No.
You have pozitive side on class B and C as well.
In class AB, a very important factor is how much power in class A will amplifier operate. You may have a class AB amplifier that will operate only first 1W in Class A or an amplifier that will operate first 20W in class A. Both are class AB amplifiers. The main reason for class AB operation is to avoid crossover distortions, who are big on class B and huge at class C.
I'm sure this was discussed on this forum.
Regards,
Tibi
You have pozitive side on class B and C as well.
In class AB, a very important factor is how much power in class A will amplifier operate. You may have a class AB amplifier that will operate only first 1W in Class A or an amplifier that will operate first 20W in class A. Both are class AB amplifiers. The main reason for class AB operation is to avoid crossover distortions, who are big on class B and huge at class C.
I'm sure this was discussed on this forum.
Regards,
Tibi
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Tibi,
in your PDF documentation:
"FQA46N15 have a regular transconductance of 36 Siemens while Exicon duble die
ECW20N20 have a maximum of 4 Siemens. This is equivalent of 9 double die
laterals. Yes, very impressive !
FQA36P15 have a regular transconductance of 19.5 Siemens while Exicon duble die
ECW20P20 have a maximum of 4 Siemens. This is equivalent of 4 double die
laterals.
In the “darkside” of Mouser are living some beasts that may leave these Fairchild’s in
dust. These are made by IXYS and a pair could be IXFK220N20X3 and
IXTK120P20T. These exhibit over 120 Siemens transconductance and are able to
transform your amplifier into a real welding machine. No joke at all. If your speakers
have never been moved before, use a pair of these and shake your world, your
walls, your neighbour …
Trough R11 and R12 will bias these devices near to class A operation. Both,
FQA46N15 & FQA36P15 Vgs - threshold voltage between 3V and 4V. I found that P
channel FQA36P15 will open around 3V, while the FQA46N15 will need
100mV-300mV more. (To have both running at same level R11 may be increased to
51ohm or more.)
Obviously Q15 and Q16 must be mounted on a large heatsink."
So, a bit confusing.
I know that the original Quad 405 output stage is class B (being part of current dumping principle) and therefore not requiring large heatsinks, though I remember that it could become quite hot with 4 ohm load.
in your PDF documentation:
"FQA46N15 have a regular transconductance of 36 Siemens while Exicon duble die
ECW20N20 have a maximum of 4 Siemens. This is equivalent of 9 double die
laterals. Yes, very impressive !
FQA36P15 have a regular transconductance of 19.5 Siemens while Exicon duble die
ECW20P20 have a maximum of 4 Siemens. This is equivalent of 4 double die
laterals.
In the “darkside” of Mouser are living some beasts that may leave these Fairchild’s in
dust. These are made by IXYS and a pair could be IXFK220N20X3 and
IXTK120P20T. These exhibit over 120 Siemens transconductance and are able to
transform your amplifier into a real welding machine. No joke at all. If your speakers
have never been moved before, use a pair of these and shake your world, your
walls, your neighbour …
Trough R11 and R12 will bias these devices near to class A operation. Both,
FQA46N15 & FQA36P15 Vgs - threshold voltage between 3V and 4V. I found that P
channel FQA36P15 will open around 3V, while the FQA46N15 will need
100mV-300mV more. (To have both running at same level R11 may be increased to
51ohm or more.)
Obviously Q15 and Q16 must be mounted on a large heatsink."
So, a bit confusing.
I know that the original Quad 405 output stage is class B (being part of current dumping principle) and therefore not requiring large heatsinks, though I remember that it could become quite hot with 4 ohm load.
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Operation class is not part of current dumping principle. You may have an amplifier running pure class A and still use current dumping.
The advantage of current dumping is that you may run an amplifier in class B or slightly biased in A (like in our case) and still have low distorsion.
Whatever class you operate, the output stage will dissipate power and you need a heatsink to dissipate the heat. The heat will be proportional with the power you operate the amplifier and load.
On original Quad405, like on current Q17, we have a class A stage that for both channels dissipate at least 12W and this is enough to make a heatsink, like the one in original 405, hot.
Regards,
Tibi
It's not a question of what I want. It's a qiuestion of conduction angles. It is class AB. There is nothing else you can call it.
EJP
EJP,
An amplifier will always run in a single class, not combination of two.
So, when an amplifier will run in Class AB, in fact will run in class A up to a level and after that in class B. For me class AB is just a description of operation in class A and then B.
We can debate this a lot, but in the end we may both been right.
Regards,
Tibi
An amplifier will always run in a single class, not combination of two.
So, when an amplifier will run in Class AB, in fact will run in class A up to a level and after that in class B. For me class AB is just a description of operation in class A and then B.
We can debate this a lot, but in the end we may both been right.
Regards,
Tibi
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Douglas Self's books, by virtue of their popularity, are responsible for a great deal of the confusion in this. Here is a quote:
"My definition of Class-B is that unique amount of bias
voltage which causes the conduction of the two output
devices to overlap with the greatest smoothness and so
generate the minimum possible amount of crossover
distortion. For a single pair of output devices the quiescent
current will be of the order of 10 ma for a CFP
output stage, or 100 mA for the EF version."
A class AB is an amp biased beyond "the greatest smoothness"
Tibi,
First of all: great project! IMO the classic current dumping principle is worth a new life! (I will order synchronous rectification boards).
Not wanting to debate the class of operation, but you caused the confusion by stating that "through R11 and R12 the output devices will be biased near to class A operation".
Well, with Iq under 100mA, the output stage is class A up to well under 100mW, class B for higher power.
Your statement in post #71 is correct; "near to class A" has no meaning.
First of all: great project! IMO the classic current dumping principle is worth a new life! (I will order synchronous rectification boards).
Not wanting to debate the class of operation, but you caused the confusion by stating that "through R11 and R12 the output devices will be biased near to class A operation".
Well, with Iq under 100mA, the output stage is class A up to well under 100mW, class B for higher power.
Your statement in post #71 is correct; "near to class A" has no meaning.
False. It can depend on the level. Some Class A amplifiers slip into AB at the highest levels. Nominally Class B amplifiers can be slightly into AB if overbiased.
This is not only false but nonsensical, because (1) you are contradicting yourself, and (2) you are stating an impossibility. (1) What you have now described is not 'always', and it is indeed a 'combination of classes', which depends on level: but (2) if you can find a Class A amplifier, conduction angle 360, which is capable of running in class B, conduction angle 180 (without a mode switch) please produce it. It would be a remarkable piece of work. And, again, it's not about you. It's about conduction angle, an empirical fact, not a personal opinion.
You can't be right when you're contradicting yourself. You have to choose.
Class A = 360 degrees conduction. Class B = 180 degrees conduction. Class AB = say 181 to 359 degrees conduction. Your claim that class B is adjacent to class A is therefore false by at least 178 degrees. This is where we cannot possibly agree.
EJP