AR tweeter replacement ????????????

[johnieo]

Hi Stan:

Thanks for your comments; it is always useful to rexamine prior work.

One issue we are facing today is quite different from that faced by rebuilders of yore: that is the continuing, slow deterioration of the urethane damping foam under the tweeter's dome. Ken's measurements showed differences between an in-service tweeter and an unused driver that had been stored in an intert environment. Eventually, these units will all deteroriate to the point where we will not like their sound. Modifications to the design made a decades ago would not have addressed this deteroriation.

Ken has made detailed measurements of the current AB Tech driver and found that it can be made to look as much like the original as possible by the addition of a 0.070 mH coil directly across the driver. As Ken commented, the original driver had wonderful dispersion. The current replacement is slightly less; probably due to its recessed faceplate.

If we examine the history of the AR tweeter, we see that the problem arose at introduction of the AR tweeter redesign for drivers used in the AR-11 and the AR-10pi. These speakers all changed the crossover design from a first order (R-C) to a second order (L-C). After examining the output of the AB-Tech driver, it confirms what we all suspected -- that the output of the post-AR-3a tweeters extended to a lower frequency than did the AR-3a. Thus, one needed the sharper slope at the tweeter's corner frequency provided by the L-C crossover. If one examines the 1991 redesign of the AR-3a - the "AR-3a Limited" - one again sees the second order crossover.

So I view Ken's conclusions as a logical extension of that design -- e.g. "what second order crossover will make the replacement tweeter look as much like the old as is possible?" Roy C's careful listening of the new versus the old confirms the measurements and modeling that Ken has done.

The aging tweeters will continue to deteriorate at an individual rate determined by their use and prior environment and at some point, they will need to be replaced. Personally, this solution will satisfy my needs; however, others may enjoy looking for alternatives. Ken noted that the AR tweeter was unique in its wide dispersion. This characteristic was a big part of the 3a's well-liked sound. A less costly alternative may be available, but at what cost to the sound? Got me.

[JKent]

>>Ken noted that the AR tweeter was unique in its wide dispersion. This characteristic was a big part of the 3a's well-liked sound.

Just a random thought, John. I recently faced the problem of trying to replace the tweeters in the Allison: Four speaker. Allison is out of business, despite a recent attempt to resurrect it. Anyway, the big problem in replacing the Allison tweets, according to Allison afficionados, is its "unique wide dispersion."

If a manufacturer comes out with a wide-dispersion tweet, I wonder if it would work with both the ARs and the Allis.

Kent

[Pete B]

>Hi...

>

>Perhaps the following report will be useful:

>

>http://www.kenkantor.com/xfer/3aHR_draft_017.pdf

>

>

>It's not for circulation yet, but it will be. (I'm totally

>buried with new venture deadlines, so am behind my hoped-for

>schedule for completing this. Sorry, all...)

>

>The AR 3/4" dome has several characteristics that are

>hard to duplicate with generally available modern drivers. Of

>note is its spectacular off-axis performance over the top

>octave. The AB Tech replacement is better than most

>alternatives in this regard. One can debate the audible

>advantages/disadvantages of dispersion at 15 KHz, of course.

>But, if you want it, the AB Tech is closer to the originals

>than you are likely to find elsewhere.

>

>-k

>

>www.kenkantor.com

Hi Ken,

I'd like to get the input impedance plots for the drivers if it's not too much trouble, or tabular data would be fine, whatever is available.

The T&S data would also be appreciated.

Looks like the AB-tech tweeters have nearly 6 dB higher voltage sensitivity than the older drivers over much of the pass band, say 8 to 20 kHz. Has anyone checked the physical properties of these drivers. Voice coil wind height, gap height etc., moving mass?

Would the later AR3-a tweeter be the one used in the AR-11 and 10pi?

Are the AB-tech A and B versions from different time periods?

[RoyC]

Hi Pete,

>Would the later AR3-a tweeter be the one used in the AR-11 and

>10pi?

No, the later 3a tweeter was a back-wired version of the original..same suspension, etc.

>Are the AB-tech A and B versions from different time periods?

I sent them to Ken at different times, but they had been acquired at the same time in a trade with Larry L. (aka "Vintage AR"), who purchases them from AB Tech. The codes on the magnets were the same.

Roy

[Pete B]

Thanks Roy, they're not very closely matched for the same part number, unfortunate.

Is the AR-12 tweeter just the 6-8 ohm version of the AR-11 tweeter, sorry for all the questions, LOL!

[RoyC]

>Thanks Roy, they're not very closely matched for the same

>part number, unfortunate.

I noticed that too. It is probably a good thing that they are being cut off at such a high frequency. The level controls help somewhat as well.

>Is the AR-12 tweeter just the 6-8 ohm version of the AR-11

>tweeter, sorry for all the questions, LOL!

No problem, Pete :-). It is encouraging that you are taking an interest in this adventure. Yes, the 4 ohm AR-11 black dome tweeter is virtually identical to the AR-12 specimen I sent to Ken. I bet the AR-11 tweeter would have been the star of the show.

Roy

[kkantor]

Sure... the impedance data is in the huge bunch of files I'll upload soon. Meanwhile, if you use the models given to calculate the driver impedance, you will get pretty darn close.

I can test the 11 tweeters later this year, no prob. I would expect them to be pretty close. But, are they readily available?

-k

[Guest treblehit]

Something a little less demanding than testing 11 tweeters.

Ken, can you remind me of the dispersion differences, brought-on by physics, between a 1" and a 3/4" tweeter?

I know I remember reading something about how much better the dispersion of a 3/4" tweeter is than a 1" tweeter everything else being equal, but I can't find it and thought you'd know off the top of your head.

Considering how much better the power handling is on a 1" driver than a 3/4" driver, and considering that a 3/4" driver is what Roy Allison spec'ed, and considering your comments on how exceptional the AR 3/4" tweeter is, even today, and how remarkable it was for its day, and just to satisfy my curiosity ->

Do you remember what the physics of the whole thing is so you can explain it (not explain it away) easily? Assuming **all else is equal** what would the advantage be of a 3/4" driver over a 1"?

Bret

[Pete B]

Thanks Ken, no I don't think the AR-11 tweeter is available. I only asked because it is the only AR-3a family type that I have here. And I'm curious to see how different it is from the AB Techs.

Thanks for the report Ken, interesting data to think about.

[kkantor]

Hi.

1- Higher frequencies have shorter wavelengths. As the wavelengths get smaller and smaller, and approach the diameter of the radiator, beaming starts to occur.

2- Directivity is all about the ratio of wavelength to driver diameter. This ratio is measured as a factor called "ka." The higher the "ka" is for a given driver and frequency, the more directional the sound will be.

3- I've created a simple table to illustrate the tweeter directivity issue. (I've also included a scan from Beranek.) This is attached here in PDF format.

3- According to Beranek, a ka of 0.5 or less is non-directional, and a ka above 3.0 is very directional, but this is only a guideline. The idealized radiation pattern for different ka values is shown in the scan.

4- I picked three sizes of tweeter, from 3/4" to 1.25". (The largest size is the effective radiating diameter of the second generation of AR 1" dome, which had a short waveguide built into the front plate.)

5- You can see that all the tweeters become directional at 20 KHz, according to Beranek's criterion. There are clear differences to be seen in the audio band, moving from 3/4" to 1".

6- There are some, myself included, who don't advocate maximum dispersion, but it all depends on the application and design goals. So, I wouldn't say that Directivity Index, by itself, is a figure of merit. But, here's the data in any event.

Hope this is useful!

-k

www.kenkantor.com

http://www.classicspeakerpages.net/dc/user_files/2765.pdf

2765.pdf

[Guest treblehit]

Thank you, Ken. That is a much, much better and more thorough explanation than I dared hope-for.

That is VERY interesting, isn't it?

When you consider how very little power there is in an 18kHz audio signal it is probably the ka transition from 2 to 3 that would be audible in the 8-10kHz+ range that would be audible (because of room reflections, or lack of them).

I think I learned a lot. Thanks again.

Now, to continue my education - is it possible that with all the "problems" of an AR-3 and AR-3a with recessed mounting in baffles, grill frames above the plane of the plate, and cabinet molding beyond the face of the grill - is it possible that one might improve the speaker (from a purely objective perspective) on-and-slightly-off axis by making the tweeter "beemier" and reducing the re-radiation of those high-high frequencies off of all these corners?

I imagine that much of the 18,000Hz re-radiated sound is extremely low in power, however. If I'm wrong, please correct me.

So, if we are hearing a difference in 3/4" and 1 1/4" tweeters, would you imagine the difference would be down in the far more audible 1kHz to 10kHz area?

Just an aside - I think it is really interesting that the 1 1/4" tweeter is preferred by some people. I wonder if this (lack of dispersion at high frequencies) has anything to do with it or if its roll-off gets more of the credit.

It's also at least interesting to note that when the 1 1/4" tweeter was in use, there was very little reproducible material above 12kHz.

Very, very interesting.

Bret

[kkantor]

B-

The "1.25" tweeter I am thinking about, with the recessed 1" fabric dome, was around for many years. It's close to what I used in the NHT SuperZero and SuperOne. It's a really, really, really good tweeter, IMO. And it was stupidly consistent when Tonegen was making it in Japan. Had that production line stayed open, I probably would have continued to use it in all the top end NHT products. I mean, yeah, the SEAS and Vifa aluminum domes were/are extraordinary performers, too. But they aren't $5.00....

There are so many differences between the 3/4" and the 1" that it is hard to know what factors one is hearing. The Fs is very different, and so the crossover point is typically very different, too.

I think that adding directionality certainly helps reduce reflection and diffraction artifacts in the top octave. The problem is that the most audibly egregious diffraction problems occur between, say, 500 Hz and 4,000 Hz. There isn't much difference between the different tweeters in this range.

-k

www.kenkantor.com

[Carlspeak]

Whether it be a 3/4 inch or 1 1/4 inch tweeter, I don't think anyone familiar with the 3a will argue about it's inherent diffraction problems. Response measurement I've made at a distance of 50 inches display peaks and valleys as the measurement microphone is moved from side to side at tweeter level. It's no wonder later designs had much thinner cabinet front moldings with the tweeter located much closer to the edge of the cabinet such as with the 'improved' model.

I refer you to the following link. Read Tom Tyson's 2/28/04 thread.

http://www.classicspeakerpages.net/dc/dcbo...ing_type=search

It's all about the music

Carl

Carl's Custom Loudspeakers

[JKent]

Off the AR topic for a moment if I may,thank you, I'm going to re-activate some old equipment that's been sitting around for years now. First my original Dyna ST-35, a PAS-3X pre-amp.

Hey Frank

ST-35 huh? Don't see many of those. I bought one (kit) in 1968 for $45 at Sound Reproduction here in NJ. Sold it about 3 or 4 yrs ago on ebay for $450! Good return on my investment! You may be interested in this: In the August, 1985 issue of Audio magazine Bascom King wrote this of the Mark Levinson ML-9: “Surprisingly, it sounds similar to the Dyna ST-35, which is a highly musical-sounding little amp (when its ceramic input-coupling capacitor is eliminated).” The Levinson was a $3,000.00 amp at that time! I wrote to Audio and the response: A very clear copy of the schematic, with notations presumably made by hand by Mr. King, showing exactly how to modify it! I never made the mod, but if you are interested, here is the schematic with notes.

cheers

somebody's friend

Kent