TB303 filter

[Hairy Harry]

The resonance feature has everything to do with the topology of the
filter.

The Moog Ladder filter and its ilk* have the property that as Q is 
increased, the peak stays at the same level as before, and the passband is 
reduced in volume.

The State Variable filter does the reverse... as Q is increased the peak 
gets louder and the passband stays at the same level.

Some hear the first as being "sounds the same" (not me) some call it
"bass is reduced as Q increases" (that's what I hear) but this is 
subjective.

The State Variable, because of the increased peak amplitude, IS likely to 
clip first. You need to reduce the input level as Q increases. The Ladder 
will not have this effect.

The MOST COOL filter in this regard (imho) is the Juergen Haible / MOTM 2040 
clone. Juergen has added a circuit to compensate the passband cut, which can 
be switched in and out. Way to go JH !

* ilk (n) 1: a small moose

H^) harry


>From: "Dan Gendreau" <gendreau at rochester.rr.com>
>To: "Jeroen Proveniers" <J.Proveniers at orga.nl>,        "synthDIY" 
><synth-diy at node12b53.a2000.nl>
>Subject: RE: TB303 filter
>Date: Mon, 31 Jul 2000 11:41:03 -0400
>
>-----Original Message-----
>From: Jeroen Proveniers
>
> >Hello,
> >recently I've made the TB303 filter just to try it out, but I have a
>problem.
> >When I turn up the resonance I also get a 'basscut' effect, the sound 
>gets
>quite thin. I
> >only used feedback to the filter, e.g only a single potentiometer, not a
>dual type, and I
> >only use the filter out (the upper side of the resonance pot). I used the
>Oakley TB3030
> >schematic. (I will build my own TB3030 very soon).
> >The TB303 uses some form of Q compensation. Without, will I get the 
>result
>I have?
> >So probably I must include a summing amplifier to sum filter out and
>resonance?
> >Anyway, I'm going to throw away this experimental board, and I'm planning
>to build the
> >Oakley VCF with a MIDI interface, but I just wanna know if I'm right...
> >JJ
>
>
>The filter in the TB3030 is based on the original Roland TB303. My 303
>sounds the same when you turn up the resonnance. The Q compensation you
>mention keeps the sound at the same relative volume level as you change the
>Resonance. Rather than making the knee of the filter louder(as Q usually
>does), it keeps the knee at the same loudness and reduces the low-pass
>level. This is one part of the character of the TB303.
>
>-Dan G.
>

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