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AR4x tweeters - An Investigation of two PRT types as replacements

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This thread has been created to share my experience with the evaluation of two currently available phenolic ring tweeter (PRT) types as potential replacements to the original AR4x tweeter. Many 4x owners have experienced the demise of their original tweeters and seek alternatives to originals that are no longer available, except as used goods thru on line re-sellers.I will cover the investigation over a number of posts to follow because the volume of test data I present will simply be too much to include in one, initial post.Many readers of this thread may not know that two versions of PRT are available.

Parts Express sells perhaps the most common one known. It's their 8 ohm p/n 270-252 http://www.parts-exp...tnumber=270-252 a nice bargain priced tweeter that conveniently has the same bolt mounting pattern as the original AR tweeter; thus making it a convenient, economical drop in replacement. This is most likely a S.E. Asia mfg. version of the original CTS PRT.

The second, and perhaps less well known source, is Midwest Speaker (p/n MT-4107-8) http://w.mawebcenter...07-tweeter.html. MW also has a 4 ohm version, but that one was not included in this investigation simply because the AR4x is an 8 ohm speaker.Although both tweeters look exactly the same, and both are manufactured in S.E. Asia, their acoustic characteristics are surprisingly quite different. As such, utilization of each requires a different HP crossover for optimum results. Otherwise, harsh, unacceptable performance could be realized by an unsuspecting AR4x owner who simply switches out the dead original tweeter for one of the PRT's. I have been able to identify xo's that yielded reasonably flat FR and pleasing to listen to sound.

[disclaimer note: the tests I share in this thread and xo's shared in subsequent posts represent one data point on each driver in my possession and may not be generally representative of these tweeter's performance characteristics]

With Kent's help (AR4x original tweeter loaner), I was able to run electrical and acoustic tests on his, the PE PRT and the MW PRT. The impedance/phase tests results below show how the 3 tweeters compare. The PE tweeter's resonant frequency is much the same as the original 4x tweeter at about 1200 hz. However, the MW tweeter's resonant frequency is 3300 hz.

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With this post I'll present some acoustic tests on each of the tweeters. They were done with the tweeter mounted in an AR4x cabinet. All tests were done on axis at about 1M using an ECM 8000 mic and presented with HolmImpulse SW, smoothed to 1/6th octave.

Below is the original AR4x tweeter borrowed from Kent. There are two curves showing the results of 2 tests. The upper curve is with no capacitor in series with the tweeter. The second is with the st'd 20 uF cap. The effect of the cap is to lower the tweeters output from 3 kHz and below, thus flattening the response somewhat by reducing the approx. 1 kHz bump. The dip at 1800 Hz is approx. the same those obtained by Zilch and Speaker Dave for this particular tweeter. I was quite impressed with the flat response and HF extension of this particular cone type tweeter sample. Kent's a lucky guy to have one that works this good.

original AR4x tweeter resp.jpg

Below is the Parts Express PRT test. This tweeter has a pronounced 10+ dB hump at 1.2 kHz - the resonant frequency point. It also has a response dip in the 2 kHz range - similar to the AR4x tweeter. Whomever, created this PRT did a pretty good job of mimicking the original 4x tweeter. Response was a bit more bumpy than the 4x tweeter. HF extension was significantly more variable and not as high as the 4x tweeter.

PE PRT 8 ohm resp no cap.jpg

Below is the MIdwest PRT test. Note how different this tweeter's response is from the others. Rolloff starts about 3.5 kHz. By the 1 to 1.2 kHz point spl is 25 dB down compared to the 4x and PE tweeters. The other, short curve is the tweeter's 3rd harmonic distortion result which was very good (~ 0.2%).

midwest 8 ohm PRT response no cap.jpg

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Now for the new crossovers needed to make the PRT's more listenable. I found the PRT's are quite efficient and do need padding down at a minimum.

The Midwest PRT did present a challenge, but in the end I was able to come up with a crossover with 5 parts, eliminating the need for the rheostat. However, one can choose to continue to use the rheostat (see PE PRT xover), but will certainly need to back off on the setting to the point where it is equivalent to the L-pad I show below (far right). With this schematic, I was able to attain the reasonably flat response show below (second from left). Limited listening tests indicated a pleasing sound.

Below is the PE PRT xover which is quite similar to original with the exception of the 0.1 mH coil in parallel with the tweeter (far left) following the 20 uF cap. I retained use of the rheostate here for the response shown below (far right) and measured it's setting at 6.4 ohms.Here is the response of the PE PRT tweeter. Note the similarities between the PE PRT and Midwest PRT responses. Each is a slight dip in the 1K to 2K region that, to me, isn't objectionable. What is objectionable is the 1K to 1.2 K hump that the original tweeter and PE PRT tweeter have in common.

Because the response measurements are gated on both response tests, disregard the information below 300 hz. THANKS!

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Now for the new crossovers needed to make the PRT's more listenable. I found the PRT's are quite efficient and do need padding down at a minimum.

The Midwest PRT did present a challenge, but in the end I was able to come up with a crossover with 5 parts, eliminating the need for the rheostat. However, one can choose to continue to use the rheostat (see PE PRT xover), but will certainly need to back off on the setting to the point where it is equivalent to the L-pad I show below (far right). With this schematic, I was able to attain the reasonably flat response show below (second from left). Limited listening tests indicated a pleasing sound.

Below is the PE PRT xover which is quite similar to original with the exception of the 0.1 mH coil in parallel with the 20 uF tweeter cap (far left). I retained use of the rheostate here for the response shown below (far right) and measured it's setting at 6.4 ohms.Here is the response of the PE PRT tweeter. Note the similarities between the PE PRT and Midwest PRT responses. Each is a slight dip in the 1K to 2K region that, to me, isn't objectionable. What is objectionable is the 1K to 1.2 K hump that the original tweeter and PE PRT tweeter have in common.

Because the response measurements are gated on both response tests, disregard the information below 300 hz. THANKS!

Carl, you might want to test the system input impedance since I'd be worried

about a dip using 20 uF with such a low .1 mH. I could be wrong but it is

worth checking.

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Here is my test of the AR4x setup I was using, except I used speaker dave's xover http://www.classicsp...15&hl=crossover

The result with speaker dave's xover looks eerily similar to the other results posted here earlier

I this with the original tweeter or the PE?

Interesting that you say the curve looks eerily similar, do you mean

to your mod or to the original? I was under the impression that Dave's

mod made a fairly large difference based on the report of his listening

tests.

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Below is the PE PRT xover which is quite similar to original with the exception of the 0.1 mH coil in parallel with the 20 uF tweeter cap (far left).

Carl,

Is the .1mh coil in parallel with the 20uf cap or the tweeter? You state it is in parallel with the cap but your schematic shows it in parallel with the tweeter. If it is in parallel with the tweeter, it is a rather radical departure from the original crossover.

Roy

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Carl,

Is the .1mh coil in parallel with the 20uf cap or the tweeter? You state it is in parallel with the cap but your schematic shows it in parallel with the tweeter. If it is in parallel with the tweeter, it is a rather radical departure from the original crossover.

Roy

Sorry for the confusing wording regarding the 0.1 mH coil. I have edited the post. Yes, it's a departure I felt was needed at the time I worked these xovers out. I can go back (when I once again have time) and recreate the PE xo w/o the coil and run another acoustic response test so you can see why I felt the coil was needed for the PE PRT.

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Sorry for the confusing wording regarding the 0.1 mH coil. I have edited the post. Yes, it's a departure I felt was needed at the time I worked these xovers out. I can go back (when I once again have time) and recreate the PE xo w/o the coil and run another acoustic response test so you can see why I felt the coil was needed for the PE PRT.

Thanks, Carl...

I'll give it a try one of these days. It is a very easy modification, especially if the 4x cabinets have the earlier #4 (.88mh) woofer inductor.

Roy

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Carl, you might want to test the system input impedance since I'd be worried

about a dip using 20 uF with such a low .1 mH. I could be wrong but it is

worth checking.

I'll try to remember to run the system imp. next time I set up this xo as noted in post #9.

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