Polyfusion VCF High Frequency Compensation circuit

[Donald Johnson]

Greetings,

(I asked Kevin Lightner about this, and he suggested I follow-up
 with a post here.)

Has anyone examined the Polyfusion LP and HP filters? It looks
like the HP filter has the correct High Frequency Compensation,
while the LP filter has 'inverted' High Frequency Compensation
(High Frequency De-Compensation). Is this right?

 thanks,
 don.

> Date sent:      	Fri, 14 Jul 2000 16:09:44 -0700
> To:             	"Donald Johnson" <djohnson at mdsi.bc.ca>
> From:           	Kevin Lightner <synthfool at synthfool.com>
> Subject:        	Re: Polyfusion Lowpass filter schematic
> 
> >Greetings,
> >First let me say thank you for publishing so much useful
> >information at Synthfool.
> >
> >I am in the process of building a VCO and found the Polyfusion
> >filter schematics on your website:
> >
> >   Polyfusion Low Pass VCF (134kjpg)
> >     http://www.synthfool.com/schematics/polyvcf.jpg
> >
> >   Polyfusion High Pass VCF (89kgif)
> >     http://www.synthfool.com/schematics/polyvcfhi.gif
> >
> >Do you know if these are correct?
> 
> 
> Yes, I've built them.
> 
> I used different FETs though.
> 
> >
> >I expected the H.F. Comp. in both schematics to feed into
> >the first control voltage summing opamp. The HP does, but the
> >LP does not. Wouldn't it provide a 'decompensation' effect in
> >the LP filter?
> >
> >I thought I'd ask you first, and if you don't know, then I'll
> >post to synth-diy.
> 
> Offhand I don't know why. I haven't really looked at the circuit 
> topology all that closely.
> They may have an issue with resonance levels vs signal levels in the 
> LP or may have another reason in mind: Polyfusion used the same PCB's 
> for both designs with extra pads allowing for the other response.
> They may have had a compromise because of this. Hard to say.
> I'd bounce it off of the DIY list.
> When I built them it was because I had the PCB's, so it wasn't so 
> much of a learning experience as it was a construction effort :)
> 
> Best-
> -Kevin
> http://www.synthfool.com
> 
> PS: Thanks for your kind comments regarding my site. :)))

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