ReliaBill Engineer

So all that comparing of the AR-4x to the mod’ed PRT has run its course for me using the AR-2ax as a test case. Now to turn my attention to the 4x speakers. The pots have some internal corrosion. I think I’m going to clean them and reuse them. Most of the corrosion is on the wiper and on the brass rivets at each end of the resistance coil. Pics to follow. Now I have to adjust the PRT impedance to match the 20 uF XO capacitor. The original 4x tweeters need some love, cosmetically. Also some loosening of the paper surround in a few spots. Easily corrected, though.

Using the AR-4x tweeter with my 2ax speakers. I have a 2.4 ohm resistor in series with the tweeter to raise its DCR to 7.1 ohms from 4.7, so the 2ax crossover won’t be affected much at all. The efficiency of this 4x tweeter is less than the PRT by quite a bit. I have the pot at 100% and it barely rises to the level of the 10” woofer of the 2ax. So I guess not the best way to listen to this AR-4x tweeter. What I can hear from it is quite smooth. I don’t know if the lower output is from age, or if that’s normal for this tweeter. But switching back to the modified PRT, it has very good definition of details and a nice robust and smooth sound. And treble is very nicely clear and extended, but not brash.

Side-by-side:

The AR tweeter. Smaller voice coil, smaller back chamber, smaller magnet than the PRT:

AR-4x. April 11 1968 on the woofers:

They’re original. Need a refoam, though. But that’s always better than someone else’s botched refoam, and usually means original woofers.

Allison made very fine speakers! I used to frequent an upscale audio shop in downtown Annapolis, MD, that sold Allison. I really loved the Model 2! I listened to my first CD in 1983 on those. Alan Parsons Project “Eye In The Sky”. It was a revelation! Im very tempted to buy a pair of 2s I saw for sale.

So my 1968 AR4x use a “#4” inductor on the woofer, measuring 1.02 mH. The capacitor is labeled 20 uF, measures 22.3 uF. According to charts I have, that equates to 1250 Hz XO frequency on the 8 ohm woofer, and around 1500 Hz on the tweeter (4.7 ohms DCR, 5-6 ohms impedance.) All drivers are in full working order in these 4x speakers. So I’ll replace the pots. Get them working with the original caps and have a listen. Later, when my parts come in, I’ll replace the caps. I want to hear the AR sound. Later, I can put my mod’d PRT in, but I have to adjust the impedance of the PRT so the 4x caps “see” the same impedance as the AR original tweeter. And vice-versa, so I can listen to the AR tweeter on my AR-2ax speakers. (That’s just for fun, and to familiarize myself with the sound of the AR paper cone tweeter.)

For those perusing this thread. The PRT 8 ohm tweeter:

Found this on-line for the 8 ohm PRT:

So, I got the 4x pair today. Neither has been molested. Both tweeters are good, and measure 4.7 ohms DCR. Pots are bad. But I have new ones. I have new caps. All the seals were in place, undisturbed. Woofers are intact, no glue failures that I can see/feel. I flexed the cones; spider looks good, surround looks good. Cone is good. Paper scrim intact behind the woofer, rock wool never disturbed. Cosmetically they need TLC. But I have a good foundation here. April 8 1968 on woofers and tweeters.

Should be here today.

Another sound check. I use this 1962 stereo album to check for resonance in cartridges. The xylophone tends to cause styli problems. Using a Shure V15-IV which is very well controlled. My phone’s ability to handle the resonance is challenged. In the room, there is no ringing or resonance.

The treated silk suspension: RoyC said this in a thread over in the AR section: ” It should be noted that the replacement PRT type tweeter does not have the same construction as the original 4x tweeter...which has a more compliant suspension. The PRT replacement is not as capable in the mid frequencies primarily due to excursion differences.” This is exactly why I gave the PRT a different suspension. The stock PRT cone outer edge is tightly secured to the phenolic ring. There is little chance for the cone to have “excursion”. To handle lower frequencies, 800 Hz-2000 Hz, the cone has to have much better excursion than above 2000 Hz. But that excursion has to be controlled, hence my use of butyl rubber treated silk. So now it has a damped, more compliant suspension. More like the original AR tweeter in that respect. In my opinion, it now has a “chance” to have a smooth roll off at the 4x crossover point of 1200 Hz

Besides seeming like the “right thing to do”, I added the wool felt and wool fibers under the cone for this reason: Fiberglass batting under the original AR tweeter cone. (borrowed pic) The original tweeter has AR’s “trademark” use of fiberglass damping pads. They used it a lot, and often, on their early mids and tweets. I chose wool. I’ve found over the years that it works well. (NASA uses melamine foam in large panels to absorbs sound vibrations.)

I have that scheduled. As I said earlier, I have a friend across town with the test equipment. He repairs amps, speakers, cassette decks and R2R tape decks. Not a tech shop, he works from his home. He’s known in the state for doing good work. He’s retired, so does it on the side. That’s also why I haven’t molested the 2nd PRT. I was his maintenance manager when we both worked for Goodyear. He was an excellent troubleshooter. I used to park him at a troublesome machine for half of a 12-hour shift, to document problems, mechanical and electrical. He would come up with a troubleshooting list that was nearly always correct. As an added feature, once I know what’s going on with the pair of 4x speakers I bought, if the tweeters work, I can test those too. Currently, I’m not aware of anyone who has measured/plotted the response of the PRT, despite it being sold as a replacement for the 4x tweeter.

The 4x has a lower crossover point than the 2ax. 1200-1400 Hz vs 2000 Hz for the early 2ax. So it should be interesting to see how this modified PRT responds. The 8” AR woofer of the 4x should be more capable of response up to 2000 Hz than the 10” of the 2ax. But that’s not what AR chose. 1200 Hz is a low crossover point for a paper cone tweeter. The paper cone tweeter has a free air resonance only slightly lower than 1200 Hz. SpeakerDave went to a 3rd order network to try to tame the tweeter hump close to its resonance. If I have the same issues, I’ll raise the crossover point to 2000-2500 Hz for both the woofer and tweeter. That way I can keep the first order network.

Thank you! Ive been playing it all day. In our basement. Allowing for some break-in time. The tweeter has opened up more. I turned it down to 70% from 75% on the level pot. It has a “big” sound, not what I’d expect from a small, inexpensive paper cone tweeter. So I’m hopeful. It fills the room nicely. I’ve played trumpet music, piano, big band jazz, various vocal with some that can exhibit sibilance on “S” sounds. All of them sound natural, easy, clear, with a wide and open sound. Even after several hours watching basketball on the TV with its sound on mute, listening to many records, the tweeter was never fatiguing or strident. (I can’t say that for the stock PRT. I had to disable it.) So I think the wool felt batting inside the cone and treated silk surround worked nicely! We’ll see when I’m able to mount it in the 4x. I have no idea if the tweeters work in the pair I have coming in.

A YouTube sound clip. Mono signal. 2ax supertweeter disabled. Mid-tweet disconnected. Output of internal mid-tweet crossover connected to modified PRT, which is mounted in foam on top of the cabinet. Mid-tweet pot at 75%.

YouTube sound clip. AR tweeter turned off, AR mid-tweet disconnected. Mid-tweet pot at 75% going to the modified PRT from the mid-tweet crossover. Mono signal from the Emmylou Harris record. Shure V15-III cartridge with original nude elliptical stylus; known for it’s excellent detail and balanced, flat response.

After listening to this modified tweeter, is it worth the effort? I’ve played 8 of my favorite LPs, and changed out cartridges 5 times. I can clearly hear the differences between the cartridges, which is as it should be. I put on the ADC 10E MKIV, ADC XLM MKIII, Shure V15-III and V15V-MR, and Pickering V-15 with the nude elliptical stylus. Each makes its own fingerprint to the music; each was clearly differentiated through this combination of AR woofer and this tweeter. None of them sounded harsh or strident. Nice amount of “”air” and a more musically effortless sound. The cartridges sounded good, the music sounded good. The stock PRT sounds “thin”, strained. Relatively “tinny” in comparison. On cymbals, blurred, not really clear and focused. Without sophisticated test equipment, all I can give are my subjective impressions at the moment. But we all know that graphs don’t tell us how something “sounds”, either. So “yes!”, I like the sound of this tweeter. It no longer has the sound of a “cheap” $17 paper cone tweeter. So yes, I think it was worth the effort on this. Now I need to hear the original AR tweeter. Hear for myself how different they are, or possibly alike they are. Anecdotally, my wife had a comment. I kept the modified PRT in the left speaker, switched back to my AR drivers in the right speaker. Went back to stereo listening. Her off-hand comment was, “Why all the work? I don’t hear a difference now.”

Ok. Good to know. Thanks! But I didn’t load photos on the AR section of my last post. I noticed no comments, but quite a few views in here. Guess I’m being stalked….

Decided to give the tweeter a test drive. Used my 2ax speakers. Disabled the super-tweeter using the pot, already had the AR mid-tweets disabled, wires disconnected and running out behind the mid-tweets. It should be a relatively good test run. The 2ax is a 2-way at its heart. The mid-tweet is a nominal 8 ohm driver (6.6 ohms DCR). I used a 60 ohm resistor across the PRT terminals reducing its DCR to 6.5 ohms from 7.3 ohms. Tweets mounted on the top of the cabinets in acoustical (leftover from room treatment panels) foam blocks. Crossover should be around 3000 Hz using the 6 uF capacitor. At the 70% Mid-pot position, this modified PRT sounds quite good! I did the same for the other 2ax speaker, but used the stock PRT. Comparison using a mono signal source, same to both speakers, switching between L and R, reveals a difference between how the 2 PRTs sound. The stock PRT is brighter, with a seeming larger suck-out in the midrange. My modified PRT has a much more present midrange and less bright, smoother treble.

Decided to give the tweeter a test drive. Used my 2ax speakers. Disabled the super-tweeter using the pot, already had the AR mid-tweets disabled, wires disconnected and running out behind the mid-tweets. It should be a relatively good test run. The 2ax is a 2-way at its heart. The mid-tweet is a nominal 8 ohm driver (6.6 ohms DCR). I used a 60 ohm resistor across the PRT terminals reducing its DCR to 6.5 ohms from 7.3 ohms. Tweets mounted on the top of the cabinets in acoustical (leftover from room treatment panels) foam blocks. Crossover should be around 3000 Hz using the 6 uF capacitor. At the 70% Mid-pot position, this modified PRT sounds quite good! I did the same for the other 2ax speaker, but used the stock PRT. Comparison using a mono signal source, same to both speakers, switching between L and R, reveals a difference between how the 2 PRTs sound. The stock PRT is brighter, with a seeming larger suck-out in the midrange. My modified PRT has a much more present midrange and less bright, smoother treble.

Just waiting for my 4x speakers to arrive. They’ll need some work. Modified PRT ready for testing. Not including curing time, took less than an hour for the modification. Pretty quick, really. I spent more time experimenting with materials than the actual mod. Over the past 10 years, I’ve spent many hours studying, researching, testing, repairing, phono cartridges, 1956-2020 vintage. You learn a great deal about materials, resonance, electrical and magnetic circuits. Just a few of the mounted cartridges. Every one has been repaired in some way. MM, MI, Ceramic, Electret.