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Definition of Class A?
Definition of Class A?
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Old 4th December 2017, 04:10 PM   #1
Bandersnatch is offline Bandersnatch  United States
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Default Definition of Class A?

And yes I am serious... Just wanted to put it down, as those infected by Marketing seem to think that the liddle bit of time in an AB amp that both sides are conducting constitutes Class A operation.

1. Full 360 degree conduction angle of the amplifying device
2. No.1 at full power to what ever the distortion spec happens to be.
3. Effective idle point in the middle of the curves across which the load line lies.

It excludes X watts Class A, XX Watts B. As the AB definition covers a bit of both sides conducting for a full 360 degrees.

For a PP amp, the two phases are acting like a bridged pair of SE amps. SE having to be full circle conduction angle...LOL

Any points I missed?
cheers,
Douglas
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Old 4th December 2017, 04:42 PM   #2
DF96 is offline DF96  England
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I think that covers it. Someone is bound to disagree!
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Old 4th December 2017, 05:30 PM   #3
kevinkr is offline kevinkr  United States
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Definition of Class A?
OT post removed
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Old 4th December 2017, 06:11 PM   #4
20to20 is offline 20to20  United States
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Quote:
Originally Posted by Bandersnatch View Post
Just wanted to put it down, as those infected by Marketing seem to think that the liddle bit of time in an AB amp that both sides are conducting constitutes Class A operation.

Any points I missed?
cheers,
Douglas
Doesn't it? My AB amp spends 99% of the time in A.
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Old 4th December 2017, 06:28 PM   #5
Bandersnatch is offline Bandersnatch  United States
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Originally Posted by 20to20 View Post
Doesn't it? My AB amp spends 99% of the time in A.
Your AB amp spends 100% of its time in AB. It is not a bit of A and a bit of B; that condition is AB. Nothing more, nothing less.

The point being is that, to which you are referring is *NOT* Class A. There is more to the definition that both conducting 360 degrees. It is bias a little bit past the middle of the curves through which the load line passes.

If there is 50 mA available to a maximum positive grid swing of X volts, a swing of -X volts from idle must not cut off that phase. It is effectively two bridged SE amps working off their respective halves of a CT primary.
cheers,
Douglas
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Old 4th December 2017, 06:42 PM   #6
20to20 is offline 20to20  United States
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Originally Posted by Bandersnatch View Post
Your AB amp spends 100% of its time in AB. It is not a bit of A and a bit of B; that condition is AB. Nothing more, nothing less.

cheers,
Douglas
My PP amp is biased so that it can run in AB IF I turn the volume control up high enough, which I rarely do. My PP amp is not biased on the center as so it would only run in A. That would create balanced clipping and cuttoff if the drive signal is too high. It will go into cuttoff if I desire and turn the volume control up high enough. The negative portion of the drive signal rarely drives the tube into cuttoff so the amp runs in A.... 99% of the time.
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Old 4th December 2017, 07:34 PM   #7
marco_gea is offline marco_gea  Italy
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Default Nelson Pass..

...who ought to know a thing or two about this topic, puts it like this:

"The bias current runs through the amplifiers at a minimum value, determining the class of operation
Class B, Class AB, or Class A.

Class B has no bias current, Class AB has a moderate bias current, and Class A has a high bias current. Class AB push-pull amplifiers are hybrids between Class B and Class A. Class AB run Class A at low power levels, and become Class B amplifiers at output currents determined by the bias."
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Old 4th December 2017, 07:40 PM   #8
scottjoplin is offline scottjoplin  Wales
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Yep, pretty basic stuff, I don't understand the point the OP is trying to make. I suspect he is yet another with an agenda and urge him to come clean if this is the case.
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Old 4th December 2017, 07:41 PM   #9
petertub is offline petertub  Sweden
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Quote:
Originally Posted by Bandersnatch View Post
And yes I am serious... Just wanted to put it down, as those infected by Marketing seem to think that the liddle bit of time in an AB amp that both sides are conducting constitutes Class A operation.

1. Full 360 degree conduction angle of the amplifying device
2. No.1 at full power to what ever the distortion spec happens to be.
3. Effective idle point in the middle of the curves across which the load line lies.

It excludes X watts Class A, XX Watts B. As the AB definition covers a bit of both sides conducting for a full 360 degrees.

For a PP amp, the two phases are acting like a bridged pair of SE amps. SE having to be full circle conduction angle...LOL

Any points I missed?
cheers,
Douglas
4. no grid current i.e. grid never goes positive relative cathode
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Old 4th December 2017, 07:43 PM   #10
Bandersnatch is offline Bandersnatch  United States
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as predicted by DF96, there are people who would disagree.

The point is that it is more than just a single quantity definition. The examples cited by marco and 20to20 are exactly single quantity. Class AB amps are only and always AB amps. The definition is wider than 'both tubes conducting for 360 degrees'.

Class A requires more than this...
cheers,
Douglas
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