newbees keep on asking questions......

[DCMagnuson at aol.com]

In a message dated 1/7/01 10:12:22 AM Eastern Standard Time, 
theoms at nettaxi.com writes:

> 
>  Have tryed to make 2 VCF on Tom cookbok.
>  Smoke didnt come out of none of them but neither worked like it was 
supposed
>  to work.
>  The first didn't work at all.
>  The second had a big self oscilattion on the resonance pot and the cut-off
>  worked like a volume :)
>  
>  Keep trying like I will.


Well, your second VCF *might* be working correctly.  The self oscillation 
thing can be fixed by swapping a resistor or two.  Some of the filters I 
built from EFM went into self-oscillation a little too easily, so I swapped 
one resistor.  

The cutoff does act *a little* like a volume knob if you're not sending a 
full spectrum signal.  For example, if your resonance is set low, and you 
turn the cutoff from high to low on a bass-sound, the bass will seem to get 
quiter as you turn it down, then it will totally disappear when the cutoff 
frequency is lower than the frequency of the tone source.  When resonance is 
set high, you'll hear the peak frequency loudly, but the other frequencies 
will be much quieter.

Try using white noise as a source, it'll be much more apparent if your filter 
is working properly (white noise is full-spectrum, so it''s pretty apparent 
if your filter is working correctly when you sweep the cutoff).  If you don't 
have a white noise generator, you could use a stereo system, and set the 
tuner to a static spot.  Not quite white noise, but it'll work.

Dave Magnuson