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.

TIA

Iain

On Mon, 13 Dec 2004 23:27:18 +0200, "Iain M Churches"
wrote:

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 obviously depends on the bandwidth of the amp, which doesn't have
much to do with 'high end' credentials. A Naim amp for instance is
specifically designed to have limited bandwidth, and will have phase
angles of around 10-20 degrees at 20Hz and 20-30 degrees at 20kHz. At
the other extreme, a Spectral will have phase angles of less than five
degrees at these frequencies. My own Audiolab 8000P measures about 5-7
degrees at 20Hz and 8-10 degrees at 20kHz (approximated due to being
measured on a 'scope, since I don't have a vector voltmeter). The
Krell is about the same. Some SS amps are DC coupled, which will give
them zero phase angle at 20Hz.

The only thing 'legendary' about the classic Krells is that they
operate in Class A at their 8 ohm rated output, and that they double
that rated output for each halving of impedance down to 1 ohm.
Otherwise, they are not especially wideband or especially low
distortion. They are however, like most good SS amps and a very few
valve amps, sonically transparent.
--

Stewart Pinkerton | Music is Art - Audio is Engineering

"Stewart Pinkerton" wrote in message
...
On Mon, 13 Dec 2004 23:27:18 +0200, "Iain M Churches"
wrote:

It obviously depends on the bandwidth of the amp, which doesn't have
much to do with 'high end' credentials.

Understood:-)
But I assumed the better the amp the better the performance.


A Naim amp for instance is
specifically designed to have limited bandwidth, and will have phase
angles of around 10-20 degrees at 20Hz and 20-30 degrees at 20kHz. At
the other extreme, a Spectral will have phase angles of less than five
degrees at these frequencies. My own Audiolab 8000P measures about 5-7
degrees at 20Hz and 8-10 degrees at 20kHz (approximated due to being
measured on a 'scope, since I don't have a vector voltmeter). The
Krell is about the same. Some SS amps are DC coupled, which will give
them zero phase angle at 20Hz.

Hmm! Thanks for the info. These are pretty impressive figs.
I have access to a phase shift generator/meter combination (digital of
course:-) made by Feedback Electronics in the UK.
That's why I was interested to obtain some kind of a yardstick.

An initial investigation of my 50W amplifier which has the first and second
stages DC coupled, shows -3 degrees at 10Hz, and -8 degrees at 50kHz.
So about the same as the Audiolab 8000P, and very close to the Spectral?
Not too shabby:-))

This amp has custom-designed Sowter transformers. A similar amp with
Hammond iron may not perform so well.

Iain

On Tue, 14 Dec 2004 11:25:59 +0200, "Iain M Churches"
wrote:


"Stewart Pinkerton" wrote in message
.. .
On Mon, 13 Dec 2004 23:27:18 +0200, "Iain M Churches"
wrote:

It obviously depends on the bandwidth of the amp, which doesn't have
much to do with 'high end' credentials.

Understood:-)
But I assumed the better the amp the better the performance.


A Naim amp for instance is
specifically designed to have limited bandwidth, and will have phase
angles of around 10-20 degrees at 20Hz and 20-30 degrees at 20kHz. At
the other extreme, a Spectral will have phase angles of less than five
degrees at these frequencies. My own Audiolab 8000P measures about 5-7
degrees at 20Hz and 8-10 degrees at 20kHz (approximated due to being
measured on a 'scope, since I don't have a vector voltmeter). The
Krell is about the same. Some SS amps are DC coupled, which will give
them zero phase angle at 20Hz.

Hmm! Thanks for the info. These are pretty impressive figs.
I have access to a phase shift generator/meter combination (digital of
course:-) made by Feedback Electronics in the UK.
That's why I was interested to obtain some kind of a yardstick.

An initial investigation of my 50W amplifier which has the first and second
stages DC coupled, shows -3 degrees at 10Hz, and -8 degrees at 50kHz.
So about the same as the Audiolab 8000P, and very close to the Spectral?
Not too shabby:-))

This amp has custom-designed Sowter transformers. A similar amp with
Hammond iron may not perform so well.

Iain


I wouldn't get too bothered by the phase response of an amplifier,
other than as an indicator of bandwidth (in a competently designed
amplifier where rolloffs are defined by single poles, -3dB = 90
degrees). Do remember that a speaker follows, with phase shifts that
are all over the place by comparison.

d

Pearce Consulting
http://www.pearce.uk.com

In article , Iain M Churches
wrote:


Hmm! Thanks for the info. These are pretty impressive figs. I have
access to a phase shift generator/meter combination (digital of
course:-) made by Feedback Electronics in the UK. That's why I was
interested to obtain some kind of a yardstick.

An initial investigation of my 50W amplifier which has the first and
second stages DC coupled, shows -3 degrees at 10Hz, and -8 degrees at
50kHz. So about the same as the Audiolab 8000P, and very close to the
Spectral? Not too shabby:-))

Are the above values with an 8 Ohm load from an 8 Ohm tap?

If you are concerned about this, I'd suggest you also check with reactive
and/or mismatched loads. FWIW Naim amps have tended in the past to have a
series resistor (about 0.2 Ohms IIRC) in their output, and then tell the
user to employ about 10 microH worth of special cable in series. (Which is
conveniently omitted from magazine measurements but is in place when using
the amp as recommended.)

Whereas many non-Naim SS amps either have direct output or have a small
series inductor. These can affect the phase response at 20kHz and mean that
values obtained into an 8 Ohm resistor are not always a good guide.
Similarly, if your valve amp has any reactance or an impedance much above
0.1 Ohms, this may interact with reactive loads and give a distinct change
in result.

How much this matters is a different matter, though. The figures you quoted
look - in themselves - perfectly respectable. Hence unless your amp has a
particularly high output impedance and/or is reactive at HF/LF I'd doubt
this matters compared to the way most speakers and rooms will affect the
phase of what reaches the listener. :-)

This amp has custom-designed Sowter transformers. A similar amp with
Hammond iron may not perform so well.

Since you are doing this, I'd be interested to know the complex output
impedance you get across the audio band as this seems to be the kind of
measurement that rarely gets done and published for *any* amp these days...

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

"Jim Lesurf" wrote in message
...
In article , Iain M Churches
wrote:
An initial investigation of my 50W amplifier which has the first and
second stages DC coupled, shows -3 degrees at 10Hz, and -8 degrees at
50kHz. So about the same as the Audiolab 8000P, and very close to the
Spectral? Not too shabby:-))

Are the above values with an 8 Ohm load from an 8 Ohm tap?

Yes, they are.

If you are concerned about this, I'd suggest you also check with reactive
and/or mismatched loads.

Not so much concerned as interested:-) The figs in themselves do not mean
much if I have no yardstick - hence ther question.



The figures you quoted
look - in themselves - perfectly respectable. Hence unless your amp has a
particularly high output impedance

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

and/or is reactive at HF/LF I'd doubt
this matters compared to the way most speakers and rooms will affect the
phase of what reaches the listener. :-)

Understood.

This amp has custom-designed Sowter transformers. A similar amp with
Hammond iron may not perform so well.

Since you are doing this, I'd be interested to know the complex output
impedance you get across the audio band as this seems to be the kind of
measurement that rarely gets done and published for *any* amp these
days...

Can you suggest the best way to ascertain these figs?


Iain

In article , Iain M Churches
wrote:

"Jim Lesurf" wrote in message
...


The figures you quoted look - in themselves - perfectly respectable.
Hence unless your amp has a particularly high output impedance

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.

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 feeback 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.


Since you are doing this, I'd be interested to know the complex output
impedance you get across the audio band as this seems to be the kind
of measurement that rarely gets done and published for *any* amp these
days...

Can you suggest the best way to ascertain these figs?

Basically the same as when measuring 'damping factor' (a term I don't
personally like as I think it is misleading).

Either:

1) Set the signal level and vary the load, and note the o/p level for two
different loads. Then use this to work out the o/p impedance. If the amp is
stable, one 'load' can be o/c as this gives you the o/p emf directly.

or

2) Ensure the amp is trying to o/p zero, but drive a signal into the
*output* via a suitable resistor. Use the other channel of the power amp to
do this if it is convenient. Then measure the drive level and the level
that appear on the o/p and work out the o/p impedance that way.

Repeat for various frequencies.

For a low feedback amp you may need to do this at high power as well as low
to see if the values change. However this may be difficult via (2) as some
amps may not like this process.

IIRC In a previous posting you say you have a phase meter, so by measuring
the signal phases as well as amplitudes you can work out the o/p impedance
as a complex value and hence determine the entire complex impedance as a
function of frequency. This can matter due to interactions with the
complex load presented by a loudspeaker. For an example of this have a look
at the model of the '303' amp and '57' speaker which is linked to my 'Audio
Misc' page (URL in my sig, below). This shows why - when the amp output
impedance isn't essentially zero - these effects can dwarf any departures
for a flat response when measureing with an 8 Ohm load. :-)

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

On Tue, 14 Dec 2004 11:25:59 +0200, "Iain M Churches"
wrote:


"Stewart Pinkerton" wrote in message
.. .
On Mon, 13 Dec 2004 23:27:18 +0200, "Iain M Churches"
wrote:

It obviously depends on the bandwidth of the amp, which doesn't have
much to do with 'high end' credentials.

Understood:-)
But I assumed the better the amp the better the performance.

That depends how you define 'batter'. The Spectral for instance has
megahertz bandwidth, so immaculate phase response at 20kHz, but is
notoriously unstable. The Naim OTOH has been highly regarded by
audiophiles for about thirty years, despite being sensitive to
capacitive lopads and demanding highly inductive speaker cable to
maintain its warranty. VCome to think of it, the phase response is a
lot worse if you include that cable, as you should since Naim consider
iot to be an essential part of the circuit driving the speaker.
Basically, there's a *lot* more to 'high fidelity' than sheer
bandwidth. Many decades ago, the brilliant Gilbert Briggs of
Wharfedale coined a very useful phrase - the wider you open the
window, the more the muck flies in!

An initial investigation of my 50W amplifier which has the first and second
stages DC coupled, shows -3 degrees at 10Hz, and -8 degrees at 50kHz.
So about the same as the Audiolab 8000P, and very close to the Spectral?
Not too shabby:-))

That's very impressive for a valve amp.

This amp has custom-designed Sowter transformers. A similar amp with
Hammond iron may not perform so well.

Impressive results explained! :-)
--

Stewart Pinkerton | Music is Art - Audio is Engineering

"Stewart Pinkerton" wrote in message
...
On Tue, 14 Dec 2004 11:25:59 +0200, "Iain M Churches"
wrote:

An initial investigation of my 50W amplifier which has the first and
second
stages DC coupled, shows -3 degrees at 10Hz, and -8 degrees at 50kHz.
So about the same as the Audiolab 8000P, and very close to the Spectral?
Not too shabby:-))

That's very impressive for a valve amp.

From the figs you gave me, it looks to be impressive for any amp.
It's a home brew too:-))
There are probably many valve amps out there that match these figs.

This amp has custom-designed Sowter transformers. A similar amp with
Hammond iron may not perform so well.

Impressive results explained! :-)
Well, yes and no. I have always believed that the most critical component
in a valve amplifier is the OP transformer. So many people splash out on a
stainless-steel laser-cut chassis, and skimp on the transformers:-(

But using a Sowter transformer does not transform a poor amplifier into a
good one, it just forms a very important high-quality last link in the
chain.


Iain

"Stewart Pinkerton" wrote in message
...
On Mon, 13 Dec 2004 23:27:18 +0200, "Iain M Churches"
wrote:

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.

It obviously depends on the bandwidth of the amp, which doesn't have
much to do with 'high end' credentials. A Naim amp for instance is
specifically designed to have limited bandwidth, and will have phase
angles of around 10-20 degrees at 20Hz and 20-30 degrees at 20kHz. At
the other extreme, a Spectral will have phase angles of less than five
degrees at these frequencies. My own Audiolab 8000P measures about 5-7
degrees at 20Hz and 8-10 degrees at 20kHz (approximated due to being
measured on a 'scope, since I don't have a vector voltmeter). The
Krell is about the same. Some SS amps are DC coupled, which will give
them zero phase angle at 20Hz.


Can you confirm that these figs are accurate?
I ask this because a chap who writes the technical column
for an audio magazine is interested to write article about
phase response, and I want to make sure that any data
I give him is 100% correct. I am sure he will give you a
mention. Fame at last:-))

TIA

Iain