Power amp phase response
 

"Iain M Churches" wrote in message

"Arny Krueger" 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.

Look at the amp's frequency response curve and use what you know
about the Hibert transform or simply draw Bode diagrams to estimate
the amp's phase response.

As mentioned before, I have access to a phase measuring set
(generator and meter) In the past, I have made measurements
using an oscilloscope.

At low levels power amps tend to be minimum-phase.

So should I take measurements at different power levels?

For a tubed amp, FR & phase response might change a fair amount at higher
levels.

Power amp phase response
 

"Arny Krueger" wrote in message
...
"Iain M Churches" wrote in message
So should I take measurements at different power levels?

For a tubed amp, FR & phase response might change a fair amount at higher
levels.

Arny. Thanks for that.
I am aware of the differences between FR (at 1W) and power band width
and have plotted these. However, I did not realise that phase response
might differ, until you mentioned it.

Iain

Power amp phase response
 

"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

Power amp phase response
 

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

Power amp phase response
 

"Arny Krueger" wrote in message
...
"Iain M Churches" wrote in message

"Arny Krueger" 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.

Look at the amp's frequency response curve and use what you know
about the Hibert transform or simply draw Bode diagrams to estimate
the amp's phase response.


The phase measuring equipment to which I have access is built
by Feedback Electronics in the UK.
It can be used in two ways, depending on whether
or not you have the additional digital meter for direct readout.

The test signal is supplied by an oscillator which has two outputs,
both with attenuators for level matching.
Without the meter unit, one proceeds as follows;
Connect output 1 from the oscillator to one input of the scope
which is in the XY mode. Adjust output level 1 of the oscillator
to give a satisfactory diagonal deflection on the scope.

Connect output 2 to the input of the device under test.
Connect the output of the DUT (via a dummy load if required)
to input 2 of the scope, and adjust the oscillator output level
to give a similar deflection in the other diagonal plane.

Then connect both signals to the scope. The resulting display
will be an ellipse. Output 2 of the oscillator has a variable
phase control, which can be adjusted until the ellipse
collapses into a straight diagonal line.
Then, read off the phase angle indicated on the
skirt of the phase control knob, and you have the answer.

If you have the digital readout unit, measurement is even simpler,
and the oscilloscope is not required.

Connect output 1 from the oscillator to input 1 of the meter unit.
Connect output 2 to the device under test.
Connect the output of the DUT (via a dummy load if required)
to input 2 of the meter unit. The latter can measure the phase
angle between a pair of signals with a 30V difference, so a 60W
amp can be measured with ease.

A led shows which signal is lagging or leading, and the digital
readout gives the phase angle. Easy and fun:-)

Iain

Power amp phase response
 

"Iain M Churches" wrote in message
...

Adjust output level 1 of the oscillator
to give a satisfactory diagonal deflection on the scope.

I meant horizontal of course!

Connect output 2 to the input of the device under test.
Connect the output of the DUT (via a dummy load if required)
to input 2 of the scope, and adjust the oscillator output level
to give a similar deflection in the other diagonal plane.

I meant vertical of course!

But the result is still the same, when the two signals are
present - a diagonal ellipse.


Iain

Power amp phase response
 

"Iain M Churches" wrote in message


The phase measuring equipment to which I have access is built
by Feedback Electronics in the UK.
It can be used in two ways, depending on whether
or not you have the additional digital meter for direct readout.

The way I do it, uses a sound card and a piece of software which is
identified in the pictures linked below.

I drive the UUT with pink noise, white noise or even music. Anything with
signal that has content in the frequency range I want to measure. I load it
appropriately. I sample the input and the output and ask the software to
plot the transfer function in terms of amplitude and phase.

You can see samples of how this works at

http://www.pcavtech.com/soundcards/LynxTWO/index.htm

(frequency response and phase response for three different sample rates)

http://www.pcavtech.com/pwramp/

(loudspeaker simulator impedance magnitude and phase plots)

Since the sound card's channels are almost perfectly identical, and are in
both measurement paths, the response characteristics of the sound card is
effectively eliminated from the results as long as it has any kind of even
halfways-reasonable response.

The pictures shown at the web site are cropped versions of screen dumps
taken under Windows XP.

Power amp phase response
 

In article , Iain M Churches
wrote:


The phase measuring equipment to which I have access is built by
Feedback Electronics in the UK. It can be used in two ways, depending on
whether or not you have the additional digital meter for direct readout.

[snip]

If you have the digital readout unit, measurement is even simpler, and
the oscilloscope is not required.

Connect output 1 from the oscillator to input 1 of the meter unit.
Connect output 2 to the device under test. Connect the output of the DUT
(via a dummy load if required) to input 2 of the meter unit. The latter
can measure the phase angle between a pair of signals with a 30V
difference, so a 60W amp can be measured with ease.

A led shows which signal is lagging or leading, and the digital readout
gives the phase angle. Easy and fun:-)

The above should work OK, although Arny's method has the advantage that if
you use a wideband signal you can recover the phase and amplitude response
across the relevant band in one measurement. This can save lots of time by
avoiding having to make loads of measurements, frequency by frequency, and
then plot the results.

FWIW I would tend either to use a sampling scope that can do FFTs
(equivalent to the method Arny describes), along with a noise/arb-gen, or
use a PSD (equivalent to what you describe) and sine-gen depending on what
is convenient. (And, these days, what kit my ex-students will allow me to
'borrow'. ;- )

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

Power amp phase response
 

"Jim Lesurf" wrote in message
...
FWIW I would tend either to use a sampling scope that can do FFTs
(equivalent to the method Arny describes), along with a noise/arb-gen, or
use a PSD (equivalent to what you describe) and sine-gen depending on what
is convenient. (And, these days, what kit my ex-students will allow me to
'borrow'. ;- )


Now you have really hit the nail on the head:-)
One has to use the means (and equipment) at ones disposal,
and be grateful for that:-)

I was very happy indeed to have the possibility to use the
phase meter set up which I described.

Iain

Power amp phase response
 

"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