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.

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.

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.

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.

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.

I guess AR let the 6’s woofer run open ended, with no coil. Apparently AR just designed around the natural roll off at the woofer’s high end. Butterworth is the natural 6 dB roll off using a coil or inductor, in most cases.

2.8 mH puts the AR woofer roll off between 400-500 Hz @ 8 ohms. But that’s kind of low for the midrange. AR liked gradual XO slopes on their early speakers. More modern speakers, like Vandersteen and VMPS, also like gradual (6 dB/octave) slopes. That 12” woofer cone, with its attached whizzer cone, would have a very difficult time producing audible output at 18 kHz. Too much mass and too much inductance in the VC. Typical usable high frequency output for that is 8 kHz.

The 15” and 12” would need to be parallel wired. The JBL would roll off at 60 Hz and above using the 20 mH inductor, about where the Philips would pick up naturally. The Philips is a 1969-1971 “full range” which will only make it to about 8,000 Hz; it’s a whizzer cone accordion surround woofer. It shouldn’t need an inductor nor capacitor.

Inductor attenuates high frequencies. Capacitor attenuates low frequencies. When used in series with the driver. Some manufacturers just let the woofer play, allowing the voice coil to act as an inductor to naturally attenuate high frequencies on its own. 60 Hz is a low frequency, requiring a substantial inductor on the woofer to roll off all higher frequencies. But your midrange and/or tweeter will be completely unprotected from being driven by too-low frequencies; distortion and likely damage to both will result. Below is a general purpose chart, that’ll get you in the ballpark for frequency using various components and standard driver impedances. It indicates a 20 mH inductor for 62 Hz @8 ohm woofer. That’s a big inductor if air core; smaller if an iron core.

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.

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.

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

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.

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.

Side-by-side:

The AR tweeter. Smaller voice coil, smaller back chamber, smaller magnet than the PRT:

AR-4x. April 11 1968 on the woofers:

They’re original. Need a refoam, though. But that’s always better than someone else’s botched refoam, and usually means original woofers.

Allison made very fine speakers! I used to frequent an upscale audio shop in downtown Annapolis, MD, that sold Allison. I really loved the Model 2! I listened to my first CD in 1983 on those. Alan Parsons Project “Eye In The Sky”. It was a revelation! Im very tempted to buy a pair of 2s I saw for sale.

So my 1968 AR4x use a “#4” inductor on the woofer, measuring 1.02 mH. The capacitor is labeled 20 uF, measures 22.3 uF. According to charts I have, that equates to 1250 Hz XO frequency on the 8 ohm woofer, and around 1500 Hz on the tweeter (4.7 ohms DCR, 5-6 ohms impedance.) All drivers are in full working order in these 4x speakers. So I’ll replace the pots. Get them working with the original caps and have a listen. Later, when my parts come in, I’ll replace the caps. I want to hear the AR sound. Later, I can put my mod’d PRT in, but I have to adjust the impedance of the PRT so the 4x caps “see” the same impedance as the AR original tweeter. And vice-versa, so I can listen to the AR tweeter on my AR-2ax speakers. (That’s just for fun, and to familiarize myself with the sound of the AR paper cone tweeter.)

For those perusing this thread. The PRT 8 ohm tweeter:

Found this on-line for the 8 ohm PRT:

So, I got the 4x pair today. Neither has been molested. Both tweeters are good, and measure 4.7 ohms DCR. Pots are bad. But I have new ones. I have new caps. All the seals were in place, undisturbed. Woofers are intact, no glue failures that I can see/feel. I flexed the cones; spider looks good, surround looks good. Cone is good. Paper scrim intact behind the woofer, rock wool never disturbed. Cosmetically they need TLC. But I have a good foundation here. April 8 1968 on woofers and tweeters.

Should be here today.

Another sound check. I use this 1962 stereo album to check for resonance in cartridges. The xylophone tends to cause styli problems. Using a Shure V15-IV which is very well controlled. My phone’s ability to handle the resonance is challenged. In the room, there is no ringing or resonance.