[sdiy] 36dB VCF output drop while increasing emphases

[Adam Inglis]

Shame the Electrix people didn't pay attention to this when they made  
the 2164-based Filter Factory. It has really nasty unmusical resonance  
at high settings and in self-oscillation.


Adam

On 20/04/2015, at 7:15 AM, David G Dixon wrote:
> You are right that the 2164 is not
> the best choice for feedback in this type of filter, because the  
> exponential
> response is exactly the opposite of what you want.  However, I  
> overcame this
> by actually using the 2164 to subtract signal from the chain, in  
> what I call
> the "reverse exponential response", and I believe that this gives a  
> much
> smoother approach to high resonance than even linear amps.  In other  
> words,
> the Doc Oc is hardwired to be in self-oscillation, and the resonance  
> amps
> decrease the overall gain from 4 to 1.
>
> As far as soft clipping, the latest incarnation of the Doc Oc uses  
> zener
> clippers on the input.  These conduct when the overall gain of the  
> input
> signal is pushed above unity.  The filter itself softens this  
> clipping.
> There are zener limiters on the feedback loop, but these are to  
> limit the
> magnitude of the sine waves in self-oscillation to 10Vpp -- they  
> really have
> little or no effect on the filter when it is used as a filter.  The  
> flat
> tops of the resulting fed-back sine waves are filtered out by the  
> filter
> stages.  Of course, the sine wave from the fourth stage is the  
> cleanest of
> all, but they're all pretty clean nonetheless.