On Sat, 06 Aug 2011 09:48:02 +0100, Jim Lesurf
wrote:

In article , Don Pearce
wrote:
On Fri, 05 Aug 2011 21:25:11 +0100, Eiron
wrote:

On 05/08/2011 17:36, Jim Lesurf wrote:

The behaviour breaks down about 5kHz because you are now just using
the central element of the array (the center disc) and so the angles
then fall with frequency in the usual way.

I thought the annuli were fed from a delay line rather than a
crossover....

They are.

Yes. But this line does roll away the HF to the outer rings as well as
introduce delays. This is partly because some power has already been
radiated by inner elements of the array. Also because each coil in the
delay line has an extra 'shorted turn' to act as a resistance. This is a
clever idea as it also cancels the dispersion that otherwise can arise with
loaded lines. The result is to reduce frequency dependence of the time
delays between elements and stabilising the apparent phase center ( 'point
source') Those who understand transmission lines will recognise this as an
interesting application of the Heavyside Criterion which was initially
devised to get transatlantic cables to work correctly back in Victorian
days! :-)

Jim is referring to the fact that the central portion doesn't get any
smaller with rising frequency, and at 5kHz its size is starting to
become appreciable.

Yes.

In principle PJW could have added one or more smaller rings to the center
and ended with a smaller center disc. Then had few more stages on the
transmission line. However I guess he didn't do this for two reasons.

Firstly, my impression from listening is that the beaming above 5k has
little real impact on imaging. Perhaps because the reality is that there
often isn't much up there or it doesn't have much effect on perception once
the region below 5k work well.

Secondly, there is some spreading, partly due to fringe fields, partly due
to the diaphragm material. So the smallest 'spot' you can actually drive
'independently' is limited.

Also, I always find that the 63 design works best when the speakers are
angled to cross in front of me, below my ear height. This makes the image
more stable and less affected by small head movements. I guess this is for
the same reason people use a similar alignment for conventional speakers.
It allows a trade off in the change in time difference and signal amplitude
at the ears at HF so that the effects tend to cancel when you move your
head sideways a little.

So overall, I think PJW got this about right.

But of course, this will depend on the room. listener, and choice of music.
My taste is probably like PJWs. The tradiational 'BBC' sound for classical
music, etc. Indeed, I still prefer Radio 3 to many commercial recordings,
and prefer the sound on the BBC Music Mag discs as well!

Personally, my only hesitations tend to be the way the 63 design exposes
things like 'grainy' or 'scratchy' massed violin playing/recordings and the
lack of heavy bass that may suit some music.

FWIW I have been tentatively feeling that the new 2805s do have a 'drier'
bass than my old 63s. But can't do any direct comparisons, so am not sure
if this is my imagination as yet. So I may start twiddling with
positioning, etc, and maybe even sometime have another try at using a sub
to find out if that can now make an improvement rather than just adding
mud!

Slainte,

Jim

It definitely is possible to add a sub without mud, it just has to be
done right. Mine is a sealed box of about 50 cubic feet (a breeze
block cupboard under a staircase) and a nice 15 inch driver with a low
Fr and plenty of throw. An Adire Tempest, I seem to recall. It sits
right in the corner of the room and rolls away from about 45Hz, so it
does't really fight the main speaker woofers at all. Takes an age to
get the phasing and levels just right, but when they are there, you
really don't notice it unless you turn it off.

d