420 go to used electronics graveyards and harvest opto-isolators every day.
The math may seem a bit heavy, but the circuits are pretty simple.
A DIGITAL MODEL OF THE BUCHLA LOWPASS-GATE
nothing in particular, but software is software. you know?
in analog, i think you can get plenty close with opamps, its just a part count/cost thing, and you lose some of the, i dunno, “grit”
and you can’t play your software mixer with a flashlight
sorry, but that’s the hand-waviest approach to modeling the vactrol that i can imagine. they just pop an integrator in there and modulate the integration time by the level a wee bit (like in any basic compressor plugin). then: “Adapting full models of photoconductor transient dynamics  for musical signal processing use is a significant task, and left for future work.”
most of the paper is about modelling the lowpass sallen-keyish circuit and the control path.
(and hey, nothing wrong with that, the simple model captures the broad behavior, just don’t go looking for too much special insight there)
in any case, i now feel extra lucky to have my little stash of VTL5C3.
analogue haven should pay you commission on this hahaha
spreading the love is payment enough. stole @tehn’s teletype studies trick – gating constant voltage w/ pulses into Mangrove’s AIR control opens fucks my shit right up.
Sold me on two. It was a “Hey, why DONT I have passive lpgs filling up the gaps” moment.
Btw, you can also buy stuff directly from MengQi, though shipping might be a bit cheaper from AH for people in the US…
Several Folktek designs (including the Mescaline and Matter) are also supposed to be full of vactrols but Mental (Mescaline’s Matter-like module) seems to be all SMD. Are there SMD vactrols ?
Not that I know of - but almost all euro contains some non-SMD elements like pots, sockets, power headers are often non-SMD, do the vactrols would be added at that point
there are SMD optocouplers, maybe there’s one with an appropriate response curve
I feel so incredibly fortunate that I snagged me a Natural Gate in that frenzy.
I have a few LPGs, had a few others in the past and nothing touches the Natural Gate. The sound is lovely, the controllability and playability are unmatched. It’s the only one where I consistently feel like I can use it with no envelope (unless I want a slowed attack). And it has that sort of accumulation effect, where rapid triggering makes it brighter than normal, which adds to its charm.
The Doepfer A-101-2 can be s almost similar in sound to Natural Gate, minus about 20% pixie dust but more rubbery and kind of… punchy in a mixed bag sort of way. The response on mine is SUPER SHORT and hyperexponential; you need a nicely controllable envelope to tame it. And it leaks quite a bit. But it has a reasonably comprehensive set of controls.
Lifeforms Dynamic Impulse Filter is also a non-vactrol LPG. It sounds nothing at all like other LPGs; it’s got a sort of saturated/driven sound that doesn’t suit some material but works a little magic on others – usually it needs a wavefolder in front to sound its best, and an envelope rather than a ping. The “dynamic impulse” bit itself is shorter than I’d like and interdependent with the other controls, so I tend not to use that input.
The LPG in Lifeforms Double Helix is like a stripped-down Dynamic Impulse Filter, but the entire Double Helix is haunted by crosstalk and other weird phantoms. The input runs through its wavefolder, which is tasty. The dynamics response is even less consistent than the DIF and it needs an envelope, but it’s easy to route one CV to hit both the LPG and wavefolder and then dial them in to taste.
Sputnik Quad VCF/VCA, which I sold to make room for Natural Gate, sounded good, with medium decay times. LPG and VCF modes had a closer-to-linear response than the VCA.
DPLPG had a sound I liked too – similar to the Sputnik but slightly longer decay – but being passive, didn’t work as well dynamically and couldn’t deal with slow attack envelopes.
I liked Make Noise Dynamix at first, but it kind of got boring for me. It’s more interesting than a VCA but still comes out flat somehow, compared to the rest.
As a circuit designer in 20xx I’ve always thought the popularity of vactrols was immensely stupid. You can trace the lineage thru Grant Richter of Wiard (whom I certainly do not think is stupid) who was the first modern designer to take up where Don’t Buchla left off. Autocorrect seems to have sensed my thoughts, as at this point I think the popularity of vactrols is an extension of the inertia in design. Aside from the issues in the supply chain, vactrols are now a novelty item and priced accordingly. £10 or similar for a dual is crazy when the same money could be spent on several SSM2164, an AD633 or even an AD534 or 734, or a THAT2180, which offer huge improvements in X and Y linearity, native exponential response in some cases, and 4 quadrant behaviour in others.
The desirable control response curve of the photocell is one point that is worth labouring on. I suspect the photocell has some memory effect, I’d be curious about the topology of the Natural Gate, which may even outperform the vactrol in this respect by specifically implementing a control curve modelled on real world phenomena.
Putting aside the belief in magic, which may not be completely misguided, control response in VCAs is interesting. It seems like some of the most advanced research has been done in DAWs to get “equal loudness” crossfade curves. I believe the psycho acoustic nonlinearity comes from the fact that loudness is perceived according to power, which is a squared term. An equal loudness contour needs to have the opposing curves sum to 1 when squared. Sine and cosine happen to do this perfectly (trig identity number 1). One of the more successful fader laws is based on this. The question is therefore one of engineering semantics, about whether to use an envelope with a linear slope and have the VCA shape it, or use a purely linear VCA and drive it from a sine source.
I’m wondering if this also marks the end of one of my favorite pieces of gear, the LA-2A. I have the real thing and the digital simulations and it still feels like ain’t nothing like the real thing baby.
This might be a rumor. But when I was a CTRL in Brooklyn they told me that make noise changed their design because the specific vactrols they used in the mmg became illegal in Europe. This might not be a “trend” but due to market control.
A bit off topic, but only because you mention that paper, I made a M4L device(s) based on it (mostly just adding a GUI and Ping device). http://maxforlive.com/library/device/4672/low-pass-gate