So I decided to get started building a synth, but wasn’t interested in modular necessarily; I own no other instruments, and don’t plan on buying any. So the first promising enclosure that popped up was this vintage aluminum case for storing old slides, 13x7x3.
My plan was to slowly build and drill holes as schematics caught my eye, just using small stripboard that I was treating as though were eurorack modules (run off of a eurorack PSU, individual power cables [not any synth standard, just 3-pin keyed 2.54mm], and all standardized sizes so I could stack, swap, etc).
I was planning on using the AI Synthesis design and used the BOM from there specifically, but realized I didn’t feel comfortable putting the secondary of the wall wart through the metal case with the jack internal, and built the MFOS wall wart schematic with a plastic insulated jack external instead. Was I right to be nervous about that, or am I being paranoid? (The outside of the secondary was my virtual ground in both cases, but my assumption is that a virtual ground fails to work as such if the + or - is lost.)
My lingering concern is my virtual ground is still through my case due to contact with pots/jacks after power up, and last night I noticed touching a jack socket affected the sound…
Conveniently, the same thrift store that had this thing, also had a brand new 4u 84hp rack yesterday, so is an all-metal case with a virtual ground, wall wart-based PSU not possible to design safely and I should scrap it now? This is more like what I want to build than a CGS/Serge clone, but thrift fate is weird like that.
Ok that was serious safety stuff, but on top of that, I’ve built a dual VCO based on Moritz Klein’s (exactly his, minus thermistors and plus one circuit using an open input on the 40106), and noticed the frequency pots affect BOTH oscillators, to a lesser extent on the non-connected pot, but substantial.
I decided to build a simple dual version of the LFO by Nicolas on electro-music, having built one already on non-stripboard before I settled on format & hoping this would be an easy success after the VCOs… only to find that it, too, has surprising results.
With a cheap scope, the saw is OK sometimes, but the skew pot response is extreme and collapses the wave with a lot of rotation left; the pulse has a harsh saw spike at its start, and has similar response to skew changes, except reaches a point after the pulse collapses that it produces negative voltage spikes. Which is extra confusing because it’s 2 copies of the same previously tested circuit, just cleaner now that I’ve gotten better at soldering & on one strip board (completely separated vertically, no missing cuts between them or anything) and they both behave the exact same (wrong) way.
Which brings me back to my DIY enclosure/initial plan for the whole thing, because the common things to all 3 circuits are the case, the power supply, and the power filtering circuit I copied and pre-loaded onto DIYLC for my stripboard layouts, which is 1N4007 diodes for polarity protection, 2 10ohm resistors for short protection, 47uf electrolytic caps and 104s for decoupling, with 104s to ground at all ICs power, as close as possible.
North Coast’s article on design mistakes mentions that the resistors as fake fuses are obviously unnecessary for diy if you check for shorts before powering, and might actually cause issues with power regulators, like in my DIY PSU, so my thoughts so far are take the resistors out, but I’m still too worried about the enclosure to want to power it up and test.
I know that’s a lot, but I generally get a little too ambitious and rotate around projects if I’m stumped and the problem seems liveable; the case/ground anxiety has that rotation stopped, so if you only had time to answer one (and a half) question after reading that novel, it’s: is my enclosure dangerous? If so, how is all that hardware mounted to an aluminum modular panel any different?
If I know the enclosure is safe, I’m going to hook up a meter and scope and watch for fluctuations at the regulators on the PSU, and remove the 10ohm resistors if so; barring missed cuts/bad joints/misplaced components, which I periodically re-check and haven’t yet noticed any issues with, are there other things I should be checking? My intuition still says it’s a greater issue than the circuit itself.
Thank you for reading, and for any insight y’all might have to share! I’ll be reading up on esoteric 4U stuff in the down time…