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?

 

A Mosfet of one part number has a wide range of spec's so matching two or four to make an adjustable frequency filter is almost impossible.
All IC tone controls filters have matched transistors. Most are single pole, a few are two poles but none are four poles.

 

So no suggestions? I'd already ruled out the MOSFET idea.

The only other idea is cascading 4 stages like this one from the NE5517 datasheet, with Vc of all 4 fed from one source:

 

Hello,

You could also have a look at a "switching capacitor filter" :
http://www.swarthmore.edu/NatSci/echeeve1/Ref/FilterBkgrnd/SwitchedCap.html

Bertus

 

I was under the impression switched-capacitor filters weren't generally used in audio applications because of noise.

 

My very low distortion oscillator uses a switched capacitor filter IC and the noise level is not measureable. The clock is 100 times the cutoff frequency so a simple single RC removes its already low level.

 

Interesting....thanks. I'll look into that.