ReliaBill Engineer

Butyl rubber surround. Not fully cured until tomorrow. I did fire it up for a few minutes, to check it out. But the process for making the surround is too labor intensive. I’m going to try a different method.

Decided it’s a better fit here. This modified PRT sounds much smoother than stock. Below, stock PRT. The clear dope constrains movement of the tweeter cone. Surround removed (below): Below, after internal damping was added, and a new (very thin) butyl rubber surround was added:

Waiting for the surround to cure. I used wool felt as batting inside.

Germane to finding a replacement tweeter for the AR-4X: Not naive at all. I’ve listened to individual drivers for more than 4 decades. Every one of them, be it a tweet, mid or woofer, has a “characteristic” sound. For the interest of those using these PRTs, a partial dissection: I removed the cone surround: I didn’t expect to see it, but these tweets use a spider! The spider secures the dome to the rear metal housing, at the cone/voice coil interface. Also, there is no internal damping (batting) inside of the cone, in the void behind the cone. No felt, no fiberglass, no poly fiberfill. The cone cannot be easily removed from the VC gap without risking damage the cone. Hard to see, but in the pics below, one can see the phenolic treated “gauze” beneath the cone. I’ll be calculating the type, amount of damping, 1000Hz to 15 kHz. Then redesigning the surround.

“I bought 2 of the 8 ohm PRTs to play around with. I don’t know if I’ll have any more luck than you did smoothing them out. But it will be a fun learning experience. I’ll test them using my Polk speakers, since it’ll be easy to adjust the mounting, and I have impedance adjusting resisters, and I’m very familiar with the original sound of these Polks. Also, the efficiency matches as well as useable FR.”

Really? Are you just trying to be difficult and defensive? Or am I misunderstanding you? So….I guess after I get these sounding smooth, I’ll have to buy a pair of 4X speakers in which to install them. But even then you’ll be skeptical, regardless of what I say, show, or even record. Likely I’ll have to send them to you for auditioning. But I’m very hesitant to do that.

Relevant to the 4X? Aren’t these PRTs being used to replace original AR tweeters in the 4X? Haven’t you yourself, and others tried to make them more suitable as a replacement? Or did I misunderstand? Listening is always the final test. Test equipment has its place, but the final test is in the listening. Right? The Polk RTA-8T speakers I’ll be auditioning these tweets in are also 2-ways. They’ll make for a good test case for these tweeters. Sink or swim.

Not in the 4X. 1988 I restored a pair of McIntosh XR7 speakers.

I said this in an earlier post: “I bought 2 of the 8 ohm PRTs to play around with. I don’t know if I’ll have any more luck than you did smoothing them out. But it will be a fun learning experience. I’ll test them using my Polk speakers, since it’ll be easy to adjust the mounting, and I have impedance adjusting resisters, and I’m very familiar with the original sound of these Polks. Also, the efficiency matches as well as useable FR.”

Just got these in. Hopefully I’ll have time tonight to make a few changes. I bought 2. One will remain unmolested.

There were extensive tests done in the early ‘80s on tweeter pistonic motion. Laser interferometry was used to analyze various cone and dome tweeters, design and materials. Cone tweeters tended to have the highest distortion caused by vibration nodes; different velocities and phase from center to outer edge. Domes had much less; even less depending on material and surround used. Paper cone tweeters just cost less than well designed domes. Paper cone tweeters tend to be used on the bottom tier speakers of any given mfr, 1950 to now. (The paper cone mid on the 2ax is another example of price point compromise.) There are exceptions, of course. The paper cone tweeter I showed above in the wharfdale W70C was replaced with a modified dome in the W70D, but the W70C was an expensive iteration of the paper cone tweet. Even that didn’t work well. Poor dispersion and a harsh character to its sound. The 4X was not AR’s flagship model. Not by a long stretch. The below tweeter, while not paper, is a very good design for a cone tweeter. It’s actually smooth and well behaved.

I disagree. Distortion usually arises from lack of pistonic motion from the driver. It can be coloration, or most often from resonance(s) in the cone or dome. It’s a lack of faithfulness of the driver’s output compared to input. 2nd and 3rd harmonics produced in addition to the fundamental signal. Cone breakup is common from paper cone tweets. The outer edge of the cone can’t keep up with the center motion driven by the VC. Out of phase output from the outer edge compared to the center of the cone often results. That’s why AR used dome tweeters in its upper line speakers, so the outer edge IS the driven part where the VC is mounted. The PRT has an outer edge restrained and a center free to move by the VC. Can you imagine a woofer with a secured outer edge instead of a surround, trying to reproduce low frequencies? Or lower mids? Or upper bass? I remember the paper cone tweeter used on the ‘66 Wharfdale W70C. It had similar issues of harshness and beaming. It was a paper cone mounted on its outer edge to a grey felt ring. I wound up replacing it with a dome tweeter back in 1982 on my father’s speakers. I couldn’t get that Wharfdale tweeter to sound good.

Inherent frequency response and behavior *is* the distortion, if it’s in the pass band. A crossover can limit distortion if it occurs below or above the XO frequencies. Even with the crossover, there shouldn’t be distortion. Rather, it should have a smooth roll off at both ends of the FR. And an inherently smooth pass band. The AR-2ax mids are harsh. AR really liked using fiberglass batting to damp distortion. I bought 2 of the 8 ohm PRTs to play around with. I don’t know if I’ll have any more luck than you did smoothing them out. But it will be a fun learning experience. I’ll test them using my Polk speakers, since it’ll be easy to adjust the mounting, and I have impedance adjusting resisters, and I’m very familiar with the original sound of these Polks. Also, the efficiency matches as well as useable FR.

Duplicate post…

Which I guess speaks well for the durability of the woofer mechanics. The voice coil can’t shed heat fast enough, and 30-1000 Hz is where the majority of music power is. Looking at the suggested wiring of the fuses, it doesn’t protect the tweeters by themselves. The fuse is seeing the total current fed to the loudspeaker. So is it going to protect the tweeter? Kind of doubtful. It’s a crapshoot whether enough current still passes to the tweeter to blow its VC before the fuse blows.

I don’t think a fuse would have protected your tweeters from the sudden *thump* of a mic input:

I’m thinking of buying a pair to play around with. Harshness comes from distortion. So have to reduce the distortion.

IMO, yes, those caps need to be replaced if original. A fuse could be added, but putting inside the cab would be a pain to replace if you blow it. I’d put it outside on the terminal plate with a fuse holder clip. Some audiophile purists would say to never use a fuse in a tweeter circuit; but these aren’t $100,000 Wilson Audio speakers, either.

Last pair of 4x tweeters I worked on had the same issue. I used an Exacto knife, new blade, to cut the glue holding the paper cone edge. Then removed the solder from both terminals. Desoldering braid worked for me. Then I cut one remaining lead. Lifted out the cone+ voice coil. Checked coil condition and continuity. Both were good. I soldered extension wires onto the originals. Threaded the extensions into the terminals. No solder yet. I then ran a small bead of thinned black RTV under the edge of the cone, centered it. While still moveable, I used a 1.2 volt battery cell to energize the leads, to center the VC. Then pressed the cone edge into the glue all around. I rechecked the centering by using the battery cell. No scraping sound. I let the glue cure. Then soldered the wires to the terminals. Pretty much what I do for repairing dome tweeters also. I like using thinned RTV because it’s reversible. If I find that I goofed, an Exacto blade can be used to cut the glue and start over. AR was fond of using fiberglass batting to damp mids and tweeters.

I’ve been comparing a new pair of KLH 5s to my fully refurbished 1965 AR-2ax speakers. Hands down, the 2ax’s are preferable. The 5s do have slightly lower bass. But in other areas of imaging, “air”, realism, FR “balance”, smoothness and extension of treble, vocals, the 2ax’s really shine!

Did you have the original diaphragms and voice coils? If not, what did you use as replacements?

True. But it’s difficult to address distortion from mechanical degradation issues when the VC windings are bad, reading discontinuity from being fried or severed. Mechanical degradation becomes secondary to a bad voice coil. This mid driver read “open circuit”:

I usually remove the drivers from the circuit, then test the drivers using a DMM set to 200 ohms. This tells me if the VC has continuity and at what DC resistance. The DC resistance tells me what impedence range the driver is in, 2/4/6/8 ohms nominal. All necessary to repair or replace the driver.

One of the mids, after arrival:

Alas, no. Neither mid has the dome+VC. I wound up getting a “forced” refund from the seller. I asked him prior to purchase what the DCR was of each. He said “open circuit”. But he failed to say that neither one has any wires to measure from! eBay agreed with me, that the seller was deceptive in his description and communications with the buyer (me). Since then I have the raw domes, but need the VCs. I may wind up having to make my own VC formers, and winding them myself. I have kapton. I have various spools of wire, 24-36 AWG. But not sure I want to go that route.