vcf5t update (EWB data)

[Harry Bissell]

Hi everybody...

I modeled the VCF5T this afternoon.  It looks promising but the Bode plots show a
limited range of cutoff frequency... maybe 2.5Khz corner for values shown. I think
the
control "phase splitter" is the problem... I think that ultimately the cutoff is
limited by the 100K resistors...

If you use the dual expo converter proposed by messrs. Juergen and Osama lately I
bet this would perform a lot better... but it would take more trannies.  This
would become a current fed diode bridge as proposed by Mr. Osama.

I'll dink around a little more later...

BTW Tom... bode plot shows that the .1uF coupling caps are way too small... try
1uF it will kick up the low end real well...

H^)   harry

Magnus Danielson wrote:

> Hi!
>
> > A "real" filter with only 3 transistors??? One of our members
> > asked me to see if  I could get this filter to work in his current
> > project. Well...I wasn't successful doing exactly what he
> > wanted ..but I  did something even better (for the rest of us
> > anyway).... I removed the opamp all together! Instead using
> > a pair of inverting transistor amps.
> >
> > The coolest thing is it works soooo much better!! That kill
> > the neighbors dog , split the sky oscillation is gone. The
> > cutoff response is very smooth. You can easily sneak up
> > on the res advancing to full on without fear you are about
> > to blow your speakers and when it does self oscillate it's
> > even (more or less) across the entire res sweep.
> >
> > The update is here. You guys should build one today!
> >
> > http://www.xavax.com/efm/cbook/pics/vcf5t-sd.gif
>
> Now I finally taken a look at this filter, and I must say that I must really
> take a closer look. First it looks a bit corny, but for instance the
> integrated expo converter and adjustable transconductance looks nice.
> One of the interesting things here is that it is basically off the shelf
> components for basically anyone. I think it is interesting also from the
> point of view of power consumption, you should have no trouble at all powering
> this from a 6R22 9V battery.
>
> Component list:
> 100     1
> 470     2
> 1k      2
> 10k     6
> 22k     2
> 47k     1
> 100k    3
> 330k    2
> 470k    3
> 10k POT 1
> 50k POT 1
> 50kTRIM 1
> 1nF     1
> 10nF    3
> 100nF   2
> 10uF    1
> 33uF    1
> 1N914   4
> 2N3904  2
> 2N3906  1
>
> Doesn't look too excessive, does it?
>
> Hm... better heat up that soldering iron I guess ;)
>
> PS. Tom, shouldn't you *really* learn to write 10n and 1n instead of .01 and
>     .001?
>
> Cheers,
> Magnus