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I grew up on 70's rock and roll, by 1980 I was in college and Reagan was president. The latter was bad news, but the good news: cool synths everywhere. One of my favorites: Sequential Pro-One and yes, it had a hyphen.
Back then I was the cool synth kid, playing in a remarkably lucrative band, I had a 5 and a 600, which both sounded great, and damn, the ProOne had that Sequential Spirit.
I see Pro-One's go for $1500-2000USD; I'm not going to buy one, I have too much vintage gear already, instead I cloned its VCF, a big part of its sound:
This is revision 3 of the initial design (here) and is almost 100% where I want it; still some minor adjustments needed for the resonance, but is otherwise good to go, and it sounds, well I think anyway, great.
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For the front panel, to nail some of the 80's design look n' feel, I used a graphics technique shown to me by one of the most talented fabricators I know: Elton at Otter Mods. Read about the SVG to Kicad graphics process here.
I will skip the rest of the usual boring fab photos, instead jumping right to the test phase:
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| worked first time (WFT), but at rev 3, it should, right? |
Altering the power rails changed the overall sound of the filter, especially bringing the V- rail to say -9 or -10VDC. Raising the rails to +/-15V didn't make a big change which surprised me a bit. I might create a starvation add-on for the negative rail for further expermentation? Stay tuned.
The one remaining issue with the filter is that the resonance, when cranked to 11, doesn't create that nice whistle-like sine wave I remember on the beloved Pro-One; instead, it creates a crunchy distortion, a bit like an MS20 filter with it's "peak" cranked way up.
that's not right, I probably missed a value somewhere?
I didn't need another crunchy VCF so I changed its current limiting resonance resistor from 200K to 330K, which topped out the resonance sound to more like a 12db/octave filter. Regardless, this filter has the nice 24db/octave Sequential fatness.
In the future I will try to chase this rez issue down, but for now I have another good sounding filter without paying a kings ransom for a vintage synth I definitely don't need.
OK--That's it for this one. Filter well and live. Until next time: don't breathe-the fumes.
