[sdiy] SVF High Pass Mode not Cutting Off

[rob at emulatorarchive.com]

Hi
Thanks for the help, I tried lower integrator cap values (220pF instead 
of 470pF) but without any change. I have plenty of Freq range. on the 
pot. A sawtooth waveform is "squashed" to a flat line as Freq is turned 
up but a very sharp spike is left at the trailing edge of the saw no 
matter what the VCO freq is. Any thoughts?

Thansk - Rob


s On 29/10/2011 13:59, Magnus Danielson wrote:
> On 10/29/2011 02:51 PM, rob at emulatorarchive.com wrote:
>> Hello list,
>> I have recently built a couple of 2-pole State Variable Filters (OTA
>> designs either using BA6110 IC or transistor current mirror OTA) and
>> both have the same problem that in Low Pass mode the frequency cutoff is
>> perfect and does its job, but in High Pass mode the filter will not
>> cutoff and go into silence. Freq pot does modify the cutoff in HP, but
>> not to full cutoff. Any ideas on whats wrong would be most gratefully
>> received.
>
> Sounds like the upper frequency limit is set too low, such that you 
> saturate too early. Shift the filter capacitors to move the filtere up 
> in frequency should require less current for the same frequency and 
> hence the saturation current would give a high cut-off to "close" the 
> filter.
>
> So, try swapping in smaller integrator capacitors to achieve this.
>
> Cheers,
> Magnus
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