active filter 'without' opamps

aaronboumans · 25th January 2009, 09:02 AM

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!

Dag · 25th January 2009, 09:26 AM

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.....

Brett · 25th January 2009, 09:41 AM

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

Pjotr · 25th January 2009, 09:55 AM

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

Calvin · 26th January 2009, 10:02 AM

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
Calvin

aaronboumans · 26th January 2009, 12:31 PM

thanks all,
I now know what to search for.
But more info is always welcome

25th January 2009, 09:02 AM	#1
aaronboumans diyAudio Member Join Date: Feb 2003 Location: deventer	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! __________________ I hate carryable audio-equipment!

26th January 2009, 12:31 PM	#6
aaronboumans diyAudio Member Join Date: Feb 2003 Location: deventer	thanks all, I now know what to search for. But more info is always welcome __________________ I hate carryable audio-equipment!

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25th January 2009, 09:26 AM	#2
Dag diyAudio Member Join Date: Feb 2005 Location: Goteborg	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.....

25th January 2009, 09:41 AM	#3
Brett diyAudio Member Join Date: Jan 2002	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

25th January 2009, 09:55 AM	#4
Pjotr diyAudio Member Join Date: Nov 2002 Location: Netherlands	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

26th January 2009, 10:02 AM	#5
Calvin diyAudio Member Join Date: Nov 2004 Location: close to Basel	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 Calvin