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AR-2X modifications?


dynaco_dan

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Hi there;

I was reading the AR-2X modification write-up done by, Sam Payne, in the AR library, under AR original classic models '54 - '74.

He removed the original Sprague 20uf 50 NP elect cap and used a Solen 10uf polyp cap in its place.

Did that change the crossover point?

What would have happened if a 20uf Solen polyp cap was used?

Why might he have gone to such a drasticly different size?

Is that required when going from elect NP caps to Polyp caps?

Obviously there is more here than meets the eye.

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Hi Vern,

>He removed the original Sprague 20uf 50 NP elect cap and used

>a Solen 10uf polyp cap in its place.

>Did that change the crossover point?

Yes, he raised it. He also replaced the tweeter, added l-pads, and replaced the fiberglass damping material with poly.

>What would have happened if a 20uf Solen polyp cap was used?

The crossover point would be lower than with the 10uf cap. Since he replaced the original tweeter and replaced the pots with l-pads, the crossover point would still be different than the original design even if he had stayed with the 20uf cap.

>Why might he have gone to such a drasticly different size?

He was basically designing a new speaker system by replacing one of the two original AR drivers.

>Is that required when going from elect NP caps to Polyp caps?

No....A 20uf cap is a 20uf cap. Differences such as esr (eqivalent series resistance), power handling, and dissipation factor may exist but that would not call for changing the value of the cap.

>Obviously there is more here than meets the eye.

...or "more than meets the ear" so to speak:-). He used an AR woofer and cabinet to build a new speaker system, and seems pleased with the outcome. He mentions at the end of his account that he changed (lowered) the crossover point again after some measurements. He probably ended up with something closer to a 20uf cap anyway.

Roy

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Hi Roy;

Thank you for your commentary.

My next question is regarding the replacement of capacitors and the adding of a small, for example, a .5 ohm 10 watt w-w resistor in series for matching the ESR?

Is this required in all or some cases?

How would one know whether they need a resistor or not?

Is this a standard practice when replacing an old NP electrolytic cap to a new Polypropylene cap?

Does the capacitor need to be on the hot or ground lead of the tweeter, in this case?

I have read where other than Polyp caps have been suggested, in other write-ups.

Is there, other than cost or availablity, a difference and or an advantage or dis-advantage to using them?

What happens if one changes the connection from plus lead to negative lead capacitor path?

I assume that a capacitor is usually in the hot lead to a driver?

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Vern,

Modern film caps, such as polypropylene and mylar, are more durable than electrolytics, and that is probably the most compelling reason to use them. There are no special requirements to replace any old cap with any new cap of equal value...and all modern caps typically exceed the power handling requirements of the old.

Trying to duplicate the parameters of the old caps can be detrimental to the scheme of things, ESPECIALLY if those parameters are actually degraded from the old caps' original specs. With that said, I and others have successfully experimented with adding small amounts of series resistance (.5 ohms or less) in line with new caps to try to emulate the character of the old caps, and subdue the harshness that some folks associate with new film caps. This is by no means "required", but worth a try if dissatisfied with the new caps in an old speaker.

Most folks seem to be happy with the sonic results, and peace of mind, achieved by simply replacing old caps with new caps of equal value, regardless of what the new caps are constructed of.

Roy

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Hi, Roy!

This business of replacing the old caps and replacing with new has me a bit puzzled.

Everyone seems to claim "harsher/brighter" sound with new caps. According to the math, however, new caps should LOWER the xover frequency (caps lose capacity as they age). Naturally, new caps offer less resistance over the balance of the driver frequency range as well.

Roy, what bothers me is that lowering the xover frequency would be perceived as "harsh". I would predict just the opposite that the speakers after re-capping would sound more "mellow".

Roy, what's really going on here?

Regards,

Jerry

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>what's really going on here?

It depends... NPE capacitors made in the sixties or even in the eighties have likely drifted far from their original value. I have seen 2x drift, Bret measured one 6-uF device as 22 uF. If leaky, water from the electrolye diffuses away and the acid concentration increases resulting the increase in capacitance. Also the internal resistance will increase. Old electrolytics left unused will need to be reformed, but after 40 years, some may never recover completely --especialy if leaky. These caps did change the crossover frequency to the point that astute listeners were offended and replaced them with newer devices.

The issue is with what were they replaced? And here you will find significant differences of opinion between listeners; the comments here relate to the mid-and hi-x-o circuits. If the NPE was replaced with a polypropylene, the sound will depend on the replacement device. Some are made very cheaply, others at considerable expense. The cheap devices use a low-quality dielectric starting material. If the original polypro film (it was formed into a thin sheet by cooling, as it was pulled from a hot liquid) was pulled too rapidly and from poor starting material, it could have a lot of pinholes or thin spots. After the aluminum has been evaporated on its surface and a voltage applied, these spots will break down. Winding this film under high tension at high speed can make it worse. When voltage is first applied to a device such as this, it will produce a lot of noise, as the aluminum is blown away from the tiny thin spots in miniature arc discharges. This noise will go away after many hours of applied voltage and all the thin spots have been destroyed--one member recommended the use of several hours of pink noise to expedite this process. In general, polypro capacitors rated for a higher voltage will have thicker dielectrics and fewer pinholes or thin spots than those rated for lower voltage. But they will be physically larger and more costly than their lower voltage counterparts.

However, there are other issues. Polypropylene can be both crystalline and amorphous -- the good stuff is about 95% crystalline and has fewer electronic traps than the poor stuff. In in the opinion of many of us who have used capacitors made from high quality material, the resulting sound is not harsh. I do not believe the harshness relates to a change in crossover frequency, but rather to the differences observed by some listeners in the brands of capacitors with which they have experimented.

These issues do not appear to be a problem in woofer by-pass capacitors. In this frequency range, no one seems to have reported any problems with less expensive capacitors. However, since NPE do have a higher internal series resistance than polypro or mylar, etc., it is best to add a half-Ohm series resistance when they are replaced. This does not seem to be as important in the mid- or hi-range x-o, as these all have series resistors as part of the circuit. The amount of series resistance in the sizes of these NPEs is small compared to the resistances of the inductors and resistors already in these paths.

Hope this helps somewhat; however, this is a topic that will always generate controversy.

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Hi John;

Thank you very much for your very thorough write-up.

It certainly helps clear up some of the mystery, regarding replacing existing upper driver caps, with newer polypropylene caps.

I've written before about, for just one example, a Stereophile reviewer would wire a pair of speakers out of phase.

Place them face to face and run sine waves or ? for maybe a week or two to burn in the speakers, caps or cables?

We need to bring a new cellphone battery up to charge before we start to use them.

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>>what's really going on here?

>

>It depends... NPE capacitors made in the sixties or even in

>the eighties have likely drifted far from their original

>value. I have seen 2x drift, Bret measured one 6-uF device as

>22 uF. If leaky, water from the electrolye diffuses away and

>the acid concentration increases resulting the increase in

>capacitance. Also the internal resistance will increase.

John, thanks ever so much for your thoughtful reply. Now, I’m beginning to understand. Seems the "harshness" is due more to the type and condition of the new caps. That is, it has nothing to do with the xover frequency.

I'm a little surprised that those old caps can actually increase in capacity as I've never seen that. I've replaced tons of bad caps over the years and by far the biggest issue is the caps LOSE capacity and at the same time they develop very high internal resistant. Eventually they appear to the signal as an open circuit. (Now I'm taking circuit caps NOT filter condensers. Anything can happen with those things. I’ve even seem filter condensers short out and take most of the power supply with ‘em.)

Anyhow, once again thanks, John, for the clearing up the “mystery”.

Regards,

Jerry

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John is the man when it comes to understanding capacitors! Thanks John.

The only thing I can add is that speaker designer, Ken Kantor (AR, NHT, Tymphany) suggested here awhile back that the main sonic difference we may be hearing between caps are differences in ESR.

On another audio message board, a fellow with the capability to measure ESR stated that he was measuring higher ESR in mylar caps than polypropylene, as well as some electrolytics. I found that information interesting.

Since most of us do not have a clue as to what is going on inside the caps we are contemplating to purchase, series resistance seems like something worth investigating. Sometimes, with luck, the solution to a mystery is the most obvious choice.

Roy

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  • 1 month later...

Hi Brad;

I saw your very thorough old versus new AR-2AX write-up in the library, very nice job done, Brad.

Near the end you discussed the AR-2X speaker system.

I assume the original AR-2X speaker came out at the same time as the old AR-2AX system did.

It would have included an aluminum 6 bolt woofer and a 3 1/2" CTS fiberglass mid/tweeter.

Do you know that crossover frequency?

It was 1,200hz with the 2 1/2" AR-4X tweeter.

I recently obtained a mint pair of AR-2X's with the AR-4X tweeter and the 6 bolt to 4 bolt adapted woofers.

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>Hi Brad;

>

>I saw your very thorough old versus new AR-2AX write-up in the

>library, very nice job done, Brad.

Thanks Vern, I would like to work over that doccument in the future to further discuss the evolution of the "new" AR-2ax. I also would like to include some pictures. It may take some time, as there is still a few unanswered questions.

>I assume the original AR-2X speaker came out at the same time

>as the old AR-2AX system did.

Correct, 1964

>

>It would have included an aluminum 6 bolt woofer and a 3

>1/2" CTS fiberglass mid/tweeter.

Correct

>

>Do you know that crossover frequency?

2000 HZ, or as they would say in 1964 2000 CPS.

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Hi Brad;

Thank you for the speedy update.

It appears that there were, in the case of the AR-2X's at least, 3 or even 4 different woofers, 2 different tweeters, 2 different caps and maybe 2 different coils.

Maybe in parallel with the AR-4/AR-4X's, caps and coils?

Possibly also the AR-1X?

And a partridge in a pear tree.

Oops wrong site.

When I have some spare time, in a while, I will be removing the woofer to see what dates, etc, I can see.

Photos if of interest, maybe even.

Of minor interest, I removed one cloth, as it was factory installed upside down.

I haven't given the other a closer look yet.

The speakers came in their original AR cartons which had some obvious wear and missing pieces.

If the cabinets had been properly packaged, they would be 99+% pristine after all of these years.

After I finish working on the flood repair and damage to my home and Wally's scanning I'll be able to relax a little more.

I informed Mark about the scanning work progress and he will also need time to see what he will do with the information.

Wally has a vast amount of really neat information that was never thrown out, thank goodness.

Unfortunately, I had thrown lots of goodies out many years ago.

With a full time job, Mark, doesn't have a lot of free time.

It would be really nice to have a mini Hifi convention soon.

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>I have never been inside a AR-2x. Keep good notes and keep us

>posted on your findings.

Hi Brad;

With the grille cloth removed from the one cabinet, I don't see a problem removing the woofer and exploring for more information, at a later date.

The cartons are matching AR-2X original's, with the serial numbers still intact.

I have seen many AR original cartons and stuffing pieces, especially when I worked at the AR local warantee depot here, many moons ago.

AR was exceptionaly generous in providing both the warantee replacement parts and packing materials, at no charge, at that time, late '60's to mid '70's.

AR seemed to invest a lot of thought into the design of the shipping cartons and stuffers.

They even wrapped the speakers in brown paper and taped an additional piece of cardboard onto the grille cloth area before packaging them.

There was an old AR ad showing a stack of 2 or 3 speakers high, AR-2AX's I believe, in their warehouse and pushing the stack over to show how strong and protective their cartons were.

I will tell another story about why this would not be an advisable type test.

I received two AR-2's recently, both basicly totally destroyed in transit.

I assume that one cabinet fell on it's face with terrific force.

You may have read elsewhere here about 1 3/8" +/- thick cabinets fronts, that was just a small section surrounding the woofers only.

There was a wooden section, nailed, but not glued, with too short of finishing nails to the inside of the front of the cabinet.

The cabinet would have appeared to be extra thick, if I had removed just the woofer, this small section is where the Tee-Nuts are installed.

In this one case, the nails came loose and the woofer and the small wooden section held tight, rolling around in transit.

It ripped all of the wires off the destroyed crossover.

The woofer was totally destroyed in the process.

The Tee-Nuts held well though, there had to be something positive about this purchase, heh?

I am very sad to see this old baby go, but not to the dump.

To be very positive for this outcome, that seller, an exceptional gentleman, is sending me, at no charge to me, replacement speakers.

As of today's dual tweeters received, I've lost about 13 out of about 15 purchased.

DOA and really poor packaging practices.

I was only wanting 2 or 3 spares.

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