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Audio amplifier output capacitor value

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ricko_uk:
Hi,
I need to drive a speaker with impedance down to 4R and am using the OPA547 (datasheet attached here).

Not familiar with audio applications but want to keep the signal as clean, noise-free and as close to the input signal as possible. I was planning to use it in a simple inverting or non-inverting configuration with only two resistors setting the gain in the classic fashion.

Two questions:
1) How do I choose the output capacitor so that I can drive speakers as low as 4R?
2) What is the logic and the calculation for choosing such value?
3) I saw some designs have capacitors in the feedback loop. Do I need any? Why?

Many thanks :)

tooki:
Caps in the feedback loop make it an active filter.

I’ve been tinkering with audio amp circuits for a few years and have come to the conclusion that, unless you expressly seek the thrill of DIYing it, it’s cheaper and better to just use a good speaker amp IC. Depending on your needs, linear or class-D will make more sense.

Benta:
No magic about the output cap. It determines the bass cut off frequency and is easy to calculate.
It's just a CR high pass filter. With a 4 ohm-loudspeaker and a 2200 uF cap, the cut off is at ~18 Hz.

Caps in the feedback loop are normally for stability, but need to be chosen carefully (meaning proper circuit analysis, not gut feeling).

ricko_uk:
Thank you Benta :)


--- Quote from: Benta on June 07, 2020, 06:21:33 pm ---Caps in the feedback loop are normally for stability, but need to be chosen carefully (meaning proper circuit analysis, not gut feeling).

--- End quote ---

1) How do I find the correct value for the OPA547 driving a 4R speaker? What is the easiest and safest way to determine it?
2) if I change the speaker to a different impedance one, do any feedback loop capacitor need to be changed to?

Thank you :)

Benta:
You don't need a cap in the feedback loop. Dunno where you got that from. The OPA547's frequency/phase response is excellent.

My comment was of a general nature.



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