ReliaBill Engineer
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Posts posted by ReliaBill Engineer
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So after a lot of listening, the 2” paper cone tweeter is a near dead ringer for the original AR-4x tweeter, sonically. It’s very close in sound. It’s not as efficient as the original, requiring turning up its output using the pot. One would have to get creative to add a face plate.
Like the original AR tweeter, the 2” paper cone tweeter suffers a bit in dispersion. Both also suffer from lack of “air” in their presentation.
The modified PRT has a lot more “air” and more “finesse” in its presentation. It adds a presentation very close to my 2ax; more effortless, smooth, natural, and not “in your face” with all genres of music, as the 4x tends to be.
Comparison: Original AR-4x tweeter to 2” paper cone tweeter
ORIGINAL
2” PAPER CONE
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Sound comparison. Original AR tweeter vs 2” paper cone tweeter. I may have the AR turned up a tad more than the 2” paper cone.
To me, the 2” is a near dead ringer for the AR original tweeter.
But neither the 2” paper cone nor the AR tweeter have the “air” of the modified PRT.
But since this thread is about a “replacement” AR tweeter, the 2” paper cone press-fit is an inexpensive alternative, and near “dead ringer” for the original. Just have to get creative for a face plate.
Original AR-4x tweeter:
2” paper cone tweeter:
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Trying out the 2” paper cone tweeter.
My first impression is that it sounds very similar to the AR 2” tweeter. More similar than the PRT, by a good bit.
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Still listening… PRT disabled.
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Another mod in work. The press-fit $1.50 2” paper cone tweeter.
A 12.5 ohm resister in parallel with the tweeter brings its DCR down from 6.05 ohms to 4.34 ohms. The original AR tweeter measures 4.7 ohms DCR. So this should play fine with the 20 uF crossover cap.
Click on the links for pics if pics not shown:
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So ready to try this out. The resultant DCR is 4.34 ohms, very close to the 4.7 ohms of my original 4x tweeters.
I’ll disconnect the mod’d PRT, then wire this directly to the wires coming from the crossover. I’ll run the wires to the trial tweeter behind the PRT tweeter seal, then screw the disconnected PRT back in place to seal the baffle. I’ll mount this trial tweeter in foam on top of the 4x cabinet.
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Bought a few (25) of the 2” paper cone press-fit tweeters to play with. $1.50 each. Built very similar to the AR tweeter. Just no face plate.
6.05 ohms DCR. Using the 12.5 ohm audio grade resistors in parallel, the DCR is reduced to 4 ohms DCR. ( I stockpiled these resistors. 40 of them.) So it should still play nicely with the AR-4X 20uF capacitor in the AR crossover. I’ll wire one up and have a listen, comparing to the original AR-4X tweeter.
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Very little difference between the 2ax and the 5, other than the dome midrange vs paper cone midrange. And that’s easily solved.
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While I have made extensive use of different woofers over the last 4+ decades, I’ve never designed a woofer from a blank sheet.
A woofer designed for an open baffle application has equal loading on front and back of the woofer cone. A sealed enclosure has very different loads front and back of the cone. A ported cabinet has a mixture.
Im just happy that brilliant folks did the hard work of the design. I don’t aspire to try.
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I use my 2 Velodyne active 15” subs. Each has a remote for dialing in phase, cutoff, volume from my listening position. I have them set for 35 Hz low pass, 0 phase. Compliments the 2ax very well.
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Sometimes the same physical principles, used in a more specific way, don’t qualify as patent infringement, and qualify for their own patent. I think of all the different patents for use of the MM principal in phono cartridges, or use of Magnetic Induction in MI phono cartridges.
Shure Bros patented the Stereo Dynetic MM cartridge in the 1950s. But countless other companies came up with their own MM cartridges shortly after.
There are multiple ways to use a sealed volume of air in a cabinet to support a woofer suspension, and tune it. Also multiple ways to design a woofer for use in a sealed cabinet.
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Why are you replacing the original rock wool or fiberglass? It’s not like it wears out.
AR-2ax:
AR-4x:
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Just doing some listening last night.
1960 track by Tommy Tomlinson and Jerry Kennedy. Tommy was Johnny Horton’s lead guitarist. This recorded 2 weeks before the car wreck that killed Johnny Horton. Tommy was badly injured. Also on the track is Boots Randolph, Chet Atkins, with Floyd Cramer and many of the “Nashville Sound” artists throughout the album.
This Mercury recording can sound bright. My phone had a difficult time. But in the room this sounds full, smooth, detailed. No hint of irritating brashness.
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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.
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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!
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After treating with clear butyl rubber. It’s not tacky, but dry.
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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:
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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.
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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!
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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.
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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.
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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.
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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.
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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.
Annoying resonance in more than 1 type of AR
in Acoustic Research
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The tweeters and midrange drivers are sealed, and have their own resonance chambers. So we’re talking ONLY about the woofer and the cabinet here.
On my resto of the 2ax and small 4x, I left all of the rock wool in place, except what I needed to remove for access to the crossover boards. Later, I replaced the rock wool as it was when new. Rock wool doesn’t wear out or degrade over time.
Piano is challenging. I use concert grand piano recordings to test styli I have rebuilt on vintage cartridges. The stylus’ ability to track piano is a key test. Resonance in the stylus assembly is laid bare by piano.
I don’t know why SO MANY people replace the AR rock wool when they refurb the AR speakers!
I haven’t had any resonance issues from the woofers in my AR speakers. The original factory placement of the rock wool works well. It was designed to work well. I still have no idea why folks remove all of the rock wool; it isn’t necessary.