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Crossover mods for the AR4x II


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Here are three more images for use in comparing the stock AR4x and eWave variants, for example:

Horizontal Polar Plots:

http://www.classicspeakerpages.net/IP.Boar...ost&id=5794

http://www.classicspeakerpages.net/IP.Boar...ost&id=5849

Horizontal Frequency Response:

http://www.classicspeakerpages.net/IP.Boar...ost&id=5778

http://www.classicspeakerpages.net/IP.Boar...ost&id=5828

Horizontal Directivity Waterfall:

http://www.classicspeakerpages.net/IP.Boar...ost&id=5798

http://www.classicspeakerpages.net/IP.Boar...ost&id=5832

Horizontal Polar Map:

http://www.classicspeakerpages.net/IP.Boar...ost&id=5791

http://www.classicspeakerpages.net/IP.Boar...ost&id=5834

Open each member of a pair in a separate window and toggle between them.

[i'll spare everyone the editorial.... ;) ]

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Polar maps of a typical 2-way, Orion, and Nathan as measured by Geddes. It's easy to see how the AR3a, AR4x, Allison, and eWave samples I have measured and presented in these pages behave with respect to directivity by comparison.

2-way is conventional pistonic drivers crossed just below 2 kHz, each evidencing collapsing directivity, ~30° beamwidth at 20 kHz.

Orion is a dipole with controlled directivity, wide dispersion (120°+ beamwidth) to ~10 kHz, and collapsing from there.

Nathan has a constant-directivity waveguide holding a moderate-dispersion pattern at nominal 75° beamwidth out to 20 kHz.

-6dB is deliniated in each by the black line:

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SO ... what is it we want to know? ;)

There's 16 total (that's two pair stacked top left, plus the two in the test mules), but only six working tweeters; just over one in three tweets survived.... :(

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Free-air impedance of 16 vintage ('68 - '88?) AR4x woofers of indeterminate provenance, all operative.

Outliers are the two "squircle" frame units with crosshatch having higher Fs, as documented in an earlier thread, and the single with a deteriorating foam "donut" surrounding the dust cap, lower Fs.

Generally, pairs are best matched. Those with ribbed cones have formed rear fiber mesh inserts covering their magnet openings at the voice coils, smooth cones have masking tape, instead. All units have mesh venting dust caps with the pole piece visible behind.

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For those requesting them, here are the files for the stock AR4x tweeter #6 without the L-pad, both with and without grille, as close as I could replicate the original measurement conditions. Protection cap is 47 uF.

HUH? The impedance depends upon whether the grille is in place or not?

Nope, apparently (check the data files to be sure), but the frequency response certainly does:

AR4x_HF__6_No_L_pad_No_Grille.frd.txt

AR4x_HF__6_No_L_pad_No_Grille.zma.txt

AR4x_HF__6_No_L_pad_With_Grille.frd.txt

AR4x_HF__6_No_L_pad_With_Grille.zma.txt

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Hey Zilch,

I've been modeling my original AR4x network in PC Designer and still find it works well with some slight revision. As you point out the impedance drops down a bit too low with the third order tweeter network. It also seems like the woofer inductance could be a little higher. 1.2mH seems like a good value. If you have the .88mH then and addition of 0.3mH would be good. (Note that my .88 measured 1.0 anyhow).

To tackle the impedance dip an additional 1 ohms in series with the 0.25mH tweeter inductor (inductor to ground) reduces the dip to about 2.5 to 3 ohms. I think most amps will be okay with that. I modeled that as 1.2 ohms for that inductor since I started with about .2ohms in my iron core sample.

3 ohms in series with the tweeter (not an L-pad but either a fixed resistor or non shunting variable R) gives about the right HF level. The end result is +-3dB over the range and has good in-phase summing on the straight out axis. These, of course, are only ideal values for the drivers you had picked but those were in the middle of your group, weren't they?

Curves below,

David

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post-102584-1278431327.gif

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It also seems like the woofer inductance could be a little higher. 1.2mH seems like a good value.

David

That would be convenient, as 50%+/- of the 4x specimens still floating around out there are likely to have a 1.2mh (AR #5) woofer inductor as original equipment. I believe the #5 coil showed up before the halfway point of 4x production, and remained in place until the end.

Roy

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These, of course, are only ideal values for the drivers you had picked but those were in the middle of your group, weren't they?

With respect to impedance, #6 tweeter is an outlier, actually, being 0.4 ohms lower than the group in the crossover region, violet here:

http://www.classicspeakerpages.net/IP.Boar...ost&id=5773

I also found it to be an outlier in resultant system forward axis location.

Woofer #1 has about 2 dB higher SPL output than #2; .frd and .zma for both are posted above. I can probably guess from their impedances which other(s) among the 16 of them here might be good matches, but it'll require more measurements to verify that. If I did all of them, it'd have to be in situ, with both near- and farfield measurements, a major undertaking.... :blink:

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With respect to impedance, #6 tweeter is an outlier, actually, being 0.4 ohms lower than the group in the crossover region, violet here:

http://www.classicspeakerpages.net/IP.Boar...ost&id=5773

I also found it to be an outlier in resultant system forward axis location.

Woofer #1 has about 2 dB higher SPL output than #2; .frd and .zma for both are posted above. I can probably guess from their impedances which other(s) among the 16 of them here might be good matches, but it'll require more measurements to verify that. If I did all of them, it'd have to be in situ, with both near- and farfield measurements, a major undertaking.... :blink:

Hard to pull a "nominal" out of the tweeter Z curves. The DCR variation isn't really so bad, but clearly some are getting damping from the silicon and some aren't. In the end the only significant tweeter variation is that the average sensitivity has a broad range. If an experimenter builds the networks with a variable pot then they can set a pair by ear and have good results.

I think my network worked best with series tweeter R only. That doesn't mean you can't use an L-pad, but should use it in a 2 terminal configuration. A straight 8 ohm power pot would do too.

The woofer network seems to be fairly "universal" too. If you have the 1.2mH network then just adding the 50uf and 6.8 ohm gets the right mid response.

The CD horn versions have excellent performance, but if a classic speaker lover wants to keep at least visual originality, then the network mods will considerably upgrade the performance.

David

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The CD horn versions have excellent performance, but if a classic speaker lover wants to keep at least visual originality, then the network mods will considerably upgrade the performance.

Agreed.

I'll put together a stock pair using woofers and tweeters selected among this lot, measure, and pass the baton to Ken. I believe I can scrounge a decent pair of stock Aetna-Pollack potentiometers from what I have, but somebody's going to have to donate a pair of logos to him for these to be complete originals....

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