[sdiy] Need help breadboarding a VCF

[harrybissell]

If you don't have the whole circuit there, the behavior of the first
integrator is normal.

The circuit relies on feedback to hold a steady DC level.  Without
feedback it will tend to "run to the rail" as you noted, and with careful
fidgeting it will give a triangle as well.

The CVR trims are to minimize the change in DC level with sudden shifts
in CV (ie center frequency).

I'd continue on until you have the whole circuit done

H^) harry

Jeff Farr wrote:

> 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
> > >
> > >
> >