Before my WP arrives, I’ve been trying to find as much information as possible about the design. In one video MQ mentions the chip used for the filters as the 2144. It turns out he’s referring to the SSI2144 chip–a 4 pole filter on an IC. The SSI2144 is a modern update of the SSM2044, a filter chip used extensively in the 80’s. The topology is a 4 pole ladder filter, and the original design was by Dave Rossum (using Bob Moog’s ladder topology) .The 2044 was considered one of the best filters available on a chip.
The datasheet has some interesting information on the behavior of the filter and some design applications. As with the Moog ladder filter the frequency band below the cutoff is attenuated with increasing Q, but they suggest a circuit that increases the filter input as the Q is increased. I’d guess MQ has implemented this–you’d want to dial in a high Q without attenuating the pass band too much. Just a guess though.
It would be interesting to know if MQ tested multiple type of filter topologies when he was prototyping the WP. It’s also possible he started with SSI2144 because it’s simpler than building a discrete circuit, and ended up liking the behavior of the filter.
The SSI2144 is used in a few eurorack VCF’s, but I don’t think it’s widely used in modular. When Dave Rossum designed his Evolution filter, he based it on the 1100 filter, an earlier ladder design he’d done. It’s worth reading about the Evolution design, because the SSI2144 has similar characteristics.
Wing Pinger arrived. Oh my gosh, this is one beautiful instrument. No question, Meng Qi inhabits the instrument like a mischievous spirit.