AR4xa rebuild fully documented.....with a twist

[RobHolt]

A number of forum members have asked that I post details of this project which I'd documented elsewhere on a uk forum.

The original project took the form of forum thread posts with comments so I've clipped out the meat of the thread and present it here as one 'article'.

Hope members find it useful.

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 plus.

Rather than repair these drivers I decided to use a newer set dating from 1979 which I'd culled from a pair of AR18. These look and work perfectly following fresh foam on the woofers, and they clearly 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_0617

IMG_0615

Cabinets after sanding back to the bare veneer and baffles repainted:

(yes thats a lot of real fibreglass in there - probably illegal today (in the uk) but it's great for a sealed box - makes you bloomin itchy though!)

IMG_0602

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.

Part 2 next, plus some measurements.

[RobHolt]

Part 2:

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 95hz to 65hz)

http://www.flickr.com/photos/trebor1966/10145836594/'>

http://www.flickr.com/photos/trebor1966/10145836594/'>universal 8 inch woofer in AR18 and AR4 cabinet

Tweeter at 0.5m on axis:

(bottom trace, the top is a different, later version of the tweeter as used is the 'S' models)

http://www.flickr.com/photos/trebor1966/10145858414/'>

http://www.flickr.com/photos/trebor1966/10145858414/'>universal tweeter 80 v 82

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

http://www.flickr.com/photos/trebor1966/10145893954/'>

http://www.flickr.com/photos/trebor1966/10145893954/'>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.

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[RobHolt]

Part 3:

Did some impedance measurements on the original cloth surround woofer (after fixing the surround) 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 in their brochure at 35Hz, and as you can see this 40 year old paper driver comes in at ........35Hz.

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:

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

[url=http://www.flickr.com/photos/trebor1966/10178837414/]

[RobHolt]

Part 4:

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 some 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.

http://www.flickr.com/photos/trebor1966/10257514034/'>

IMG_0654

http://www.flickr.com/photos/trebor1966/10257406206/'>

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Another coat of blackboard paint on the baffles:

http://www.flickr.com/photos/trebor1966/10257642495/'>

IMG_0662

First application of oil/wax mix:

http://www.flickr.com/photos/trebor1966/10257668905/'>

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:

http://www.flickr.com/photos/trebor1966/10257403733/'>

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]

Part 5:

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

http://www.flickr.com/photos/trebor1966/10312481406/'>

DSCF1246

http://www.flickr.com/photos/trebor1966/10312138134/'>

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http://www.flickr.com/photos/trebor1966/10312107774/'>

DSCF1249

http://www.flickr.com/photos/trebor1966/10312482983/'>

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http://www.flickr.com/photos/trebor1966/10312572235/'>

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First listen - Nice bass! - surprisingly deep, solid and waffle free.

[RobHolt]

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.

http://www.flickr.com/photos/trebor1966/10380926124/'>

ar3

http://www.flickr.com/photos/trebor1966/10380947235/'>

ar2

http://www.flickr.com/photos/trebor1966/10380922824/'>

ar1

This shows a close-in on the weave:

http://www.flickr.com/photos/trebor1966/10380975436/'>

ar4

[RobHolt]

Part 6:

So how do they sound?

At the moment I've just been swapping them for the usual Quad ESL63s so they run from the Quad amp kit but the load is benign and sensitivity average so any decent 15 watter upwards is fine IMO - probably less if used in small rooms or no more than averagely loud SPLs.

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 overall 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 had 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. Don't see many good ones in the uk but they aren't expensive so I'll keep an eye out.

Just to close, I did briefly do a side by side with the original 4xa drivers (managed to get one woofer and tweeter working) in one cabinet using the old crossover and these later drivers in the other using their standard 6uf capacitor. They sound surprisingly alike, very similar at the bottom end and the very top but with the older drivers being a little more forward in the mids - very clearly everything was on the same page sonically and if you like one sound you'd like and feel instantly at home with the other. This I think is great news since, certainly here in the uk, the model 18 was incredibly popular with many thousands sold. People tend to sell them off cheaply due to rotting woofer foams so there is always a plentiful supply of otherwise perfectly good drivers, and they make good candidates for revitalising the older more classic 8" 2 ways with the better cabinets and build.

[ar_pro]

Excellent job, Rob - they look very nice.

There was a '65 Stingray at the Carlisle, PA Corvette show a couple of years back, and the owner had taken a similar approach, substituting modern parts & technology where possible.

The resultant "resto-mod" very much kept the spirit of the original, while improving performance & appearance in every way.

[RobHolt]

A little update.

If you look at the 1m response plot above you'll see its decently even, but the bass driver shows a rising trend in the 500hz-2khz region.

The original AR4ax of course used a large air cored inductor on its bass driver, so I decided to try this on the 18 driver and see what happened. Now I know this driver was built to run 'open' but it does have some peaking at the top, and of course the 18 is renowned as a rather forward sounding 'rock monitor'. In use I find it sounds well projected and direct but also a little cuppy and hollow on vocals.

Inserting the inductor addresses the excess energy at the top of the bass driver and in the process introduces a little baffle step compensation.

Used this way, the cuppy quality is banished and traded for a slightly softer, warmer overall balance.

However, if you unwind 30 turns from that large inductor you get a nicely flat mid response with just a hint of presence dip - it sounds very nicely balanced indeed.

Here's the response before and after adding the full inductor - with 30 turns removed the dip is just a little shallower.

Tweeter here is set to the mid point:

AR bass inductor

You can also get a smoother top end, particularly around the bottom of the tweeter range by reducing diffraction effects. Use some 8mm thick acoustic foam and cut so that it fits snugly within the grille cut-out, then punch a hole slightly larger than the tweeter cone area.

8mm thickness brings the foam flush with the grille cloth:

IMG_0907

[djcheung]

Hi Rob,

I know this is an old post.

I am in the process of restoring a pair of AR3a and looking for the grill cloth material.

May I ask where did you get yours from?

Best regards,

David.

[JKent]

David,

Rob is in London and unless you're in Europe his source may not be the best for you.

Please check the AR3a restoration manual in the Library http://www.classicspeakerpages.net/library/acoustic_research/original_models_1954-1974/original_models_schematicss/restoring_the_ar-3a/ . It is THE resource for your restoration and it will tell you the best material to use: Irish Linen from online seller 1-2-3 Stitch.

That fabric is very high quality and absolutely authentic. But it's not cheap.

For cheap, get a 40% off coupon for Michael's (coupons online or in the mail) and get a roll of Charlescraft Irish Linen (actually made in Poland now), in "Tea". It's in the needlepoint section and a roll is $9, or $5.40 with the coupon. It's lighter-weight and a higher thread count than the linen from 1-2-3 but you can't beat the price.

Good luck with the restoration.

And Rob--Beautiful work!

Kent

[djcheung]

Hi Kent,

Sorry for not including my location.

I am actually located in Cambridge, UK.

I have refoamed, changed the caps and cleaned off a non-original gasket that was stuck on the cone to make them look like the old version drivers.

Refinished the cabinets with BLO.

And the previous owner painted the grill cloth black.

Best regards,

David.

[stan461]

Absolutely beautiful work!

What brand of stands are you using and how tall are they?

Thank you.

[RobHolt]

Hi Rob,

I know this is an old post.

I am in the process of restoring a pair of AR3a and looking for the grill cloth material.

May I ask where did you get yours from?

Best regards,

David.

Hi David,

I ended up changing the cloth again to the type sold by ARMan based in the US.

He sometimes sells via eBay.

It has virtually no measurable effect on the HF response.

[RobHolt]

Absolutely beautiful work!

What brand of stands are you using and how tall are they?

Thank you.

Hi Stan and thanks for the comments.

The stands are Target HR high mass types and are 24". I didn't buy them especially for the ARs and just had them for use with various other (much) smaller boxes. I think the smaller 2 way ARs work better on a tall stand - gets the bass/mid up high which is important for dispersion with a relatively large mid driver as you want to be on axis for best response.

Just landed a nice pair of Teak AR7s so those will next for the refurbishment. Those have all working drivers