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New AR style loudspeaker design


RobHolt

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Below I describe the results of a new project to build a loudspeaker with more than a hint of AR legacy. I hope some will find it of interest.

This project has been long in gestation but finally we have a finished product.


It is offered here since it might be of interest to the audio DIY community and is not being produced as a commercial product. Therefore you are free to take as much or as little of what follows as you wish - it’s free!

I would like to thank my good friend Simon Ashton (Tenson) for his help with the project and for the many hours spent using his CNC machine :)
Perhaps I should start with the origins of the idea.
There were two driving forces for this. Firstly, I’m a fan of Acoustic Research loudspeakers and hold the opinion of the early AR designers Edgar Villchur and Roy Allison in high regard. Both understood the importance of good power response and room integration, and ISTM that many modern loudspeakers are quite poor in this regard. Many have excellent on-axis response but revert to the proverbial dogs breakfast off axis. Poor crossover design is one factor, but the unfortunately popular marriage of (relatively) large bass/mid drivers to tiny tweeters, combined with a crossover point set too high means many loudspeakers are hobbled from the off. The recently posted video of the Floyd Toole lecture illustrates many of these points well.


The second consideration behind the project where the excellent results I’d heard from the Cotswold Sound Systems BMR drivers. These offer the combination of a clean and extended response coupled with point source radiation in a fairly efficient and inexpensive driver. I’d heard such a driver in the budget priced Cambridge Aero 2 and heard enough good points to realise that an essentially excellent BMR driver was being held back by a small MDF ported box with less than great tuning.
So the idea was born. A classic AR style stand-mount, with acoustic suspension closed box bass, coupled to the BMR at a low crossover point. The styling is also based around classic AR - notably the best selling AR4 with which this new design shares cabinet height and width.
But which bass driver to use?
Sealed boxes were once popular for loudspeakers but sadly no longer. The quest for efficiency and boosted LF response means we have very few drivers suited to a closed box. The Vifa unit used in the old JPW and Heybrook designs is still available and would work well, but why not use original AR drivers? - they sold hundreds of thousands of models over many years using what is in many ways a generic AR 8” driver. In fact the AR7, 6, 17, 18, 28, 18S and 8S and others all used close variants of this basic unit. The differences tend to be in very top end of the response - not important here as the crossover point is so low. The AR6 & AR7 have a slightly lower Fs, but again 30Hz v 25Hz matters little in this cabinet.
 So the design uses the AR bass drivers, which are in plentiful supply but will need restoring with a re-foam kit. These drivers are perfectly designed for the small-medium sealed box.
The Cotswold 2.5” BMR is rated for use above 250Hz and runs all the way to 25kHz my measurements show. It has very even output both on and off axis, other than a kink in the response at 10kHz after which it recovers quickly. In this design the crossover point sits at 350Hz and is 1st order electrical.

 The use of a small super tweeter is the icing on the cake and you could build this ‘speaker as a 2 way with little impact on performance.
The tweeter is used to control the very top end of the BMR response above about 12kHz by slightly reducing on axis output while lifting the off-axis output by a couple of dB. It is not included to extend response into the bat region.


The crossover is simple - 2nd order electrical for the bass crossing to 1st order for the BMR at 350Hz, with the tweeter coming in gently on a 1st order crossover at a very high 15kHz.
Therefore the lower mids all the way up to the limits of audibility are handled principally by a single driver, avoiding the traditional crossover point in the audibly critical upper mids.

 The cabinet is 22 litres internal volume and is constructed from 18mm birch ply. The choice of ply is important as it gives a sonically clean cabinet. I don’t recommend the use of MDF - the Devil’s wood, as it usually has a very obvious signature thanks to poor energy decay characteristics. In short, it rings.


The grille design is somewhat unusual as you can see. The complex CNC’d patterning of the cutout which includes edge chamfering is designed to minimise diffraction effects, and the traditional AR style natural linen cloth is accounted for in the frequency response characteristic - this ‘speaker is designed for use with its grille.


There is one final twist - the BMR runs open-back, with the rear output (attenuated) venting through the back of the cabinet. The BMR chamber runs between the front and rear baffles and is countersunk into both, forming a tight front-rear brace. There is also a brace behind the bass driver which braces the sides and rear panel. The countersunk edges of the chamber are glued to the baffles to preserve the sealed box loading for the bass driver.
The BMR chamber is filled will long hair wool and capped-off with a thin open cell foam disc for a neat appearance. You can vary the stuffing to taste - it’s main effect is to alter rear output level. The rear output pulls the radiation pattern toward a cardioid response in mids, further improving the in room power response characteristics.
The internal stuffing is also long haired wool in preference to BAF wadding or foam - use plenty filling the entire space but don’t compress the fibres down hard. You need the air to flow around the wool fibres. Wiring is 5 amp lamp flex - nothing heavier is required and I recommend avoiding very thick cabling as it can stress the driver terminals, and the heat required to solder such wire can damage the driver connections particularly on the tweeter and BMR.


The badges and labels are of course mine to give a finished product but you can of course use anything you wish - or none.


The cabinet is veneered in light ash veneer to compliment the exposed ply edge. I like exposed ply so this seemed a good choice. The cabinet is sanded smooth, the ply edges given several coats of shellac sanding sealer for a smooth finish, then the entire cabinet given several coats of Briwax polish. This gives a soft sheen and can be reapplied and buffed at will. The front and rear baffles are sprayed matt black - use many light coats on the rear panel for a good finish.

The terminals are dual purpose and take 4mm plugs and bare wire.

That’s it for now as I’ve prattled on for long enough.
The name - AR22 Tribute is exactly what is says, a modern homage to a classic loudspeaker form in a 22 litre box.
Drivers:
Acoustic Research 8" paper
Cotswold Sound Systems BMR46NA8Y
Vifa OT19NC00-4
LOTS of pictures follow showing various stages of build from CNC-ing the ply panels and phenolic tweeter mounts, to cutting the BMR tube, grille frames and assembly.
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Electrical stuff and measurements:
System impedance and phase angles:
Crossover point:
On axis response: (Green trace with grille)
30 deg off axis - horizontal:
2m med-far field on axis:
Some comments on the measurements:
These are made in room on tall stands well clear of boundaries.
They show good correlation between the close on axis, off axis and far field responses. The microphone is known to lift by 2-3dB above 10kHz and I haven't loaded the correction file so the very top end is a little flatter than shown here.
An essentially flat, neutral result is obtained with the grille in place, or a brighter balance if used nude.
The load is pretty benign but does hit 5 Ohms in the upper bass. This won't trouble any decent amplifier but you probably want to avoid very weedy tube amps with high output impedance.
Plans next......
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Hi, many hanks for the comments.

I agree it would be unacceptable to call it the AR or Acoustic Resrearch.....something loudspeaker.

But this just uses the letters in a model number. Plus there was never an AR22T.

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Extraordinary, Rob - what an outstanding effort!

I've read through your post twice, and copied it to my iPad; now I'm going to sit back & go through it again, while relaxing with some tunes.

Please let us know your listening impressions, especially regarding the speaker's performance vs. your expectations.

Also, I wouldn't worry about it being "OK" to put the "AR 22Tribute" label on your loudspeaker - the intent & usage should be clear to anyone; if it poses a conundrum to some 55th Century archaeologists, peeling back the layers of ancient London, well, that's their problem. ;)

Again, just superb work on a very creative project!

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Hi Rob,

Nice speakers !

But may I ask where did you get the grille cloth material from?

David.

David,

Check the AR-3a restoration guide for recommendation re the best fabric.

Wichelt 18 CT linen in "lambswool" should be available on both sides of the pond.

Over here the best source is 123 Stitch http://www.123stitch.com/cgi-perl/abcSearch/search.cgi

-Kent

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Wow, I always thought my Spendor SP1 looks good but your AR22t really looks gorgeous! The use of a wide band midrange-tweeter, IMO, will positively contribute to the realism of vocal and instrumental music. For people on the other side of the pond, could you recommend a US version of Cotswold Sound Systems BMR fullrange?

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Hi Rob,

I have purchased some from Vintage-AR before for my AR3a but they came in a shade of yellow and apparently it is the right colour.

But it is completely different from what you have on your pictures.

I will put up a picture later.

David.

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Wow, I always thought my Spendor SP1 looks good but your AR22t really looks gorgeous! The use of a wide band midrange-tweeter, IMO, will positively contribute to the realism of vocal and instrumental music. For people on the other side of the pond, could you recommend a US version of Cotswold Sound Systems BMR fullrange?

http://www.cotswoldsoundsystems.com/index.php

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Hi Rob,

I have purchased some from Vintage-AR before for my AR3a but they came in a shade of yellow and apparently it is the right colour.

But it is completely different from what you have on your pictures.

I will put up a picture later.

David.

Hi David,

The shade does seem to vary as does he weave.

The cloth I purchased 12 months ago is whiter and more open weave.

Both look excellent though.

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