[sdiy] Understanding Q in filters

[Magnus Danielson]

Dan Snazelle skrev:
> 
> So i have been trying to find and or (modify) a low pass filter with variable Q that only uses one or 2 opamps (at most a TL074).
> I have tons of books that get into filters but most of them are NOT aimed at using them for music.
> 
> Don Lancasters book is excellent but one problem i have with almost all of them is figuring out WHERE in the schematic to add a pot to control the level of Q (or damping). often Q is mentioned, but is described as being changed with capacitor choice, NOT resistor value, which would obviously make it hard to control with a pot.
> 
> SVF desings seem to be a bit too complex for what i am making right now, which is something with ideally very low parts count.
> 
> heck even a link to a datasheet that explains changing Q would be great

Not all filters fits the bill, which you have discovered. State-variable 
filters as well as ladder type filters or other multi-stage filters with 
external feedback for Q is however fairly easy to modify.

After many years I finally picked up Hal Chamberlin "Musical 
Applications of Microprocessors" and sure enough it was found within a 
minute on how to do what you asked for. He proposes the use of an 
exponential converter to provide a useful Q-range of 0,5 to 500.

His design is CA3080 based, but could be trivially converted to NE5517 
or LM13700.

> filters seem to be an area that a man could spend his life on.....

So very true.

> thanks
> 
> and happy holidays
> 
> 
> 
> 
> --------------------------------------------
> check out various dan music at:
> 
> http://www.myspace.com/lossnyc  
> 
> 
> http://www.soundclick.com/lossnyc.htm
> 
> 
> http://www.indie911.com/dan-snazelle
> (or for techno) http://www.myspace.com/snazelle 
> 
> 
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