Hello All, I recently was gifted AR 8B bookshelf speakers. Of course the foam surrounds are dry rotted and I have the kit to refoam the woofers.
While I am doing that I got inspired to tweak the crossover thinking that i can improve the sound over the original. The woofer is running free-range with no LP filter. The tweeter uses a 5.0 uF capacitor only. Per what I posted below, it sounds like this was done as a cost-cutting effort and not necessarily to deliver the best sound. I'm thinking a 12 db per octave LP crossover at 2000 Hz is a good idea to prevent distortion. According to AR, the tweeter has an "effective" rolloff of 18 db at 2000 Hz using the single 5.0 uF cap. Should a more sophisticated HP filter be used incorporating an induction coil and a cap, and should the HP filter use a 12 db/octave slope as the tweeter was designed to have a natural rolloff giving an effective 18 db/octave slope? Or should the HP filter use a 3rd order filter? Please advise, the more I look into this the more I am getting confused! Thanks!
While I am doing that I got inspired to tweak the crossover thinking that i can improve the sound over the original. The woofer is running free-range with no LP filter. The tweeter uses a 5.0 uF capacitor only. Per what I posted below, it sounds like this was done as a cost-cutting effort and not necessarily to deliver the best sound. I'm thinking a 12 db per octave LP crossover at 2000 Hz is a good idea to prevent distortion. According to AR, the tweeter has an "effective" rolloff of 18 db at 2000 Hz using the single 5.0 uF cap. Should a more sophisticated HP filter be used incorporating an induction coil and a cap, and should the HP filter use a 12 db/octave slope as the tweeter was designed to have a natural rolloff giving an effective 18 db/octave slope? Or should the HP filter use a 3rd order filter? Please advise, the more I look into this the more I am getting confused! Thanks!
There is no need to incorporate an inductor in the tweeter high pass filter as the combination of the natural roll-off built into the tweeter and the electrical roll-off caused by the series capacitor supplies a more than adequate acoustic roll-off slope.
Such minimalist crossover designs can work very well provided the drivers are engineered with natural roll-offs to suit, as Acoustic Research appears to have done in this case.
In my opinion, adding additional crossover components may negatively effect the integration of the drivers in the AR8B.
To justify a crossover modification, you would need to view the frequency response of the speaker in its original state in order to reveal any potential weakness that might be solved by adding additional crossover components.
If the tweeter has a natural roll-off of 12 dB/octave then adding a 2nd order, 12 dB/octave filter could result in an acoustic roll-off of 24 dB/octave.
Adding a 2nd order, 12 dB/octave filter to the woofer could also result in an acoustic roll-off of 24 dB/octave.
That may be a direction in which to experiment, but I fear it could result in a reproduction that is less 'alive' than that of the original configuration.
Hopefully this, my second contribution, will give a bump to your thread and encourage a vintage AR loudspeaker enthusiast to chime in!
I'd say that the xover frequency at 5kHz is high enough, and the paper cone tweeter robust enough, that a simple cap replacement is all that's required
Some info here: https://blog.speakerworks.com/2017/08/acoustic-research-speaker-repair-ar-8b.html
Some info here: https://blog.speakerworks.com/2017/08/acoustic-research-speaker-repair-ar-8b.html
Note that there is a contradiction in that one source says the AR8B has a 2,000 Hz crossover while the other says a 5,000 Hz crossover.
The 2,000 Hz figure is substantiated here:
The 2,000 Hz figure is substantiated here:
2khz crossover frequency would require a pretty heavy cone and some more inductivity than usual.
2 khz would be very low crossing for a tweete with 6db.
Natural roll off at 4khz for a bass driver is more realistic
2 khz would be very low crossing for a tweete with 6db.
Natural roll off at 4khz for a bass driver is more realistic
2 cents >
I have read that "liquid cooled" / ferro-fluid type tweeters dry-out over time and may actually create a 'goo' in the voice coil gap.
This would not only effect frequency response, but also output efficiency 😕
I have read that "liquid cooled" / ferro-fluid type tweeters dry-out over time and may actually create a 'goo' in the voice coil gap.
This would not only effect frequency response, but also output efficiency 😕
if its dried out there should be too much output of the tweeter at resonance and a bump in the frequency response visible.
This could be cured with some filtering.
But the voice coil can also stuck and then there is a dip in the response
This could be cured with some filtering.
But the voice coil can also stuck and then there is a dip in the response
This begs the question, what did a pair of AR8B speakers sound like in original, factory-fresh condition? Do we know?
We would need to know that in order to provide a reference against which to gauge the effect of any crossover tweaks.
We do have a contemporary review to go by (the one in post #8), but in it I see no criticism of the crossover arrangements.
In the comment section we are told, "Our measurements showed that, within the inherent limitations of its 6-inch woofer diameter and compact overall dimensions, the AR8B is a remarkably good sound reproducer. This was completely confirmed by listening tests".
I suspect you'll be able to tell that I'm a great advocate of "If it ain't broke, don't fix it"!
P.S. I presume you have tested the tweeters to see if they are working properly, since contributors have warned against the effects of the cooling liquid having potentially dried out over time.
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