# Class AB audio amplifier Help?

Discussion in 'Homework Help' started by faz17, Feb 19, 2010.

1. ### faz17 Thread Starter New Member

Feb 19, 2010
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HI,
Ive been reading over some previous post and am struggling to design a 10W class AB audio amplifier.

any help would be appreciated.

2. ### hobbyist Distinguished Member

Aug 10, 2008
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A class "A" amp is biased so that it conducts all the time, it has No distortion. But very INefficient, uses a lot of power.

A class "B" amp is very efficient in that it has no bias at all, and conducts when the input signal is large enough to cause the transistor to conduct. It conducts only half the input waveform.

Because of the inherent Vbe of a transistor, there is distortion called crossover distortion, that occurs.

So a class "AB" amp, is designed to be biased as a combination, of the efficiency of a class "B", and the less distortion of a class "A", so that it conducts the full input signal, with less to no distortion, but during idle periods, it uses a lot less current, so power is conserved.

3. ### Audioguru New Member

Dec 20, 2007
9,411
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First, what will be the speaker impedance?
Do you know how to calculate the supply voltage?
Have you selected suitable output transistors? Which ones?

A class-AB 10W amplifier is extremely simple. What design problems do you have?

4. ### faz17 Thread Starter New Member

Feb 19, 2010
14
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I'm using an 8 ohm speaker but haven't chosen suitable transistors as of yet , being a 10W I'm assuming the Supply voltage is V^2/r

I'm not sure about how to go about designing it, its been a few years since i last designed an amplifier.

5. ### Audioguru New Member

Dec 20, 2007
9,411
896
Why not use an amplifier IC? The schematic for everything it needs is in its datasheet. Its distortion will be 0.1% or less.
If you design your own amplifier without knowing how then its distortion might be 20% or more.

10W into 8 ohms is a peak to peak voltage of 25.3V. The output transistors are emitter-followers so they do not saturate but have a loss of maybe a total of 4V. So the power supply must be at least 29.3VDC.

6. ### faz17 Thread Starter New Member

Feb 19, 2010
14
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Yeah an IC sounds the best approach. Thanks

7. ### faz17 Thread Starter New Member

Feb 19, 2010
14
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Hi, ive been looking for Audio Amplifier ICs and am struggling to find 10W class ABs for 8 ohm Impedance

8. ### Audioguru New Member

Dec 20, 2007
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An LM1875 amplifier IC produces 10W at clipping into 8 ohms when its supply is plus and minus 16V.
A schematic and pcb design are in its datasheet.

A TDA2030A amplifier IC is almost the same.
A schematic and pcb design are also in its datasheet.

They operate in class-AB for no crossover distortion.

Feb 24, 2006
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10. ### Audioguru New Member

Dec 20, 2007
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The TDA2009A produces only 5W to 7W per channel into 8 ohms, not 10W.

Feb 19, 2010
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12. ### Audioguru New Member

Dec 20, 2007
9,411
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The TDA9297SA does not produce 10 Watts per channel. It produces 10 Whats per channel into 8 ohms at a horrible-sounding 10% distortion.
One graph shows it beginning to clip when the output is only 3 Watts per channel and a 12V supply.

It seems to have a problem dissipating heat since it has a Mickey Mouse plastic case and uses a "clip" to hold it on a heatsink.
It does not have a differential input. It is a stereo bridged amplifier that drives both wires of each speaker for almost 4 times the power as a single stereo amplifier.

A TDA7240A has a metal tab that is bolted to a heatsink and produces 12 real low distortion Watts into 8 ohms when its supply is 14.4V. It is a single mono amplifier.

13. ### faz17 Thread Starter New Member

Feb 19, 2010
14
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Thanks, so if i was to use the TDA7240A chip, and I have a differential input, how could I set this up, would I require two chips per differential input sets.

14. ### Audioguru New Member

Dec 20, 2007
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A stereo amplifier has two channels with two amplifiers and two speakers. A differential input is completely different.

A TDA7240A is a single mono amplifier. For stereo you need two of them.

15. ### faz17 Thread Starter New Member

Feb 19, 2010
14
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I understand that, but im getting a differential input for my amplifier stage, dose that mean i have to take a different approach

16. ### Audioguru New Member

Dec 20, 2007
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Very few amplifiers have differential inputs.
Why do you need an amplifier with differential inputs?

17. ### faz17 Thread Starter New Member

Feb 19, 2010
14
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Im trying to amplify a signal for a bluetooth chip, which gives a differential output.

What effect to the signal quality would occur if i were to try one output to ground (-ve) and the +ve to the input of the TDA7240 chip.

18. ### Audioguru New Member

Dec 20, 2007
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I don't know which bluetooth chip you have. I don't know what it is supposed to drive (an earphone?) so I don't know why its output would be differential. Do not short one output wire to ground.

It will have a normal single-ended output if you use a coupling capacitor from one output wire and ground. The input of the TDA7240A IC must not be connected to 0V with a resistor and must not have a DC voltage applied.

19. ### faz17 Thread Starter New Member

Feb 19, 2010
14
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Im working with a Wt32 bluegiga bluetooth module and am to drive an 8 ohm active Loudspeaker.

Just to confirm you are suggesting connecting one of the Differential Audio outs from the bluetooth chip to ground via a coupling capacitor. The other wire can be connected to the input of the TDA7240A like normal.

20. ### Audioguru New Member

Dec 20, 2007
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I don't know what is "an active Loudspeaker".
The TDA7240A has an output of only 9W into 8 ohms with a 14.4V supply. ST-Micro made a mistake by saying their TDA7297SA produces 10 Whats into 8 ohms when they should have said into 4 ohms. Then I copied the error.

NO!
Never short an output to ground with a piece of wire or with a capacitor.
One wire has a capacitor-coupled signal to the input of the amplifier IC and both have grounds that are connected together.