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"Discrete Class A"?
Old 10th May 2006
  #1
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bobby_z's Avatar
 

"Discrete Class A"?

What does it mean? It's kind of a popular phrase among very expensive preamps and monitor controllers..
Old 10th May 2006
  #2
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dave-G's Avatar
I'm no EE, but to oversimplify:

discrete: no ICs (chips) are used .. individual (discrete) components or potted opamps made with individual components

class-A: one amplifier does the entire signal; both swings of the waveform (+ and -) .. AB means one amp does the positive side, and another does the -

It's not like a USDA grade, or the letter-grade in the window of an LA restaurant. It's just a nomenclature

-dave

Last edited by dave-G; 10th May 2006 at 01:55 PM..
Old 10th May 2006
  #3
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Infernal Device's Avatar
 

Dumb question... are 1073's "Class A"?
Old 10th May 2006
  #4
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dave-G's Avatar
Quote:
Originally Posted by Infernal Device
Dumb question... are 1073's "Class A"?
.. and discrete.

(yes)

-dave
Old 10th May 2006
  #5
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bobby_z's Avatar
 

So, umm, what makes "discrete class A" amps better than IC opamps?
Old 10th May 2006
  #6
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thebri's Avatar
Class A Also has a better slew rate, as it is always on. This also means it uses more power to do it's task, unlike class B and AB that take power as they need it. Thus Class A sounds "louder" as some people say, but at the cost of electrical efficiency.

There is a killer, extra verbose explination of all this stuff on www. vacuumtube.com, but I don't remember where on that mess of a site it is.
Old 10th May 2006
  #7
Gear Maniac
 

Quote:
Originally Posted by bobby_z
So, umm, what makes "discrete class A" amps better than IC opamps?
Generally speaking, IC's (especially cheap ones) have less headroom and distort in a far less pleasing manner.

There's some very expert info on this from Hutch, Manley's chief design genius...

http://www.manleylabs.com/mantra.html
Old 10th May 2006
  #8
Gear Addict
 

Quote:
Originally Posted by thebri
Class A Also has a better slew rate, as it is always on. This also means it uses more power to do it's task, unlike class B and AB that take power as they need it. Thus Class A sounds "louder" as some people say, but at the cost of electrical efficiency.

.
Class A does not necessarily have better slew rate nor does it always sound "louder". Slew rate is far more related to circuit parasitic capacitances, transistor parameters, drive current capability etc.
Old 10th May 2006
  #9
Gear Maniac
 

Also, Class A designs should perform better distorsion specs than the others - amplifying the signal on boths polarities have a significant role in reducing distorsions.
Old 10th May 2006
  #10
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nosebleedaudio's Avatar
 

Where a well designed Class A amp should shine is on low level detail, it doesnt have to be driven hard to come alive.
Old 10th May 2006
  #11
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beechstudio's Avatar
 

1081's are class A/B and still smoke! So I don't think a piece of gear has to be 'class A' to be totally pro. It's more of the quality of components in the signal path .(or lack there of) that make the difference. IMHO
Old 11th May 2006
  #12
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Tim Farrant's Avatar
 

Quote:
Originally Posted by dave-G
..class-A: one amplifier does the entire signal; both swings of the waveform (+ and -) .. AB means one amp does the positive side, and another does the -
Dave, this is sorta correct, but lets not confuse Class A with "push pull" circuits. A push pull circuit can also be Class A. Class A is best defined as a circuit where the elements are never turned off. In a normal linear power amplifier, you have a push pull output stage where one device handles the negative swing and another the positive (Class AB). The point at which one device takes over from another is set by the "bias". If the bias is raised to such a point that neither device turns off, then it's Class A.

The only real disadvantage with Class AB is there is a glitch as one device takes over from another, called crossover distortion.

Neve 1073 is single ended Class A, ie, as you say, one device handles the whole signal. In our Class A stuff, we use push pull circuits (sometimes referred to as "differential") which are biased as described above.

Most opamps are Class A up until the output stage which is biased Class AB. But - and this is very important regarding Class AB circuits - the output stage will remain in Class A if the load it is driving draws less than half the bias current. This means, if the opamp has a high load impedance, it will be still run in Class A.

Bearing this in mind in opamp design, you can achieve very good results with these devices, so just because it's an opamp does not mean it's going to sound bad - it's only going to sound bad if the designer has not used the device in a sensible manner.

Sorry for the long post, hope it helps.
Tim.
Old 11th May 2006
  #13
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T.RayBullard's Avatar
 

DAV preamps use ICs, and well, yeah..they are the heat.
Old 11th May 2006
  #14


The first stage in almost every pre-amp is class A - there is really no good way to make a differential pair operate any other way.

Pre amps that use an input transformer and no diff pair are almost invariably class A on the first stage as well.



-tINY

Old 11th May 2006
  #15
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lawrence_o's Avatar
There is also a HUGE disadvantage in class A. The circuits form a lot of heat so when you take the Neve discrete consoles, they warm up so high that when using them every day, after about 5 years you'll have to recap them as the conductors dry out.

I think good ICs can also do the job and will we hear it? Hmmm, doubt it. Verify e.g. Neve versus SSL. Both are good, sound different but both are top-end gear.

Regards.
Old 11th May 2006
  #16
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DeepSpace's Avatar
 

In the words of Rupert Neve:

Quote:
...With any push/pull amplifier, there will be a point at which there's a transition from the negative half-cycle amplifier to the positive half-cycle amplifier, or vice-versa. Even if it isn't a very obvious one, even if the amplifier is biased into Class A/B, that transition is enough to produce some high order harmonics. It is those high order harmonics which make the difference to the sound. They are the crossover distortion...
[from the Greg Simmons interview with Rupert Neve, as reproduced in Vol 1 Issue 2 of AudioTechnology magazine (the above quote appears on page 60)]

To put it into context, I do not think there is any contradiction between the above quote and the remarks posted above by Tim Farrant. Rupert's reference to push/pull configurations refers to the common practice of biasing the complementary components assymetrically (ie offset symmetrycally against each other) to increase headroom (but with the expense of distortion produced at the crossover point when one device ceases and then subsequently recommences operation).

So, the point of Class A designs is that they provide one way to avoid crossover distortion - and the point of discrete componentry is that it provides one way to achieve a Class A design. But as Tim has said, it is not the only way and op amps can indeed be used in ways which minimize or eliminate the negative effects of the A/B or push/pull operation of their output stages.

One method is the one Tim mentions, to control the impedance of following stages such that the output stage is unable to pass voltages which exceed the available supply current in either polarity (effectively keeping it within the limits of Class A operation because it never reaches the threshold at which the Class B 'mode' of the A/B configuration is invoked). Another is mentioned by Rupert Neve in the article quoted above, where he describes a technique involving applying a bias voltage to the non-inverting output of an op-amp to drive the devices assymmetrically and displace the point at which crossover artifacts will be introduced.

So I fully agree with Tim's observation that "just because it's an opamp does not mean it's going to sound bad" and would add that just because it is Class A and discrete does not mean that it is going to sound good either. In both cases it rests on the skill of the designer (not to mention the quality of the components etc etc!).

Last edited by DeepSpace; 11th May 2006 at 12:58 PM..
Old 11th May 2006
  #17
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dave-G's Avatar
Quote:
Originally Posted by Tim Farrant
Dave, this is sorta correct, but lets not confuse Class A with "push pull" circuits. A push pull circuit can also be Class A. Class A is best defined as a circuit where the elements are never turned off. In a normal linear power amplifier, you have a push pull output stage where one device handles the negative swing and another the positive (Class AB). The point at which one device takes over from another is set by the "bias". If the bias is raised to such a point that neither device turns off, then it's Class A.
Thanks for the clarification, Tim.


Now where's my EQ?!!
-dave
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