"Trevor Wilson" wrote in message
...

"Iain M Churches" wrote in message
...
I would be interested to know the phase response
of a high end SS amp (OK, I know, they are ALL high end:-)
at 20Hz and say 20kHz, or more points if the info is
available.

If someone can supply me with some figs (from say the
legendary Krell:-) I cold use these a benchmark for the
measurements of my own amp.

**It is simply not a problem to attain 0o at 20Hz and 20kHz, with a decent
bandwidth design.


--
Trevor Wilson
www.rageaudio.com.au

Can you point us to a commercially produced amp that meets those
parameters? That would be useful.

Iain

"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










Also bear in mind - as implied by Arny's comments - that these values may
well be dependent upon the output power level, particularly with amps that
have low levels of feedback. (Which is implied by having a high output
impedance unless the feedback is from the primary.)

None of the above may matter much in use. Depends on circumstances. But it
can mean that values measured using an 8 Ohm load don't really tell you
what will happen when you connect to a speaker.

In article , Iain M Churches
wrote:

"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 .

OK. :-)

[snip some discussion of damping factor]



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 have had similar experiences/regrets. I also now regret I did not keep a
much more careful 'diary' of what I did 20-30 years ago as I now sometimes
struggle to recall results, discussions, etc. Loads of info familiar to me
at the time, but which I have now largely forgotten. Peril of old age, and
youth being wasted on the young. :-)

[snip]

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.

I would agree. Although I feel the term 'damping factor' is quite
misleading as it allows people to assume that the main effect is in damping
the mechanical resonance of the loudpeaker, when I think this is generally
not the reason for its main audible effects.

[snip]

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.

Yes. I think that is an important point. It shows why - once the
amplifier's damping factor exceeds about 20 any further increases may not
really have any effect - as your estimate below implies.

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.

Although I can't comment on the specific values for a given speaker, I'd
agree with the implication. That once we have an amplifier output impedance
which is somewhat less than the series resistance of the speaker unit we
should find that the effect on 'damping' becomes much the same as we
continue to reduce the amp's output impedance. i.e. if your only concern is
damping the loudspeaker resonance then a very low output impedance isn't
required.

However I'd say that the real effect resides elsewhere... :-)

Consider a loudspeaker whose load impedance varies from, say, 5 Ohms at
some frequencies up to, say, 32 Ohms at others. Then consider using this
with an amp that has a damping factor of, say, 20 (ref 8 Ohms).

The speaker is nominally designed on the basis of being fed with a voltage
source (i.e. of negligable source impedance). This has been the standard
practice in the domestic audio business in my experience.

The damping factor translates into a nominal amp output impedance of 0.4
Ohms.

At the frequencies where the speaker has an impedance of 5 Ohms, the 0.4
Ohms between the nominal EMF voltage of the amp output combined with the
speaker load impedance acts as a potential divider, and the voltage
appearing on the speaker terminals will be 5/(5+0.4) = 0.926 of the level
which would appear there if the same amp had a zero output impedance. This
corresponds to the power changing by -0.67 dB.

Whereas, at frequencies where the speaker has an impedance of 32 Ohms, the
corresponding drop will be 32/32.4 = 0.987 or -0.1 dB.

The result is variations at the 0.5dB level in the response.

If the speaker impedance varies more than this, and if the speaker and amp
have reactive components in their impedances, the variations may be larger.
This also has implications for the phase response which would need to be
considered if this is felt to be an issue.

Hence we can find that the simple interaction between the amplifier's
output impedance and the frequency dependence of the speaker impedance can
make slight but audible changes to the overall response. When listeners are
unaware of this mechanism it can then given them a 'cue' which may affect
their judgements. They may also assume the audible differences are due to
something else.

How much this matters, and whether the result is felt to be 'better' or
'worse' will depend on the circumstances. However it is a 'hidden' variable
which people tend to ignore or misunderstand and complicates the issue so
far as I am concerned.

FWIW One of the amps developed and sold by the company I used to work for
had an output impedance that not only went low, but became slightly
negative in the mid-bass region. This replaced an earlier SS design amp
that had an output impedance that approached an Ohm. :-) (I did not do
the initial design for either of these.) You may not be surprised to be
told that the two amps were felt to sound quite different in many
circumstances. (Although I'm sure there were other contributions to this.
:-) )

My personal preference tends to be to have a low output impedance. If
someone wants the changes that a higher value gives it is easier to add a
few tenths of an Ohm in series than it is to get a lower output impedance
from a commercial design. I tend also to prefer to remove hidden variables
which are essentially uncontrolled.

That said, although my pref is to go for amp o/p impedances below 0.1 Ohm,
I see no reason to dismiss amps that have relatively high impedances. They
may work very nicely, and give results someone prefers in relevant
circumstances. The typical effects are quite small, anyway. However for
these reasons I tend to avoid the term 'damping factor' as for reasons you
have indicated yourself. It tends to draw attention away from the more
likely reasons for any audible effect in many cases.

An extra issue with low feedback is that the output impedance may be level
dependent as well as frequency dependent, and this further complicates the
issue...

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

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

"Iain M Churches" wrote in message
...

"Trevor Wilson" wrote in message
...

"Iain M Churches" wrote in message
...
I would be interested to know the phase response
of a high end SS amp (OK, I know, they are ALL high end:-)
at 20Hz and say 20kHz, or more points if the info is
available.

If someone can supply me with some figs (from say the
legendary Krell:-) I cold use these a benchmark for the
measurements of my own amp.

**It is simply not a problem to attain 0o at 20Hz and 20kHz, with a
decent bandwidth design.


--
Trevor Wilson
www.rageaudio.com.au

Can you point us to a commercially produced amp that meets those
parameters? That would be useful.

** http://www.me-au.com/sound.html

If the link does not get you to the right spot, click on 'ME850'.


--
Trevor Wilson
www.rageaudio.com.au

In article ,
Chris Morriss wrote:
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.

I'd like to know were a full range driver that would satisfy most on here
can be bought?

--
*Young at heart -- slightly older in other places

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:

I'd like to know were a full range driver that would satisfy most on
here can be bought?

Bose?

runs.. ducks... hides....


--
Teal'c: "Dr Jackson's preliminary electroencephalogram proved anomalous"
O'Neill: "I dare you to say that again"

In article ,
Tim S Kemp wrote:
I'd like to know were a full range driver that would satisfy most on
here can be bought?

Bose?

runs.. ducks... hides....


As well you should. ;-)

--
*I must always remember that I'm unique, just like everyone else. *

Dave Plowman London SW
To e-mail, change noise into sound.

In message , "Dave Plowman (News)"
writes
In article ,
Chris Morriss wrote:
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.

I'd like to know were a full range driver that would satisfy most on here
can be bought?


I was thinking more of the one-amp-per-speaker philosophy using active
crossovers.
--
Chris Morriss

In article ,
Chris Morriss wrote:
I'd like to know were a full range driver that would satisfy most on
here can be bought?


I was thinking more of the one-amp-per-speaker philosophy using active
crossovers.

Well, yes, but then the amps aren't required to be full range...

--
*The only difference between a rut and a grave is the depth.

Dave Plowman London SW
To e-mail, change noise into sound.