Any blind listening tests on Class A vs Class B amps?

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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. 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

Archived from groups: rec.audio.tech (More info?)

On Sat, 3 Sep 2005 07:11:16 +0100, Don Pearce <donald@pearce.uk.com>
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

Archived from groups: rec.audio.tech (More info?)

On Sat, 3 Sep 2005 09:11:53 +0100, Don Pearce <donald@pearce.uk.com>
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 <no@2spam.com>
>> 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

Archived from groups: rec.audio.tech (More info?)

On Sun, 04 Sep 2005 09:25:27 GMT, Tim Martin wrote:

> "Don Pearce" <donald@pearce.uk.com> wrote in message
> news:6mupi03blf0p$.s9pvdjy1mm6$.dlg@40tude.net...
>
>> 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

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"Tim Martin" <tim2718281@ntlworld.com> wrote in message
news:b4zSe.2723$vC4.107@newsfe4-win.ntli.net
> "Don Pearce" <donald@pearce.uk.com> wrote in message
> news:6mupi03blf0p$.s9pvdjy1mm6$.dlg@40tude.net...
>
>> 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.

Archived from groups: rec.audio.tech (More info?)

On Sun, 04 Sep 2005 09:25:27 GMT, "Tim Martin"
<tim2718281@ntlworld.com> wrote:

>
>"Don Pearce" <donald@pearce.uk.com> wrote in message
>news:6mupi03blf0p$.s9pvdjy1mm6$.dlg@40tude.net...
>
>> 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

Archived from groups: rec.audio.tech (More info?)

"Stewart Pinkerton" <patent3@dircon.co.uk> wrote in message
news:iknnh1ta504e65n8ehi8d2o0ss6difst6d@4ax.com...

> 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

Archived from groups: rec.audio.tech (More info?)

On Wed, 07 Sep 2005 04:12:22 GMT, Ben Bradley
<ben_nospam_bradley@frontiernet.net> wrote:

>On Mon, 05 Sep 2005 15:39:42 GMT, "Tim Martin"
><tim2718281@ntlworld.com> wrote:
>
>>
>>"Stewart Pinkerton" <patent3@dircon.co.uk> wrote in message
>>news:iknnh1ta504e65n8ehi8d2o0ss6difst6d@4ax.com...
>>
>>> 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

Archived from groups: rec.audio.tech (More info?)

"Tim Martin" <tim2718281@ntlworld.com> wrote in message
news:ixfUe.2982$yF2.168@newsfe6-win.ntli.net

> 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.