Doepfer

Hi Tom,

> i just bought a wasp filter module.  it sounds very cool.
> i am curious if anyone could explain to the principle it operates 
> upon?  i dont fully understand how electronics work.  i understand 
> that op amps are basic circuits that are in many electronics.  i 
see 
> a lot of op amp ICs when i go to a local store that sells 
electronic 
> parts.

Filters can appear complex and hard to grasp for a newcomer to 
electronics (certainly did for me!). However the Wasp filter is in a 
standard configuraton, called a 'state variable filter', only as you 
know, uses digital inverters rather than op amps. (The state variable 
filter generally uses three op amps: one as an inverter, and two as 
integrators.)

> how does this circuit use "digital inverters" as analog op amps, 
and 
> why won't it self oscillate?

On their own, with no feedback, op amps generally have very high 
gain, around 200,000 times, which is 'tamed' by using negative 
feedback to bring it back down to reasonable levels (say unity to 100 
or so). A digital inverter is essentially the same: as the input goes 
from high to low (say), the output will (very suddenly) switch from 
low to high - if you draw this out, the slope of the output would be 
very steep, which translates into a high gain. Using digital 
inverters rather than op amps makes it hard to use the negative 
feedback to tame the high gain, and so to bias the outputs to keep 
them swinging around the mid-point, without shooting off either to 
the high or low state, and I suspect this has a lot to do with why it 
won't self-oscillate (I've been trying to work this out myself, off 
and on when time allows, for some time).

  the res all the way up does create a 
> very grotesque and fantastic sound.  my understanding of filters is 
> that the resonance is feeding the filter back into it self, thus 
> oscillating at the tuned frequency (cutoff).

This is correct, and is called positive feedback - the signal fedback 
gets bigger and bigger, eventually bursting into oscillation (and 
controlling the amplitude then becomes a real problem). If you search 
the web you'll should find some tutorial-type sites, but when it 
comes down it, some studying will be necessary to properly understand 
how and what filters do!

Tim