Replacement AR-4x Tweeter: A Mod In Work

[ReliaBill Engineer]

The AR tweeter.

Smaller voice coil, smaller back chamber, smaller magnet than the PRT:

 

 

 

 

 

 

 

 

 

 

 

[ReliaBill Engineer]

Side-by-side:

 

 

 

 

 

 

 

[ReliaBill Engineer]

Using the AR-4x tweeter with my 2ax speakers. I have a 2.4 ohm resistor in series with the tweeter to raise its DCR to 7.1 ohms from 4.7, so the 2ax crossover won’t be affected much at all. 
 

The efficiency of this 4x tweeter is less than the PRT by quite a bit. I have the pot at 100% and it barely rises to the level of the 10” woofer of the 2ax. 
 

So I guess not the best way to listen to this AR-4x tweeter. 

What I can hear from it is quite smooth. I don’t know if the lower output is from age, or if that’s normal for this tweeter.

But switching back to the modified PRT, it has very good definition of details and a nice robust and smooth sound. And treble is very nicely clear and extended, but not brash. 
 

 

 

 

 

[ReliaBill Engineer]

So all that comparing of the AR-4x to the mod’ed PRT has run its course for me using the AR-2ax as a test case.

Now to turn my attention to the 4x speakers. The pots have some internal corrosion. I think I’m going to clean them and reuse them. Most of the corrosion is on the wiper and on the brass rivets at each end of the resistance coil. 
 

Pics to follow.

Now I have to adjust the PRT impedance to match the 20 uF XO capacitor. The original 4x tweeters need some love, cosmetically. Also some loosening of the paper surround in a few spots. Easily corrected, though.

 

 

 

 

 

 

[ReliaBill Engineer]

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

 

 

[ReliaBill Engineer]

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.

 

[ReliaBill Engineer]

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. 

[K6cdxkms]

Following with intense interest,

I have 3 pair of AR4x's to go through.

One of the pair I have already changed out the caps and cleaned the pots, refinished the cabinets, the sound was better than when I got them but they still need help.

 Mid/tweeter  is muddy...at least to me it is.

I liked what you did to the 2ax  tweeter, I could really tell the difference ..even when coming across on a computer.

Looking for a reasonable pair in terms of $ this last week.

Thanks for posting what you are doing.

 

 

[ReliaBill Engineer]

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.

[K6cdxkms]

When you do the check use the sound track from post on March 25. 

I think that track gives a reasonable broad dispersion across the spectrum of what should come out of a speaker.

Maybe its just me ,but I always believed that the speaker should represent the actual music as played by the artist,  If someone wants a more muted/muddy  sound, then adjust the controls to distort to their preference.

Thanks for the work you are doing.

Now i guess it time to cover up with some good old asbestos blankets due to the heat from the flames that will be coming my way.

Ken

[ReliaBill Engineer]

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.

[K6cdxkms]

Something that I have noticed, been coming to this board for over the last 9 years, do not post very much and just started to post again this week, last time I posted was in 2015 When rebuilding a pare of 4x speakers,   and cannot find my posts. No Matter.

Back to what i was going to say,,, I have noticed that many have apparently died, left or no longer post, a few have stayed and provide good to better info up to a point, but few give any sound with their restorations, improvements or modifications, only charts.  

I remember back in the day what I thought AR speakers sounded like but due to age and where I have been over 50 years the ears are not what they used to be, maybe they are what they used to be and the speakers are not , then again I have been told that I have selective hearing loss as well as selective db gain, I found both useful for staying alive.

Never the less, I find myself going back into the 4x's that I rebuilt along with 2 more pair that I picked up due to internal parts and surrounds that need help.

I never was happy with the mids and highs of the rebuilt 4x's I did and hope to brighten them both up a little with out the harsh highs of the new speakers sold today.

I was very impressed with what you accomplished with the 2ax speakers and hope you can accomplish the same with the 4x's.

Maybe I need to find a set of 2ax's and rebuild as you did.  

[Aadams]

I, along with most of the “lurkers and monitors”,  await the operating 4x portion of your project, especially the response curves you spoke of earlier.

[ReliaBill Engineer]

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. 

[Aadams]

Maybe I misunderstood your objective in naming the thread Replacement AR-4x Tweeter: A Mod In Work.  

If your objective was to create a mod to satisfy yourself then your thread is done. 

But, I recall earlier, you stated that you would get response curves from "a friend across town" to show us the difference between a stock PRT and your mod.  You also stated that you would lend your finished mod PRT to another qualified forum member for subjective evaluation of the results in a working AR4 model.   I bought what you said.  Maybe I read too much into your words but they seemed like straight forward English at the time.

 

[ReliaBill Engineer]

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.

[meta_noia_fot]

I’m also one of your “lurkers and monitors.” What engagement did you want with last night’s post? Was I supposed to ask what toppings you got on your pizza pie?

[ReliaBill Engineer]

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. 

[genek]

The problem with YouTube vids, or any sort of uploaded audio file, is that what the listener hears is the original source (in this case, your modded tweeter) filtered through two other transducers (the mic and whatever speakers or head/ear phones the listener is using). And that's excluding any effects of the recording app, streaming compression, etc.

This is why the majority of us put our faith in measurements. Preferably, comparative tests of mods and originals taken with calibrated equipment, though we know that most people doing mods and tweaks don't have access to an actual audio lab with an anechoic chamber.

[ReliaBill Engineer]

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.

[genek]

The PRT is sold as a replacement driver. But the data that you discovered and posted (a valuable contribution to the site, btw) shows that it really is not a suitable replacement and never has been.

Your project is quite interesting to follow. But in the absence of measurements, the finished result will have no more credibility that the stock PRT did before we saw the data you posted. Possibly even less, since you are still the only person who has posted a first hand listening impression.

[ReliaBill Engineer]

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:

 

[genek]

It isn't just about replacing the AR tweeter with another AR tweeter, or about replacing it with a different tweeter that replicates the sound of the original. Even a mod that is intended to alter the original sound can only be verified to be an improvement with objective measurements. Otherwise, you are in the same subjective territory as people who are convinced that their $5000 speaker cable sounds better than 14ga lamp cord.

[ReliaBill Engineer]

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.

[genek]

You actually need objective measurements and blind listening tests, because the single most powerful influence on listener preference is probably confirmation bias.