Based on the definition, class B amplifier should have 0 bias current. However, I have not seen any hifi amplifier with a pure class B design (with 0mA bias), even labeled with Class B. They all have some bias presented, such as 10mA.
Couple Question.
1. Is amp with 0 bias output stage possible? It is efficient, runs cool. Why nobody use it.
2. It seems to boil down to the distortion. We have solved the switching noise of class D, why nobody solve the switching noise of class B? Maybe somebody did, I just don't know.
Couple Question.
1. Is amp with 0 bias output stage possible? It is efficient, runs cool. Why nobody use it.
2. It seems to boil down to the distortion. We have solved the switching noise of class D, why nobody solve the switching noise of class B? Maybe somebody did, I just don't know.
quad 405 no bias in the output devices its been done 1975
I tested them less than 0.01 percent distortion at 1 khz
loads glowing white at 100 watts
Trev
I tested them less than 0.01 percent distortion at 1 khz
loads glowing white at 100 watts
Trev
№3 Klass AB + C and Current dumping. Original in magazine Funkschau 1977, Heft 25, z.130-134 .
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It is possible: see this (very simplified) example:
Would like to build super simple mini transistor amplifier
Small artefacts remain, but they can practically be polished off if you use an antisaturation diode on the steering transistor and a pre-bias diode in series.
The operation is quasi-pure class B, but with a small "keep alive" constant current.
Note that the circuit is not purely theoretical: I have used it in a PA system, where quality was not paramount: intelligibility, reliability and low, conntrolled quiescent current were.
I never went as far as using it for my personal HiFi, but why not?
Doubt a Hi Fi amp is labelled as "Class B", care to show some example?
You mean lowering output bias to 0mA instead of 10mA?
What´s the point?
An amp will run as cool at 10mA as in textbook Class B compared to dissipation when it´s really working, so it´s not worth it.
Running behind a *perceived* advantage which actually brings nopractical benefits, while low level crossover distortion is well percived and annoying.
It WAS solved for ages, just pass insignificant 10mA at idle and solve it for good.
Don´t search for a non solution to a non problem.
That is a smart trick in your design, steering current with a single transistor.
I did some research on class B.
Basically, you need to enclose the output stage in a small negative feedback loop to cover the artifacts of the switching noise. The loop has to be small (few components), otherwise it will end up an oscillator.
Most opamps have limited bandwidth, the slew rate is not fast enough to hide the switching noise. I will try to avoid using an opamp to drive the class B stage.
In Douglas Self's book, "Audio Power Amplifier Design Handbook", the blameless design is labeled as class B. The classification seems to be different than what is told on textbook.
The one of the purpose of this discussion is to find out what is point to go pure class B. I am fine if there is no point to go pure class B for HIFI.
pure class B this is the zero point. It is easy to imagine, difficult to obtain in reality.
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Not everyone agrees on what class B means: zero bias current or just enough bias current to let the two sides take over from each other gracefully? Douglas Self uses the latter definition, which is also the historically correct definition as far as I know. Zero bias current is then class C, because it results in dead zones and hence in conduction angles below 180 degrees.
Anyway, that's just semantics.
Anyway, that's just semantics.
What you are talking about is a Class C amplifier.
When D. Self is talking about Class B he means what others are calling a perfect Class AB.
From D. Self:
"Class-B is subject to much misunderstanding. It is
often said that a pair of output transistors operated
without any bias are ‘working in Class-B’, and therefore
‘generate severe crossover distortion’. In fact, with no
bias, each output device is operating for slightly less
than half the time, and the question arises as to
whether it would not be more accurate to call this
Class-C and reserve Class-B for that condition of quiescent
current which eliminates, or rather minimises, the
crossover artefacts."
Stein
who listens at 100 watts?
would not distortion be rising towards lower watts?
what was the distortion at 0.1 watt? 1 watt?
There's a historical example of no bias current in the output stage. It was the Dynaco Stereo 120. Same design was also in the Stereo 80 and the SCA80.