My first class AB amplifier

[audioguru]

An amplifier with a high open-loop voltage gain can use a lot of negative feedback to reduce the gain to a useable amount, reduce the distortion to be extremely low and increase the bandwidth. Haven't you heard about opamps? An OPA134 opamp has an open loop voltage gain of typically one billion. With its gain reduced to 10 with lots of negative feedback then its low frequency distortion is only 0.0008%.

Your compounded output transistors are called Sziklai pairs. They have no voltage gain but instead have a high current gain.
Their high current gain allows the driver transistor to have a low current so that it doesn't overheat and so that it can have a high value collector load resistance so it can have a higher voltage gain.

 

[Nigel Goodwin]

An amplifier with a high open-loop voltage gain can use a lot of negative feedback to reduce the gain to a useable amount, reduce the distortion to be extremely low and increase the bandwidth. Haven't you heard about opamps? An OPA134 opamp has an open loop voltage gain of typically one billion. With its gain reduced to 10 with lots of negative feedback then its low frequency distortion is only 0.0008%.

Your compounded output transistors are called Sziklai pairs. They have no voltage gain but instead have a high current gain.
Their high current gain allows the driver transistor to have a low current so that it doesn't overheat and so that it can have a high value collector load resistance so it can have a higher voltage gain.

Actually Sziklai (how do you pronounce that?) pairs can, and usually do, have voltage gain - the example he posted however didn't.

For a nice example of an amp using them have a look at:

Sziklai - Complementary Feedback Power Amp Output

 

[ben7]

I want to make it have an RMS output of 20watts into 8ohms.

So, If we want to have a 20watt RMS output, the output will have to be 40watts peak.

40W x 8ohms = 320

√320 = 17.88854381999832, or about 18V required at the load

18VDC/8Ω = 2.25A peak

The + and - supply rails would need to be able to each handle half that value of Amps, or 1.125A minimum

So how would I figure out what is the + and - supply voltage needed?


According to https://www.electro-tech-online.com/threads/audio-amplifier-diagrams.113553/#post929153

 

[ben7]

Here is my updated circuit with a Sziklai pair output stage.

From the useful link that Nigel gave me it seems that a Sziklai pair is good for Hi-Fi amplifiers.

Should R11 and R12 be 220Ω like shown in the diagram?

 

[audioguru]

I want to make it have an RMS output of 20watts into 8ohms.]

The output waveform must be almost 18V peak but the drivers for the output transistors, the output transistors and their emitter resistors have losses so maybe the power supply should be plus and minus 22V.

Now that you have compound output transistors with high current gain, you can reduce the current in the driver transistor Q3 by increasing the value of R8 and R9 to 1k ohms or 3.3k ohms. Then the driver transistor can be a little one and can have a high open-loop voltage gain. Also, the value of C5 can be reduced.

Maybe you are wasting time not looking at the many 20W into 8 ohms IC amplifiers that are available. The LM1875 and the TDA2050 produce 20W into 8 ohms with a plus and minus 20V to 22V supply (at low distortion) or produce severely distorted square waves at 40W.

 

[ben7]

Maybe you are wasting time not looking at the many 20W into 8 ohms IC amplifiers that are available. The LM1875 and the TDA2050 produce 20W into 8 ohms with a plus and minus 20V to 22V supply (at low distortion) or produce severely distorted square waves at 40W.

Well, I have built a TDA2005 amplifier before, but it is no fun using them unless you know how complex they are inside!

Too bad the power transformers cost $50.

 

[Nigel Goodwin]

Well, I have built a TDA2005 amplifier before, but it is no fun using them unless you know how complex they are inside!

Too bad the power transformers cost $50.

The transformers for a similar spec discrete amp would cost just the same, as they are the exact same transformers.

 

[ben7]

The transformers for a similar spec discrete amp would cost just the same, as they are the exact same transformers.

Yes, I know that, I will have to check my power transformers to see their output voltages.

 

[ben7]

Do you think I could use a + - 24 volt supply instead?

 

[audioguru]

Do you think I could use a + - 24 volt supply instead?

A higher voltage creates an amplifier with more output power and more heat.
The transistors must be able to survive the high voltage and be cooled well so they do not overheat. They must be able to provide the higher current.
The power supply must be able to supply the power.

The total supply is 48V but the output transistors have losses so the output signal is probably 42V p-p which is 14.85V RMS. The power into an 8 ohm speaker is about 27W at clipping. The power into a 4 ohm speaker (it current is nearly doubled) is about 45W at clipping.