>> I've seen in many Sallen Key circuits people stick the input signal into >> various points to generate some different responses, but always the high >> pass is only 1 pole. > > i haven't seen that with the SK. for HPF, i've only seen it with the the > R's and C's swapped. like with > http://sim.okawa-denshi.jp/en/OPseikiHikeisan.htm .
I'm referring to the MS20 and Steiner Parker filters, and also copies of those (eg Doepfer A106), they don't do a 2 pole high pass, so can't generate a proper notch or peaking response. MS20 v2: You can see here in the first filter they stick the input into the base of the second cap (C2): http://machines.hyperreal.org/manufacturers/Korg/MS-synths/schematics/KLM-307.GIF Steiner Parker: Also here, although this is a partially differential design, they do the same thing, a bandpass is generated by putting the input to the base of the first cap, the high pass into the second cap (middle one) - which gives a correct bandpass response, but again only a 1 pole high pass: http://www.cgs.synth.net/modules/cgs35_syntha_vcf.html . The last cap is a differential input of the first one, so they both join to get filtered by the middle cap non-differentially. Andy -- dupswapdrop -- the music-dsp mailing list and website: subscription info, FAQ, source code archive, list archive, book reviews, dsp links http://music.columbia.edu/cmc/music-dsp http://music.columbia.edu/mailman/listinfo/music-dsp
