[sdiy] Need help breadboarding a VCF

[Jeff Farr]

Well, I got this circuit to work quite well on a breadboard, sounded
pretty nice too.  Looking over the design I assume this is a 2-pole
design.  I am curious about exactly how the capacitor in the feedback
loop works as an integrator, and what exactly the "CVR Reject" trims
do.  Also, while scoping the output I notice that the dc level of the
waveform jumps around, even inverting the waveform sometimes.  This
seems odd.. particularly considering that it sounds correct.

Now, I'm commiting this to a PCB, and some odd troubles have arisen. 
The output of the first integrator (this is ENS VCF #1, a standard
3080 OTA filter) is stuck at -15v.  I've scoped the signal to that
point and all is well.  While adjusting the coarse tune the output of
the 3140 will jump between a positive voltage and the -15v. 
Strangely, in the time between the two voltages (if I turn the knob
verrrry slowly) the scope shows a rather nice triangle wave...  While
breadboarding a circuit fragment I found I could replicate some of the
measurements by disconnecting pin 3 from ground, but the continuity
checker says all is well on the board.. I'm stumped

On 8/15/05, Jeff Farr <moogah at gmail.com> wrote:
> Ok.  All the 3140's checked out, the 3080's look 'ok', with the output
> voltage between 4-4.5v.  Using the math you provided this actually
> looks about 1v low on average tho.  I re-assembled the filter on the
> breadboard today, in general things look much better, however, it
> still doesn't work.  First, I'm connecting the power for each IC in a
> daisy chain, is this OK?  It seems the 071 at the output doesn't work
> unless I run wires directly from the bus, otherwise turning on the
> power results in a DC offset that appears to block any signal (even
> from jumping over the feedback resistor) are the other IC's
> interfering with the power here?  Is this a sign of another
> malfunction?  I've also ignored everything but the LP output for now,
> skipping the 100k resistors on the other three lines, are there any
> caveats here?  Thanks for all your help, that last email did more for
> me than several chapters of the text I've got.
> 
> On 8/15/05, Ian Fritz <ijfritz at earthlink.net> wrote:
> > At 07:14 PM 8/14/05, Jeff Farr wrote:
> > >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.
> >
> > Great!  (The voltages were rough guesses ... er ... approximations).
> >
> >
> > >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.
> >
> >
> > Here are my ideas for quick/dirty testing.
> >
> > Start out by making a simple voltage source.  For example, take a 10k
> > resistor, connect one end to the power supply (+) voltage and connect the
> > other end to ground through a forward-biased diode.  This gives you about
> > .65 V at low impedance at the junction.
> >
> > To test the 3140, take a 10k and a 100k resistor and make a gain of 10
> > inverting amp.  (If you are not sure how to do this, google up an opamp
> > tutorial -- there are zillions available.)  Connect the .65 V source to the
> > input and check the output for ~6 - 7 V negative.  Understand, though, that
> > the 3040 has an unprotected FET input and it can be degraded by static even
> > though it seems to be working.
> >
> > The 3080 is more complicated, since it has a voltage input, a current input
> > and a current output.  Try something like this:  Divide the above voltage
> > source down to ~.06V with a 4.7k - 470R voltage divider.  connect the .06 V
> > to the (+) input.  Connect the (-) input to ground through a 470R
> > resistor.  Connect the programming input (pin 5) to ground through a 27k
> > resistor.  Connect the output through a 10k resistor to ground.
> >
> > The program current is ~ 13V / 27k ~ .5 mA, so the output current is given
> > by the magic OTA formula as
> >
> > Iout = 19.2 * .06 * .5 = .6 mA.
> >
> > Into the 10 k output resistor this gives an output voltage of ~ 6 V.
> >
> >
> > >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).
> >
> > Could be blown.  Try the above test to be sure.
> >
> >
> > >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).
> >
> >
> > Again, google yourself up an opamp tutorial.  Another good source is the
> > ENotes Handbook and the various Application Notes.  All oriented towards
> > synth applications. Another source is the National AN-20 and AN-31
> > documents.  I think you can find these online.
> >
> > If you have trouble understanding where the math comes from, a lot of it
> > comes from Ohm's law and Kirchoff's law.  You are expected to know how to
> > use these readily, and details cannot be put in a text, as they would take
> > up too much room.  Again, use any elementary electronics text or google for
> > tutorials.
> >
> > "Hope this helps"
> >
> >    Ian
> >
> >
>