# Audio Amplifier Design HELP

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#### Remix23

Joined Mar 11, 2016
23
Hey guys,

I was designing an audio amplifier in my own time to enhance my electronics skills and I am having issues with the transistor part of the circuit. The specs are as follows:

Designing it with a +VCC with 0V supply
Frequency Range 20Hz - 20Khz
In-Band Sensitivity of 2V/mV maximum input of Plus-minus 15mV
In-Band input impedance will be 50k with the output load of a minimum of 2K

The power output is 20W RMS to an 8 Ohm speaker with less than 1% THD

I have calculated the gain for each op-amp, I am using three LM741 op-amps, I know the gain bandwidth is 1Mhz thus all the op-amps will be 2000. I have calculated the resistors and capacitors.

I'll attach my circuit but as from my research, I need the transistor set up the same as the second picture with additional supply. It will be an AB amplifier btw.

I am struggling to figure out how I am supposed to calculate the values for the transistor section.

I would appreciate some assistance, please.

Thanks

https://www.electronics-tutorials.ws/amplifier/class-ab-amplifier.html

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#### Zeeus

Joined Apr 17, 2019
616
Hey guys,

I was designing an audio amplifier in my own time to enhance my electronics skills and I am having issues with the transistor part of the circuit. The specs are as follows:

Designing it with a +VCC with 0V supply
Frequency Range 20Hz - 20Khz
In-Band Sensitivity of 2V/mV maximum input of Plus-minus 15mV
In-Band input impedance will be 50k with the output load of a minimum of 2K

The power output is 20W RMS to an 8 Ohm speaker with less than 1% THD

I have calculated the gain for each op-amp, I am using three LM741 op-amps, I know the gain bandwidth is 1Mhz thus all the op-amps will be 2000. I have calculated the resistors and capacitors.

I'll attach my circuit but as from my research, I need the transistor set up the same as the second picture with additional supply. It will be an AB amplifier btw.

I am struggling to figure out how I am supposed to calculate the values for the transistor section.

I would appreciate some assistance, please.

Thanks

View attachment 194095View attachment 194095

https://www.electronics-tutorials.ws/amplifier/class-ab-amplifier.html
Hi : don't really know much

How about including the transistors in the feedback loop so you can set what the dc voltage of the emitters of the transistors will be...right now, don't think it's possible to know beforehand

#### Remix23

Joined Mar 11, 2016
23
Don't think
Hi : don't really know much

How about including the transistors in the feedback loop so you can set what the dc voltage of the emitters of the transistors will be...right now, don't think it's possible to know beforehand
Don't think that's possible, i just need to know how i am supposed to calculate the values of the resistors and capacitors in the transistor stage.

#### MisterBill2

Joined Jan 23, 2018
19,398
You can find a whole lot of amplifier circuits at the "Schematicsforfree" site, including a whole lot of commercial equipment. Looking at circuits that work is often very educational, and in this case, free.
Also, the 741 opamp is a very old design, much newer designs that are both lower noise and lower distortion are available. and also cheaper. And they have models available if you then need to use a modeling program.

#### Remix23

Joined Mar 11, 2016
23
You can find a whole lot of amplifier circuits at the "Schematicsforfree" site, including a whole lot of commercial equipment. Looking at circuits that work is often very educational, and in this case, free.
Also, the 741 opamp is a very old design, much newer designs that are both lower noise and lower distortion are available. and also cheaper. And they have models available if you then need to use a modeling program.
I've done a lot of work using the LM741 with this design so I'd rather just use this for now and then use the experience and use a better more efficient op-amp as you mentioned. Can you see the second picture I uploaded? That's what i am trying to incoporate into the final bit.

#### ronsimpson

Joined Oct 7, 2019
3,204
How about including the transistors in the feedback loop

how i am supposed to calculate the values of the resistors and capacitors in the transistor stage.
For C2 Speaker (RC) F=1/(2x3.14xRxC) Where R=8 ohm load and C=2200uF. At that frequency and below the output of the amp shifts from going to the speaker to being across C2. This is one place where you have a low frequency roll off problem.

I looked at the graphs for the TIP42. In one place it looks like the transistor has a current gain of about 50 at 2A. In another place it looks like it needs 40 to 70mA in the base for 2A in the collector. Your R8 & R12 need to supply 50mA and it won't. (when the output voltage is near the supply)

#### ronsimpson

Joined Oct 7, 2019
3,204
Here is an example. T2, T4 together has very high current gain. The op-amp only uses a mA to control the output stage.
Note this (like most internet schematics) has an error. R17 is not 5.6 but 5.6k.

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#### Audioguru again

Joined Oct 21, 2019
6,782
I agree that the 52 years old 741 opamp is awful for audio since it is noisy (hiss), has an output swing that is too small and has trouble at high levels producing frequencies above 11kHz.
I also agree that R8 and R12 must have a much lower value that an opamp cannot drive.

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#### Audioguru again

Joined Oct 21, 2019
6,782
I spotted this problem above:

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#### Audioguru again

Joined Oct 21, 2019
6,782
The 2200uF output capacitor reduces the output level a little (-3dB) at 9.1Hz. We cannot hear a vibration below 20Hz.

#### Remix23

Joined Mar 11, 2016
23
The 2200uF output capacitor reduces the output level a little (-3dB) at 9.1Hz. We cannot hear a vibration below 20Hz.

That's the new circuit, I know this op amp is crap but it is a challenge to do.

I am having difficulty figuring out the formulas to calculate C6 and all the resistors and capacitors in the transistor section.

I know i need another supply at C6.

#### MisterBill2

Joined Jan 23, 2018
19,398
You need to include AC negative feedback if you intend to use the output section as shown, and even with a better circuit you need the feedback to reduce distortion. The reason is that there are several areas of rather non-linear operation as Q3 turns off and Q4 starts conducting, as well as Q1 not being completely linear. Negative feedback helps reduce the distortion some. As drawn the amplifier will not sound very good at all.

#### Audioguru again

Joined Oct 21, 2019
6,782
Why are you just guessing?
Look here:

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#### Remix23

Joined Mar 11, 2016
23
You need to include AC negative feedback if you intend to use the output section as shown, and even with a better circuit you need the feedback to reduce distortion. The reason is that there are several areas of rather non-linear operation as Q3 turns off and Q4 starts conducting, as well as Q1 not being completely linear. Negative feedback helps reduce the distortion some. As drawn the amplifier will not sound very good at all.
Would you be able to show the drawing of that please. Or if possible add to the circuit.

My concerns are getting the correct component values for the output section. The components within the three stages are correct. Just the transistor stage and not sure how to calculate it.

Is my design so far okay apart from the ac negative feedback?

#### Remix23

Joined Mar 11, 2016
23

#### Audioguru again

Joined Oct 21, 2019
6,782
The design of the output stage will not work.

#### MisterBill2

Joined Jan 23, 2018
19,398
Would you be able to show the drawing of that please. Or if possible add to the circuit.

My concerns are getting the correct component values for the output section. The components within the three stages are correct. Just the transistor stage and not sure how to calculate it.

Is my design so far okay apart from the ac negative feedback?
What is missing is anything to stabilize the bias points of the three power transistors. Not sure why I didn't see that sooner, but that is going to be a problem. So it may sort of work for a short time, but not very well. Each stage needs to be stable for the DC static condition, even before it is driven to produce an output. If the gains of the two output devices are not identical the output will not work right.

#### DickCappels

Joined Aug 21, 2008
10,246

Please let me know if this will be a problem, and if not I will lock this thread in about 12 to 15 hours from now.

#### MisterBill2

Joined Jan 23, 2018
19,398