analog circuit for time-alignment? there's Bessel filters and what else? - diyAudio
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Old 6th January 2007, 08:12 AM   #1
m8o is offline m8o  United States
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Default analog circuit for time-alignment? there's Bessel filters and what else?

(moderator, please move this if it is in the wrong forum)

Hello all. Wondering if those in the know can confirm something. Is there any other [ or better ] way to produce an ANALOG time delay stage then to use a Bessel Low Pass filter with a cutoff frequency well above the upper bandwidth point of the driver?
Think trying to time-align a tweeter horn to a mid-horn where the two are about two feet apart [Klipsch La Scala].

As I'm building a custom constant-current gainclone tri-amp for it, and will be making extensive measurements of each horn+driver in the speaker with a sophisticated tool I have, I'll be able to determine the exact time-alignment difference between the tweeter/mid/bass horns. I wish to design in some compensation into my design ... without resorting to digitizing and using a DSP.

Is making use of the uniform group delay provided by a Bessel filter circuit stage, and cascading them as needed until I reach the desired delay, the only/best solution in the "analog" domain?

Thanx much for your thoughts.
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Old 6th January 2007, 09:06 AM   #2
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There is a thing called an allpass filter which changes the phase and can be used for time delay Mentioned on linkwitz's site here:
http://www.linkwitzlab.com/filters.htm#4
To get the delay you want, you would have to cascade many such stages, its probably worth reconsidering your decision not to go digital.
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Old 6th January 2007, 04:56 PM   #3
m8o is offline m8o  United States
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Nice Link! Thank You.

If I have to I may. I'll give it some time trying analog but digital may end up the case.

Is it true that time-alignement really only audibly exibits itself in the cross-over region?

P.S. am I making an incorrect assumption that Siegfried Linkwitz in the same Linkwitz as in Linkwitz-Riley?
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Old 6th January 2007, 05:13 PM   #4
m8o is offline m8o  United States
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Nice ... take a look @ all those extra sections on the tweeter:
http://www.linkwitzlab.com/images/graphics/38xo_eq1.gif

I read on another site how using a Linkwitz/Riley without delay compensation negates benefits of using Linkwitz/Riley. I wasn't sure how "right" that was though, or if I could rely on that (or quote it ever if need be). I see now under the "Delay Correction" item on the link above that I can! "Active crossover circuits that do not include phase correction circuitry are only marginally useable. "

There may be hope for the analog route yet....

This page is useful for someone that's been out of this game for decades like me, annotating all the sections of the complete x-over circuit:
http://www.linkwitzlab.com/xo_eq.htm

consort_ee_um, this circuit has -everything- I need as an x-over for my constant-current tri-amp project I'm embarking on for my Klipsch La Scalas (the LM4780TAs arrived yesterday ), including delay compensation and selectable tilt control. I could not be more jazzed! Thank you. And thank you Mr Siegfried Linkwitz if you read this board!
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Old 6th January 2007, 06:18 PM   #5
gootee is offline gootee  United States
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Default Re: analog circuit for time-alignment? there's Bessel filters and what else?

Quote:
Originally posted by m8o
(moderator, please move this if it is in the wrong forum)

Hello all. Wondering if those in the know can confirm something. Is there any other [ or better ] way to produce an ANALOG time delay stage then to use a Bessel Low Pass filter with a cutoff frequency well above the upper bandwidth point of the driver?
Think trying to time-align a tweeter horn to a mid-horn where the two are about two feet apart [Klipsch La Scala].

As I'm building a custom constant-current gainclone tri-amp for it, and will be making extensive measurements of each horn+driver in the speaker with a sophisticated tool I have, I'll be able to determine the exact time-alignment difference between the tweeter/mid/bass horns. I wish to design in some compensation into my design ... without resorting to digitizing and using a DSP.

Is making use of the uniform group delay provided by a Bessel filter circuit stage, and cascading them as needed until I reach the desired delay, the only/best solution in the "analog" domain?

Thanx much for your thoughts.

There is a variant response-type, called "Linear Phase .05-degree Equiripple Error", which can be implemented with allpass, bandpass, lowpass, or highpass characteristics, using many different analog filter circuit topologies.

One trick is to vary the number of poles, so that you can simulataneously get the cutoff frequency you want (or phase-reversal freq, for allpass) AND the group delay that you want. i.e. Using a higher-order filter (more poles) can allow getting a higher cutoff frequency while still increasing group delay.

You can download the free FilterPro software, from TI's website. It lets you specify the response type, frequency, and filter topology type, et al, and then designs an opamp-based filter for you and plots the amplitude, phase, and group delay. Very handy!

I recommend that you also use LT-Spice (free from http://www.linear.com) to simulate the resulting designs, since you can get much better accuracy from the plots, that way.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html
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Old 6th January 2007, 06:53 PM   #6
m8o is offline m8o  United States
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Thanx for all that.

Yep, I'm an LTSpice user and subscribe to their Yahoo! group. I'm re-learning a lot of things as I go.
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Old 6th January 2007, 08:23 PM   #7
Nordic is offline Nordic  South Africa
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I only started using it 2 nights ago... mind posting link to the group?
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Old 6th January 2007, 11:45 PM   #8
m8o is offline m8o  United States
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http://tech.groups.yahoo.com/group/LTspice/
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Old 7th January 2007, 02:33 AM   #9
m8o is offline m8o  United States
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I just discovered Tina and other then the 100 node limit for $39 it does a great job; almost everything of the expensive version. Have a look at it @ tina.com. The free TI version only does 2 opamps and 20 nodes tho...
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Old 7th January 2007, 09:20 AM   #10
djk is offline djk
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A three-way horn speaker cannot be aligned with all-pass filters.

At the risk of repeating myself:

A three-way horn speaker cannot be aligned with all-pass filters.

Why?

The tweeter needs the most delay, then the mid. The time delay is proportional to the frequency. For the high tweeter crossover points you will need so many opamps for the delay that you will have a sea of noise.

Buy a digital crossover.
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