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In Reply to: RE: your statement still isn't logical. posted by morricab on February 18, 2025 at 08:58:43
Show the measurements of the amp in question. Demonstrate how it is possible for a Class B amp to not have zero crossing distortion when it is clear that the transistors are going into cutoff and it is highly non-linear. The disjointedness of the handoff between transistors in Class B, where one is going into cutoff and is non-linear and the other is also non-linear (but with a different transfer function in that region) upon turn on is inherent in the devices and the Class B bias.
A power amp is considered class B if the output transistors go into cutoff at the zero point. So how could such an amp have no zero crossing artifact?
The answer is the driver transistors drive the speaker at very low levels. They don't go into cutoff like the outputs do. At higher levels the outputs are doing the work so the driver section doesn't get hot although heatsinks on driver transistors are common.
Any technician that has seen emitter resistors open up in the output section knows to expect shorted outputs, but you might be surprised to find that if the drivers are OK, an amp damaged in that way might play just fine at a low volume. The driver transistors are the bit you're not thinking about.
In a class B amp if you want it to be musical you'd simply be careful about how the driver section is designed. I've pointed this out several times but not this specifically. I hope this helps your understanding.
Follow Ups:
"The answer is the driver transistors drive the speaker at very low levels. They don't go into cutoff like the outputs do."
At that point (for the duration of that low level operation) the amp is not a Class B amp.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Clearly that isn't the case.
An amp's class of operation is defined by its output circuit only.
"By this metric, all solid state amps are class A."
I need you to explain that please.
Are you saying that all SS amplifiers drive the speakers with the driver transistors at low levels?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Not all but quite a few.
A lot depends on design. If you look at an amp with a symmetrical design, you'll see driver transistors direct coupled to the outputs.
Just like the output transistors, the drivers have emitter resistors that are connected to the loudspeaker output. At low levels the driver transistors are perfectly capable of driving the speaker. What level depends on how well the device is heatsinked as well as the series resistance of the emitter resistor.
I put myself through college working on consumer gear so I've seen many amps that still played despite having damaged output transistors. We're not talking a lot of power here, just milliwatts. But that's all that's needed to allow for a smooth transition, if you were to take advantage of this aspect in a class B design.
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