[Patrick Turner]

People have said lots of things about amplifier distortions, and how
NFB makes the distortion worse, or sound worse, or both,
or that the number of harmonic residue after NFB is applied is more
complex
and containing a higher number of higher order harmonics.
I have even said that the main harmonic distortion component in a PP amp

is the 3H.

I make no apology for saying that just about all the above general
statements are
not necessarily 100% truthfull at all.

I have just up-graded my pages on the 8585 to include better schematics
with a somewhat
"better" and simpler front end preamp stage ahead of the power amp.
This latest 8585 version is shown at

http://www.turneraudio.com.au/8585-a...ober-2006.html

This amplifer version was supplied to a guy wanting to run 3 pairs of
stacked Quad ESL57
which all been carefully re-built by a Melbourne man, and one of
Australia's Quad ESL experts, Mr John Hall.

If one examines the distortion graphs carefully where I have analysed
the spectral content
of the THD, the second harmonic is the most prominent, even though this
design has
low bias current for the KT90 and is a class AB amp.
I first though that the 2H would either become less or more if the EL84
driver triodes were swapped
since the 2H they make because they are never quite perfectly matched
will either cancel or add to 2H
produced in the V2 power amp input stage or because of impefect 2H
cancellation in the output stage.
But swapping the EL84 around made no difference.
Both channels behaved the same with predominantly 2H THD content, so I
have assumed
the 2H is mainly due to the use of the 12AU7 V2 triode for the input
stage of the power amp.
The 12.5% CFB operation of the output stage even with low bias is so
blameless and distortion free
that the driver stage distortion is greater.
However, the THD is not objectionable in being too high; in fact its
very low for a tube amp
and benign, since at normal levels there is only 2H followed by 3H in
significance
and all other H are well down and inho, they are of zero significance.

Consequently to the THD tests shown at the page I have re-tested the amp
without the blobal
NFB connected and all harmonics of significance were simply larger by
the predicted amount
of about 10dB.
At 80 watts into 5 ohms, without global NFB, the THD was 0.75%, with 2H
being the main H.
The CFB arrangement appears to work very well on reducing the 3H in
comparison to plain UL operation
or any other config. Mr Walker has been proved right again!

Its a pity that so little information about tube craft exists at present
at r.a.tubes; what happened?
Did the US elections distract everyone?
I hear there is a wake on at Dubya's house, rock down, he's handing out
free booze
to drown his sorrows.

Anyway, I have composed this message on my latest aquisition, a 2003 PC
with windows XP,
and with such luxurious items within such as an 80gb HD, wow, sure won't
have to worry about
filling up hard drives in the forseable future....

I have set the composition of this email for plain text.
Also the text size is 11, but on my newer 17" monitor screen it displays
it at what looks like 7, and it a PITA for
me to read and the display is as easy on my eyes as the older 17"
monitor run with W98 and where
the text display is larger and easier to read.
I only know a little about triodes, and hardly anything about PCs, and I
cannot seem to
adjust the size of website texts and email texts as displayed to a
larger size.

I welcome comment about the 8585, PC use, and if ppl are really
desperate,
a few comments might be permissable after addressing amplifier issues
that are
about the Lame Duckness that the US President may have just become.
Micheal Moore is already harping about giving the Democrats a very hard
time about Iraq.
But something has to give in Iraq, there is great unrest both in Iraq,
and in the US, and the US could not keep
spending 2 billion bucks a week for the next 20 years "while the job is
being done",
while also losing another 20,000 soldiers.

I just recently read a new 1,500 page book by Robert Fisk, and the
Middle East has a truly unfortunate
history, and the awkward facts Mr Bush & Co and US mainstream press
prefer not to mention are mentioned in all their
gory details.
I have no magical solutions to offer the protagonists in the ME; I
suggest they all
let their armour rust and listen to more music, but human frailties and
preponderance
of vice in the human character will keep the cruise missles and
Kalashnikovs
in good working condition......

Anyway, afaiac, tubes do magical things with music, and there is not any
really
great reason to **** up the magic ability with distortions, and
lessening distortions
by mild applications of NFB around amplifiers that are substantially
linear, low noise,
wide bandwidth to begin with even without global NFB does no harm if we
mainly want to use the
NFB to reduce the output resistance of the amp to get an acceptable
damping factor.

Patrick Turner.