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I am experimenting with Op-amp for making a audio amplifier.

I am using this circuit. Audio amplifier

I want to know if the values of C424 and C118 are correctly chosen here or do they need to be change? I hear small salt-and-pepper type noise in the audio. I wonder how can that be removed?

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Are you sure that you need 20x amplification? In my experience, the line out voltage levels can actually be higher than what you get from "headphones out".

You can experiment with the output capacitor a little... not sure what bass response your headphones have, and your "line out" from the audio codec - but you can speculatively try increasing the output capacitor to a couple hundred uF.

The noise can be in your line out signal, or can be introduced by grounding. You haven't mentioned how your phones amp is going to be powered.

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This is an inverting amplifier with a mid-band gain of \$\mathrm{A_v=-10k/500=-20V/V}\$. Since this is an inverting amplifier, beats may be heard a bit odd because the push of a beat turns into a pull (think of the movement of the speaker's outer cone).

C424-R2 pair brings a high-cut at \$\mathrm{f_c=(2\pi \ 10k\ 80p)^{-1}\approx199kHz}\$, which is way beyond the audible top-end limit but shouldn't be a problem here.

And R3-C118 pair brings a low-cut at \$\mathrm{f_c=(2\pi \ 33\ 100\mu)^{-1}\approx48Hz}\$. We might expect this to be under 20Hz but the headphone may not be able to generate sound (e.g. sound muddy) at those low frequencies. So 48-50Hz seems acceptable.

So, the output signal (at pin-7 of the chip) will be 20 times the input signal (i.e. 8Vpp = 2.8Vrms). Assuming the headphone's impedance is 32 Ohms, the output power will be Pout=2.8² / 64 = 0.12W, and half of this power (~60mW, a reasonable value) will be transferred to the headphone. So, no problems in terms of electronics design.

I hear small salt-and-pepper type noise in the audio.

Maybe you should check the wiring and setup (i.e. supply, decoupling capacitors, etc.). If you post a photo of the setup in the question then it'll be easier to help.

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It is a strange choice of opamp as a headphone amp. 20mA current limit might be ok into high Z cans, but will be a bit limiting into a big pair of over ear low Z cans, and it is far from being particularly quiet. Paralleling up a couple of 5532s thru suitable resistors seems likely to bother be quieter and cheaper.

Is your codec really ok driving into 500R? I would be surprised.

Av = 20 seems excessive for this application, try making the 500 ohm input resistor more like 5k, the codec will be happier and the lower gain should still be quite adequate for cans (which usually want less then a volt or so up 'em).

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  • \$\begingroup\$ How does the value of C424 can help in removing the salt and pepper noise? \$\endgroup\$ – scico111 Oct 29 at 13:09
  • \$\begingroup\$ It doesn't, what it does is help to stabilise the opamp by compensating the capacitive loading of the inverting input. \$\endgroup\$ – Dan Mills Oct 29 at 13:32

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