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Old 25th January 2009, 10:02 AM   #1
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Default active filter 'without' opamps

hello
I was wondering, is it possible to make an active filter with 'normal' components? I mean a bufferstage, then some coils/caps/etc... and then another bufferstage.
So coils/caps can be very smal and no interference from the speakers themselfs and therfore a better and simpler design. And also less opamps in signalpath.
Is it a stupid thought and/or are there any items here on diyaudio?
Thanks!
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Old 25th January 2009, 10:26 AM   #2
Dag is offline Dag  Sweden
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This is a classic:
http://www.neilmcbride.co.uk/naxocircuits.html
(terrible schematics but correct...rewrite if you don't understand)

and you dont even need coils.....

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Old 25th January 2009, 10:41 AM   #3
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Yes, easily. here is a link to John Broskie's Tubecad article which describes how to do it with tubes as cathode followers. Substitute JFETs as source followers and you're away. Nice software too for small $.
Broskie - Tube Based Xovers
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Old 25th January 2009, 10:55 AM   #4
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Yes, and it is widely done in the past. A second order Sallen-Key filter only needs a 1x buffer as the active component. And that is exactly what an emitter c.q. cathode follower is.

http://en.wikipedia.org/wiki/Sallen_Key_filter
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Old 26th January 2009, 11:02 AM   #5
Calvin is offline Calvin  Germany
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Hi,

the unity-gain Sallen-Key topology simply relies on an active element with gain 1. It sounds rather weird to me to use OPs with their complex internal structure and OL-gains in the millions what a simple buffer would do anyway, because of its nature!
So you can use a simple transistor-buffer in emitter- or sourcefollower mode. Instead of the emitter/source-resistance I´d opt for a simple transistorized constant-current-source. Have a look at Erno Borbely´s articles about simple SE-JFET-stages. Imho You can´t get any better sonically than that.

jauu
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Old 26th January 2009, 01:31 PM   #6
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thanks all,
I now know what to search for.
But more info is always welcome
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