In message , Iain M Churches
writes

"Jim Lesurf" wrote in message
...
In article , Iain M Churches
wrote:

"Jim Lesurf" wrote in message
...


Thanks for your reply, Jim. If you don't mind I will cut it into
sections so that I can read and digest the information which each
contains before replying .


The damping factor is 20, which for a valve amp very respectable:-)

Given the qualification in the statement, yes. :-)

However bear in mind that a damping factor of 20 for an 8 Ohm load implies
an output impedance of 0.4 Ohms. If this kind of value exists across the
audio band then you can expect interactions with a typical loudspeaker's
impedance to produce changes in the power-frequency response that can be
very audible, and changes in the phase-frequency response that may be far
higher than the values with an 8 Ohm load. Hence I'd tend to regard such a
damping factor as being a bit low, but despite that I'd expect the results
to sound fine in most cases.

Over the years, I have been involved in the measuring of quite a lot
of push pull UL valve amps on a comparative basis. I have also
been fortunate enough to have worked alongside or met
some good valve amp builders or designers
(ex. the legendary Jim Kerr of Kerr McCosh)
Sadly, I was young at the time, and did not ask many of the questions
to which I would now like the answers:-(

I even saw Henry Leak, when my Dad took me along to
return his TL12 amp for a valve change. We went to the factory in
Acton London W3. I remember seeing rows of ladies in their forties,
nimbly assembling circuit boards, which still are pristine by any
standards.

Only a few of the best sounding amps have large amounts of NFB.
15dB seems typical. Some using pentodes at the front end have a lot
more.

Most of them have a DF between 10-15, so I thought my fig of 20
was very respectable. Both Tremaine and Olson stated that in tests,
a listening panel were unable to detect changes in increase in DF
above about 12. I know that SS amps can easily achieve a much higher
figure.

DF is an interesting parameter, and I feel it has a lot to do in explaining
why amplifiers with a similar FR or PBW sound so different.
Arthur Radford built a series of speakers for studio use, with valve
amplifiers built in on the bottom of the stand. The speaker impedance
was 24 Ohms, with a corresponding output transformer.
This way, he could achieve a greater DF. They did
sound very good indeed:-)

Normally we express DF in terms of the ratio of the internal output
impedance to the load impedance. (and so arrive at the figure of
0.4 Ohms which you quoted above) DF = Zload/Zout

But according to Tremaine, the equation is more complex,
and should also take into consideration the DC resistance of
the voice coil, which is a limiting factor.

So the equation becomes DF = Zload/(Zout+Rvc).
Assuming a Rvc=6 Ohms, a DF of 16 calculated by the first equation
becomes 1.23 using the second equation, and even with zero output
impedance, a DF of infinity in the first equation gives us only 1.33
when calculated in the second.

ref: Tremaine, pp1120


Kunniottaen!
Iain





If the amp is feeding a speaker directly, with no crossover, then a very
low output impedance is not required, because the dc resistance of the
voice coil is dominant. The same cannot be said for multi-unit speakers
with complex crossovers, where the interaction of the non-linearity (in
both frequency and level) of the valve amplifier output Z, and the
crossover can cause problems.

--
Chris Morriss