Need help designing headphone amplifier circuit

Question

I have to design a headphone amplifier circuit. We are told the impedance of the headphones and the minimum voltage they require and also the voltage source. After researching I have found that the LM386 is a popular choice. I've seen many circuit diagrams but what I don't understand is how to pick the voltage gain needed for the amplifier.

I originally thought that you would need to know the input audio signal voltage and when you multiplied that by the gain that would need to give the minimum voltage required by the headphones, but it appears that what matters is the current which I don't get.

And are you able to design such a circuit without knowing about the audio input signal? If you do need to know about it what is it you need to know about it and why?

I would greatly appreciate any help as I am very lost. Thanks

Answer 1

Voltage in x gain = voltage out. The current that flows in the headphones is V/Z where Z is impedance of headphones. Z is a little frequency dependent.

Input current to amplifier is voltage in/ input impedance. Input impedance is usually 1k to 1Mohm (generalism warning).

Yes, you need to know the input voltage. You need to know the maximum it can attain and the nominal RMS value that would produce (say) a comfortable nominal listening level in the headphones. The headphones will have a SPL (sound pressure level) characteristic that you need to research. Also research what would be a comfortable SPL level. You then begin to know what the voltage level ought to be across each speaker in the headphones.

You might also consider hard-clipping to prevent acoustic shock (should your amplifier be capable of delivering much higher voltages to the head phones).