And we get the term breadboard from people putting nails in a wooden board
I started out in the early 60's with brass upholstery tacks and a pine board with a couple of tube sockets screwed to it. Then I learned to cut the terminal strips out of a discarded TV set and screw them onto the board.
It's great to hear that a "pro" isn't laughing at me.
Everybody starts somewhere. The fact that you made something that works on the first try is an incredible accomplishment. I have seen senior students in engineering school do far worse. "Pro" is a relative term. I have designed, and laid out the PC boards for complex things like cell phones (it was my day job before the layoff), but I am mostly a hardware guy. In the software world, I'm still a rookie.
I will definitively go through the process of learning Eagle
I use Eagle for most of my personal projects. For a project where you are only going to make one unit, especially something large like this, a PC board may not make sense. It would cost too much, that's why I used the blue perf board. There are actually two pieces glued together, because that's the biggest ones I had. It is possible to make your own PC board directly from Eagle without even making Gerber files. Google the "toner transfer method" of making a board. I use that a lot, especially if I'm not sure its going to work.
You really get 13bits resolution with no hysteresis and averaging and with multiplexes???
I have not actually made any measurements on exactly how much resolution I'm actually getting. I needed to be able to sweep a VCO without actually hearing the discrete frequency steps, since I am simulating a 70's vintage analog music synthesizer. I just kept cranking up the resolution until I got a smooth sweep. It needed " analogReadResolution(13); " to sound like an analog VCO. Keep in mind that the chip in all the Teensy 3.X's have two only A/D converters. The A/D pins are already multiplexed inside the chip, so we are actually multiplexing twice. I do not externally multiplex the pins that are directly controlling the emulated VCO's since they respond to the musician's controller playing. I multiplex the inputs from the pots since they are only changing synthesis parameters, and a small delay or stutter is not a big deal. I got all that working with a Teensy 3.2 which was pretty much maxed out, so I did not use averaging or hysteresis. I have switched to the Teensy 3.6, so I may try them in the future, but not until I get all my other code and hardware working.
How much is the voltage actually jittery in percentage?
I send some of the data out the serial port and watch it in a terminal window for debugging purposes. Any A/D converter will exhibit random changes in the LSB, so I pushed that bit out beyond the limits of audibility by going to 13 bits of resolution, I can see the LSB, and sometimes the adjacent bit bouncing around in the serial data, but I can't hear a perceptible pitch change, so I don't care.
but considering it's an audio application I guess you can cope with continuous data.
I don't plan on ever sending any knob data out the USB port, so I don't have that constraint to worry about. My box is actually on the other end of the wire. Eventually I will need to plug a controller, MIDI keyboard, or computer into my synth, and play it over MIDI. Got to get the sounds all working before I even think about USB or MIDI.
I did have a color scheme that made sense
Mine went out the window when I put that multi colored ribbon wire in. I originally built the box for a Teensy 3.2, but ran short of processing power and I/O, so I ripped out the original processor board and made the new one that's in there now. Since it fit's either a T3.2 or a T3.6 (and maybe a T3.5) I needed more pots and connections, hence the 60 conductor ribbon cable. I still have room for a few more pots too!
cost at volume to big companies such as Behringer, you really think something like 1/10th?
I used to work at Motorola designing cell phones and two way radios. When you are making a million phones a month, yes. Many common parts cost us about 10% of the single piece price from a distributor, and sometimes less. We had purchasing people whose full time job is negotiating cost, delivery and scheduling a particular "commodity" years in advance. I don't know how big Behringer is, or exactly how they buy parts, but I'm sure they get pots far cheaper than we do. The pots I used cost me about USD $1.20 each when I bought 50 of them from Mouser. I could believe that a large company gets them for $0.12 each.