[sdiy] Need help breadboarding a VCF

[Ian Fritz]

At 10:24 PM 8/11/05, Jeff Farr wrote:
>Ok, thanks for the info.  Once I was home for work I started
>re-building the circuit (this is the 3rd time).  I found a connection
>that was wrong and took the time to solder leads to the pots I would
>need which cleaned up the breadboard quite a bit.  Now, by connecting
>R35 to negative supply I get 1.9v to 3.2v across it through the range
>of the coarse tune pot.  Working through the math (3.2v/18k ?) this
>corresponds to a current delta of ~.0008a.

I calculate that the output current should go from near zero to ~.7 
mA.  The ratio of high to low current should be about 200,000.

You need to check the output swings of IC-1 (+/- .2 V), base of T1 (+/- .18 
V) and the base of T2 (-.4 to -.8 V).


>Seems a bit on the low
>side to me, however, I realized that 8/1.8 resulted in ~5 octaves so
>perhaps I made a dumb error in the math and things are working
>correctly.

What's "8/1.8"?


>I should mention that I didn't bother to connect any other
>control input (no CV, fine tune, etc)  and I don't have any 100ohm
>pots for the CV trim (just using a 10ohm resistor as a voltage divider
>with the 560), perhaps this affects things as well.  While looking
>over datasheets I realized that the LM301a is prolly alot closer to
>the LM307 than the LM714 is.. (duh?).  I also don't have any AD821's
>(or any PNP pairs) so I'm working with 2 3906's.

That's all OK.


>I haven't hooked up
>the actual filter yet as I'm going to take this one sub-circuit at a
>time.

Good approach.

Let us know what you get for the above measurements.

   Ian