[sdiy] Replacing CA3080 with CA3046 ??

[karl dalen]

--- Ian Fritz <ijfritz at earthlink.net> wrote:
> At 09:57 AM 4/9/05, Scott Gravenhorst wrote:
> >karl dalen <dalenkarl at yahoo.se> wrote:
> > >
> > >--- Antti Huovilainen <ajhuovil at cc.hut.fi> wrote:
> > >> On Sat, 9 Apr 2005, Tim Parkhurst wrote:
> > >>
> > >> > http://www.idg.se/personal/bergfors/bergfotron/VCA.htm
> > >> > This comparison shows that actually the lowly 13600 makes a pretty
> darn
> > >> good
> > >> > VCA. This list includes the 3080 , the SSM2024, 3046 and 3086-based 
> > OTAs,
> > >> > amongst others. It doesn't include the 3280 or 5517 however.
> > >>
> > >> What is the reason for the higher offset of the LM13600 circuits
> compared
> > >> to CA3080?
> > >>
> > >> Antti - Who will need some 40 VCAs for one project...
> > >
> > >The thing with the 13600 and 700 is that the offset is not constant,
> > >it responds in a somewhat eliptical fashion depending on varying Iabc,
> > >from some 500uV to 25mV and back, i tested this long time ago about
> > >12-15 13700 they all shows this in varying amount. Used the classic PNP
> > >Iabc source expo.
> >
> >That's interesting.  Is there an active method to compensate, like using one
> >OTA with 0 volts on it's inputs, but varying it's Iabc the same as another
> OTA
> >to generate a correction voltage to cancel the offset?
> >
> >Or would this be useful only if the other OTA is part of the same package as
> >the compensator?
> 
> I recently looked at this offset problem quite closely.  I made a discrete 
> OTA with high quality parts, but found I couldn't do any better that the 
> CA3280.
> 
> The offsets for the 13600's that I looked at were all over the map -- even 
> between the two units on one chip.  This will make your compensation idea 
> difficult, I'm afraid.

I can only agree to what Ian says here, its difficult
since the variation is not in a simple linear scale.
I remeber i did some essential experiments to try to 
compensate for this but it turned out to be cheaper
and less of a hassle to leave it as is.

Altough my app for the VCA back then was not critical as
for Ians caos generator still it was a bit dissapointing
not to get it at least constant!

Are you building a vocoder Antti?

Reg
KD