# Class B and Class AB amplifier dissipation when no signal?

1. ## Class B and Class AB amplifier dissipation when no signal?

I am going to do an audio amp.

Class A dissipates significantly when no music is playing.

What is the dissipation of Class B and Class AB when there is no signal?

http://en.wikipedia.org/wiki/Amplifier
wiki and others didnt tell of this.

2. ## Re: Class B and Class AB amplifier dissipation when no signal?

A class B output stage has no current. A class AB output stage has a small bias current. Of course, the rest of the driver circuits for either output stage will have some no-signal bias current. The actual value for these currents depends upon the circuit design and the output power of the amp.

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3. ## Re: Class B and Class AB amplifier dissipation when no signal?

The class AB amplifier is an improvement on the class B. Class B is plagued by crossover distortion (or null distortion).

Class AB allows you to overcome the problem of crossover distortion, by turning up the bias on the transistors so that each is just barely turned on with no signal.

When you want a lot of power, however, it is all too easy to turn up the bias to such a degree that it wastes a lot of current, and overheats the transistors. A more elaborate biasing arrangement is needed.

I have a Youtube video which shows the concepts of class B operation. It visually shows current moving through the wires at different stages of operation.

It shows an arrangement where the NPN is uppermost, and one where the PNP is uppermost.

http://www.youtube.com/watch?v=nLMC8j_8ys4

4. ## Re: Class B and Class AB amplifier dissipation when no signal?

thanks,
as we know, Class AB operates the transistors in linear mode, so they heat up much......how much bigger and heavier is a class AB's heatsink compared to a heatsink for a class D amp?

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5. ## Re: Class B and Class AB amplifier dissipation when no signal?

I'm not aware of true class B PAs used for audio applications, in so far post #1 is asking a theoretical question, I think. Quiescent current and power dissiption of class AB is a matter of trade-off. In most cases, the idle power dissipation is small compared to maximum power dissipation and in so far doesn't add much to the heatsink size.

Class D is a different topic.

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6. ## Re: Class B and Class AB amplifier dissipation when no signal?

Class D heatsink weighs only half of Class B heatsink?
(equivalent sound power)

7. ## Re: Class B and Class AB amplifier dissipation when no signal?

A linear class-AB amplifier gets hot when it is driving power into a speaker because it is a variable power resistor between the power supply and the speaker.
The output Mosfets in a class-D amplifier switch completely on (no voltage aross them so they do not get hot) and switch completely off (no current in them so they do not get hot). The output Mosfets in a class-D amplifier get warm because they switch at a high frequency.

Most class-AB amplifiers heat with about 50% of the max output power. 100W out makes 50W of heat.
Good class-D amplifiers heat with about 10% of the max output power. 100W out makes 10W of heat.

8. ## Re: Class B and Class AB amplifier dissipation when no signal?

The Blackstart ID 100TVP guitar amplifier weighs 22 pounds (9Kgs).
Since its only 100W, can you definetely say that its a Class AB amplifier, and not a Class D amplifier?

Blackstar video where they discuss weights (near end)
http://www.youtube.com/watch?v=o6LjuyzUTXk

Blackstar ID:100TVP amplifier
http://www.richtonemusic.co.uk/produ...FaLHtAodWBYAiQ

9. ## Re: Class B and Class AB amplifier dissipation when no signal?

Originally Posted by treez
The Blackstart ID 100TVP guitar amplifier weighs 22 pounds (9Kgs).
Since its only 100W, can you definetely say that its a Class AB amplifier, and not a Class D amplifier?
It uses old fashioned vacuum tubes (valves) so the Huge and Heavy power transformer must heat them.

Class-D amplifiers are solid state and are fairly new. Their power transformer is much smaller because it does not need to heat the filaments in vacuum tubes.
I think some class-D amplifiers use switched-mode power supplies that use a small lightweight power transformer with a ferrite core.

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10. ## Re: Class B and Class AB amplifier dissipation when no signal?

Originally Posted by treez
The Blackstart ID 100TVP guitar amplifier weighs 22 pounds (9Kgs).
Since its only 100W, can you definetely say that its a Class AB amplifier, and not a Class D amplifier?
It uses old fashioned vacuum tubes (valves) so the Huge and Heavy power transformer must heat them.

Class-D amplifiers are solid state and are fairly new. Their power transformer is much smaller because it does not need to heat the filaments in vacuum tubes.
I think some class-D amplifiers use switched-mode power supplies that use a small lightweight power transformer with a ferrite core.

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11. ## Re: Class B and Class AB amplifier dissipation when no signal?

It uses old fashioned vacuum tubes
Unlikely. A 100 W tube amplifier would be more than 9 kg. The advertising say "loud as valve" Like many recent guitar amps, it apparently does a valve sound emulation.

Maybe it's class AB and transformer power supply, may be something different. What's the guessing good for? The requirements of a guitar amp are quite different from a (HiFi) audio amp anyway.

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12. ## Re: Class B and Class AB amplifier dissipation when no signal?

What's the guessing good for?
..its fascinating that people are still using mains transformers. There must be a reason, i assume the 9kgs is due to the 50Hz trafo.?

Maybe 50Hz trafos are used because with them, you dont have to conform to standby regulations?

13. ## Re: Class B and Class AB amplifier dissipation when no signal?

The GEEtar amplifier says True Valve Power and it can simulate the distortion of 6 valves. So I guess it sounds like a valve amplifier.

I built my first amplifier in 1961 from a Heathkit. It had valves. I took it to a MacIntosh amp clinic where it was tested and it had 25% distortion. They wanted to sell me a MacIntosh very expensive amp but I replaced the inexpensive valves instead. Then its distortion measured 0.5% at mid frequencies. I replaced the valves every 3 months to keep the distortion low.

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