ReliaBill Engineer

The thin butyl sealant is fully cured. Pushing in on the dust cap of the cone, the surround is flexible and moves evenly all around the cone edge. Now to mount these in a pair of 4x speakers. But I’ll need to get the original 4x speakers working fully. The pots are corroded. Fortunately, I have spare parts from my 2ax refurb; 2 new pots and some metal film polypropylene capacitors. I will be able to listen to the original tweeters, then compare original to the stock PRTs, original tweeters to my modified PRT, and stock PRT to modified PRT. If all goes well, I’ll take these over to a friend for some measurements of impedance curves and FR plots.

Letting the sealant cure.

Exactly why I just posted my comment to genek. Distraction from WHAT??? Isn’t this thread all about a MODERN replacement tweeter for the 4x??? Speculation? Conjecture?? Wouldn’t that apply to ANY and ALL modern tweeters you care to name?? So why get defensive? Why be so trigger happy?? Why be so contrary? It’s as if you’re waiting to pounce on any and all suggestions, no matter what or from whom. Sheesh! All I’m doing is starting from one particular tweeter that has long been suggested, and sold, as a replacement tweeter. So if I’m successful in making it sound more agreeable, how is it a BAD thing?? SMH

Too much pushback in here, for either approach or reason.

Instead of a butyl “roll” edge, I’m trying a treated edge. High quality, high thread count silk. Treated with butyl rubber. High flexibility but sealed and damped. I used parchment paper so the treated silk just pulls off after fully curing. Later I’ll cut what I need for the tweeter. Silk is bad about fraying; treating it first will allow me to cut it without fraying. The 2 rods were used to keep the parchment paper flat. It wants to curl.

I already started a thread in Mods and Tweaks yesterday. I guess you didn’t see it. I never said I was setting out to “improve an original” tweeter. Where did that come from?? I said I was working on improving the PRT. Go back and read my posts and see the pics. Nowhere do I show or comment about improving an original AR 4x tweeter! SMH

Nothing defensive in my post at all. Just me scratching my head at your very negative comments.

I started with a butyl rubber roll edge. But it turned out to be too stiff: So I removed it. Now I’m working on a different surround. It’s curing as we speak. It should be more flexible, yet damp well.

The new PRT: After removing the factory surround. I decided to add some acoustic damping under the cone, to attenuate the back wave. Black wool felt and a little wool fiber: After the felt is installed:

In rough shape, but intact: These will be my baselines for comparative listening. After some refurb, of course.

How much easier would SpeakerDave’s crossover have been if the response anomaly wasn’t there? That’s all I’m saying. And what if the PRT sounded more like the original AR tweeter? I certainly understand the “If it ain’t broke, don’t fix it!” proverb. But to me, it’s “broke”. I thought you might encourage the attempt. Guess not. So is the following an attempt to shame me somehow?? Reaching over the fence to get some type of agreement or “consensus”? Is that what we do in here? “To Gene's point...I just had a conversation with "Vintage_AR" (Larry Lagace), and told him about this thread. He just laughed and said he sells "lots of them" and has never had a return or a complaint. I then wentto the Parts Express website. Here is the description:” And the above doesn’t agree with the below very well: ”This tweeter is certainly not a perfect replacement, especially when used in a pair next to an original. Unfortunately, there are really no other "drop-in" options. There was some experimentation with crossover changes for this tweeter mentioned in the forum quite a few years ago, but I'm not aware ofanyone who implemented any of the suggestions.” Selling “lots of them” doesn’t mean it’s the better option. It means it’s a convenient, “viable” option. Looking at prices of AR-4x pairs with the PRTs installed seems to drop the asking price considerably. Of course, it means the pair isn’t all original; and there are plenty of comments about the PRT being a compromise. But I see no reason for my attempts to be disparaged, any more than SpeakerDave’s work to be.

I decide to move my efforts over to the Mods and Tweaks section. I would hate to fail and keep posting in here. Plus, it really doesn’t belong here.

I decided it would be best to discuss this in here, rather than in the Acoustic Research section. This way it can be seen without potentially hijacking a thread, nor competing with genuine original AR tweeter discussions. In essence, when the original 4x tweeters blow, or are otherwise damaged and non-functional, many replace them with the Phenolic Ring Tweeter (PRT) sold in various outlets such as Parts Express, eBay vendors, Amazon, etc. However, while the PRT fits nicely, and is cost effective (around $20), it doesn’t perform the same as the original AR tweeter. The small AR-4x speaker is a nice sounding legacy speaker, despite its small size. A common failure in these is the tweeter. Out of pure curiosity I decided to buy a pair of these PRTs and have a look at them. They are reasonably well made. But a bit “harsh” in my opinion. Can these be made to be better sounding? Can they more closely mimic the performance of a genuine AR-4x tweeter? That’s what I want to find out. I may fail or succeed. Original 4x tweeter: PRT, stock: Typical listing:

I’m a bit more than surprised…. These PRTs have been offered up as “replacements” and “temporary replacements” for many years for the AR 4x tweeters. But yet, nowhere have I been able to find a representative FR and impedance plot for them. How is that possible? Again, if I’ve overlooked it, I apologize in advance. The AR tweeter FR plot:

FWIW…. I just bought a pair of 4x speakers. All intact, never worked on. So now we’ll see…. Supposedly both woofers work, tweeters intact, but may or may not work. Prime for repairing the originals (if required), and testing and comparing these PRTs.

I’ve been trying to find comparison FR and impedance plots of the PRT vs an original AR tweeter for the 4x. I either overlooked it, or I just can’t find it. Anyone know where they are?

I looked at that one, plus many more. Just based on appearance, they are very obviously not AR. There are a few paper cone tweeters that look a lot like the AR tweeter, but they all require pretty major mounting mods. I was going off of RoyC’s comments about the PRT sounding similar and not requiring major mods to mount in the baffle.

I think you’re being a bit too defensive, RoyC. The reasons I decided to experiment with this tweeter were already stated. But to recap, here are my reasons:

A smooth tweeter is a smooth tweeter. No “extreme” aberrations in the FR and a smooth, well controlled impedence curve, makes designing a crossover a breeze. I saw how much work Speaker Dave put into the 4x tweeter XO in an attempt to tame its response. (In combination with the woofer.) It would have been easier if the tweeter had a smoother FR response, don’t you think? My experience says “yes!” The mention of dome tweeters was only to illustrate a point. Not so much to state definitively the merits of one over the other. Pistonic motion was the point of the comparison I was making. Unconstrained (but damped) motion of a tweeter cone/dome provides the best chance for smooth FR output. Im still in the process of documenting my work on this tweeter. I haven’t even solidified my own process, yet alone listened to final results. So of course I haven’t provided any instructions to the readers. But in the end, it should be up to the readers to decide if they want to try it themselves, no? I think so. Yet already it seems a judgement has been made about the merits of doing this, and its level of complexity. Prejudged, if you will. I’ll finish this tweeter, then mount it for audition, and do some critical listening. I’ll compare it to the untouched unit. If it passes that test, I’ll have it tested with plotted results, mine and the original. I think that’s fair. BTW: I’m no quack. I’ve been doing this kind of work for many years. I was a product and equipment design engineer, engineering manager for 6 companies. Now as a reliability engineer for NASA SLS, it is my job to identify root causes of failures and non-comformance, predict and quantify failures, as well as design solutions. It’s very challenging in aerospace vehicles. But with audio components, a certain artistic flair helps.

Well, it wouldn’t fit under a “restore”, to the AR purists. Not using an AR dome, and not an AR voice coil would make it a Mod, no?

Ok. Maybe a place for Replacement Replacements? Mods to Replacements? You want me to start the “Mod To Replacement 4x Tweeter”?

My Amazon order arrived today. So tonight I hope to finish work on the tweeter. I removed my butyl roll edge. Looked great, but not the damping I needed. Didn’t Speaker Dave make his theory, progress and calculations before he published his schematic and “How To”, and even his listening observations? Pretty sure he did. This thread is about a replacement tweeter, not an original AR tweeter. So I’m not blaspheming an original AR component here. In a previous post I offered to send RoyC these modified PRTs for evaluation. And RoyC’s modified PRT was linked to previously in this thread. So I’m hardly doing anything or saying anything without precedent. So far what I’ve said/shown is what I’m doing, why I’m doing it, and final product is in progress. Testing will come later. That’s why I’m leaving one PRT untouched. While I don’t have a full suite of test equipment, I do know someone across town who does. Im a bit surprised at the hypersensitivity. I’m not in the market for selling my services or products, as others in here are. Once I’m done and satisfied with the results, I’ll freely pay to send these to RoyC for his observations and critique. It won’t hurt my feelings if he gives me a “👎”. Like you said, my saying they sound great carries no weight or credibility. As for the 2 midrange drivers, that’s a longer term project. They are both missing the original “guts”. No VC, no domes. So it’s more than just a restore; it’s a new fabrication. But according to you, when I’m done, it will be fruitless, since I don’t have the “street cred” to say they sound good and are viable repairs/replacements.

There’s more going on in this tweeter than you realize. First, RoyC said this PRT has a sound similar to the 4x AR tweeter, but some find it more harsh. That’s where I started. I also read through every post, every page in Tweaks And Mods of the 4X crossover efforts by Speaker Dave. That gave me some background. Other background I have is repairing and modifying countless tweeters over the last 40+ years, so I’ve seen many of the best out there. Also, I’ve taken apart, repaired, improved many phono cartridges; they are merely the reverse of a speaker driver. A driver converts an electrical signal into mechanical sound energy; a phono cartridge converts mechanical energy into an electrical signal faithful to the mechanical movements of the stylus. One is the reverse of the other, yet all the same principles apply. Also, I have dual degrees in mechanical and electrical engineering. After examining the PRT I see why it has distortion, sounds harsh. The cone is rigid-mounted. (The same problem an aged phenolic dome tweeter has in the 3a and 2ax speakers!) The clear dope on the outer edge of the PRT is way too stiff; the cone fights itself to move. It has a powerful motor (magnet and voice coil) so the voice coil is going to move while the outer edge of the cone cannot. That’s a recipe for distortion! It also has no damping for the back wave produced when the cone center moves. A tweeter is like any other driver or a phono cartridge; it’s a tuned motor system. The PRT has a motor (VC and magnetic gap) and a spring. The motor moves the cone. It moves the cone against a spring; the spring is the accordion spyder, a flat sheet of phenolic-treated gauze in the case of this tweeter. To get proper motion, the cone surround provides damping that resists bouncing of the spring-loaded VC. In this case, the surround of the PRT is STIFF and WAY over damped, to the point that the cone can’t move properly. The phenolic ring is an insulator for the VC wires. It allows running the VC wires on the surface of the mounting plate without electrically shorting the wires. AR used black electrical tape to achieve this on the early 3a and 2ax front-wired tweeters (and mids on the 3a). What I’m going to do is retune the surround on the PRT cone. Allow it to move, but match the damping of the surround to the spring force of the spider. It’s exactly the same as matching a shock absorber to the spring of a car’s suspension. In that case, a car will bounce uncontrollably on its springs when it rides over a bump in the road; the shock absorber allows one bounce, then return to neutral position. Same for a properly tuned speaker driver’s cone suspension. Thats why RoyC’s attempt to tame this PRT failed. He added his dope to the existing dope. Adding dope to a surround that is already too stiff won’t accomplish anything.

I’ve been working (designing, building, testing) crossovers since 1981. I wrote my EE thesis on the crossover I built in 1983 for my dad’s Wharfdale W70C speakers (1966). If you start with a driver with a smooth, well-behaved impedance and response curve, watch where your crossover frequencies and fundamental resonance are, it’s relatively easy. Back in 1983 I had to plot those curves by hand. Since the Linkwitz-Riley wasn’t out yet, I used a combined Butterworth-Chebyshev 2nd order 3-way response curve. But truthfully, these early ARs had/have very fundamental crossovers. Inductor on the woofer for low pass, high pass capacitor on the tweeter. The only more simple crossover than these is no crossover. Forgot to include these pics of the PRT tweeter. Installing the wool felt inside the cone, to absorb/damp any sound from the back wave. Also just a small amount of loose wool fiber. Below, the wool felt installed:

I’ve got materials coming to me via Amazon. If all goes well, I may buy 2 of the 4 ohm units and work on those. What I failed to effectively communicate to RoyC is that if these sound good in my Polks, they’ll also sound good in the 4X, or in any speaker, regardless of the crossover. Speaker designers and mfrs always start with a good sounding driver. Even they can’t make a silk purse from a sow’s ear. If they try, they will wind up spending lots of R&D time and increase the parts count in the crossover trying to tame a so-so driver. I think of the extra steps AR used to form the metal grill, add a fiberglass pad, then glue the grills onto the 2ax midrange cone driver, just to attempt taming its squawky nature from using a low cost driver.