[sdiy] isolator vs low/band/high pass filter

[Byron G. Jacquot]

>I need again your knowledge and explanations.. I'm wondering how works
>an isolator, and if it can be made using several filters in parralel..
>If i get it, it's just a sound source splitted into three different
>frequencies, and for each one a volume control ?
>SO, is it a bad idea to use three filters without resonance control to
>do that ? I"ve got some lm13600 spare, and was thinking about it.
>
>If somebody here has already built an isolator.. i would be interested
>in talking about that !

I think the usual topology is a crossover circuit followed by a summer that reassembles the bands.

With a high quality crossover, the desired behavior is that the re-combined output is identical to the input.  

One very common crossover design is the Linkwitz-Riley, realized as opposing pairs of Sallen-Key filters.  For higher isolation, the 24-dB versions are probably better.  If you don't need to change the frequencies, you can make them with standard op-amps.  If you want them to be tunable, the OTAs are probably the easiest solution.

Sigfried Linkwitz has a lot of good information on his webpage...start there, and keep ckicking around!

http://www.linkwitzlab.com/crossovers.htm


--Byron Jacquot