[sdiy] Noninverting integrator / 2-opamp Biquad

[fmg]

jhaible at debitel.net wrote:
> 
> Remember the discussion a few days ago?
> 
> Now I found that this odd little circuit
> - NIC creates negative reistance to undampen
> a passive RC circuit - has been used in the
> famous Orban Equalizer 672A.
> 
> That's a mix between graphic and parametric EQ:
> Parametric, but with small sweep range for each
> band. This is more than compensated with the
> use of 8 of such parametric bands!
> 
> (Absolute) bandwidth can be adjusted independent of
> center frequency, and all this with ordinary pots
> (no dual pots, as in a 3opamp biquad)
> Center tapped pot for gain, but probably just to get
> an exact 0dB detent.
> 
> www.orban.com  - I think they have a download area.
> 
> JH.
> 

Jurgen,

I don't know if we are talking about the same think (haven't the
Orban schem), but for negative resistance I took a model from
an Analog's application note and did some simulations with it.
The sim showed a weird gain/phase response. I really would like
to breadboard a few "cells" to see how it works with real components
values and how useful it could be to keep torturing harmonics.

For reference:
 Analog Devices AN-281 "Passive and Active Analog Filtering" -
 Frequency Dependent Negative Resistor (FDNR) - pag.4

Fabio Gonzalez