Corsynth is a company based in Spain that has just introduced their first product: the MU-formatted C101 OTA-based lowpass filter. Nice to see a new player in the game, especially in the 5U formats, which the past few years has been taking a back seat to the Euro-format world.
OTA-based filters have a different sound and different characteristics from the more widely known transistor ladder filter, a design and sound usually associated with Moog. The classic Roland analog synths such as the System 100, the Jupiter-4 and -8, and the Junos used OTA-based filters, and so people who hear an OTA-based filter often describe it as the "Roland sound". OTA filters aren't very common in the modular world; the only other one I know of currently available in any 5U format is the Synth Tech MOTM-440, and it's not really a "typical" OTA filter because Paul Schreiber designed his own discrete OTA circuit for it. I have a 440 and it's a rather different beast. Judging from the sound samples on Corsynth's Web site, the C101 captures more of the classic, smooth Roland filter sound.
Here's a photo of the module, borrowed from their Web site:
As you can see, it has some interesting features. The most important ones are the voltage-controlled resonance and the 1V/octave control input. Getting any analog VCF to properly scale to 1V/octave is no mean feat, and it allows for some interesting tricks. There's obviously using the VCF as a VCO by turning the resonance up into self-oscillation, but the 1V/octave input also allows for things like harmonic tracking, e.g., you can run the output of a VCO into it, and feed the VCO's control voltage into the 1V/octave input. Then, you set the cutoff knob so that the cutoff centers on a harmonic of the VCO's output, and the filter will stay with that harmonic as the VCO frequency varies.
The two FM inputs, each with its own attentuators, will save you from having to use a CV mixer when doing complicated modulation. There's the two audio inputs, one with an attentuator, and the oversize cutoff frequency knob is a nice touch. Panel graphics are legible and easy to understand.
This thread at Muff's has a good discussion, and more photos, including a photo from the rear. The physical layout is interesting. There's a board that sits behind and parallel to the panel, Dotcom-style. But unlike most of the Synthesizers.com modules, this board carries only the panel controls. The core of the circuitry is on a daughter board behind the panel board, and connected to it via a ribbon cable. Probably the main motivation for doing this was to make it possible to adapt the design to other formats. The construction appear to be all through-hole, with all of the ICs socketed, which makes for easier repairs and mods. The board has both Dotcom and MOTM power headers. I've asked a question on the thread about what OTA was actually used, if it was the LM13700 or something else. (Unfortunately the CA3080, which was used for this type of application for years, is out of production and no longer available.)
The C101 is a pretty good-looking module, and a type of circuit that doesn't appear often in the modular world. The Web site lists the module at a price of E210, which currently translates into U.S. dollars at $290.