ReliaBill Engineer

Listening to big band jazz (Glenn Miller) and Bobby Hackett on the Epic (Columbia) label. These can be bright and brash. Trumpets sound “bloomed” and a bit harsh on both the AR tweeter and the stock PRT. With the modified PRT brass is smooth, detailed with “bite” but not harsh. Even Al Hirt sounds nice! And his horn can sound irritating at times. Brushed cymbals and cymbal strikes come through clearly and with nice metallic shimmer.

Forgot to add that I will be changing out the AR paper-wax cap and using metal film p-p caps. Also I will add a “dressing” to the modified PRT, so it looks more like the AR tweeter. Not paint, in case anyone is wondering. And unrelated to the tweeter, the AR woofer surrounds will be sealed, as well as cabinet refinished and new grills installed.

After extensive listening, I would say this modified PRT is a major success story. It maintains the character of the original AR tweeter; It sounds very much like it. But the AR tweeter has a certain “graininess” to it that I find taxing over time. It lacks depth in its presentation. I would add that the unmodified PRT tweeter mirrors that graininess in its presentation. Neither the AR nor stock PRT are “pleasing” over time. Turning down the tweeter level helps with reducing listener fatigue, but then (for me) frustration sets in because detail is muffled and the overall presentation is too warm and “chesty”. Doing the same with the stock PRT ends with the same frustration. I can see why vintage_ar parts added dope to the surround of the PRT, in an attempt to “tame” it. Not sure how well that works, since the source of the “graininess” is not eliminated; it’s coated over with dope. Other than aesthetics for both my modified PRT and the 4x speaker, my work here is done. I accomplished my goal and proved my theory and point. Start with a smooth driver, and the crossover becomes easy. It’s very difficult to design a crossover to “fix” a problematic driver. This PRT can be made to be smooth and detailed while keeping its overall sound character; a sound character very similar to the AR tweeter without its flaws. From vintage_ar parts, regarding their replacement PRT tweeter for the AR-4x:

Listening to recordings with vocals. Very smooth and natural. Bass is quite good for this small speaker, but that’s all AR’s doing. (But I will be treating the surround later. Ultra thin butyl rubber. But not a sticky formula; it will cure dry to the touch. I don’t like a forever tacky compound that attracts and holds dust and fuzz/hair.) Still playing with the level control. About have it dialed in now. This tweeter has more “air” to the sound than the AR tweeter. While it reveals high treble extremely well, the midrange is smoother and more balanced. Not at all bright. Not irritating at all. Smooth and pleasant. Emmylou Harris:

I put more info in Mods & Tweaks. But here is the mod’d PRT. With original AR wax-paper cap and original batting, new pot:

So….. I mounted the modified PRT in place of the original AR tweeter. Gave it a listen. I had to add a parallel resistor to the PRT to match the DCR of the AR tweeter. 4.6 ohms AR vs 7.4 ohms PRT. End result is the PRT measures 4.75 ohms. After listening, I got the results I was expecting. The PRT is considerably more efficient, so I turned down the level control, from 80% to 40%. Played with it some, going up and down from 0 to about 65%. Too much volume from the tweeter above that. What doesn’t come across well in the video is a more “full” and wider sound from the PRT.

I’m not used to seeing so much very fine dust when using fiberglass in a sealed box. Abundant here. The texture of this is very grainy and more crumbly than fiberglass. More easily compressed than fiberglass. Fact is, I had to be careful when putting it back in before reinstalling the woofer. It compresses very easily. It’s exactly the same batting as was used in my 1965 2ax’s. So what’s the verdict? FG or rock wool? (Doest’t matter for these tests, since I haven’t changed it.)

It’s what was in these originally. Looks identical to the rock wool insulation installed in my attic back in the 1950s; house built in 1931.

Replaced the corroded pot. But all else is original. Original drivers, original capacitor (20 uF spec’d, measures 22.3 uF), original rock wool batting. 1.03 mH inductor. So this is my baseline for comparing a replacement 4x tweeter. I think that’s a valid comparison. Not a “shot in the dark” comparison. When this original AR tweeter is damaged, or beyond repair, and replacements are few and far between, or perhaps above your price point, what do you do? Thats the question asked in this topic.

So here is one of the 4x speakers. The only part not original is the pot. I had to change that out. But original 20 uF cap, which measured 22.3 uF. Untouched tweeter and woofer. Set at the “dot” setting. (I marked the original pot position and wiper setting at the “dot” on the XO board prior to removing, 11.3 ohms on mine.) Again, my amp is set on “mono”, so both stereo channels are being played through the single speaker. I think my modified PRT is going to sound very close to this original tweeter.

The 2ax mid-tweet is not the same as the 4X tweeter. So kind of pointless. Fits the hole, yes. But fitting the hole doesn’t mean it’s a replacement. As has already been mentioned in here. That’s why I’m modifying the PRT. But welcome to the party! And yes, I have a pair of 4X, so not pointless. The impedance are not the same! My 4X tweeter measures 4.7 ohms DCR. My 2ax mid-tweet measures 6.6 ohms DCR. I used the 2ax as test bed, just to listen and compare. Just to listen for the PRT’s (modified and not) overall sound from 1400-20K, since it plays well with the existing 2ax crossover values. It’s a very valid comparison. I know the 2ax inside and out, so I could compare the PRT to the AR sound. Listen for problems from the PRT as it does the work of both the AR mid-tweet and super tweeter. And a low crossover point near the free air resonance of the PRT. I do this as a hobby, at present. So I can afford to put in a lot of time, and it doesn’t cost me shop time. I don’t have to pass the time/cost on to customers. Scottie, you run a business, as of February 2024. Comparing 4X to the 2ax: 4x tweeter: 4x tweeter vs PRT: 2ax mid-tweet. Not the same as the 4X. Very different! There is a definite reason that AR went to the trouble of adding fiberglass batting to this mid-tweet and creating a cage to hold it. Also in using a glue to hold the cage that absorbs/damps vibration. Epoxy is too hard, and will not damp vibration transmitted into that aluminum expanded metal cage. Epoxy is difficult to remove without causing damage. The fiberglass pad is there to attempt better dispersion from the paper cone, and damp resonance that it has inherently. Removing the pad will not help it to mimic a 4x tweeter.

Thank you Gene for the context. This is a hobby for me. A great break from a highly technical world in which I work. I would love, at some point, to invest in measurement tools. It has to meet with the WAF; she doesn’t understand much of my work, nor my hobby. In a WebEx meeting now, where NASA has changed the timelines (again) for delivery of the 2nd and 3rd SLS vehicles for Artemis. It would be great if there was a consolidated repository for various test results on work done on AR legacy and heritage speakers over the years. I promise I will make every effort to provide some measurements, warts and all. Good and bad. Sink or swim. I can’t make promises on a time frame right now.

Dentritic. Broadly means branching out. But doesn’t apply here. What we have here is pretty straight forward: 1: Discussion of a replacement 4X tweeter. PRT was mentioned, with its faults. 2: Decision to get a pair of PRTs, look at it. 3: Decide to change the PRT and listen to it. Compare to unmodified. Use 2ax as a “test bed”. 4: Decision to purchase a pair of 4X speakers. To listen to. To compare PRT to original. Fix speakers first. So there you have it. All the dentrites.

6 dB/octave first order low pass network is quite “long” in its roll off. You’d have to go to a 2nd order filter to get a steeper slope of 12 dB/octave. Here is another chart that shows capacitor and inductor values to get that 12 dB/octave roll off. Inductor in series, capacitor in parallel with the woofer terminals. Top of chart shows wiring for low pass woofer.

Scouring through many posts and pages on AR speakers. Even the AR3/3a document. I see very few comprehensive measurements of output from the drivers and speakers, before and after refurbished. Or contemporary (when new) charts comparing to refurbished woofers, mids and tweets. Same for those refurbishing original AR drivers, particularly dome mids and tweeters. I find that a bit curious. Especially by those calling for exhaustive tests on a budget replacement tweeter. Maybe I looked in the wrong places. Just lining up what to test and how.