Active filter - substracting signals

Kees · 18th October 2003, 06:58 PM

When you look at active XO design, you always see that the high and the low pass have their own, separate filter.

Why not make just 1 high pass fiter for the tweeter.
For the bass speaker, just invert the high pass signal and add it to the original signal.
This makes the system immune for tolerances on parts as well.

Basically it is the same idea as a passive series filter.

Are there people who have tried this?

cocolino · 18th October 2003, 07:36 PM

Hi Kees,

take a look at this thread (and particular for posts and attachments from Charles aka "phase_accurate")
Active Subtractive XOs

BTW: use the forum search and You`ll find even more useful stuff on this topic

seangoesbonk · 18th October 2003, 08:04 PM

There are a couple circuits using this technique in one of Randy Slone's books:
http://books.mcgraw-hill.com/cgi-bin...ectioncode=ENG
What you have to remember is that a high-pass filter (for the tweeter) changes the signal's phase with respect to the original signal. You have to use an "all-pass" filter on the original signal (to shift its phase, without affecting its frequency response) in order to subtract the high-pass filter output from it.

Hope that made some sense...

Kees · 18th October 2003, 10:13 PM

Thanks for all the replies.
The article from Pass covers is what I was looking for.

It's not clear to me why the all pass filter is needed. In the article from Pass I can't find one either.

Nelson Pass · 19th October 2003, 03:04 AM

I aims to please.

planet10 · 19th October 2003, 09:10 AM

Quote:

Originally posted by Kees
It's not clear to me why the all pass filter is needed. In the article from Pass I can't find one either.

In a subtractive XO, the derived output is always 1st order. If you add an allpass then you can get symetrical slopes... the very few comments on the sonics of the higher order symetrical XOs have been less than stellar thou...

dave

Pjotr · 19th October 2003, 09:31 AM

Hi

Keep in mind that the accuracy of the subtracting network directly affects stop band attenuation for the substacted branch. If it is 1% off, stop band attenuation can be no more than 40 dB. If it is 5% off, it is no more than 26 dB!

Cheers

18th October 2003, 06:58 PM	#1
Kees diyAudio Member Join Date: Oct 2002 Location: The Netherlands	Active filter - substracting signals When you look at active XO design, you always see that the high and the low pass have their own, separate filter. Why not make just 1 high pass fiter for the tweeter. For the bass speaker, just invert the high pass signal and add it to the original signal. This makes the system immune for tolerances on parts as well. Basically it is the same idea as a passive series filter. Are there people who have tried this?

18th October 2003, 07:36 PM	#2
cocolino diyAudio Member Join Date: May 2002 Location: Bavaria (south of veal sausage equator)	Hi Kees, take a look at this thread (and particular for posts and attachments from Charles aka "phase_accurate") Active Subtractive XOs BTW: use the forum search and You`ll find even more useful stuff on this topic __________________ Christoph *STEAL the BEST - INVENT the REST*

18th October 2003, 08:04 PM	#3
seangoesbonk diyAudio Member Join Date: Jan 2002 Location: Vancouver, BC, Canada	There are a couple circuits using this technique in one of Randy Slone's books: http://books.mcgraw-hill.com/cgi-bin...ectioncode=ENG What you have to remember is that a high-pass filter (for the tweeter) changes the signal's phase with respect to the original signal. You have to use an "all-pass" filter on the original signal (to shift its phase, without affecting its frequency response) in order to subtract the high-pass filter output from it. Hope that made some sense... __________________ My girlfriend's sub is bigger than mine...

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18th October 2003, 10:13 PM	#4
Kees diyAudio Member Join Date: Oct 2002 Location: The Netherlands	Thanks for all the replies. The article from Pass covers is what I was looking for. It's not clear to me why the all pass filter is needed. In the article from Pass I can't find one either.

19th October 2003, 03:04 AM	#5
Nelson Pass The one and only Join Date: Mar 2001	I aims to please.

19th October 2003, 09:31 AM	#7
Pjotr diyAudio Member Join Date: Nov 2002 Location: Netherlands	Hi Keep in mind that the accuracy of the subtracting network directly affects stop band attenuation for the substacted branch. If it is 1% off, stop band attenuation can be no more than 40 dB. If it is 5% off, it is no more than 26 dB! Cheers

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