? Active crossover with constant phase ?

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HI everybody !!
I need some help. There is this design of active crossovers using time shift filters. This crossover has no difference in phase between high and low frequencies. There used to be an article in the Elektor magazine. But I can´t find any info anymore. If anyone knows a link or something please post.
Thanks, Ilias.
Phase Linear Cross-Over


Elector published a book High-end Audio Equipment in 1995 and the article you want ( Active phase-linear cross-over net network) is in that book.

If you have any problems finding it please e-mail me.


P.S. Nelson Pass also published an article on subtractive (linear phase) cross-overs and I believe this is available on the Pass Labs web site.

[Edited by jam on 07-02-2001 at 12:25 PM]
Ah, but there's a difference between a crossover which is
"linear phase" and one that's "phase coherent at the
crossover point". The first reproduces (at least in theory)
a transient perfectly, while the second simply ensures that
the two drivers are moving in the same direction at the
same time at the crossover frequency. Linkwitz Riley
crossovers are in the second category; here (I think!) we
are talking about the first kind, which is much harder to
put into practice.


[Edited by Alex M on 07-03-2001 at 09:24 AM]
Phase coherent

The design above is almost perfectly phase coherent across most of the audio band (there's some minor deviation at very low frequqncies). The only reason for specifically mentioning crossover point was this is often the worst area.

Worth a peek.

Helpful software


Since you're obviously into SPICE, you may find the following application useful. It's 'Filter Wiz' available in LE or Pro versions (http://www.schematica.com/).

There are partly working demo's available, its' a very easy to use wizard-style application, that allows the design of active filters in Butterworth, Chebyshev, Inverse Chebyshev, Elliptic or Bessel / Thompson alignments.

One can force filter orders, or define by pass / stopband performance. It's great for creating crossovers, since one can easily prototype a design, fine tune in SPICE modelling and just build the final design.

Saves a lot of time with a soldering iron!

The Elektor publication, "Build your own High-End Audio Equipment" is a compendium of earlier Elektor designs published in their monthly magazine. Hence, in India, the Phase Linear Crossover article appeared in October 1986 and has also appeared in the former book.

The design uses fourth order Linkwitz-Riley filters (Low Pass) and also All Pass to simulate the time delay and then applies subtraction to obtain Band Pass and High Pass. This technique was an adaptation of the same used by Lipschitz and Vanderkooy.

Elektor also published another Phase Linear Crossover using only the subtraction method, but this time using fourth order L-R High Pass filters. If memory serves me right, this appeared in January 1985 in the Indian publication. This has however, not been included in their compendium.

The point is that I have tested both filters and have working boards even today. The filter that I have later referred to, has better sonics that the earlier mentioned one. But there seems to be a 'hole' somewhere about the crossover point. The one that Promitheus is referring to, which I have mentioned first, suffers from a slightly 'smudged' sound. I guess this may have to do with group delay. The dynamics and slam are present but the mid-band is slightly smeared. This is discernable only when compared with a 'better' filter. On its own, the Elektor design in worth the try.

The 'better' filter that I have mentioned is an adaptation of a circuit featured in Electronics World and was titled "Precise X-over". This was a two way design, that I configured as a three way. The simulations I performed on Electronics Workbench were exactly the same as published in terms of slope and phase. The slopes are between fourth and eighth order, something like sixth order. Within the pass band, the phase remains the same and group delay is minimal, since second order L-R filter configuration is used. The sonics of this filter is out of the world. I use them with a full horn loaded three way system for outdoor use and the results are more than just impressive. And to boot, I am only using a combination of LF353 and NE5532 opamps. Better opamps would probably sound even better.

Give this a try, seriously.
Subtractive filters


I am a firm believer in subtractive filters but they have one drawback, the derived response is only 6db/octave but a subtractive filter will always sum to perfect phase.

I think the problem you are experiencing is there will a slight bump in the frequency response of one of the slopes and this gets worse with higher order filters.

Your choice of drivers is more critical using a subtractive filter but done right I have not found a better filter design. The best texts I have found on the subject are Nelson Pass's article and some old National Semiconductor application notes.


[Edited by jam on 07-04-2001 at 01:24 PM]
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