I'm working on designing an audio line mixer (well, it's been on the shelf for a few months but I'd like to pick it up again) and I'd like to put in variable 4-pole high pass filters that would be controllable from a single knob to two filters (a stereo signal).
After I learned a bit about MOSFETs in a class last term, I tried simulating a filter with a MOSFET as a voltage controlled resistor, but I couldn't get it to work at a low enough frequency due to the limited resistance range. Since this is for audio applications, I'd like a filter that can go at least down to 30Hz, possibly as low as 20Hz. Upper limit isn't as critical, probably somewhere around 500Hz. I thought if I replaced the resistors in an active filter design with MOSFETs, and controlled those with a single source, it would accomplish it.
What other options would I have for varying two 4-pole filters with one control, without a lot of deviation in frequency between the two? I could go with an OTA-based filter, but that requires one OTA for each stage. Plus, a lot of the OTA filter info I've seen is kind of confusing. Any other common methods that could maybe use fewer components?
After I learned a bit about MOSFETs in a class last term, I tried simulating a filter with a MOSFET as a voltage controlled resistor, but I couldn't get it to work at a low enough frequency due to the limited resistance range. Since this is for audio applications, I'd like a filter that can go at least down to 30Hz, possibly as low as 20Hz. Upper limit isn't as critical, probably somewhere around 500Hz. I thought if I replaced the resistors in an active filter design with MOSFETs, and controlled those with a single source, it would accomplish it.
What other options would I have for varying two 4-pole filters with one control, without a lot of deviation in frequency between the two? I could go with an OTA-based filter, but that requires one OTA for each stage. Plus, a lot of the OTA filter info I've seen is kind of confusing. Any other common methods that could maybe use fewer components?