Electronics > Beginners

Adding filters to class AB audio amp

<< < (6/8) > >>

d4n13l:

--- Quote from: Audioguru again on August 19, 2019, 02:35:00 am ---Throw away that horrible class-B amplifier with its awful crossover distortion. Use an opamp made for audio to drive the output transistors.
If C5 has the extremely high value of 10uF then the 10 ohms resistor in series with it will overload the amplifier above only 1600Hz. An LM386 IC amplifier uses a 0.05uF capacitor in series with 10 ohms for its Zobel network. Then it begins loading the amplifier output at very high ultrasonic frequencies where the speaker is a very high impedance and the amplifier will try to oscillate without the load.

EDIT: I forgot about the capacitor to ground you added at pin 2. It does nothing at pin 2 because this inverting input has the input signal cancelled by the negative feedback. Make a lowpass filter parallel with the feedback resistor.

--- End quote ---

I'll take the advice thanks, but I'm following this schematic for educational purposes, I started with class A amps now I'm working with this to get some reference points, for example, I wanted to know how bad would the audio actually sound, and I used this op amp because is what I had available at the moment (and thought its high slew rate would be good for this configuration) I actually need to do some research into audio amps I have not idea how they are improved for audio, also I didn't know how loud would be sound actually be with the speakers I got, stuff like that.

and of course you learn stuff along the way

fourfathom:

--- Quote from: d4n13l on August 17, 2019, 05:54:12 pm ---

--- End quote ---

You can improve the crossover distortion by adding a resistor from the transistor bases to the transistor emitters.  The idea is to let the opamp drive the output directly (through the resistor) while the circuit transitions through the crossover region.  Instead of a the opamp needing to compensate for a 1V crossover deadband, it now only has to correct for a gain-slope change (which is easier).  The resistor value depends on the opamp, and the load, but you might try 50 Ohms or so.  This doesn't get you "golden ear" distortion levels, but can make a significant difference.

Zero999:

--- Quote from: GerryR on August 18, 2019, 01:55:26 pm ---
--- Quote from: Zero999 on August 17, 2019, 08:56:39 pm ---I've done an LTSpice simulation. Note that I didn't include the Zobel network or decoupling because it's not necessary for a simulator, which uses ideal components. Of course they should be included in real life.

Look at the horrible crossover distortion....

--- End quote ---

The OP's schematic shows a 100 nf (.1 uf) cap, not a 10 uf like in your model.  It is a ~160 Hz hi-pass into the op-amp.  If the OP wants a lower response, then changing to 1 uf will drop it to ~16 Hz.  I believe the 10 uf will cause other problems at the low end.

--- End quote ---
Yes, I increased the capacitor size for full range response. I think the title of this thread centres around the OP's misunderstanding of how the circuit works. I don't believe the OP actually wants any filtering. If so, he should have given the desired frequency response. Look at the size of the capacitor on the output stage. Even though it's good to oversize decoupling capacitors, it beyond the joke to have a cut-off of 9Hz after an input stage with a cut-off of 160Hz.

What problems do you think having a 10µF capacitor will introduce? It's generally good practise to make DC coupling capacitors much larger than the bare minimum because it reduces phase shift and attenuation at the lowest frequency of interest. The only downsides are a bigger audible click at turn on it taking longer for the steady state condition to be reached. 10µF and 10k is an RC time constant of 100ms, so it will be fairly close to steady state after being turned on of 0.5s.


--- Quote from: fourfathom on August 19, 2019, 06:10:27 am ---You can improve the crossover distortion by adding a resistor from the transistor bases to the transistor emitters.  The idea is to let the opamp drive the output directly (through the resistor) while the circuit transitions through the crossover region.  Instead of a the opamp needing to compensate for a 1V crossover deadband, it now only has to correct for a gain-slope change (which is easier).  The resistor value depends on the opamp, and the load, but you might try 50 Ohms or so.  This doesn't get you "golden ear" distortion levels, but can make a significant difference.

--- End quote ---
I'm not sure what you mean. Are you talking about biasing it more towards class AB? That will reduce crossover distortion, but using resistors along makes it sensitive to supply voltage and temperature variations. A better way is to use diodes or another small transistor with a VBE multiplier. If both transistors will be biased so they simultaneously conduct (true class AB operation) then emitter resistors are required to prevent thermal runaway.

John B:

--- Quote from: Zero999 on August 19, 2019, 08:18:10 am ---
--- Quote from: fourfathom on August 19, 2019, 06:10:27 am ---You can improve the crossover distortion by adding a resistor from the transistor bases to the transistor emitters.  The idea is to let the opamp drive the output directly (through the resistor) while the circuit transitions through the crossover region.  Instead of a the opamp needing to compensate for a 1V crossover deadband, it now only has to correct for a gain-slope change (which is easier).  The resistor value depends on the opamp, and the load, but you might try 50 Ohms or so.  This doesn't get you "golden ear" distortion levels, but can make a significant difference.

--- End quote ---
I'm not sure what you mean. Are you talking about biasing it more towards class AB? That will reduce crossover distortion, but using resistors along makes it sensitive to supply voltage and temperature variations. A better way is to use diodes or another small transistor with a VBE multiplier. If both transistors will be biased so they simultaneously conduct (true class AB operation) then emitter resistors are required to prevent thermal runaway.

--- End quote ---

It's a resistor from the output of the op amp to the emitters of Q1 and Q2.

Zero999:

--- Quote from: John B on August 19, 2019, 08:53:18 am ---
--- Quote from: Zero999 on August 19, 2019, 08:18:10 am ---
--- Quote from: fourfathom on August 19, 2019, 06:10:27 am ---You can improve the crossover distortion by adding a resistor from the transistor bases to the transistor emitters.  The idea is to let the opamp drive the output directly (through the resistor) while the circuit transitions through the crossover region.  Instead of a the opamp needing to compensate for a 1V crossover deadband, it now only has to correct for a gain-slope change (which is easier).  The resistor value depends on the opamp, and the load, but you might try 50 Ohms or so.  This doesn't get you "golden ear" distortion levels, but can make a significant difference.

--- End quote ---
I'm not sure what you mean. Are you talking about biasing it more towards class AB? That will reduce crossover distortion, but using resistors along makes it sensitive to supply voltage and temperature variations. A better way is to use diodes or another small transistor with a VBE multiplier. If both transistors will be biased so they simultaneously conduct (true class AB operation) then emitter resistors are required to prevent thermal runaway.

--- End quote ---

It's a resistor from the output of the op amp to the emitters of Q1 and Q2.

--- End quote ---
Sorry, I should have reread your reply more carefully. It was obvious that's what you meant.

Yes, it will soften the crossover distortion, but it'll still be there. I doubt it'll do much in this application because the op-amp would have to supply a significant output current. It's more effective in applications where the output current is under an order of magnitude greater than what the op-amp can supply.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod