[sdiy] Need help breadboarding a VCF

[Ian Fritz]

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