Spiracle
Finishing Elytra gave me some confidence when it comes to designing and working with PCBs. This is why I didn’t shy away when the idea for Spiracle came to mind: A 4 channel, fully analog audio mixer. The idea spawned from necessity; I have a bunch of hardware instruments, but my audio interface only has 1 input. My brain kicked into the same thought process it did for Elytra: Why buy when you can DIY.
I started by learning a bit more about opamps. I did some research when I started work on synth0, but I didn’t get deep enough to be able to design my own mixer.
I did still have some opamps left over from synth0, so with the help of my scope and function generator I started testing my design on a breadboard. I started small by just testing a follower like in the picture and built from there.
Once I was happy with the circuit design, I started designing the PCB. Just like with Elytra, I went with the curved tracks:
But as you may have noticed, I added teardrops on the holes and pads for extra roundness. And that’s right! Pads! I dared to do SMD work for the first time.
My local electronics store sells 1206 SMD components. Since they’re quite big, I was able to solder them by hand, using an iron. Not the cleanest job, but as I have learned, using lead free solder is quite a bit more difficult than leaded.
My hopes were that I would solder it all up and it would just work. But that was not the case. When I first tried it, all it did was nothing but heat up quickly. It took a few hours of debugging to figure out what was wrong: I soldered the battery leads backwards. And I will not admit fault. I will blame KiCad. The battery header footprint in KiCad is what confused me.
Turns out, the pin which is filled in black in the silkscreen, is marked with a triangle and uses a square pad is in fact the + voltage pin, and not ground. I’ve grown to learn that the square (as opposed to round) pad represents ground, and every other clue was pointing me in that direction too. But no, it was backwards, so I was feeding the opamps reverse voltage.
Thankfully, no opamps died from that, or so I thought. The right channel of the master bus didn’t work. Which is really weird, because the left channel was perfectly working. I couldn’t have half-killed the chip could I?
After many more hours of debugging it was left as my only option. Fixing it would involve buying a new NE5532 chip, desoldering the old one and putting in a new one. Which is easier said than done.
During summer break, I stayed with my parents, which don’t have any electronics stores nearby. This meant I had to wait until I moved back to the city where I go to university until I could get a new chip.
Cleanly desoldering an 8 legged chip soldered with lead free solder is also not super easy with my little plastic solder pump.
This year I became an intern at my uni’s IT services, which is next to the lab technicians’ office/workshop. Since there’s good vibes between our two departments, I asked to use their desoldering gun, and they kindly let me.
This made the job a breeze. Hold the tip over the pad, press the button that engages the pump, and the pin is free.
I tried running audio through the board after replacing the chip it and… It worked. Turns out the chip was half broken, which to me is super weird. If one opamp got cooked, shouldn’t the other one have gotten cooked too? They’re on the same die!
Now that it was fully working I could print the fader caps (is that their name?) and the power button cap, as well as attach the battery holders to the back.
And about the back! I have to give a special thanks to arf20. They provided the faders, which are really great quality faders used in professional mixers. This saved me quite a bit of cost, since they’re quite pricey. This is why I added a little “sponsored by arfnet” badge on the silkscreen:
And yes, that’s a caterpillar. Just like with Elytra, I wanted to have a big artwork on the silkscreen. This time I asked my friend flow to make something for me. I wanted it to be bug inspired again, so I asked for a caterpillar. flow took it quite literally.
Overall I’m happy with how it turned out. It sounds crystal clear, and If you overdrive it, it responds non-linearly, which adds some nice warm saturation. Here’s a lazy little demo I made for this article:
I also made a case out of laser-cut plywood and 3D printed parts using the laser cutter at uni:
I designed it so it’s the minimum volume possible:
And the 3D printed bottom pieces are printed flat and then folded:
