AW,2:Re: AW: Another VCF Finished

[gstopp at fibermux.com]

     So you are saying that a working unity-gain OTA integrator can be 
     built as follows:
     
     input to 3080 (+) input
     3080 output to cap to buffer
     buffer output to 3080 (-) input
     
     without the 100K/220 linear-mode input resistor dividers on the 3080 
     inputs? Is this to utilize the negative feedback to keep the 
     differential input voltage on the 3080 to zero, thereby eliminating 
     any non-linearities? Is there any danger to the 3080, during 
     slew-limited clipping for example?
     
     Curious minds want to know.
     
     Also regarding feedback in the highpass cascaded-differentiator 
     filter, there will be some corner peaking, but not nearly as 
     well-defined as in the lowpass case, before the high-frequency 
     oscillation begins. Note also that the ultrasonic oscillation is 
     inaudible, although it may wreak havoc on tape deck record head bias 
     oscillators during recording!
     
     I haven't actually built a 4-pole highpass myself so all this talk is 
     from reading. It is also interesting that the CEM 3320 data sheets 
     show resonance control in the highpass configuration - maybe I'll go 
     back and read about that.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: AW,2:Re: AW: Another VCF Finished
Author:  Haible_Juergen#Tel2743 <HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de>
at ccrelayout
Date:    4/16/96 12:20 PM

>You can handle large signals without any attenuating resistors with this 
>design, so signal-to-noise ratios can be tens of desibels better than with 
>the other designs. Also low frequency distortion is lower with this design.
     
Very good point. Though it's not "ten's of decibels" IMO, the effect 
is remarkable. It's like a built-in preemphase / deemphase system. 
Many OTA-based phasers also take advantage from this.
     
> Wrong! What about the resonating high-pass filter in Korg MS-20 ?
     
The MS-20 High pass is a completely different story, as it is not a 
multiple-Lag-with-overall-feedback design.

Anyway, resonance on the OTA-design Gene is speaking of *is* 
possible to some degree. If memory serves, the FORMANT filter 
(which seems to be very similar to the one which Gene built), 
does provide resonance in HP mode. But resonance on these
High pass circuits is indeed much more critical in Hp than in LP, 
and exactly due to the reasons which Gene pointed out.