Ahoy!
This time, I wanted to refine the quick prototyping idea discussed midway through this previous post:
- Minimal breadboarding--I hate breadboards
- Hookup wires for fast modifications/fixes to switches, input jacks, and potentiometers using JST connectors
- Panel mounted hardware
- SMD 1206 parts--easy to place, easy to change; heat the part up, remove it, drop another value in.
- SOIC IC's
- PAIA Frac Format--bigger than dinky Eurorack--not as rotunce as large format.
The idea: if I came up with something really cool I would port it to another format....Frac isn't dead, it just smells funny.
This time I put this workflow to the test with EFM's take on Korg's 770 VCF:
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| Frac me up Scotty? Nope. |
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| A real 770....could the 1999 VCF design from DIY pioneer Tom Gamble get me the sound of classic 700/770 synthesizers? |
I heard a B3 guy play a Korg 770 at a music festival a few years ago and thought it sounded--bodacious.
So how did this project go?
The good news: the entire fabrication/modification process went much much quicker than using through-hole, skiff construction, or breadboards.
Fabrication of the module in its entirety--this was not a super low parts count VCF--took about two hours.
Since the quick prototyping challenge was met I could make simple changes to the EFM design post-fabrication fast, fast, fast.
The bad news: this particular 770 filter sounded really lame.
Yeah, I made modifications and swapped in different component values trying to bring this filter into the land of the living; I banged away on the prototype for about three hours then gave up.
PASS ON LOWPASS
I couldn't get LowPass section to completely block audio without greatly impacting its overall frequency response.
So either I ended up with a constant "wah" or a modulated maximum cutoff frequency that was too low to be audibly pleasing.
This drove me nuts.
Changing rail voltages altered this poor performance, so I tried doing wacky things with diodes to drop the power rails by .6, 1.2, 1.8 etc volts.
This helped but I never got things to a place I liked.
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| Diode between rails and V--; instant drop from -15V to -14.3 |
The LP filter input brought the audio signal down at output about 16db (!); output amplitude got slightly better with a decent amount of resonance added; but amplifying the circuits output back to near unity with an additional op amp made the output too noisy/hummy/buzzy.
If I cranked the "Q" pot past about 80% I got horrible sounding distortion at output. This was not cool MS20 Daft Punk distortion: this was butt-ugly distortion.
The bandpass setting, which I don't recall a Korg 770 having, sounding, well, oddly bad ("badpass"?)
The highpass sounded (almost) OK? It might have made sense to ditch the LP and BP sections and just focus on that with some mods? Well, maybe, but maybe not, the HP didn't sound that good.
Overall the filter had a feminine quality, was a Seinfeld low talker, and didn't punch; to me, it definitely didn't capture any of the je ne sais quoi of old school King Kong Korg.
FABRO-CATION
However the fabrication workflow felt like a win and I might do it again.
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| Boards in bags from the blog's faithful sponsor, PCBWAY. Even when my projects sound really bad these guys do really good. Help out this blog and check 'em out. |
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| Project consisted of 3 PCB's: front panel, jacks board, and main board. Also a cheapo stratocaster guitar pickup selector to choose LP/HP/BP. Why not? |
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| The jacks board is 4x 35RAP Switchcraft 3.5mm mono jacks, spaced 600 mil apart. This small PCB might find its way into other projects. |
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| I used the SMD goo-n-tweezers + hotplate for the lions' share of soldering. Read more about this solder technique midway through the previous post here. This is the "before" picture: sloppy, right? |
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| Ah! better! The goo-n-tweezers method is pretty forgiving since the parts center themselves during soldering. I had to solder wick one side of the op amp and resolder D2 by hand, other than that, all the soldering came out great. |
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| Next I added the hookup wire and panel mount pots. I had everything right the first time, no pots where CCW was 100% , no input jacks hard-wired to outputs. |
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KORG ME UP, SCOTTY?
So, where do we go from here? If I felt I could get anywhere near the cool Korg 7xx series sound with this VCF I would have kept working. But I wasn't on the same planet.
I put kicads, gerbers, libraries etc. on github: here.
If anyone wants to fork this dork, or do whatever you want: do whatever you want.
I also posted the jacks board (which i figure I will use in other projects) to the PCBWAY community site: go here.
I am going to strip this one down for its reusable parts and move on. This module's death may allow others to live.
Over and out captain. See ya next time.
