Restoring the Acoustic Research AR4xa loudspeaker

Discussion in 'Hi-Fi and General Audio' started by RobHolt, Oct 7, 2013.

  1. RobHolt

    RobHolt Moderator

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    I like Acoustic Research loudspeakers. The acoustic suspension woofer loading, the very simple crossovers, the bespoke in-house built and designed drivers, the Villchur and Allison design expertise, the classic look.
    I also like the idea that all AR models are designed for close to wall use, and while this isn't always ideal in every way, it does lend a degree of consistency across the range.
    The sometimes crude looking paper drivers perform very well and require the minimum of external filtering, and while that's not something I see as essential from an ideological perspective, it always feels 'right' :)

    The late 60s and early 70s were the best days for AR and many of the designs improved on the early work.
    There is often some confusion over the acoustic suspension (AS) principle and it's often mistakenly confused with Infinite Baffle (IB).
    AS requires very low compliance in the woofer spider and surround, relying on the restoring force or air-spring effect of the air contained within a sealed box.
    With IB you are still relying on the spider and surround providing the dominant restoring force. Suitable drivers for AS loading will typically have Fs (free air) resonance below 30hz and be capable of good excursion. Back when the original AR loudspeakers were introduced, drivers required huge cabinets for decent LF extension and the distortion performance was very poor due to nonlinearities introduced by the driver suspensions. Ed Villchur's idea was to eliminate the effect of such suspensions, reducing distortion and allowing reasonable bass extension from relatively small boxes, though these are still far larger than what's considered small today!

    The 4xa is a rework of the earlier 4 and 4x with the main difference being the choice of tweeter. Each successive version shrank the paper cone/dome giving better top end extension and dispersion. Crossover frequency is quite low at 1.8khz but entirely sensible when crossing to an 8" woofer. Again, the correct engineering choice and the larger than usual tweeters have no problem crossing this low.

    This particular pair of ARs were bought two years ago in a charity auction for £20 and have been sitting unloved since awaiting my attention. The bass drivers work but the coated cloth surrounds have detached from the woofer chassis in places and the tweeters didn't work. The cabinets were strong and intact. Quite scratched and with the usual corner dents. But hey, these are 40 years old.

    Rather than repair these drivers I decided to use a newer set dating from 1979 which I'd culled from another similar AR model. These look and work perfectly following fresh foam on the woofers, and they share design very closely with the older drivers with the tweeter in particular being essentially the same thing but with a larger magnet assembly.

    Here are the drivers:

    [​IMG]
    IMG_0617

    [​IMG]
    IMG_0615

    Cabinets after sanding back to the bare veneer and baffles repainted:
    (yes thats a lot of fibreglass in there - probably illegal today but it's great for a sealed box - makes you bloody itchy though!)


    [​IMG]
    IMG_0602

    [​IMG]
    IMG_0606


    Next job will be fitting a new HF level control and wiring + the crossover cap.
    Then some new grille cloth, brass badges and finally oiling the cabinets.

    See you again next week for part 2, plus some measurements.
     
    RobHolt, Oct 7, 2013
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  2. RobHolt

    Tenson Moderator

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    Interesting that the tweeter is very much like some of the more modern designs like the seas 22TFF which are similar to ring radiators where the surround forms a significant portion of the radiating surface. I bet they would benefit well from a small phase plate in the middle :)

    Are you sure the drivers can be swapped in to this speaker, is the crossover the same?

    Looks like a nice sanding job how long did that take?!
     
    Tenson, Oct 7, 2013
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  3. RobHolt

    Dev Moderator

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    As Simon said, nice sanding job. I wish I had the patience for sanding.
     
    Dev, Oct 7, 2013
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  4. RobHolt

    RobHolt Moderator

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    Took best part of an evening, with lots of stops for tea :)

    The tweeters are virtually the same as the old ones - same cone/dome and motor but slightly larger magnet. They measure very closely too.
    The bass driver shown above is just an updated version of the original and again the response traces are very similar, including the system resonance when both drivers are fitted to the same cabinet. AR themselves fitted the driver as an official repair if the old drivers blew and it later became known as the 'universal' AR 8 inch driver.

    Crossover is just a 5uf cap on the tweeter. The woofer runs direct.
    I've taken loads of measurements but here are the essentials. The woofer was tested direct as intended, the tweeter via a 10uf cap to protect it from LF. All measurements are made with the cabinet pulled to the middle of the room, so the bottom end should tail down for what is a boundary design. The dip at around 140hz is room related:

    Bass unit at 0.5m on axis:
    (the two traces show the change to LF by transferring the driver from the 10ltr cabinet to the 19ltr AR4xa - the system resonance drops from a 95hz to 65hz)


    [​IMG]
    universal 8 inch woofer in AR18 and AR4 cabinet


    Tweeter at 0.5m on axis:
    (bottom trace, the top is a different version of the tweeter)


    [​IMG]
    universal tweeter 80 v 82


    Combined at 1m - axis between the drivers: Both speakers to show pair match:


    [​IMG]
    AR18 drivers in 4xa cab


    I really like the clean and smooth roll down on these drivers with no shelving or bumps that need further attenuation in crossover.
     
    RobHolt, Oct 8, 2013
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  5. RobHolt

    RobHolt Moderator

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    Another update.

    Did some impedance measurements on the original cloth surround woofer and the later foam version. Primarily to check that Fs (free air resonance) is comparable as this is very important for this design. AR spec the original 4ax at 35Hz, and as you can see this 40 year old paper driver comes in at ........35Hz.

    [​IMG]



    Next up the replacement woofer with foam surround.
    Just a tad lower (which is good) at 33Hz and nicely comparable to the original, though a tad lower impedance overall:


    [​IMG]


    Finally the tweeter. Not much of note here only that it matches the original and the resonance appears very well damped:


    [​IMG]
     
    RobHolt, Oct 10, 2013
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  6. RobHolt

    Tenson Moderator

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    That's an unusually flat impedance for a tweeter! Do you think it might have some impedance flattening components inside?

    Good to see you got a way to measure impedance, did you make a cable to do it or just crack-clip everything? Careful if you crock-clip as I've had dodgy readings from that with everything coming out some ohms higher.
     
    Tenson, Oct 11, 2013
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  7. RobHolt

    RobHolt Moderator

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    What what what!........we'll have no bodging here!

    I did a little jig, then I made this ;)


    [​IMG]
    IMG_0651 by trebor1966, on Flickr

    [​IMG]
    IMG_0648 by trebor1966, on Flickr


    No amp needed - the Edirol FA66 sound card headphone out provided enough poke to play quite load, and I used 25R as the series/sense resistor so it doesn't get taxed too hard when driving a load.
     
    RobHolt, Oct 11, 2013
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  8. RobHolt

    RobHolt Moderator

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    Another update.

    Parts arrived for the crossover - what there is of it!
    Three position heavy duty selector, resistors and 6uf Solen PP cap.

    The 4xa used a variable series resistor on the tweeter and many AR systems use just series resistance to adjust tweeter level rather than the usual L-Pad. This works ok with such simple crossovers where most of the response shaping is done in-driver. Rather than use a pot I decided to use a three position switch giving flat, -1.5 and -3db shelving.
    AR have traditionally referred to a gentle down-tilted on axis response as 'normal' when using a wide dispersion HF driver.
    I think more than 3db of cut is OTT. For anyone wanting resistor values I used 2R2 & 3R3.


    [​IMG]
    IMG_0654

    [​IMG]
    IMG_0658


    Coat of blackboard paint on the baffles:


    [​IMG]
    IMG_0662

    First application of oil/wax mix:

    [​IMG]
    IMG_0673

    AR always had the quirky habit of attaching the printed instruction page to the roughly finished back panel with glue. Looks a bit odd but hey it's the logical place for instructions!
    My instructions were in poor state so a very kind chap over at the Classic Speaker Pages took a high resolution photo of his back panels for me, and with a little photo touch-up we have new instructions duly attached to the panels:


    [​IMG]
    IMG_0681

    Next up will be new grille covers. AR uses open weave Irish linen and I managed to find some which looks correct - at least in the photos, we'll see when it arrives :) .......
     
    RobHolt, Oct 14, 2013
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  9. RobHolt

    Tenson Moderator

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    Looks really smart with black baffles :) What is the stuff to the right of each tweeter? Looks like a control panel or something?
     
    Tenson, Oct 14, 2013
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  10. RobHolt

    RobHolt Moderator

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    Older AR tweeters had the connections on the front plate, so the lead-out wires were taken to a front panel terminal panel. They later adopted the rear mount terminals like everyone else. It was never an issue in those days since the grilles were actually glued in place and not intended to be removed.

    Like this:

    [​IMG]
    artweet
     
    RobHolt, Oct 14, 2013
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  11. RobHolt

    RobHolt Moderator

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    Cabinets looked fine after finishing. No need to add any stain, the oil/wax brings out the colour nicely.


    [​IMG]
    DSCF1246

    [​IMG]
    DSCF1253

    [​IMG]
    DSCF1249

    [​IMG]
    DSCF1247

    [​IMG]
    DSCF1250


    Nice bass - surprisingly deep, solid and waffle free.
     
    RobHolt, Oct 16, 2013
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  12. RobHolt

    Dev Moderator

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    Good work Rob. They look great.
     
    Dev, Oct 16, 2013
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  13. RobHolt

    RobHolt Moderator

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    Thanks Dev.

    Linen arrived today for the grilles and it's the right weave but it's too yellow. Household bleach and the washing up bowl to the rescue!
     
    RobHolt, Oct 17, 2013
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  14. RobHolt

    felix part-time Horta

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    Lovely work, Robert. And a fine spin-off article on the drivers in the reference section :)
     
    felix, Oct 17, 2013
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  15. RobHolt

    rjr513

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    Outstanding work Rob and I like the Switch adaption rather than the Pot, I used
    similar when i recapped my AR3a,2ax,& 5 using a 12 position step switch with
    combination of 20 series /parallel resistors,,would upload image but have exceed
    my upload quota it's telling me
     
    rjr513, Oct 20, 2013
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  16. RobHolt

    Labarum

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    Nice work, Rob.

    I wish I had the skill - and the patience.

    What do you use to drive them? How do you like the sound?
     
    Labarum, Oct 20, 2013
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  17. RobHolt

    RobHolt Moderator

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    Thanks Brian. It was a very enjoyable project.

    At the moment I've just been swapping them for the ESLs so they run from the Quad kit but the load is benign and sensitivity average so any decent 15 watter upwards is fine.
    Impedance wanders around between 5.5 and 10 Ohms if you ignore the woofer peak, and sensitivity is 86dbw.

    Two things characterise the sound - excellent bass and coherence.
    The bass end is tight, punchy and compact in nature. It doesn't roll out from the cabinet and envelop the listener as on many modern large-ish ported designs, but instead sounds propulsive and well metered. The drivers integrate well both in terms of giving a good response but also in character. They also allow for a very wide listening area and the sound is very out of the box. Not ESL levels of transparency, scale or finesse but this type of simple loudspeaker has it own positives. If I has to sum them up - funky little boxes of fun does it nicely.

    Eventually they are going to run in my workroom system once I've properly refurbished a Pioneer SX737 receiver, and I'd like to source an AR XA turntable (on which the the LP12 is actually based). Don't see many good ones in the uk but they aren't expensive so I'll keep an eye out.
     
    RobHolt, Oct 20, 2013
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  18. RobHolt

    Labarum

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    You see quite a few active 8" two way studio monitors, so there must still be merit in the configuration, but hardly any domestic 8" passives.

    Here's one that may be interesting

    http://www.stirlingbroadcast.net/sb88.html

    I wish . . .
     
    Labarum, Oct 20, 2013
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  19. RobHolt

    RobHolt Moderator

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    Last update on the project.

    Grilles now have some fresh open linen on them.
    You need something lightweight and open for best results and I found something suitable in a light oatmeal natural colour. it's even got small flecks and blobs in the weave like the original material.


    [​IMG]
    ar3


    [​IMG]
    ar2


    [​IMG]
    ar1


    This shows a close-in on the weave:


    [​IMG]
    ar4
     
    RobHolt, Oct 20, 2013
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  20. RobHolt

    Labarum

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    Impressive - modern boxes are all stuck in a rut with black grills, even if the wood is almost a white oak or ash!
     
    Labarum, Oct 20, 2013
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