Restored Acoustic Research AR7

Landed these recently for £30 and they were in great physical shape, bar the usual rotted woofer surrounds and a few superficial scratches. The vast majority of AR7s were clad in vinyl wrap so I was lucky to find a pair in real teak veneer. Fortunately AR used an oiled finish on most of their cabinets which makes restoring the finish a little easier IMO.

For the driver foam I was aware that the driver in the AR7 has a lower Fs (free air resonance) than some other AR drivers and therefore needed a very soft surround.
I decided to try the type supplied by http://www.audiofriends.nl since this claims to be the correct type with compliance carefully matched to the particular loudspeaker.
If they are correct the Fs should sit around 25h..... well after re-foaming, 24.4Hz will do very nicely and the driver is working perfectly to factory spec. Also when fitted into the AR7 cabinet, the system resonance is 64Hz, again perfectly to factory spec:

Here's the driver's impedance showing the Fs and minimum impedance at 5.5 Ohms:



AR7 bass driver refoamed - Copy by trebor1966, on Flickr


Here's the system plot showing the driver in box at 64Hz and confirming the crossover at 2khz, again to spec.



AR7 system imp - Copy by trebor1966, on Flickr


The cabinets were in great condition but had a few superficial nicks, so I gave them a very light sanding and refinished with some Feed n Wax. One very liberal coat left to sit on the veneer overnight, then buffed and another coat added, left for half an hour or so and buffed.

The covers were excellent. Looking behind the brass badge showed that the exposed cloth hadn't discoloured so the covers were left alone.
The badges had turned black due to adhesive from the plastic protective covering which had never been removed. Wire wool followed by Brasso has them looking like new.
Backs were fine and have the speckled Euro finish. Level switch was cleaned with Deoxit and the old cap replaced with a new Solen.

More to follow but for now some pics:


Before with disintegrated foams:



AR7 by trebor1966, on Flickr


IMG_0956 by trebor1966, on Flickr


Driver re-foamed:



IMG_0958 by trebor1966, on Flickr



IMG_0963 by trebor1966, on Flickr


IMG_0964 by trebor1966, on Flickr


IMG_0965 by trebor1966, on Flickr


IMG_0967 by trebor1966, on Flickr


IMG_0969 by trebor1966, on Flickr


IMG_0971 by trebor1966, on Flickr


More including response plots to follow.

 

Here's the response at 1m.
This was taken with the 'speaker pulled into the middle of the room and raised 26" from the floor.
The dip at 160hz or so should be ignored - its a room effect, likely a ceiling of floor bounce cancellation:


AR7 1m by trebor1966, on Flickr

This shows the difference between measuring on the tweeter axis and the bass driver axis. The latter is clearly preferable, indicating that you want the 7s up high if listening reasonable near-field.
The red trace is the tweeter axis:


AR7 tweeter and bass axis by trebor1966, on Flickr

Overall there is some mid rise apparent and that is audible. Interesting to note that the larger boxed AR6 used the same drivers but drove the bass unit via an inductor to pull down the upper end of its range. Looks like it would benefit the 7 too. Perhaps it was omitted on cost grounds given this was a small entry level model. You can't have everything for the original $70 price, and that's not bad at all for a single 6uf cap as the crossover.

 

Lovely restoration Rob - well done - and bear me in mind should you wish to sell them, or better, the 6's

The tweeters on all these AR's was set slightly down back then for genuine reasons to do with 1960's-1970's records I believe. The balance of the 7's was designed for shelf mounting - no speaker stands on those days - so shelf mounting would have lifted the mid bass up to better balance the upper mid.

 

Hi David,

Yes tweeter level is set a little lower than on the later models. It only really sounds correct in a more modern context with the tweeter switch set to 'flat' IMO. The 'normal' position drops everything by another 2dB.

I can see the logic - AR were big on power response and overall room response and these designs have very good dispersion, so tend to sound livelier at the top than the on axis responses would suggest.

You get first dibs if they go, though I think they'll be in the permanent collection.

 

Lovely vintage found and restoration, Rob. Still much to learn from some of those old boxes of yore.

In passing, I've got two pair of Heybrooks at home right now, and I am enjoying every piece of them!

Regards,

Vincula

 

Hello... i read this thread today, congratulations for the very good work. I did not know about the particular "very soft" foam of Ar7 woofers.. i am restoring mine, and i measured 35 Hz as FS after refoam , so i think it si not perfect, thank you for the useful info.. perhaps did you measured also T&S parameters? Thanks!!!