[sdiy] Need help breadboarding a VCF

[Jeff Farr]

Ok, I've got the expo working, the voltages at the base of T2 are a
little off, but the overall operations appear well in range.  Now, I'm
moving on the the actuall filter circuit and would like some simple
circuits to use either a CA3140 or 3080 in to be sure that I didn't
blow any of the IC's during the first attempt.  I can't seem to get a
3140 to act as an inverting amplifier, reading with the scope shows
what appears to be a huge DC offset (the trace isn't even on screen). 
The effect also seems 'capacitive' such that a signal slowly returns
to my speakers after power-off (traveling over the feedback resistor,
I assume).  I've begun digging into the application notes for the
3140, but many of the circuits listed there aren't much simpler than
the filter I'm trying to build.  I'd also like to know of any good
texts on opamps, something comprehensive that shows not only the math,
but how the math is derived.  I've got the "student manual for the art
of electronics" but it assumes I'm familiar with more terminology than
I am (same with electronotes, really).

On 8/12/05, Ian Fritz <ijfritz at earthlink.net> wrote:
> At 11:47 AM 8/12/05, you wrote:
> >... I've made all the measurements you've
> >mentioned.  The PS voltage to the 301 looks good.  The rest are as
> >follows:
> >
> >IC1 pin 6 to:
> >ground       .74 - 1.18
> >neg supply 13.96 - 14.41 ....
> >
> >measuring pin 2 of IC1 to ground shows a steady 1.92v, adjusting the
> >coarse tune doesn't affect this.  Also, while looking over the
> >datasheets for the 301 I see that there is almost always a small cap
> >from pin 1 to 8, what does this do?
> 
> OK, your IC-1 circuit is not working.  If you use a 301 you *must* use the
> compensating cap.  In this case it is especially important since the gain
> is less than one.  Don't you have something like a TL071, LF351, etc. you
> can use?  You might be better off with the 741.  :-)
> 
> Anyway, put in something 30 pF or more for the compensating cap, and just
> to be sure put a similar cap around the 2k feedback resistor.
> 
> The reason you are getting such flakey results is almost certainly that
> IC-1 is oscillating.  Pin 2 is a virtual ground and should read zero.
> 
>    Ian
> 
>