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

Tom Wiltshire tom at electricdruid.net
Thu Aug 27 11:37:10 CEST 2015


On 27 Aug 2015, at 10:04, Mattias Rickardsson <mr at analogue.org> wrote:

> Hi Andrew et al.,
> 
> On 27 August 2015 at 10:24, Andrew Simper <andy at cytomic.com> wrote:
>> 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.
> 
> the nice thing with doing it the 303 way is that the resonance pot (or
> VCA) is doing both jobs; providing resonance feedback as well as
> boosting the output of the filter. No extra boost pot (or VCA) is
> needed before the filter. On the other hand, it might end up less
> "fat" than when adding the boost before the filter, as Richie just
> mentioned before.
> 
> When it comes to the gain drop, shouldn't it be possible to add more
> poles & phase shifts somewhere else in the frequency range in order to
> avoid some of the cancellations?
> 
> /mr

What happens if you stick an extra unity-gain inverter at the end of the filter?

Then you'd be able to use positive feedback for the resonance, and adding the input signal into the resonance VCA (or pot) wouldn't change the tone so much (since it'd be a simple sum, rather than the difference of the two signals). Does this make sense, or do I need more tea?

Tom




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