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Old 29th June 2008, 11:56 AM   #4121
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Default altec 288c field coil

recently someone sold at ebay a Altec 288c transformed to field coil. It went for expensive money.Interesting the advantage the seller describes over the normal Altec driver.

There is a site on the internet dedicated to make field coil drivers DIY

Angelo
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Old 29th June 2008, 09:27 PM   #4122
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Default Richard C. Morgans - Horn Optimization


University of Adelaide:
Optimization Techniques for Horn-Loaded Speakers by Richard C. Morgans

Abstract, Table of Contents

Chapters 5 - 8

12th International Congress on Sound and Vibration:
The Sound Field at the Mouth of a Horn
by Morgans, Hansen, Zander, and Murphy

More Links to Technical Articles - thanks, Angelo!

Highly Recommended Book:
Loudspeakers for Music Recording and Reproduction by Philip Newell and Dr. Keith Holland, Lecturer at the Institute for Sound and Vibration Research at the University of Southampton.
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Old 1st July 2008, 09:42 AM   #4123
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Hi folks, just want to get your quick reaction.

I've been looking through the 2nd and 3rd harmonic distortion curves for the Beyma 18G50, the 18G550, and the 118Nd/w. I'm looking most closely at the 100 ~ 200 Hz region, and giving the 3rd harmonic about 10~15 dB more importance than the 2nd - which is nearly inaudible by comparison.

Based on those criteria, the 18G50 has notably low 3rd-harmonic distortion in the 100~400 Hz region, a remarkable -60 dB below the main signal - that's 0.1% distortion! True, the 2nd harmonic is substantially larger at -45 dB (about 0.5%), but the audibility of 2nd is much lower.

What undermines this cheery picture is looking at the distortion of the 18G550, a presumably similar driver. Now the 2nd and 3rd harmonic have changed places, with the 3rd harmonic gradually rising towards a peak around 450 Hz. Not so good - but of more concern, what constructional difference accounts for the reversal? A slightly misaligned voice coil that isn't quite centered in the gap? Or an outright mislabeling of the two harmonics - although this seems unlikely for a machine-generated graph.

Minor stuff, I know, but weird data like this sort of undermines confidence in the published information. Maybe the twin spiders are assembled slightly differently in the two drivers, and this shows up in the distortion data.

I was also looking at the BMS 18N850v2, with its very long Xmax figures. The distortion being measured at 1000 watts (!!!) makes it hard to compare to the Beyma figures measured at 1 watt - if drivers were amplifiers, the distortion could be approximately scaled by level, with 30 dB less drive level most likely yielding 30 dB lower distortion. But drivers do not behave like amplifiers, especially in terms of nonlinearities, so any guess of distortion at 1 watt is strictly a guess.

Paul W has been using the BMS 18N850 successfully in his quasi-cardioid speakers, so there's one vote of confidence for this driver - and it has attracted favorable comment over in the Subwoofer forum as well. Assistance Audio is selling them for US$350 each, which isn't a bad price for something on this performance level.

P.S. I haven't forgotten the TD drivers, but I don't have the nerve to commission a custom 18" driver before the design gets more solid. I'll try the off-the-shelf pro stuff first and see how it works out.
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Old 1st July 2008, 11:11 AM   #4124
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I emailled Jack at Assistance re the 18N850v2 about 6-8 weeks ago (still no response since then) and was told there were none in stock, nor had there ever been. Hope you're more successful.
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Old 1st July 2008, 11:54 AM   #4125
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Quote:
Originally posted by Lynn Olson
Hi folks, just want to get your quick reaction.

I've been looking through the 2nd and 3rd harmonic distortion curves for the Beyma 18G50, the 18G550, and the 118Nd/w. I'm looking most closely at the 100 ~ 200 Hz region, and giving the 3rd harmonic about 10~15 dB more importance than the 2nd - which is nearly inaudible by comparison.

Based on those criteria, the 18G50 has notably low 3rd-harmonic distortion in the 100~400 Hz region, a remarkable -60 dB below the main signal - that's 0.1% distortion! True, the 2nd harmonic is substantially larger at -45 dB (about 0.5%), but the audibility of 2nd is much lower.

What undermines this cheery picture is looking at the distortion of the 18G550, a presumably similar driver. Now the 2nd and 3rd harmonic have changed places, with the 3rd harmonic gradually rising towards a peak around 450 Hz. Not so good - but of more concern, what constructional difference accounts for the reversal? A slightly misaligned voice coil that isn't quite centered in the gap? Or an outright mislabeling of the two harmonics - although this seems unlikely for a machine-generated graph.

Minor stuff, I know, but weird data like this sort of undermines confidence in the published information. Maybe the twin spiders are assembled slightly differently in the two drivers, and this shows up in the distortion data.

I was also looking at the BMS 18N850v2, with its very long Xmax figures. The distortion being measured at 1000 watts (!!!) makes it hard to compare to the Beyma figures measured at 1 watt - if drivers were amplifiers, the distortion could be approximately scaled by level, with 30 dB less drive level most likely yielding 30 dB lower distortion. But drivers do not behave like amplifiers, especially in terms of nonlinearities, so any guess of distortion at 1 watt is strictly a guess.

Paul W has been using the BMS 18N850 successfully in his quasi-cardioid speakers, so there's one vote of confidence for this driver - and it has attracted favorable comment over in the Subwoofer forum as well. Assistance Audio is selling them for US$350 each, which isn't a bad price for something on this performance level.

P.S. I haven't forgotten the TD drivers, but I don't have the nerve to commission a custom 18" driver before the design gets more solid. I'll try the off-the-shelf pro stuff first and see how it works out.

With ARTA I have measured 3 different Beyma drivers and they all measure nearly the same as the what they publish in response, distortion and impedance.

IOW I have found them to be consistent with my little stash.

My experience with Beyma drivers is they also sound good and are well made.

Hope this helps.
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Old 1st July 2008, 03:38 PM   #4126
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Quote:
Originally posted by Lynn Olson

What undermines this cheery picture is looking at the distortion of the 18G550, a presumably similar driver. Now the 2nd and 3rd harmonic have changed places, with the 3rd harmonic gradually rising towards a peak around 450 Hz. Not so good - but of more concern, what constructional difference accounts for the reversal? A
The labels are not computer generated in all likelihood - the distortion labels are wrong on the 18G50 graphs. Look at where the peak in the distortion is versus where the peak in response in the fundamental is. There will typically be a peak in HD2 at half of a peak in the fundamental response and a peak in HD3 at 1/3 of a peak in the fundamental response, as the fundamental response is raising the level of those harmonics.

In general, the distortion spectrum could be changed by using different spiders, surround geometries, magnetic system geometries - without seeing the BL, Cms and Le vs x curves, it's tough to say if they're the same or not.
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Old 1st July 2008, 05:26 PM   #4127
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Default Re: Richard C. Morgans - Horn Optimization

Quote:
Originally posted by Lynn Olson

University of Adelaide:
Optimization Techniques for Horn-Loaded Speakers by Richard C. Morgans
Lynn - thanks for this, a fascinating study. I will have to get the whole paper. The first person that I have read that understands the problem.
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Old 1st July 2008, 05:48 PM   #4128
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Quote:
Originally posted by Lynn Olson

P.S. I haven't forgotten the TD drivers, but I don't have the nerve to commission a custom 18" driver before the design gets more solid. I'll try the off-the-shelf pro stuff first and see how it works out.

Hi Lynn,

I actually have everything in line here for an 18" driver, minus a phase plug. That may or may not be quick to get done here but is the only thing holding up the process. I have a surround that will do 18mm and am planning for a driver in the range of 13-14mm Xmax. Should be able to have something easily in the range of 95dB or more with the full copper sleeve on the pole.

John
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Old 1st July 2008, 06:56 PM   #4129
SunRa is offline SunRa  Romania
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Quote:
Should be able to have something easily in the range of 95dB or more with the full copper sleeve on the pole.
I hope this is not off-topic. The copper sleeve also covers the top of the pole piece? (as visible from the phase plug?). And what is the thickness you discovered to be the best trade-off in terms of demodulating the flux/maintaining the efficiency?

Also I've noticed that other manufacturers are using demodulating rings (al or cu) in the gap, but not on the pole piece, rather on the front plate/motor pot side. What is your experience with this.

I hope I am not getting too much into design/technical details, but I find these things interesting.

Thank you!
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Old 1st July 2008, 08:37 PM   #4130
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Quote:
Originally posted by SunRa


I hope this is not off-topic. The copper sleeve also covers the top of the pole piece? (as visible from the phase plug?). And what is the thickness you discovered to be the best trade-off in terms of demodulating the flux/maintaining the efficiency?

Also I've noticed that other manufacturers are using demodulating rings (al or cu) in the gap, but not on the pole piece, rather on the front plate/motor pot side. What is your experience with this.

I hope I am not getting too much into design/technical details, but I find these things interesting.

Thank you!
Well I can't give away all of our secrets. In reality, the thicker you can go to get the Bl you need, the better it is. We use all Ceramic 5 or Y35 magnets if you go by chinese grade numbers. Most companies use Y30 because they are about 20% cheaper, but also 10% less strength. By doing this, we get back about an extra 10% in flux in the gap, and then turn down the pole and apply copper until we get back to where we want to be. It's all tradeoffs and optimizing what we need. The higher the Bl you start out with the better, as it means you can add more copper.

The sleeve covers the entire pole, and the pole is extended from .5" to 1" past the top plate, depending on the driver and excursion. Having the shorting ring directly in the gap is by far the most effective at reducing distortion and inductance vs at the bottom of the pole, or above and below the top plate. Putting it on the entire pole keeps inductance more linear, as even when the coil begins to run out of the gap upwards or downwards, it is still next to the same amount of copper. If you simply do this on the ID of the gap plate, as your coil begins to ride out of the gap, it is near less and less of the shorting ring the farther it goes and as a result inductance is not linear.

John
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