"Peter Chant" wrote in message
...
On 12/10/2014 07:46 AM, gregz wrote:

You could try a little series resistance on the woofers
to push up the Qts
and be more suitable for closed box?


Interesting. This might actually reduce the influence of
the amp and
cables. I have one book which I cannot find which was
obsessed with
measuring the amp impedance and matching the speaker to
the amp.


I can understand that approach with a valve amp where there
is an output transformer involved, but matching with a
transistor amp? Eh? The main factor with a transistor amp is
having a power supply that could deliver the (often short
term) currents needed. JLH proved it could be done easily by
having a regulated PSU. In the '80s I built a dual mono
power MOSFET amp with regulated supplies a la JLH and tests
showed it would do 110W into 8R and 220W into 4R - which is
what it is all about. [I would still be using it today save
it has developed a dc offset problem on one channel and I
have never had the time to sit down and find out why.] I had
a pair of the Bailey designed transmission line speakers at
the time (rather like the Imhoff TLS80] and that could
rattle windows at 10 paces with that amp!

A few decades ago there was an obsession with getting the
amp output impedance as low as possible to increase the
damping factor, until someone - could have been JLH or Doug
Self or someone like that - proved that it is easy to over
damp a circuit. Indeed the lower the amp output impedance
becomes the more effect the resistance of the
interconnecting cable has which was why I believe there was
a move to direct amping. I believe it was only with
increasingly powerful computer modelling that it was
discovered that much could be achieved by better driver,
cabinet, and particularly crossover design and at the same
time more could be made of signal level filtration so bi or
tri-amping became popular and to an extent survives today
(although very little in the UK from what I read.) As a
result of the work on crossovers, in my very limited
experience you will often find a small series resistor on
the output of the bass section of a crossover.

Now I know this will be contentious, but from what I heard
years ago the Motional Feedback speaker marketed by Philips
did far more to achieve purity of (bass) sound than anything
I've ever heard. I remember going to one of the hi-fi shows
in Harrogate probably in the '70s. I walked into the
ballroom at the Old Swan Hotel (of Agatha Christie fame) as
I could hear what I thought was a brass band playing -
possibly Grimethorpe - so you can imagine my surprise when
all I saw was two MFB loudspeakers. Staggered was not the
word. The only reason that I can think they never took off
was (a) the price which compared with the price of some
so-called hi-fi kit these days would now be seen as cheap
and (b) because it was done by Philips who were not
perceived to have hi-fi capability. I often wonder what
would have become of the technique if the design had been
done by someone like an early Linn?



--
Woody

harrogate three at ntlworld dot com