Power Amplifier Project

[Epicdemise]

I am currently reworking the circuit to work with 2 darlinton pairs and will post it soon. I tried them yesterday but could not bias them correctly, so we will see ifi can make it work today. Thanks for all the help so far.

 

[Epicdemise]

I am not really sure how to bias the resistors with the darlington, so R3-R6 are kind of just place holders.

 

[ronsimpson]

Does it work? When you get this working we can take the next step.
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It should work but it is too slow.

 

[Epicdemise]

No it doesn't work. The current isn't working and because of that the voltage is off. I've looked all over for examples but I can't really find any that really help me understand how to bias this configuration.

 

[ronsimpson]

The circuit you posted looks good. Can you measure some voltages and we can find what is wrong?

 

[Epicdemise]

Node 4 is being covered up by the speaker output. I'm sorry if I did not get the voltages needed.

 

[audioguru]

Tt doesn't work because you shorted the output oid the opamp to the speaker.
R3 should have the same value as R4.
The output transistors need base-emitter resistors to quickly turn then off when required.

 

[Epicdemise]

lol, I can't believe I shorted it and didnt even realize it. Why did you add the other two resistors? I've seen it in some schematics but not all.

 

[ronsimpson]

A resistor from B to E helps turn off the transistor.
If you take away the base current the transistor will turn off but slowly. By using a resistor the base if pulled down (0 volts) faster and the transistor will turn off sooner.
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Now you have the amp working. It is too slow because of the op-amp. See the graph below. (this is from the ua741) It shows that at 1mhz the max gain is 0db or 1. At 100khz the max gain is 20db or 10. At 10khz the max gain is 40db or 100.

You need a gain of slightly above 100 at 20khz. So the amp is too slow.

Options: use a faster amp OR make the output transistors have voltage gain so the op-amp needs less gain.

 

[Epicdemise]

Alright after doing that it works great at the 100mV into 10k ohm and now it is clipping for the 500mV into 1k ohm.

 

[Epicdemise]

Ok, I'll check that out. Thanks for the help guys.

 

[Epicdemise]

yeah it seems like I will have to add some voltage gain to the output transistors for the 500mV

 

[ronsimpson]

Alright after doing that it works great at the 100mV into 10k ohm and now it is clipping for the 500mV into 1k ohm.

What? 100mV for 10k is fine but if you drive at 500mV you need to change the resistor to 50k not 1k.

 

[Epicdemise]

So I thought about the volume control in the completely wrong way. From the original post you will see I have to be able to have an avg of >10watts from 100mV to 500mV, but I can choose which student wants to take the volume control. But it will still effect my amp because it is technically my R1 of my op amp driver.

 

[ronsimpson]

You need a volume control. Input is one end. Ground is the other end. The wiper goes to the 10k resistor.
With the wipe all the way up your 100mV will go directly to the 10k resistor. You will get full power out.
With the wiper at 20% the 500mV in will be dropped to 100mV and goes to the 10k resistor . You will get full power out.

 

[ronsimpson]

 

[Epicdemise]

Okay, cool. So I've never worked with a volume control on a simulating so I am going to want the pot to also be 10k correct?

 

[audioguru]

I do not know if your simulator produces 500mV peak or 500mV RMS.
Since the gain of the amplifier was WAY TOO HIGH then its output was clipping like crazy.

With a gain of 130 and an input of 100mV peak then the output should be 13V peak which is 9.2V RMS which is 10.6W into 8 ohms.

The TL071 is a pretty good audio opamp. Its open-loop gain at 20kHz is about 200 so there is still enough gain after 130 times to reduce high frequency distortion.

 

[ronsimpson]

10k is fine. You should probably have a capacitor between the pot and the 10k resistor to block DC. How do you choose the capacitor value? This cap will set the low frequency cut off point (20hz).

 

[Epicdemise]

I figured out yes it does work that way. Umm I am curious if there is a way to get some extra voltage out of my circuit or if I need to add something to get the full power from the 500mV without clipping (As seen in the waveforms)