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Old 18th June 2013, 07:58 AM   #21
Jmmlc is offline Jmmlc  France
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Quote:
Originally Posted by marco_gea View Post
Hello,

In response to some insightful comments made by Jean-Michel Le Cléac'h (here and on other French-language forums), I have made new simulations trying to further improve on the TAD crossover.

This is what I have come up with:

- Woofer low pass: 6th order Bessel at Fx * 1.25 (-6dB of attenuation at Fx)
- Tweeter high pass: 2nd order Butterworth at Fx * 1.3 (-6dB of attenuation at Fx)
- Offset = 0.4 * c / Fx

The result is even better phase match and a smoother Group Delay curve, still with a practical offset between Woofer and Tweeter.

Comments welcome.

Marco
Hello Marco,

I just came back from a one week geological field trip and see your message.

Congratulations! This version of the crossover is IMHO far better than the original one.

The group delay curve is more constant in the frequency range inside which our hearing system is the most sensible to phase distortion.

If the response curve "in coincidence" is smooth then this crossover may be called quasioptimal!


Best regards from Paris, France

Jean-Michel Le Cléac'h
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Old 18th June 2013, 01:22 PM   #22
jmbee is offline jmbee  France
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Jean-Michel,

"If the response curve "in coincidence" is smooth then this crossover may be called quasioptimal!"

Then you can call it a quasi, as the two ways are very closely phase-matched
around Fc : axis and "coincidence response" nearly the same

crd
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Old 18th June 2013, 02:12 PM   #23
Jmmlc is offline Jmmlc  France
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Hello Jimbee,

From the graph you gave, yes, I can see a very flat summed response and a common -6dB attenuation for the HPF and LPF, meaning that the 2 loudspeakers work in phase at the crossover cut-off (here defined at -6dB).


May be this will give some ideas for improvement for the TAD engineers...

Best regards from Paris,

Jean-Michel Le Cléac'h
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Old 18th June 2013, 04:37 PM   #24
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Seems worth noting that electrical delay and acoustic center shift are only equivalent directly in front of the driver.
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Old 18th June 2013, 06:02 PM   #25
Jmmlc is offline Jmmlc  France
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Quote:
Originally Posted by dumptruck View Post
Seems worth noting that electrical delay and acoustic center shift are only equivalent directly in front of the driver.
Hello,

If the horn propagates parallel isophase wavefronts then the acoustical center will still be at the same apparent (curved) distance from the listener's ear inside the angle inside which the horn radiates . So for HF horns and LF classical loudspeakers the method consisting in cancelling the shift in distance by the group delay of the LPF (if the group delay is most constant) seems relevant.

Best regards,

Jean-Michel Le Cléac'h
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Old 18th June 2013, 07:46 PM   #26
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Hmm, I am having trouble understanding how that could work (I mean work ideally - I do understand how it works out okay as a crossover design strategy).

For an idealized 2-way system like this with a horn and a delayed LF in front of it, where is the point that the listener could rotate around on the horizontal axis and maintain a constant apparent distance to both sources?
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Old 19th June 2013, 09:26 AM   #27
Jmmlc is offline Jmmlc  France
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Quote:
Originally Posted by dumptruck View Post
Hmm, I am having trouble understanding how that could work.

For an idealized 2-way system like this with a horn and a delayed LF in front of it, where is the point that the listener could rotate around on the horizontal axis and maintain a constant apparent distance to both sources?

Hello,

Because in horns the wavefront are curved and for a given wavefront at the mouth (for a good horn ) the travel from the diaphragm of the loudspeaker is the same in every point of the wavefront....

Then , when the listener rotate around the horn, the apparent acoustic center moves maintaining the same distance from the listener ear to the apparent point from which the waves seems to come.

For a non horn loaded loudspeaker, this is different as the apparent acoustic center doesn't move...

This is why most simulations of the lobing due to the crossover is a very rough aproximation when it come to a 2 ways system consisting of a classical LF loudspeaker and a HF horn.

For sure an optimisation of the relative positions of the HF horn over the LF loudspeaker is still needed...

Best regards from Paris, France


Jean-Michel Le Cléac'h
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Old 13th September 2013, 01:21 AM   #28
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Quote:
Originally Posted by marco_gea View Post
Hello,

In response to some insightful comments made by Jean-Michel Le Cléac'h (here and on other French-language forums), I have made new simulations trying to further improve on the TAD crossover.

This is what I have come up with:

- Woofer low pass: 6th order Bessel at Fx * 1.25 (-6dB of attenuation at Fx)
- Tweeter high pass: 2nd order Butterworth at Fx * 1.3 (-6dB of attenuation at Fx)
- Offset = 0.4 * c / Fx

The result is even better phase match and a smoother Group Delay curve, still with a practical offset between Woofer and Tweeter.

Comments welcome.

Marco
Marco
Have you tried your crossover ?
If not, I intend to do so in coming months.
Thank you very much for sharing your design.

Pierre
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Old 13th September 2013, 09:07 AM   #29
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Actually, I did, yes.
But I found out the hard way that it is practically impossible to obtain a true 2nd order high-pass transfer function for the horn, even when crossed over at Fx > 2*Fc. The phase shift around Fc is just too great, and this ends up messing the phase response of the high-pass and making it closer to a 4th order. Thus, it is impossible to attain the theoretical phase matching shown in the simulations. Alas, such is the real world vs. the ideal!
So I settled for a compromise solution that is, remarkably, much closer to the original TAD version. Low-pass: 6th order L-R; High-pass: quasi-3rd order Butterworth (transitioning to 4th order below Fc). With the right physical offset, this provides a good compensation of the LP group delay, and subjectively a very good integration between the two speaker units. Ideal? No. Practical and good enough? I would venture to say yes (at least for me).

Marco
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Old 13th September 2013, 10:12 AM   #30
nar is offline nar
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You could also try a quasi_optimal of another kind composed of Bessel_18 on the high pass with a Linkwitz_24 on the low pass finding different frequencies and offset adjust.

For example, about LP 500 Link4, HP Bessel 700 -3dB -180°, offset 200mm.
This gives in theory a good correspondance between on-axis and off-axis response.
You need 1'4 or 2' compression driver I guess

Don't be afraid to fiddle with values ...

I found measuring curves with my horn + compression driver to exhibit a 18 Bessel curve when passive XO designed and adjusted for Link24@-6dB.
For now I try a quasi_optimal 4/4 Butt24/Link24, but real life often calls for practical adjustments ...

A crossover for a horn mixed with direct radiation speaker is always a challenge !

Best regards,

nAr
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