Electronics > Beginners
Adding filters to class AB audio amp
mikerj:
--- Quote from: soldar on August 17, 2019, 08:59:53 pm ---VR2 provides negative feedback. You can put any filtering network in its place to adjust frequency response.
--- End quote ---
Adding additional,variable phase shifts into the feedback is likely to turn this into a oscillator, and even if it could be made stable it will undoubtedly make the crossover distortion even worse. Filters should go before the amp.
Circlotron:
R4 and C7 are there to keep a load on the amplifier output at high frequencies. The speaker impedance goes up with frequency meaning less and less load and the chance of high frequency oscillations and instability. The impedance of C7 comes down with frequency, and if R4 and C7 are sized right the amplifier will see a constant load regardless of frequency. I have seen typical values of 100nF and 10 ohms.
John B:
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.
d4n13l:
ok I'll work on the other stuff next, now I get the Zobel network (made by R4 and C7) and why is there but in order to calculate the cap and resistor values you need the inductance value of the speaker, at least according to https://diyaudioprojects.com/Technical/Speaker-Zobel/, however I can't find an easy way of calculating it, does anyone now a practical way for doing it or people just take approximate values?
GerryR:
Speakers, I believe are rated for impedance at 1 KHz (8 \$\Omega\$ @ 1 KHz), so you can measure the resistance with an ohm meter and calculate the inductance at 1 KHz.
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