circuit would the pins go? There are two logical places:
- In series with each input. This would make possible expanders that do per-input attenuatuation or muting. That means 7 “send” pins, plus 7 “return” pins, plus ground, which adds up to 15 pins, which you’d probably do as 2x8 pins, the same size as the larger variant of the Eurorack power connector.
- Added to each output. This would enable chaining multiple modules together without using patch cables. For that, you’d need 2 input pins, plus 2 output pins, plus ground, meaning either 5 or 2x3 pins.
For 7:2, I had to split the circuit across two boards: a front board with all the jacks and a bit of passive per-input circuitry, and a back board with the active circuitry and the power connector (you can sort of see what this looks like in the photos in the first post).
Now, this is hard to know without having seen the schematic, but if you wanted to add expander pins of the first variety, they would need to go on the front board (because on the back board, the signals are already mixed to two buses). But the front board is completely packed - there is simply no space for anything that large. Of course, that could be solved by making the board larger, but then it would no longer fit behind a 3hp panel. Another problem, as identified by @Leverkusen, is that the circuit would still need to work with no expander connected, which means you’d need another 7 jumpers just to make the module work, and if you lost any of them, you would also lose an input.
Expander pins of the second variety don’t present a problem in terms of size: there’s fewer of them, and they would go on the back board, which can be made a fair bit larger without affecting the width of the module. However, making the circuit work equally well with and without an expander connected is trickier here - the expander pins would either need to be buffered multiples of the outputs (which would require more circuitry), or take advantage of the switching on the output jacks, so that inserting a cable in the output jacks would prevent the signal from going to the expander pins. That in turn would require a few more pins connecting the front and back boards. Doable, but not without drawbacks. (And, you could get the same functionality using e.g. a Unity Mixer, at a “cost” of two-thirds of an hp per module.)
