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Modules I'm working on: VCO, VCF, Teensy 3.6 multitool, etc.
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Modules I'm working on: VCO, VCF, Teensy 3.6 multitool, etc.
MickMad
Hey everyone,

I've been working on some Eurorack modules for the past year or so and I want to share working prototypes with you synth-heads smile

So far I've made the following:

- CORE, a 21HP digital multitool based on the Teensy 3.6, with 8 CV outs, 8 GATE outs, 1 SYNC IN, 1 SYNC out, USB device port, MIDI in, MIDI out, 4 encoders and a 2.2" display
- AVCO, an 8HP VCO based on the AS3340 (CEM3340 reissue), with 3 tuning contols (OCTAVE, COARSE, FINE), CV in and a switch to select between normal frequency range and LFO range, FM in with attenuator and switch for lin/exp FM, SYNC in with switch for Hard/Soft, PWM in with attenuator and manual control for initial PW, and 4 waveform outputs (pulse, saw, tri, sine)
- AVCF, a 10HP VCF based on the AS3320 (CEM3320 reissue) in 24dB/oct LPF mode, with a 3 channel input mixer, switches to enable/disable channels 2 and 3, manual FREQ and RES controls, and FREQ in, ENV in and RES in with attenuators
- 2AVCA, an 8HP dual VCA based on the AS3360 (CEM3360 reissue), each VCA has an IN attenuator, a CV input with attenuator and switch for lin/exp control, and a manual BIAS control for the volume.

As a necessity, I also designed a 2HP passive mult, and a 4HP active mult with hi precision, transparent buffers (called PM and AM respectively).

Here's a quick video where I use the CORE to send a (very familiar) sequence to 2 AVCOs and to send an envelope to both AVCF and one 2AVCA channel; the notes sent from the CORE, as well as its ADSR parameters, are controlled via USB-MIDI through Reason.



There's still some refinements I got to do on the modules, for example: the AS3360 doesn't like using the exponential control scheme reported in the original CEM3360 datasheet, so I might either have to remove the feature or change chip; the AVCO when used in LFO mode glitches a bit when I turn the main frequency control knob all the way up, I guess it's a reference current issue that doesn't like seeing a timing cap 50 times bigger than the usual one, but I guess it's also a non issue since the LFO should be used in, well, low frequencies; also, I don't really like the soft sync feature on the 3340 and I might as well remove that and put a negative/positive/both polarity selector instead; the filter is pretty much spot on expect for a couple of PCB quirks; the CORE on the other hand requires still a lot of development on the software side, and maybe on the hardware side too since I am currently using the Teensy's own 2 DACs, with 12 bits resolution, that I then mux and S&H out to 4 channel each (total of 8), and these DACs don't really like being clocked at more than 48KHz so that gives a final sample rate of about 12KHz for each channel, so yeah I might switch to more expensive 16 bits DACS or at least to faster DACs for a better SNR.

Anyway, I really hope you'll like them smile I'm planning a crowdfunding for these puppies (I'd start with the VCO) and your feedback will be really, really appreciated

Cheers,
Mick

EDIT: I finally managed to embed the video in the post, thank you @melodydad
EDIT 2: changed the title to something more relevant
Jaytee
Are these going to be available as DIY projects? I assume so, since you posted it here, but it wouldn’t be the first time someone came here to show off prototype modules they DIYed but were planned to be sold as complete modules.
MickMad
Yes, my idea is to propose barebone PCB, partially assembled PCB for the less experienced (I use mainly 0603 passives, SOT-23 transistors and diodes where needed, SOICs for most ICs with the eventual QFN package), and fully assembled modules. There would be a panel option for the DIY versions too, of course, but I would also release the panel layout so that anyone could make their own panels too, in case they don't like PCB panels or in case they don't like my design (I hope that's not the case smile )
Jaytee
Amazing, sounds like you have all the bases covered. Can’t wait to see more.
Dave Kendall
thumbs up
rctr
Looks very interesting!
MickMad
Hi there, I got a small update about the VCO, I am planning for manufacturing and I am looking at some improvements.

First one is that I am dropping the LFO switch and its associated bigger timing cap, the big timing cap doesn't work properly at high frequencies (probably due to the wrong reference current which is set for a 1nF cap), and more importantly because the 1nF cap alone works at extremely low frequencies. Here's a pic of the triangle out at the lowest frequency setting with the normal timing cap selected, no CV in:



My scope's time scale is not big enough to hold not even HALF of the wave cycle, so considering that as you can see in the picture it took about 500s (10 timescales x 50s) to drop from 5V to 3V (1/5th of the total 10Vpp output), it would take 2500 seconds or about 41 and a half minute to complete a cycle... that is 0.0004 Hz!

Second one is about the sync, as I mentioned in the first post I might modify the current sync circuit and fix it to Hard Sync, I personally don't use the soft sync, and I don't actually like how it sounds, so to get more gnarly sounds I am thinking of either keeping the switch and use it to select between positive hard sync and negative hard sync, or see if I can source a ON-ON-ON switch to do a positive/negative/both polarities sync. Meanwell has one in their sub-miniature switches catalogue, the same ones stocked by Thonk; I already asked Steve if he could source them but he is really busy so it's going to take a while before he gets back to me. In the meantime, I'll hack a circuit on one of the prototypes to see how it sounds.

Lastly, I would also like to know how many people would be interested in bare PCBs VS PCBs with preassembled SMD VS fully assembled module; I want to see how many people are really interested in an intermediate-to-advanced SMD DIY project with tight components placements and with somewhat small parts (99% of the passives are 0603s, there's a couple of 0805s, a couple of SOT-23s, then SOICs for the ICs, and small SMD trimmers too, with the Bourns 3224w package); this is a board best done with a solder paste syringe and a hot air gun or reflow oven.
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