ReliaBill Engineer

Today, off and on, I’ve been making adjustments to the tweeter level. Sealing up the woofer surround changed the balance of sound between the woofer and tweeter. I had to increase the tweeter level, meaning (most likely) the upper midrange of the woofer increased, drowning out the tweeter level from before. The sealant is fully cured now on the woofer surround. The bass is more articulate and tighter in the lowest registers. Midrange is increased also.

Just had a listen. Sealing the surround made a BIG difference! The bass is much deeper and tighter. Even the lower midrange is more articulate. I knew the surrounds were leaking air. They didn’t pass the “leak test” when the woofer was pushed in and released. They do now. They return slowly to the neutral position. In my opinion the woofer looks better, too. This is turning out to be a real “gem” of a small bookshelf speaker!

After treating with clear butyl rubber. It’s not tacky, but dry.

Tonight I cleaned the cloth surround of the woofer. I used a microfiber towel and isopropyl alcohol. I have a body mist bottle that’s filled with 91% IPA. I spritzed the towel, then the surround, and gently wiped the surround. Lots of dirt came off! The microfibers grabbed the fuzz, small hairs. Looked so much better after the cleaning! Next to apply a very thin treatment of clear butyl rubber. Before cleaning: After cleaning:

Having examined and listened to both now, the original AR tweeter, the stock PRT, (and my modified PRT), what I said previously has been verified. Since I enjoy my modified PRT (just last night I reinstalled the AR tweeter and listened for several hours) I plan to take the AR tweeter and perform a dissection/autopsy on it. Take pics for documentation purposes, and then reassemble it. It’s the best way to remove the corrosion from around the surround and repaint it. Likely I’ll be selling the pair of AR tweeters. So it may help others to know how to disassemble this tweeter, or just how it’s made; the guts. And re-assembly.

That 1-3/8” phenolic dome tweeter would be very expensive to manufacture today. Thick, large steel faceplate doubles as the outer gap pole. But makes for an outstanding tweeter!

One very interesting thing occurred to me as I was listening to this modified PRT, (post cap replacement) vs the original tweeter and the stock PRT. It also occurred to me when I was using this modified PRT on the 2ax, in place of both the original cone midrange and original phenolic dome tweeter pair. In fact, I stated it when I was listening to the 2ax using this modified PRT. This modified PRT sounds so close to the 2ax mid+tweeter it’s just uncanny! It possesses that articulate and smooth nature of the AR upper tier speakers. Conversely, the original AR-4x tweeter does *not* have the characteristic sound of those upper tier AR speakers! The original AR 4x tweeter is somewhat brash and has (to me) an unwelcome “in your face” quality to its sound. Also, it beams, having less dispersion off-axis. Using the pot to lessen this brash sound results in a more numb, warm and chesty sound that I don’t care for either. There’s no middle ground. I’m pretty sure this is the sound SpeakerDave was trying to tame using crossover mods for the original 4x tweeter.

Today I disconnected the original 20 uF capacitor and added the metal film polypropylene cap. I took a pic of the pot position and measured the resistance at optimum setting, just for my own record keeping. Then I replaced the back cover on the pot. I like these 15 ohm 25 watt pots. Beefy, robust and smooth. The position shown is for the PRT tweeter. The wires from the original cap have been cut: Listening after the cap was replaced gave the expected, and somewhat unexpected results. More crisp clarity from the modified PRT with the new cap. But yet still smooth and effortless. No fatigue at all.

Adding anything to the tweeter dome material will change its mass. Increased mass of tweeter domes is most often detrimental to the dome’s ability to reproduce high frequencies. So yeah, you can add some dope or “coating” of some kind to a tweeter dome….but are you really going to improve it? Change it, yes. But improve it? Perhaps not. Likely not.

I didn’t have to replace the tweeters in my 1965 2ax’s to get full tweeter FR and output. I redesigned the tweeter surround. The magnets are VERY powerful, so I knew greater output was possible from these 2.3 ohm (2.3R) voice coils. AR tied the domes down, inside and outside of the domes, to RESTRICT output. By redesigning the surround, I kept the AR sound and parts, and just made it more “capable”. The level pots allow one to tailor the tweeters’ output from zero to too much. It’s amazing to hear just how good the AR tweeters are, when they are allowed to produce reasonable output! So my AR’s don’t suffer from restricted and “soft” tweeter output. Neither my 2ax’s nor my 4x’s. A simple twist of the level pots allows original AR sound, or a modern AR sound. Just up to your tastes.

I’ve had very good results with these. Different from soft silk domes. They are available in various diameters and DCR, 4 and 8 ohm.

The vender sells, doesn’t manufacture. They are made in China. Good luck getting the Chinese to custom manufacture 2 or 4 domes for you, at 2 ohms DCR .

I’ve used many such domes for various repairs of tweeters. While they may “fit” the tweeter face plates and VC gaps, don’t expect them to sound like what they are replacing. More goes into a dome tweeter design and performance than just a VC and dome shape. The only real success story I have is using some China-made domes to replace the SL2000 tweeter domes made in the 1980s by Polk. But then, those Polk soft Mylar domes had a pretty bad FR issue.

I don’t get the attraction to these MS “augmentation” boxes. Just an angled baffle box with two (4?) 8 ohm tweeters. And probably 2 bipolar electrolytic caps and a level control. Could probably use any one of 100’s of tweeters out there today. Guessing crossed over above 3 kHz and 10 kHz, switchable, even though it says “range”.

But that’s not what MA says about its MS tweeter array. It also speaks about ON AXIS response, and the KLH and AR limited FR:

This second 4x in the pair is going to need a lot of work! Everything measures ok. Just aesthetically not too pleasing.

Micro Acoustics was best known for their cutting styli used to master the lacquer discs for records. For me, there is an interesting tie in here. I have 4 MA cartridges. I had to repair all 4. Snapped cantilevers and/or broken elastomer tubes on the resolver plates. That’s where I learned to work with butyl rubber. I have the MA QDC-1, 2002, 3002, and 309. Each one is a superlative cartridge! Beryllium cantilevers on the 2002/3002 and nude diamonds. Aluminum cantilevers on the other 2. Some of the very best cartridges ever made; they are unaffected by R and C loading. Very linear and dynamic, reproducing minute detail from the grooves. l got ridiculed for it, but I rebuilt the dome tweeters in my 1965 2ax speakers. I rebuilt them to greatly increase the tweeter output by re-engineering the tweeter surround using butyl rubber. My reasoning was that I have the level controls to turn the volume up or down to my own tastes. I love the sound from those phenolic dome tweets! Crystal clear and liquid smooth. I looked at those MA tweeters. But it would have been a waste of money for me. I have my tweeter pots adjusted so I don’t want for more treble. I keep the pots at 80% setting.

Maybe tonight I’ll get the caps changed to metal film p-p. I expect this to exacerbate the problem with bright tweeters on the 4x, both AR and stock PRT. The old AR wax-paper caps tend to “mellow” the tweeters. After that, I’ll apply butyl rubber to the woofer surrounds. As they are, they leak air. I used a rubber tube against the surround and was able to pass air through the rolled surround. I’ll snap a few pics as I apply it. Aesthetically, there won’t be much change in appearance from the untreated surround. But I prefer that. I don’t want a shiny appearance nor a sticky substance on it. Nor a change in the color. I don’t use toluene. I use a more gentle, slower evaporating solvent. It allows better “flow” and soaking of the fabric.

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.)