Electronics > Beginners
Adding filters to class AB audio amp
Audioguru again:
--- 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.
--- End quote ---
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.
Audioguru again:
This thread title says "adding filters".
Why? To cut low frequencies to make an adult male voice sound like a chipmunk and to muffle high frequencies in speech and music like an old telephone?
GerryR:
--- 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.
d4n13l:
--- Quote from: Audioguru again on August 18, 2019, 01:02:52 pm ---This thread title says "adding filters".
Why? To cut low frequencies to make an adult male voice sound like a chipmunk and to muffle high frequencies in speech and music like an old telephone?
--- End quote ---
Hi audioguru,
Well, since the intended purpose of the amp is to work with audio I'd say the obvious answer is to restrict the amp output to the appropiate 20hz to 20kHz bandwith. But I'm actually doing this to learn how they work so is more of and excersise, and since I happend to have a bunch of sort of crappy small speakers I'm thinking it would be a good idea to implement some kind of bass cut.
d4n13l:
Going with GerryR's note on speakers having 8 ohms at 1kHz I came out with a value of 10 ohms for R5 and ~10uF for C5 for the Zobel network.
As for the filters, I take it C6 and the speaker are making a high pass filter so a better value for C6 would be 1000uF as to allow freq over 20hz?
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?
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version