[sdiy] how to calculate cutoff in an OTA filter?

[Chris McDowell]

The subject says it. How do we calculate the cutoff frequency in a filter with OTAs, or VCAs for that matter. Maybe this is more obvious than I think but I'm not making sense out of it.

So for a state variable filter, if transconductance, gm = 1/R, then R = 1/gm, then why is the cutoff not just 1 / ( 2 * PI * (1/gm) * C) ? 

Cheers and thanks in advance,
Chris