Restoring the Acoustic Research AR4xa loudspeaker

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

These will be at Scalford in March so if anyone wants a listen, do pop in.
 
According to Richard Dunn (NVA), who worked for AR in the UK in the early 70's, the ones you appear to have (US made?), were much better put together and with superior cabinets too..
 
RobHolt said:
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_0654


IMG_0658


Coat of blackboard paint on the baffles:



IMG_0662

First application of oil/wax mix:


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_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 :) .......
Click to expand...

good afternoon rob,

recently i bought a pair of ar4xa speakers too and i am about to start restaurating them.
searching the internet for information i came to your report of your restauration. i think you did a very good job.
my set is from 1976 and has the latest tweeter type and looks very good also. i'm only missing 1 of the two logo's and the information on the backside of the cabinet.
i have to refoam the woofer but i can do this myself and do some work on the filters.
i have the original cloth but it is kind a dirty.. i hope i can clean them.
can you please help me with the picture of the information on the bach of the cabinet?

regards,
stefan
 
Click to expand...
good afternoon rob,

recently i bought a pair of ar4xa speakers too and i am about to start restaurating them.
searching the internet for information i came to your report of your restauration. i think you did a very good job.
my set is from 1976 and has the latest tweeter type and looks very good also. i'm only missing 1 of the two logo's and the information on the backside of the cabinet.
i have to refoam the woofer but i can do this myself and do some work on the filters.
i have the original cloth but it is kind a dirty.. i hope i can clean them.
can you please help me with the picture of the information on the bach of the cabinet?

regards,
stefan
 

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