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.75" AR-2ax Paper Dome Tweeter Dissection

Pete B

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John (ARjohn) kindly sent me a .75" AR-2ax tweeter for dissection/analysis.

The entire magnet assembly is potted into a plastic cup that is also the

mounting flange. The number 561-74??, is printed on the back. The last

two digits are blurred. probably 29. This driver was most likely manufactured

in the 29th week of 1974. Did these ever have a metal frame (cup)? Would like

to hear about the evolution of these drivers if anyone can comment.

The plastic mounting flange was cracked in shipping unfortunately, even

though it was well packed by John.

There's about 1/4" of what looks like normal copper enameled magnet wire

left but I have read about aluminum wire being used in the 3a version. Did

the 2ax tweeter also use aluminum wire?

I commented about this tweeer in another thread where I mentioned that

there are 3 globs of foam (looks like urethane) attaching the dome to the

frame. I also stated this in another thread: What I didn't notice is that there

is also a clear, cellophane looking layer where a normal tweeter's suspension

would be. It seems to me that this layer is probably there to keep foreign

matter out of the magnetic gap as I doubt that it provides much restoring

force. The solid lead in wires being routed through the globs of foam which

would help to keep the flex evenly distributed across the wire to reduce

breakage. RoyC kindly commented to confirm these points. It has also been

mentioned that the clear coating was probably a butyl rubber material.

There is about a .25" hole in the clear rubber edge of this tweeter, where the

voice coil windings can clearly be seen. Other than the whole in the clear edge

and the broken lead in wires this tweeter looks to be in very good shape. The

hole could probably be easily fixed, assuming that new lead in wires can also

be attached. I could then measure the T&S parameters. The untreated hole

acts as a leak in the chamber behind the dome.

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  • 3 weeks later...

Roy has confirmed that the .75" 3a and 2ax tweeters both used copper

wire not aluminum. There are threads about aluminum wire but it was

not used in these drivers as I incorrectly stated above.

There's enough wire left to repair this driver. I'm wondering what gauge

and type of lead in wire AR used? Was the solder joint just held down

with the black electrical tape that is commonly seen?

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  • 3 months later...

T&S Data is questionable since there is hardly any peak in the impedance curve:

There was a clear peak at 2200 Hz, but it was very small due to perhaps damping

in the gap, or perhaps crumbled foam has dropped into the gap:

AR-2ax .75" Dome Tweeter:

Rvc = 5.62 ohms

Fs = 2223 Hz

Qt of .99

Qm = 1.05

Qe = 18.23

Zmax = 6.88 ohms

Zmin (passband) = 6.79 ohms

Z (at 10k) = 7.34 ohms

Lvc = 39.4 uH at 10 kHz

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I have two more samples from my own AR-2ax's, they also show a very minor but

measurable peak. Fs is certainly not very consistent and completely different from

the previous sample.

AR-2ax .75" Dome Tweeter:

Unit = PLB #1

Date SEP 28 1971

Rvc = 5.60 ohms

Fs = 4767 Hz

Qt of .53

Qm = .63

Qe = 3.6

Zmax = 7.06 ohms

Zmin (passband) = 6.79 ohms

Z (at 10k) = 6.86 ohms

Lvc = 28.7 uH at 10 kHz

AR-2ax .75" Dome Tweeter:

Unit = PLB #2

Date SEP 28 1971

Rvc = 5.56 ohms

Fs = 5396 Hz

Qt of .47

Qm = .54

Qe = 3.55

Zmax = 6.92 ohms

Zmin (passband) = 6.84 ohms

Z (at 10k) = 6.93 ohms

Lvc = 30.2 uH at 10 kHz

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