diyAudio - Entries for March 20, 2013
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Active elliptic anti-imaging filter

Posted 20th March 2013 at 03:18 AM by abraxalito
Updated 20th March 2013 at 03:30 AM by abraxalito (Added FR)

Passive filters rock for SQ, no doubt about it but I'm still curious how good sounding an active DAC I/V post filter might be. So I've figured out an almost equivalent FR active version of my 7th order LC elliptic filter. This active elliptic has been designed using LTSpice's FilterCad program giving the pole/zero positions, then the Williams handbook of filter design helped me translate those numbers into a working circuit. Its using what Williams calls the VCVS 2nd order section based on a twin-T network to realize the zeroes.

My first attempt at an active elliptic filter was using gyrators but that proved very hard (practically impossible) to get stable with CFB opamps due to their HF gain peaking. VFB opamps I ruled out at the start for inadequate SQ - its not hard to make gyrators stable with them. Hence this approach which promises to work with CFBs though I'd guess I'll probably need to add series Rs between the stages in practice. Nothing built yet but thought I'd...
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