[sdiy] Re:

[René Schmitz]

Hi Todd and List,

The Sallen Key topology is very popular. There are a number of circuits 
that can be reduced back to this. The main differences is how the 
voltage dependant tuning is archeived.

You would use variable resistors, because thats easier to handle. 
Variable caps have low capacitance (350p) and the dual ones usually have 
a common terminal (the rotor which is usually grounded in RF circuits to 
avoid hand capacitance to come into play.) And you don't have voltage 
control. (If thats not important use a dual pot.)

Varying the resistors can be done by replacing them with LDRs. Buchla 
did that. I used that in a version with tubes :-) Or by using diodes 
instead, Korg MS50 and the Steiner Parker filter do it that way. Check 
Ken Stones page for the latter. Then there is the Korg 35 filter (check 
Jürgen Haibles and Martin Czech's pages) where the variable resistors 
are done with transistors. A version with OTAs is the MS20 filter.

Resonance can be archeived in this kind of filter by makeing the gain of 
the amplifier variable. By RA and RB. (More than 3 and you get 
selfoscillations.)

Cheers,
  René

Todd Goodenow wrote:
> Ok so lets say you take a simple circuit design like
> so
> http://www.ecircuitcenter.com/Circuits/opsalkey1/opsalkey1.htm
> 
> basically a single op-amp low pass filter. Now since
> the frequency of the filter is set by one of the
> capacitors, is it possible to use a variable capacitor
> for variable frequency? For resonance could one set up
> a feedback loop with a potientiometer controlling
> amount? I'm new to this stuff but trying to figure out
> my way along. I want to build a simple filter just to
> get started (i don't want a kit either, I took a good
> physics course and want to understand this stuff from
> the ground up).
> 
> thanks, todd
> 
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-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159