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Crossover mods for the AR4x


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David,

Great job - and a superb thread. It's getting me thinking about refining the crossovers in my AR-7's. The tweeter is dead-flat above 2.2k, but there's a bump of about 5 db centered at 1.7k. My AR-17's, measured under identical conditions, are flat right though the midrange. I know the AR-7's should be measuring better!

Looking forward to further posts on your AR-4x's.

Best Regards,

Rich W

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Here I played with the external network values until I got a reasonable fit to the response of the internal part.

David

David,

What method do you use to interface your external crossover network with the speaker drivers? Does it require putting holes in the cabinet?

Best Regards,

Rich W

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David,

What method do you use to interface your external crossover network with the speaker drivers? Does it require putting holes in the cabinet?

Best Regards,

Rich W

I've just removed the tweeter level control and run a pair of wires through the hole. The woofer network is still connected through the input terminals.

David

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"The story so far..."

We've been dissecting an AR4x and trying to improve the crossover while keeping the cabinet, drivers and external appearance stock. Last time a 2nd order network on the tweeter gave a much more classic bandpass than the stock 1st order, and mated pretty well to the woofer. Midrange response was much flatter, only marred by some residual dip at 2k. Phase blend between the units was a little off and I had a suspicion that a 3rd order tweeter network might help in two ways: It would allow a little more control of tweeter shape to maybe fill in the 2kHz hole, and the extra phase shift would pull the units better back into phase from their current no-mans land.

Regarding the shape control that comes with order, higher order means more degrees of freedom, more bends that can be put into the curve. First order can only sag and rolloff. Second order can bump upwards, then reverse direction and roll off. Third order can sag, then bump then fall again. The challenge is to put the inflection points in the right place. A third order network would have a better chance of EQing the 2kH dip in the raw tweeter response by placing its corner bump at 2kHz.

Attached is an early attempt at a third order tweeter network. I seemed to be stuck with either too little at 2kHz or too much at 1300Hz. Although other measurements showed the phase blend to be better, total response flatness wasn't any better than the previous 2nd order filter.

post-102584-1267323308.jpg

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I was able to get a pretty good shape out of the tweeter on its own with the right network. This curve has a 22uF entrance cap, a .25mH inductor to ground, then 10uF to a 4ohm resistor and finally the tweeter. (Need to learn how to draw circuits.) The previous hole at 2kHz is gone. That means that the network's corner bump is just the right height and placement to fill it in.

The problem is that adding the woofer will still give us the bump just below crossover.

post-102584-1267324287.jpg

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I had started this project thinking that the woofer response, with its single inductor crossover, was fully adequate and wouldn't need changing. But matters weren't cooperating. I added and pulled out a pair of wires from the woofer terminals so that I could experiment with a second order woofer network.

This would generally be a large capacitor across the woofer terminals. Alternativley a capacitor in series with a resistor (both across the woofer) could give something less than second order. Usually that is called a Zobel network, but that term only properly applies if the capacitor and inductor are carefully choosen to exactly flatten the woofer's inductance rise. Alternativly if the resistor is 1 or 2 ohms, rather than a Zobel it is becomes a mechanism for softening the corner (reducing corner Q). I've used it here as a mechanism to pull down the top of the woofer's region. Best value in the end was 100uF and 5.6 ohms. Its effect was to pull down the 600 to 1500 Hz region by 2 to 3 dB.

post-102584-1267325146.jpg

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Here is the same curve with phase. The phase lags an extra 20 or 30 degrees, further proof that it is degenerate 2nd order, more an EQ trick than an increase in order.

post-102584-1267325533.jpg

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So lets put the pieces together and see what we've got. First the individual woofer and tweeter with phase curves.

This phase blending is as good as it gets. The microphone was placed at 1 m and exactly midway between woofer and tweeter. A good phase match at this position means that the crossover polar will be symmetrical with no peak above or below axis. Don't confuse this with linear phase. Such a system can't be linear phase, with the woofer significantly deeper than the tweeter. We just want the most seamless blending of the units.

From about 900 to 3kHz the phase curves lie one on top of the other. Beyond those frequencies one or the other of the units has fallen to a low enough level that any phase deviation doesn't matter.

The corner shapes are pretty good although there is a little bit of asymmetry and maybe a touch too much overlap.

post-102584-1267326509.jpg

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And here is the system with elements combined (blue curve). Because the phase blends are good the blue curve is everywhere greater than the red and maroon individual sections.

post-102584-1267326705.jpg

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This curve pair shows the effect of the added woofer elements (the shunting 100uF and 5.6 ohms). Blue is without, red is with. This should reduce a little midrange promenance. We'll see, when we get to the listening tests.

post-102584-1267327041.jpg

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And finally a comparison of the original network (red), the best 2nd order network (blue), and the new 3rd order network (green). we've been able to further reduce the variation and can now fit most of the curve into a 4 to 5 dB window. Note that I did the latest measurements in a different room and position, so the low frequency room effects (especially from 100 to 200Hz) are different.

Perhaps its a more complex network than absolutely necessary, but there are many speakers on the market that would pursue this complexity in order to achieve the best overall result.

So far lll of this has been done by measurement, but there will be a listening phase next. I like to get the sections working and blending as well as possible and then fine adjust a few key components by listening test. Certainly the tweeter resistor and the new woofer damping resistor would both nicely control level for a couple of octaves in their respective bands. They will be tweaked if needed.

post-102584-1267327754.jpg

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Almost forgot...

Carl had asked for a nearfield woofer curve. I don't have an impedance curve but it looks like roughly a Q of 1 or so at 65 - 70Hz. The woofer and network inductance tends to obscure the exact Q.

post-102584-1267328174.jpg

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Dave, when you're satisfied that you have everything just so, can you post wiring diagrams of your "final solution?" Some of us don't do well trying to figure out how to mod or build circuits based on written descriptions. :)

It seems like an awful lot of trouble to go to to improve what was Acoustic Research's least expensive loudspeaker of the time, clearly their maximum compromise of performance for the sake of economy. When AR set about building a better model, they took an entirely different route, they built three way systems recognizing the inherent difficulties of matching only two drivers to cover ten octaves.

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It seems like an awful lot of trouble to go to to improve what was Acoustic Research's least expensive loudspeaker of the time, clearly their maximum compromise of performance for the sake of economy. When AR set about building a better model, they took an entirely different route, they built three way systems recognizing the inherent difficulties of matching only two drivers to cover ten octaves.

40 years after the fact, there's not much else to do with them, is there? It's not as if you can just run out and buy AR's latest version at Best Buy.

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40 years after the fact, there's not much else to do with them, is there? It's not as if you can just run out and buy AR's latest version at Best Buy.

You can take a idea and develop it to its most absurd limits. The idea of a two way 8" acoustic suspension small bookshelf louspeaker went from the $57 AR4/AR4x to one costing hundreds of dollars probably Pete Snell's model K to one costing many thousands of dollars such as Peter Qvortrop's Audio Note Model K. It would be interesting to see how the best version of Audio Note's expression of this idea which I think costs over $15,000 stacks up against AR4x whether in its original form, as tweaked here, or as tweaked using other means such as a graphic equalizer. There might not be much difference.

http://www.audionote.co.uk/products/speakers/an-k_01.shtml

A somewhat larger 2 way 8" ported design model E sold in the past in versions costing up to $125,000 per pair. This in part due to two outboard capacitors in separate enclosures made of pure silver.

http://www.audionote.co.uk/products/speakers/an-e_01.shtml

The use of Russian birch for the cabinets is claimed by the manufacturer to enhance the sound by acting like a sounding board in the construction of a musical instrument such as a guitar due to the way it resonates. I've heard it and wasn't impressed.

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It seems like an awful lot of trouble to go to to improve what was Acoustic Research's least expensive loudspeaker of the time...

Absolutely, but this is a hobby, an interesting waste of time by definition.

In the end I'll have some totally stock looking AR4's with performance as good as most any 2-way system built in the 60's up through the modern era.

In Hot Rod terms: a Resto-Mod!

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Dave, when you're satisfied that you have everything just so, can you post wiring diagrams of your "final solution?" Some of us don't do well trying to figure out how to mod or build circuits based on written descriptions. :)

Will do. Zilch is already helping on this. :)

David

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It would be interesting to see how the best version of Audio Note's expression of this idea which I think costs over $15,000 stacks up against AR4x whether in its original form, as tweaked here, or as tweaked using other means such as a graphic equalizer. There might not be much difference.

I don't know about you, but being able to take a 40-year-old speaker that sold for $59 each and make it sound not much difference from a modern pair of $15,000 speakers just by updating its crossover doesn't sound like a particularly absurd limit to me. Although it probably will end up moving this thread to mods and tweaks.

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Almost forgot...

Carl had asked for a nearfield woofer curve. I don't have an impedance curve but it looks like roughly a Q of 1 or so at 65 - 70Hz. The woofer and network inductance tends to obscure the exact Q.

Thanks Dave for the woofer curve. It's pretty much like what I've obtained. Also, nice work on fine tuning the xover. When you get it tweaked after your listening tests I hope someone else who can take measurements (Zilch? you there?) tries your new xover to confirm the effect. I don't have a 4x. Otherwise I'd give it a shot.

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I ran across this curve on the Stereophile website. It illustrates a point made way back in post #2 about the problem of a first order network on the tweeter. While first order (a series capacitor) should give a 6dB per octave electrical rolloff, this only works well when the load is a resistor. When the load is a tweeter and its realworld impedance curve, then things won't be so tidy. At the tweeter's resonance its impedance will go quite high. This is a "light load" on the network and the voltage will rise also (in the graph the bump at 500Hz). At frequencies above that, the tweeter impedance will drop to near its DC resistance and voltage at the tweeter terminals will fall. Finally, at highest frequencies the inductance will bring the tweeter impedance up again and the voltage will rise again.

One solution for this would be a Ferrofluid of the heavy damping type. It would drop the impedance rise at resonance (lower Qm) and give a more classic highpass.

For these reasons text book crossovers are never very successful. A software based approach must always bring driver impedance into the calculations as the load on any proposed network.

David

post-102584-1267457858.jpg

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Dave, when you're satisfied that you have everything just so, can you post wiring diagrams of your "final solution?" Some of us don't do well trying to figure out how to mod or build circuits based on written descriptions. :P

Which L Pad are people using from PE? I've read both good and bad about using these.

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Which L Pad are people using from PE? I've read both good and bad about using these.

Download "Restoring the AR-3a" from the library and check the section on level controls. The parts and procedure for replacing original pots with L-pads are the same.

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Is this close to a wrap?

[Having found their missing AR4x tweeter pair, the West Coast contingent is itching to flex its mettle.... :P ]

I want to build up a pair of crossovers and see if the values are a "best fit" to both tweeters and woofers that I have. Then document the inductor resistances and confirm polarities, etc. (Hey, there's a lot of pressure to get these things right!)

But don't let me stop you from doing your own mods. You are more than capable!

David

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