[sdiy] Module Placement - Dist > Filter or vice versa

[Roy J. Tellason]

On Thursday 18 November 2004 02:54 pm, Scott Gravenhorst wrote:

> >I don't recall running across that one either,  though I might have and
> > just don't remember...

> I don't believe it's a terribly popular idea.  AFAIK, it's only been
> discussed in theory so far, but the René Schmitz 4069 VCO proves that the
> integrator concept works very well.

I imagine it would.

<...>

> >That sounds pretty nifty.  I'm really surprised that there doesn't seem to
> > be any simple way to control amplitude with voltage,  and hasn't been for
> > as long as I've been looking.  How low of a signal level are we looking
> > at there?  I never did get much of a handle on OTAs though I have a few
> > to mess with here,  both 3080 and 3094...

> A quick look at OTA voltage controlled state variable filters reveals a
> 100:1 voltage divider feeding the OTA input.

For the signal?  And what kind of signal levels are feeding that?

> I'm looking at a +/- 50 millivolt AC input signal after the attenuator. 
> This level satisfies the H11F1 distortion requirements.

I've not done terribly much with this stuff,  and am too used to thinking of 
the sorts of amplitudes one finds running around in synths,  10VP-P or 
something.  The rest of it is hi-fi stuff at "line level",  which is what,  
several hundred mV?  And I've worked with pro sound stuff,  but not in a lot 
of years,  most of that was not too far from there and with lots of headroom.

I'm just a little concerned about things getting buried in the noise,  or 
similar.

> >> I'm still sitting on a design myself, 4069UB for the integrators and
> >> H11F1 for the resistive elements.  Schematic is done, gotta get my ass
> >> in gear and breadboard the thing.  I chose H11F1 rather than 4007 or
> >> 4069 because the H11F1 eliminates virtual ground problems AND the H11F1
> >> distorts symmetrically (when driven with more than 100mv Vds) according
> >> to my own testing.

> >Sounds interesting,  I'd like to hear more about this once you get around
> > to getting  going with it.

> As am I...  (as I kick myself squarely in the butt).

(Trying NOT to envision this...  :-)

> René Schmitz http://www.uni-bonn.de/~uzs159/ has a design for an opamp
> based VC SVF filter that uses 4007 N transistors as voltage controlled
> resistors.  Osamu Hoshuyama http://www5b.biglobe.ne.jp/~houshu/synth/ has a
> CMOS synth site that uses OTAs made from CMOS for an experimental VCF, but
> the results apparently are less than wonderful due to nonlinearities that
> can't be compensated and these CMOS OTAs can be tricky to implement because
> resistor values can't be cast in stone as would be desired for projects
> that others might build.

I'll have a look,  but later.  I'm already getting way too sidetracked today, 
between the interesting links I keep bumping into in various emails and 
several preschool grandkids running about the place...