ReliaBill Engineer

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.

No idea what that means. Veiled cynicism? I’m not sure. Next 3 days will be overcast and raining here. So I should be able to spend time on this project, huddled away in the garage. I’m anxious to hear the original 4X configuration. The only thing I’ll be changing is the pot and driver seals; original cap, original rock wool damping, original drivers. So I’ll continue on. It was never my intent to publish a white paper on this with complete Hirsch-Houck lab results. Neither is this a preamble to a patent application.

I have no confirmation bias at present. Just comparison listening. I learned decades ago that just because I had a hand in the work, doesn’t make it better than another sample, or an original sample. I learned to try again until I got it right, or achieved what I set out to achieve. I worked doggedly for 6 months on a new stylus for the GE VR1000 cartridge. Originals were awful. Only when the stylus passed my tests and comparisons did I call it “good”. Then I sent one to a friend in Phoenix who had worked on the cartridge for 3 years trying to improve the stylus. He gave it a good listen over a week. He said it was the best sounding, best performing stylus for the VR1000 he’d ever heard. He already had a stylus business and began selling mine.

I have yet to *hear* any comparisons on speaker cable. But TONS of test measurements are out there. Meaning, *objective* measurements don’t tell the whole story either. Objective measurements have been used for decades to quantify cartridges, CD players, cassette decks, turntables, speakers, etc. And here we are. Still debating. There should be a clear winner based on objective measurements by now.

Again, I previously said what you reiterate above. For those insisting on replacing an AR tweeter with an AR tweeter, none of the discussions in this thread apply, at all. This is only for those needing, or desiring to replace the AR tweeter, either due to non-working AR tweeters, or not liking the AR tweeter. The original tweets can get pricey: If you look at the response curves, assuming both are typical, the PRT has a decent chance of replacing the original AR tweeter. Possibly enhancing the listening experience for those wanting a smoother treble, and flatter overall response than offered by the original AR tweeter. There are similarities between the 2 curves. A rise between 10 and 20 kHz. An up slope between 2k and 1kHz. SpeakerDave’s curve with original AR cap: Parts Express’ curve of the stock PRT:

I’ve already discussed the shortcomings of YouTube videos in at least 3 other threads. I mentioned it just a few posts back in here. But the alternative is *nothing*. In this case, I view something as a better indicator than nothing. At this point I’m not going to purchase calibrated mics and software to test driver output. Skeptics will be skeptics. Computer, ADC, cables, software, room/chamber, etc all come into play. All can be second guessed and nitpicked, if one desires to. This is a low cost ($17) PRT driver that can be improved. This is not a $1,350 woofer we’re dealing with here in a $50,000 speaker. The PRT is already a replacement driver, not a repaired AR original tweeter. If I was testing and comparing original drivers before and after a major repair, I could see the skepticism. That isn’t what’s going on here.

Just posting for the lurkers and monitors (thread watchers) that I was taking a night off. The speakers are in my garage workshop, so if I’m in the garage, I’m not with my wife. I wasn’t asking for “engagement” or comments on the pizza. (You know better.) If there are long lulls in posts, some might think I’m “finished”, or lost interest in the project. But if you want to make comments or have questions about pizza, go ahead and ask! I won’t be offended.

Indeed. But I have to add that the forum member was not receptive to being sent anything. And what I also said was that I’m not finished. I have yet to listen to the 4x with original tweeter, and compare to the modified PRT. All I said was that I’m satisfied with the modified PRT. That part was accomplished. I feel I have succeeded there. I feel like you’re jumping the gun, in a rush to find fault.

I have been told by skeptics in here that YouTube vids don’t help. To that I ask, “Why not?” What else can I do to help people hear what I’m hearing? YouTube vids are far from perfect; I know that. But at least it’s something we can go by. It’s not nothing. And per your comment (K6cdxkms), that was my goal: To smooth out, possibly correct, the faults of the PRT tweeter. But unlike a few in here, I recognize what distortion is, and its root causes. I first dealt with it in repairing cartridges and especially in their stylus assemblies. I started with ADC cartridges and styli (1963-1983), then moved to GE VR cartridges made in 1957-1960. Cantilevers that weren’t straight, suspensions that were hardened or manufactured incorrectly. Pole pieces that were out of alignment. Hearing resonance that blurred the sound, or resulted in greatly increased groove noise, or in truncated frequency response. I spent a couple of years experimenting with different elastomers, consulting with my colleague at Goodyear Tire and Rubber, a chemical engineer and compounder. He gave me free access to any elastomer used there. And yes, I was rehashing work already done by cartridge makers over decades. This work in my hobby directly relates to speaker drivers, which are just larger versions of transducers used to play records. Mechanical to electrical, electrical to mechanical transducers. In a driver, there is an electrical motor; consisting of the voice coil and magnetic gap. There is a cone/dome, mounted on the motor assembly, and the motor assembly is mounted to a spring, the spider. This assembly will bounce when driven to move by an electrical signal. There has to be a damper to prevent this bouncing. The damper has to be tuned to the spring force of the spider, in exactly the same way a shock absorber is tuned to the spring force of a car’s suspension; one bounce, then return to neutral without bouncing. Thats what I’ve done to the PRT. The factory damping is too stiff. So I removed the factory damper and replaced it with a more compliant, but damped surround. No crossover can properly damp a cone/dome mechanical assembly; that damping must be built into the driver. I did the same for the phenolic dome tweeter of the 2ax. For the PRT, the result is a driver that has wider dispersion and exhibits none of the strained sound of the factory original. In long listening sessions, it exhibits a smooth, more natural sound without being frequency constrained at the top end, nor at the bottom end of its range.

Well, many more lurkers and monitors of this thread than participants. I’ve gotten my share of skeptics in the Acoustic Research thread discussing a replacement tweeter for the 4x. But that’s fine. I’ve proven that what I’ve done works. “Low cost tweeter, so why bother?” I’ve even been laughed at. Not the first time. I have a long track record of doing the unconventional. Yard work today. Final Four basketball now. Making an online order for pizza now.

I still have to replace the pots in my 4x’s. Maybe tonight. I’m going to listen using the stock capacitor, since it’s still within spec. I will replace it, though. With a low output from the original tweeters, it will render the speaker as muddy sounding. I won’t know if the tweeter has relatively low output until I replace the pots and have a listen. I mounted the stock PRT yesterday on the 2ax and listened for several hours at elevated volumes. Initially it sounds like it renders more “detail”. But I found it fatiguing after a while. Even with it turned down using the pot. I switched back to the modified PRT. The difference is very noticeable. It’s smoother, with no listener fatigue at all. Yet still renders detail. The upper treble is “sweeter” and keeps its extension. Lower midrange blends seamlessly with the 10” woofer. The stock PRT sounds more “squawky” and forward.

I guess AR let the 6’s woofer run open ended, with no coil. Apparently AR just designed around the natural roll off at the woofer’s high end. Butterworth is the natural 6 dB roll off using a coil or inductor, in most cases.

2.8 mH puts the AR woofer roll off between 400-500 Hz @ 8 ohms. But that’s kind of low for the midrange. AR liked gradual XO slopes on their early speakers. More modern speakers, like Vandersteen and VMPS, also like gradual (6 dB/octave) slopes. That 12” woofer cone, with its attached whizzer cone, would have a very difficult time producing audible output at 18 kHz. Too much mass and too much inductance in the VC. Typical usable high frequency output for that is 8 kHz.

The 15” and 12” would need to be parallel wired. The JBL would roll off at 60 Hz and above using the 20 mH inductor, about where the Philips would pick up naturally. The Philips is a 1969-1971 “full range” which will only make it to about 8,000 Hz; it’s a whizzer cone accordion surround woofer. It shouldn’t need an inductor nor capacitor.

Inductor attenuates high frequencies. Capacitor attenuates low frequencies. When used in series with the driver. Some manufacturers just let the woofer play, allowing the voice coil to act as an inductor to naturally attenuate high frequencies on its own. 60 Hz is a low frequency, requiring a substantial inductor on the woofer to roll off all higher frequencies. But your midrange and/or tweeter will be completely unprotected from being driven by too-low frequencies; distortion and likely damage to both will result. Below is a general purpose chart, that’ll get you in the ballpark for frequency using various components and standard driver impedances. It indicates a 20 mH inductor for 62 Hz @8 ohm woofer. That’s a big inductor if air core; smaller if an iron core.

So far…. Ive been listening to the modified PRT tweeter for many hours. I’ve compared it to the original. I’ve compared it to the early AR 2ax hard dome tweet and mid-tweet. I’ve compared it to the unmodified PRT and there is a MAJOR difference in sound! I’m pretty sure the modified PRT will sound very close to, but preferable over the original AR 4x tweeter. (But perhaps not?) The big difference will be its smoother character, lower resonant frequency and extended treble response. Both the stock PRT and AR tweeter exhibit a “spitting” “S” sound on vocals in the 2ax. The modified PRT is clear without that sibilance.

Well. I tried. Not going to work. Too much pitting from corrosion on the center contact disc. You can see it here, where metal is gone from that area of the center disc.

Wire brush. Vinegar. Wire brush+vinegar. Isopropyl alcohol rinse.