Electronics > Beginners
Adding filters to class AB audio amp
GerryR:
--- Quote from: d4n13l on August 18, 2019, 10:27:18 pm ---.... But I'm having trouble as to where to put the low pass filter, maybe at pin 2 of the op amp as seen in the diagram? which would be 795nF (closest I have is 1uF) for C7 to cut frequencies below 20hz, or 63nF if I wanted to cut the bass at 250hz?
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The 100 nf and 10 K is a high pass with a break frequency of ~160 Hz; nothing below that (at 6 db per octave) is going to "get in." Change the 100 nf or the 10 K to change what you "let in" to the amp at the low end.
John B:
--- Quote from: Audioguru again on August 18, 2019, 12:57:56 pm ---
--- Quote from: John B on August 18, 2019, 05:24:52 am ---Not only will the output transistors have crossover distortion, but with only a 12V supply to the op amp, the output of the op amp itself is likely to have crossover distortion above a certain voltage output swing. It will likely need loading down to the negative rail, ground in this case. Something like 2k-10k ohms depending on how much output swing you want.
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The only opamps I know that have crossover distortion at ANY supply voltage (within spec's) are the LM324 quad and LM358 dual (they use the same opamps) because they are designed for very low idle current so they are missing the biasing of their output transistors that eliminates crossover distortion. A load on the output causes one of the output transistors to operate in class-A eliminating the crossover distortion.
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Mea culpa. I probably got bitten by the fake bug again. As not to derail this thread, I documented it here: https://www.eevblog.com/forum/beginners/tl07x-tl08x-ebay-fakery/
d4n13l:
--- Quote from: GerryR on August 18, 2019, 11:17:06 pm ---The 100 nf and 10 K is a high pass with a break frequency of ~160 Hz; nothing below that (at 6 db per octave) is going to "get in." Change the 100 nf or the 10 K to change what you "let in" to the amp at the low end.
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Ok thanks, but that takes me to my original post, I'm having trouble analyzing the filter in series like that, all literature that I've seen would take the output of the filter from between C1 and R2.. so are VR1 and C1 making a variable low pass filter as well?
Audioguru again:
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.
fourfathom:
--- Quote from: Audioguru again on August 18, 2019, 12:57:56 pm ---The only opamps I know that have crossover distortion at ANY supply voltage (within spec's) are the LM324 quad and LM358 dual (they use the same opamps) because they are designed for very low idle current so they are missing the biasing of their output transistors that eliminates crossover distortion. A load on the output causes one of the output transistors to operate in class-A eliminating the crossover distortion.
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Doesn't adding this resistor at the output merely move the crossover distorting to a different place? Of course with small signals the modified crossover point won't be reached, so the distortion as a % of the signal will be improved.
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