[sdiy] Need help breadboarding a VCF

[Ian Fritz]

At 08:58 PM 8/19/05, 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.

Correct. (See the second sentence in the text.)


>I am curious about exactly how the capacitor in the feedback
>loop works as an integrator, and what exactly the "CVR Reject" trims
>do.

The output from the 3080 is a current, and as this current flows into the 
left side of the cap, the voltage on the right side increases.  This 
voltage is V = Q/C (capacitor equation) and the charge Q is the integral of 
the current (definition of current).

The trimmer compensates for asymmetries (offsets) in the OTA, to make its 
operation as close to ideal as possible.

>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.

Hmmm ... not good.  :-)


>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.

Sounds like the feedback loop is not connected properly, as Harry has 
already suggested.  The integrators *will* ramp to the rail without the 
full circuit.

   Ian