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Don Pearce
 
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Default Any blind listening tests on Class A vs Class B amps?

On Fri, 02 Sep 2005 22:38:33 -0700, hoarse with no name wrote:

Class A amps seem to be universally considered to provide better sound
than Class B amps. Has this ever been put to a real test?


As far as I am aware, there are no class B amps around these days. Anybody
know of any?

d
  #2   Report Post  
Stewart Pinkerton
 
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On Fri, 02 Sep 2005 22:38:33 -0700, hoarse with no name
wrote:

Class A amps seem to be universally considered to provide better sound
than Class B amps.


Not in this Universe! :-)

Has this ever been put to a real test?


Yes, and even into a very tough but highly transparent load (Apogee
Duetta Signatures), my pure Class A Krell KSA-50 mkII sounds identical
to my low-bias Class AB Audiolab 8000P.

The only significant difference for the DIY enthusiast is that it's
quite difficult to make a traditional Class A amplifier that sounds
bad, this is not so true of Class AB, where many pitfalls await the
unwary constructor. Ultimately, there are many more important things
about amplifier design than mere class of operation.

--

Stewart Pinkerton | Music is Art - Audio is Engineering
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Stewart Pinkerton
 
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On Sat, 3 Sep 2005 07:11:16 +0100, Don Pearce
wrote:

On Fri, 02 Sep 2005 22:38:33 -0700, hoarse with no name wrote:

Class A amps seem to be universally considered to provide better sound
than Class B amps. Has this ever been put to a real test?


As far as I am aware, there are no class B amps around these days. Anybody
know of any?


No, but most amateurs describe Class AB amps as Class B. There's no
real confusion here, I think.
--

Stewart Pinkerton | Music is Art - Audio is Engineering
  #4   Report Post  
Don Pearce
 
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On Sat, 3 Sep 2005 07:16:14 +0000 (UTC), Stewart Pinkerton wrote:

On Fri, 02 Sep 2005 22:38:33 -0700, hoarse with no name
wrote:

Class A amps seem to be universally considered to provide better sound
than Class B amps.


Not in this Universe! :-)

Has this ever been put to a real test?


Yes, and even into a very tough but highly transparent load (Apogee
Duetta Signatures), my pure Class A Krell KSA-50 mkII sounds identical
to my low-bias Class AB Audiolab 8000P.

The only significant difference for the DIY enthusiast is that it's
quite difficult to make a traditional Class A amplifier that sounds
bad, this is not so true of Class AB, where many pitfalls await the
unwary constructor. Ultimately, there are many more important things
about amplifier design than mere class of operation.


True enough. Provided both amplifiers are competently designed, there is no
audible difference. The big difference between A and B (or AB) is that I
really don't want to be using a Class A amp during the summer.

I've just had a quick read through Doug Self's book on this subject, and
his view (which he supports very persuasively) is that Class A is not the
panacaea to linearity that many believe it to be.

d
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Stewart Pinkerton
 
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On Sat, 3 Sep 2005 09:11:53 +0100, Don Pearce
wrote:

On Sat, 3 Sep 2005 07:16:14 +0000 (UTC), Stewart Pinkerton wrote:

On Fri, 02 Sep 2005 22:38:33 -0700, hoarse with no name
wrote:

Class A amps seem to be universally considered to provide better sound
than Class B amps.


Not in this Universe! :-)

Has this ever been put to a real test?


Yes, and even into a very tough but highly transparent load (Apogee
Duetta Signatures), my pure Class A Krell KSA-50 mkII sounds identical
to my low-bias Class AB Audiolab 8000P.

The only significant difference for the DIY enthusiast is that it's
quite difficult to make a traditional Class A amplifier that sounds
bad, this is not so true of Class AB, where many pitfalls await the
unwary constructor. Ultimately, there are many more important things
about amplifier design than mere class of operation.


True enough. Provided both amplifiers are competently designed, there is no
audible difference. The big difference between A and B (or AB) is that I
really don't want to be using a Class A amp during the summer.

I've just had a quick read through Doug Self's book on this subject, and
his view (which he supports very persuasively) is that Class A is not the
panacaea to linearity that many believe it to be.


Yes, I swithered as to including Self's 'blameless' Class AB designs
in my post, but erred on the side of brevity - for a change! :-)

While a Class A PP design has no switching discontinuity, it certainly
can have 'crossover distortion' in the sense of nonlinearity of
transfer function. As Self points out, a *carefully* designed Class AB
output stage can have an optimised bias point which makes it even more
linear than an equivalent Class A design. I have designed and built
several amps on this principle, using Hitachi MOSFETs biased at 120mA
per device.
--

Stewart Pinkerton | Music is Art - Audio is Engineering


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Tim Martin
 
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"Don Pearce" wrote in message
...

As far as I am aware, there are no class B amps around these days. Anybody
know of any?


Presumably this "high end audio manufacturer" makes Class B amplifiers:

http://www.norh.com/docs/amps/

"... 99% of all audio amplifiers today are Class B. Class B amplifier can be
built today so that its distortions are well below what the human ear can
detect and nearly to the point where it is unmeasurable.
Many amplifiers call themselves Class A/B. In reality, very few are. Early
Class B amplifiers had a problem known as switching delay. In a class B
design, a transistor works 50% of the cycle while another transistor works
50% of the cycle. In early class B amplifiers, there was a distortion
created between the time the devices were switching back and forth. Some
people referred to this distortion as notch distortion because there was a
notch appearance on an oscilloscope between the two waveforms.

Class A/B was created to leave the transistor conducting while the second
transistor was conducting. This created an overlap between the two signals.
The problem with this approach is that it created its own distortion called
gumming. This means that the signal would get a little fatter where the two
devices were both conduction.

Today, if you look at a properly designed Class B amplifier on a scope, you
will see no switching distortion."




  #7   Report Post  
Don Pearce
 
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On Sun, 04 Sep 2005 09:25:27 GMT, Tim Martin wrote:

"Don Pearce" wrote in message
...

As far as I am aware, there are no class B amps around these days. Anybody
know of any?


Presumably this "high end audio manufacturer" makes Class B amplifiers:

http://www.norh.com/docs/amps/

"... 99% of all audio amplifiers today are Class B. Class B amplifier can be
built today so that its distortions are well below what the human ear can
detect and nearly to the point where it is unmeasurable.
Many amplifiers call themselves Class A/B. In reality, very few are. Early
Class B amplifiers had a problem known as switching delay. In a class B
design, a transistor works 50% of the cycle while another transistor works
50% of the cycle. In early class B amplifiers, there was a distortion
created between the time the devices were switching back and forth. Some
people referred to this distortion as notch distortion because there was a
notch appearance on an oscilloscope between the two waveforms.

Class A/B was created to leave the transistor conducting while the second
transistor was conducting. This created an overlap between the two signals.
The problem with this approach is that it created its own distortion called
gumming. This means that the signal would get a little fatter where the two
devices were both conduction.

Today, if you look at a properly designed Class B amplifier on a scope, you
will see no switching distortion."


I don't put too much store by that article. In fact I believe it is
entirely backwards in that many amplifiers that call themselves class B are
in fact class AB. Even a milliamp of bias current in the output devices is
enough to make this true, and holding an output pair at exactly zero bias (
no reverse bias or forward bias) is really hard - far easier to allow just
a little forward bias to ensure that the true horror of both transistors
turning off at the mid point can be avoided.

As for seeing switching distortion on a scope - well, if you could do that
you would have an amplifier of unimaginable horror. This statemenrt also
leads me to believe that this article is not written with great technical
expertise. Bujt then you say that this is a high-end audio manufacturer, so
this is no great surprise.

d
  #8   Report Post  
Geoff Wood
 
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"Tim Martin" wrote in message
...

"Don Pearce" wrote in message
...

As far as I am aware, there are no class B amps around these days.
Anybody
know of any?


Presumably this "high end audio manufacturer" makes Class B amplifiers:

http://www.norh.com/docs/amps/

"... 99% of all audio amplifiers today are Class B. Class B amplifier can
be
built today so that its distortions are well below what the human ear can
detect and nearly to the point where it is unmeasurable.



I would not buy anything of a "high end audio manufacturer" who does not
evidently understand what most spotty electronic geek students learn in
their first semester (if they didn't know already).

geoff


  #9   Report Post  
Arny Krueger
 
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"Tim Martin" wrote in message

"Don Pearce" wrote in message
...

As far as I am aware, there are no class B amps around
these days. Anybody know of any?


Presumably this "high end audio manufacturer" makes Class
B amplifiers:

http://www.norh.com/docs/amps/



"... 99% of all audio amplifiers today are Class B.


Absolutely totally and completely false.

Here is a true statement:

99.99% of all audio amplifiers today are Class AB

Class B amplifier can be built today so that its
distortions
are well below what the human ear can detect and nearly
to the point where it is unmeasurable.


Nope, a true class B amplifier is rare thing because it
depends on precise biasing that cannot be controlled well
enough.

Many amplifiers call themselves Class A/B.


Yes, and there's nothing wrong with that.

In reality, very few are.


The author is living in an alternative universe.

Early Class B amplifiers had a problem
known as switching delay.


Some did.


In a class B design, a
transistor works 50% of the cycle while another
transistor works 50% of the cycle. In early class B
amplifiers, there was a distortion created between the
time the devices were switching back and forth. Some
people referred to this distortion as notch distortion
because there was a notch appearance on an oscilloscope
between the two waveforms.


This would be true except that almost zero SS amps have been
built this way.

Class A/B was created to leave the transistor conducting
while the second transistor was conducting. This created
an overlap between the two signals.


So far so good.

The problem with this
approach is that it created its own distortion called
gumming.


I've heard of a lot of weird stuff, but I've never heard of
gumming distortion.

This means that the signal would get a little
fatter where the two devices were both conduction.


Spare me!



Today, if you look at a properly designed Class B
amplifier on a scope, you will see no switching
distortion."


Probably, you will because true class B is a single, very
exact point and its hard to hold.


  #10   Report Post  
Don Pearce
 
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On Sun, 4 Sep 2005 18:38:51 -0400, Arny Krueger wrote:

The problem with this
approach is that it created its own distortion called
gumming.


I've heard of a lot of weird stuff, but I've never heard of
gumming distortion.


Hee hee! I think he is perhaps referring to Gm doubling during the phase
when both output trannies are conducting.

d


  #11   Report Post  
Stewart Pinkerton
 
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On Sun, 04 Sep 2005 09:25:27 GMT, "Tim Martin"
wrote:


"Don Pearce" wrote in message
. ..

As far as I am aware, there are no class B amps around these days. Anybody
know of any?


Presumably this "high end audio manufacturer" makes Class B amplifiers:

http://www.norh.com/docs/amps/

"... 99% of all audio amplifiers today are Class B. Class B amplifier can be
built today so that its distortions are well below what the human ear can
detect and nearly to the point where it is unmeasurable.
Many amplifiers call themselves Class A/B. In reality, very few are. Early
Class B amplifiers had a problem known as switching delay. In a class B
design, a transistor works 50% of the cycle while another transistor works
50% of the cycle. In early class B amplifiers, there was a distortion
created between the time the devices were switching back and forth. Some
people referred to this distortion as notch distortion because there was a
notch appearance on an oscilloscope between the two waveforms.

Class A/B was created to leave the transistor conducting while the second
transistor was conducting. This created an overlap between the two signals.
The problem with this approach is that it created its own distortion called
gumming. This means that the signal would get a little fatter where the two
devices were both conduction.

Today, if you look at a properly designed Class B amplifier on a scope, you
will see no switching distortion."


No, the author is clearly clueless, and does not understand the
meaning of Class B - despite having quoted it at the outset.
--

Stewart Pinkerton | Music is Art - Audio is Engineering
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Stewart Pinkerton
 
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On Mon, 5 Sep 2005 06:05:03 +0100, Don Pearce
wrote:

On Sun, 4 Sep 2005 18:38:51 -0400, Arny Krueger wrote:

The problem with this
approach is that it created its own distortion called
gumming.


I've heard of a lot of weird stuff, but I've never heard of
gumming distortion.


Hee hee! I think he is perhaps referring to Gm doubling during the phase
when both output trannies are conducting.


Yep, and the better designers don't do this at all - they set the bias
point so that the Gm of one device has fallen off to about half the
usual value, at the crossover point.
--

Stewart Pinkerton | Music is Art - Audio is Engineering
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Tim Martin
 
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"Stewart Pinkerton" wrote in message
...

No, the author is clearly clueless, and does not understand the
meaning of Class B - despite having quoted it at the outset.


Quite possibly; here's what it says on the company's web page describing
one of their amplifiers:

http://www.norh.com/products/leamp2/index.html

"Class: Class A JFET gain stage, CLASS A/B bipolar stage"

But ... but ... :-)

Tim


  #14   Report Post  
Ben Bradley
 
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On Mon, 05 Sep 2005 15:39:42 GMT, "Tim Martin"
wrote:


"Stewart Pinkerton" wrote in message
.. .

No, the author is clearly clueless, and does not understand the
meaning of Class B - despite having quoted it at the outset.


Quite possibly; here's what it says on the company's web page describing
one of their amplifiers:

http://www.norh.com/products/leamp2/index.html

"Class: Class A JFET gain stage, CLASS A/B bipolar stage"

But ... but ... :-)


Now THAT is totally different from "Class AB": "CLASS A/B" is
pronounced "Class A over B." It's sort of like X(A/N)^TH.
Okay, so I just admitted I read a silly fantasy novel (but at least
I haven't read the whole series). It's no worse than that
manufacturer's website, and at least it's advertised as fantasy.

Tim


  #15   Report Post  
Stewart Pinkerton
 
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On Wed, 07 Sep 2005 04:12:22 GMT, Ben Bradley
wrote:

On Mon, 05 Sep 2005 15:39:42 GMT, "Tim Martin"
wrote:


"Stewart Pinkerton" wrote in message
. ..

No, the author is clearly clueless, and does not understand the
meaning of Class B - despite having quoted it at the outset.


Quite possibly; here's what it says on the company's web page describing
one of their amplifiers:

http://www.norh.com/products/leamp2/index.html

"Class: Class A JFET gain stage, CLASS A/B bipolar stage"

But ... but ... :-)


Now THAT is totally different from "Class AB": "CLASS A/B" is
pronounced "Class A over B." It's sort of like X(A/N)^TH.
Okay, so I just admitted I read a silly fantasy novel (but at least
I haven't read the whole series). It's no worse than that
manufacturer's website, and at least it's advertised as fantasy.


Ah well, that's a horse of a different colour. Beware daemons, they
are likely to be allied with the evil god Set....................
--

Stewart Pinkerton | Music is Art - Audio is Engineering


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Tim Martin
 
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So; suppose we have a sound reinforcement system, with an ordinary class AB
amplifier, whose manufacturer says it can deliver say 250 watts per channel
or more at some distortion level. And we are using it for a pub band rock
performance, and towards the end of the evening, it's at or close to full
power, with speakers putting out 120dB or so.

Os this amplifier working in Class A some of the time and Class B some of
the time, or what?

Tim





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Arny Krueger
 
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"Tim Martin" wrote in message


So; suppose we have a sound reinforcement system, with
an ordinary class AB amplifier, whose manufacturer says
it can deliver say 250 watts per channel or more at some
distortion level. And we are using it for a pub band
rock performance, and towards the end of the evening,
it's at or close to full power, with speakers putting out
120dB or so.


Ah, the real world. ;-)

Os this amplifier working in Class A some of the time and
Class B some of the time, or what?


A typical power amp is working Class B (only half the output
stage conducting at any one time) almost all the time, with
a short trip though Class A (both halves conducting) in a
small region around each zero-crossing, related to current
passing through the output stage.

It's usually not a big thing that this zero-crossing is
often a different point than a zero crossing in the input
signal or the output voltage.


  #18   Report Post  
Geoff Wood
 
Posts: n/a
Default


"Tim Martin" wrote in message
...
So; suppose we have a sound reinforcement system, with an ordinary class
AB
amplifier, whose manufacturer says it can deliver say 250 watts per
channel
or more at some distortion level. And we are using it for a pub band rock
performance, and towards the end of the evening, it's at or close to full
power, with speakers putting out 120dB or so.

Os this amplifier working in Class A some of the time and Class B some of
the time, or what?


Class A/B doesn't mean Class A sometimes, and class B sometimes. It means
that the output devices are biased somewhere between Class A and B, and that
a quiescent current does flow all the time - the device/s never fully switch
off as in class B, but are not on full all the time as in Class B.

It's a bit like gray - between black and white, but never black or white.


geoff


  #19   Report Post  
Sparky377
 
Posts: n/a
Default Any blind listening tests on Class A vs Class B amps?

Even worse, the author may actually have a clue but deliberately wrote the
article the way it was written. My guess is the marketing department is the
guilty party.
They may know the truth, or at least was told such by the engineering
department, but decided the truth doesn't make their product look better
than the competition.

You know how it goes; There's lies. Then there's damn lies. Then there's
statisticians. And then there's the marketing department.

Sparky377

"Stewart Pinkerton" wrote in message
...
On Sun, 04 Sep 2005 09:25:27 GMT, "Tim Martin"
wrote:


"Don Pearce" wrote in message
.. .

As far as I am aware, there are no class B amps around these days.
Anybody
know of any?


Presumably this "high end audio manufacturer" makes Class B amplifiers:

http://www.norh.com/docs/amps/

"... 99% of all audio amplifiers today are Class B. Class B amplifier can
be
built today so that its distortions are well below what the human ear can
detect and nearly to the point where it is unmeasurable.
Many amplifiers call themselves Class A/B. In reality, very few are. Early
Class B amplifiers had a problem known as switching delay. In a class B
design, a transistor works 50% of the cycle while another transistor works
50% of the cycle. In early class B amplifiers, there was a distortion
created between the time the devices were switching back and forth. Some
people referred to this distortion as notch distortion because there was a
notch appearance on an oscilloscope between the two waveforms.

Class A/B was created to leave the transistor conducting while the second
transistor was conducting. This created an overlap between the two
signals.
The problem with this approach is that it created its own distortion
called
gumming. This means that the signal would get a little fatter where the
two
devices were both conduction.

Today, if you look at a properly designed Class B amplifier on a scope,
you
will see no switching distortion."


No, the author is clearly clueless, and does not understand the
meaning of Class B - despite having quoted it at the outset.
--

Stewart Pinkerton | Music is Art - Audio is Engineering



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