[sdiy] Ladder filters and gain drop, that old chestnut

Andrew Simper andy at cytomic.com
Thu Aug 27 10:24:24 CEST 2015

If you check the 303 schematic you will see gain compensation at the
output, there is a boost in the output signal level with increased
resonance to make it not as quiet. You can do this boost anywhere you
want, at the input, at the output, or a bit of both. Another trick is
to feed the input signal into the same resonance gain circuit, but
this changes the frequency response of the resonance signal, making it
high pass instead of low pass, so this changes the tone considerably
of the resonance, and so the filter.



On 27 August 2015 at 05:11, Gordonjcp <gordonjcp at gjcp.net> wrote:
> I've been playing around with a diode ladder filter a bit recently, and it really does seem to lose a hell of a lot of gain at moderate resonance settings.  Now the low output level isn't a hassle since I've got the potential for enough gain in the following VCA stage.  It would be nice, though, if the output didn't go so nuts at *low* resonance settings - I estimate there's a good 10dB between self-oscillation and "no feedback".
> So I was wondering, what influences this?  I noticed that the TB303 doesn't seem to suffer from it so badly, among others.
> If I set the feedback level to be constant and quite high, would controlling the resonance by reducing the gain in the differential amp give a decent swing of resonance without the output increasing to silly amounts?
> --
> Gordonjcp MM0YEQ
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