The Curtis CEM3320 IC was a much loved VCF-in-a-chip in its 80s heyday. Known for its smooth operation, it was used in a whole heap of classic synths: Sequential Circuits used it in a bunch of theirs, the Crumar Spirit employed it, the PPG Wave 2, the LinnDrum and Linn 9000, a handful of Oberheims, and on and on. This 4-pole 24 dB/Oct low pass-filter is what gave so many of those synths a large part of their character, their X factor. AI Synthesis has taken that chip, or a modern reproduction of it at least, and followed in their footsteps/circuit paths, with their new AI024 X filter.
AI Synthesis already has one filter in their line, the AI0004 OTA filter (the same structure found in the KORG MS-20), so adding another with a different flavor is a nice addition. Looking at the AI024 X filter and its two PCBs, it would make a great DIY project for a beginner or seasoned pro. There's plenty of space for all the through-hole components without the need to cram anything, and the kit even includes chip sockets for the 3320 IC and the TL074 op amp. If you’ve ever fried any chips soldering them directly to the board, you too will appreciate these chips, which can save you some heartache. The module I received for testing came pre-built, so there were no tears or wounded chips on my end, but so long as I was even remotely confident with a soldering iron, I wouldn't hesitate to put this one together.
There's not a whole lot to the architecture of 024. There is a single Input and Output, and for controls there's a Cutoff and a Resonance, with CV inputs and attenuators for both. 024 can self-oscillate and you can use it as a trackable VCO with its 1V/Oct input for this purpose, though you may have to calibrate it via a trimmer on the back of the module, as I did. It took a minute to dial this in, as noted in the build guide, but the calibration was pretty simple.
While there's nothing extravagant here—no crazy faceplate design or flashing lights—024 doesn’t disappoint with its sound; it's smooth, like butter. It also seemed to hold on to lower notes better than a lot of other filters, with the low end of my signal being mostly retained. Modulation of the Cutoff CV with a sawtooth wave for both the input signal and modulation signal, and with the resonance high, 024 produced some beautiful filter sweeps, slightly stepped with plenty of character and an overall warm tone. While 024 doesn’t necessarily howl, it can definitely produce some nice whining; two words not normally associated with each other, but sonically pleasing in this instance, though if you want a growly, mean filter, you might want to look elsewhere.
I found the controls on 024 to be really responsive, and unlike a lot of unruly filters that are out there, it was very reasonable to handle and satisfying, too. There are a few sweet spots here. My favorite being in the top end of the frequency sweep with the resonance at a moderate amount, at the point where 024 would just start stepping out; literally and figuratively. It steps so nicely, it’s no wonder all of those famous synths of yesteryear used the 3320 chip in their designs.
Abe Ingle keeps adding excellent, classic-based modules to his AI Synthesis line. Whether you’re just starting out looking to DIY your modular into existence, or you’re looking for more flavors in the form of a VCF to add to your rig, the AI024 X filter could be your X factor, too.

Price: From $15-$140

aisynthesis.com