TB303 filter

[Tony Allgood]

> When I turn up the resonance I also get a 'basscut' effect, the sound gets
> quite thin.

This is typical of many a ladder filter, but does seem to be more pronounced
in the diode ladders. However, what you are actually hearing is the overall
pass band level being lowered as resonance increases. This does sound good
in many cases. The effect can be minimised by using the feedback signal from
the wiper of the res pot to boost the output of the filter. The TB303 does
this by simply adding the two signals together at the VCA. My SuperLadder
does the same but adds the signal in a simple op-amp mixer circuit.

In my VCF-1 design, I actually increase the level of the signal sent to the
filter in the first place. So at high res, the signal into the filter is
higher than at low resonances. Clipping levels remain unaltered in this
process, since clipping is more prevalent at low resonance settings. This
could be done with a VCA, or as I have done in the VCF-1, by a dual gang
pot. The disadvantage in doing this with a VCA is that additional circuitry
is placed in the audio path. By adding the resonance VCA in the output, like
the SuperLadder design, you can be sure that the VCA only has an effect at
higher resonance settings.

In my opinion the TB303 filter is lacking in comparison to the Moog Ladder.
That is noisier, thinner and not as musical. Although I have been very
impressed by the System-101 synth by Roland which uses the same diode ladder
arrangement as in the TB303.

Further details on my site: www.techrepairs.freeserve.co.uk/projects.htm

Cheers,

Tony