[mick]

On Thu, 15 Nov 2007 05:08:12 -0800, Andre Jute wrote:

snip

Looks like a clever design. Gordon Rankin used to work with the Diatone,
no longer available but in its day a Japanese street legend, and he
always said the trick is to build the box right and not to try to
extract too much at the low end. This Chang idea of effectively putting
the box on top of a dedicated stand which is also a guide to couple the
longer waves to the floor is a pretty trick way of accomplishing two
separate functions (balancing the midrange, and getting more bass from a
small driver than it really wants to give).


I've just been playing with the first one. It's not easy finding
somewhere to even stand it in my shed! Stuffing seems to be important &
I've also tried a couple of values of resistors & a very small inductor.
The top end seems to be rather strident & harsh, but that may not be
helped by the surroundings, amp & source (DVD player).


It's years since I've been on their site but Fostex themselves published
some very clever designs on a principle devised by Harry Olson where,
instead of angled planks like in my Fidelio-type horns (so difficult to
saw right that I buy factory woodwork and then alter it), ever shorter
planks are glued together flat in a step formation, and all junctions
and joints are perpendicular. The structure is thus easy to build and
very stiff so that unwanted resonances are almost non-existent. I've
forgotten the name of the Japanese chappie who took this method and
refined it tremendously with a gazillion designs for just about every
Fostex driver. You'd expect a nasty comb response but in fact the
waterfall is nicely smoothed off for the meterheads and super on the
ear.


Those designs are still on their site. I did consider building one, but
this version took less wood. ;-)


I'll have a better idea in a couple of days when I've had a first
listen. :-) Hopefully I'll be able to resist gluing them together long
enough to get some photos!

Absolutely essential. Enquiring minds want to know!


:-)


Unfortunately, the FE126e speakers, which I originally ordered, appear
to be unavailable at the moment so I've used their shielded cousins.
That has probably cost me some sensitivity. The 126s are supposed to be
94dBW and the 127s 91dBW at 1m, but the bass on the 126s is uneven and
benefits from some series resistance. That loses some power and must
surely make the damping worse, so perhaps I've not lost much.

So much depends on the boxing anyway, 3dB this way or that on a spec
sheet isn't a big deal.


That's why I decided to accept them rather than wait. I'd already got the
boxes cluttering the shed up too.


snip

The 417A should be run with 18mA or more of current; they can take it.
The more current you put on the driver, the less bother you will have
with Miller capacitance.


The mains trannies are an unknown quantity, but by my estimation should
be capable of about 100mA, so I can manage 18 to 20mA ok. I've done a
quick breadboard using LED biasing at 15mA for the 417A. No proper
testing, but it looks promising so far even using an old el-cheapo OPT
for testing.


I have plans to change the output bottle to 6C4C or 6B4G dht (much
like 6.3v 2A3)

I had a matched quad once that a Joenetter sent me when he retired from
tubes. I gave them to a guy who built an Audio Innovations First PP 2A3
clone because I thought them a bit small -- this was before I decided
that triode PP makes the finest sound, when I was just starting out in
SE. I shoulda kept them; his amp is super with horns and even reasonably
sensitive bookshelf speaks, and an absolute liquid- sound delight on my
Impresario speaker. It is also a match made in heaven with ESL57. Andy
Evans was explaining to me the other day on UKRA that the same AI
"First" design is still in production under another name.


Hmmm... I just happen to have a pair of 5k7 a-a OPTs here, which have
been put to one side "for a rainy day". Perhaps I need 2 pairs of DHTs to
play with!

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info http://mixpix.batcave.net