I’m not an EE, but a visual artist who has made many projects in hardware and software (sometime with the guidance of an EE, most of the time not)
Limited technical knowledge, but lots of hacker knowledge 
I’m happy to help answer questions here too!
Thanks y’all (@corpusjonsey, @rvense, @bcfrench9, @bmoren)! I have done a decent amount of electronics assembly, and designed some simple circuits with microcontrollers, but I haven’t done much of anything with audio.
I’m looking at adding a stereo send and return to the Befaco Hexmix. You can peep the full schematic here.
The signal runs through a mess of op amps to do most everything the mixer does. Ultimately I know I need to boost the singal to 5v and then bring it back down the expected level. I’m planning to pull the resistors at the end of each of the channel inputs (R22, R27, …), add stereo switches to determine whether they continue on their extant circuit path or get sent to a summed stereo send and return. There’s an unused channel 7 on an expander connecter that I would use for the returns (B_MIX_7_L/R). It’s also worth noting the jumpers on the schematic aren’t on the actual final module.
I’ve done enough research to understand loosely how op amps work, but they do so much it can be hard to really wrap my head around. If my reading informed me correctly, the following “Return” circuit from the Hexpander is using the op amps in an “integrator” configuration where the capacitor limits gain? Essentially the feedback on the negative line of the opamp allows the level of the signal to change.
The stereo switch determines whether the return signal is line level or modular level, and then the op amp brings the signal down to 800mVp.
So my first questions:
Please assume I have little idea what I am talking about (true!). I’m sure I’m using incorrect terminology to describe things. Corrections welcome!
Next bit: The power supply!
I know that if I add op amps, I need to manage the power. I was proud of myself for figuring out the 4/8 lines were referring to the power lines for the op amps while I was in the shower yesterday. 
On the other hand, I have no idea what the network of capacitors and resistors are doing in this diagram. I’m not sure what this is called, so I can’t look it up. Any pointers would be welcome!
Thanks in advance!
I wouldn’t think of the capacitor as limiting gain in the integrator configuration. The capacitor wired from the negative terminal to the output defines an integrator, allowing for the integration of the applied signal (e.g. dc offset voltage applied >> ramp). The feedback resistor in the configuration essentially applies a filter, below which the integrating behavior is limited.
And the second schematic snippet shows bulk decoupling caps.
Regarding the power supply, the capacitors are called bypass (or decoupling) capacitors to filter out noise on the DC line (https://en.wikipedia.org/wiki/Decoupling_capacitor). The resistors create voltage dividers that divide down the rails (+12V) to some other voltage (https://en.wikipedia.org/wiki/Voltage_divider). Looks like they’re used to bias an LED/s?
I’ll add that I have an old copy of “Practical Electronics for Inventors” that explains a lot of stuff pretty well, so thanks for the terminology!
So on reading, the rule of thumb is to use a pair of 100nF decoupling capactitors per analog IC and 1 per digital IC. There are 21 op amps, so that makes sense! Also a pair of what I’m guessing are tantalum capacitors for the two polarized ones? There’s also an extra one in there, I guess? I’m not sure why as there don’t appear to be any digital IC’s…
I wouldn’t worry how many there are. It can’t hurt to have more than enough in this case (besides cost, it’s negligible).
That is fair. Just trying to make sense of things. This is also a circuit designed by a person, so I get that people have preferences for how they will approach a problem rather than, “This is the rule.”
Another way for me to inquire; here’s the summing amplifier for all of the channels:
What’re people using these days for fine dithering of LED matrices? I used FadeCandy a few years ago to great effect. Working with Processing was a blast.
You see a small cap like that sometimes for “stability”, though I’ve seen claims the TL072 doesn’t need it. I have no maths or other science to back this up, but I’ve seen quite a few inverting buffers in schematics and this configuration is common.
As with decoupling, I think it could be something the designer just did because that’s how they always do it.
If you want to dig deeper into this, there is a book called “Small Signal Audio Design” by Douglas Self. It’s not a beginner’s book (a lot of it is over my head, haha) but it goes through common circuits like this from the perspective of designing pro audio gear like mixers and preamps.
Seconding the book “Small Signal Audio Design”. @grey as said above the capacitor acts as a gentle low pass filter, making the op amp stable by introducing lead compensation.
The first paragraph of section 7 (Lead Compensation) does a good job at explaining why it might be needed: http://www.ti.com/lit/an/sloa020a/sloa020a.pdf
LTSpice was already open so i quickly simulated the circuit stepping that capacitor from 5pF to 100nF. You can see the -16dB gain line, with all the responses deviating at different points. You’re not wanting to roll off high frequencies as such by using this cap, just make sure that the op amp does not oscillate.
I’d recommend getting your head around LTSpice, or some equivalent. Its good for experimenting with basic audio circuits
Thank you for the explanation. I’ve ordered the book. There was a single reasonably priced copy on Amazon. 
Also thank for the link to the TI doc. I read the spec sheet, but was unaware of the more general document. I’ll also peep LTSpice.
