Speaker Amplifier circuit problem - need help

[Smoke44]

Hi,

I have to design a speaker amplifier circuit for my car (E30-SEDAN1994). We are told the impedance of the speakers and the minimum voltage they require and also the voltage source. After researching I have found that the LM386N-4 is a popular choice. According to a datasheet of LM386N-4. 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 speaker, 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.

[earthwormjim]

Nice homework assignment...

For simplification, you can treat the speakers as resistive loads, and use their specified impedance. Then you can treat the output as a voltage problem, or a current problem, it really doesn't matter since your load is basically ohmic. I would treat it as a voltage problem personally.

Determining the current gain would be a little awkward, your input signals are going to be high impedance, so you would need a very high current gain. You honestly shouldn't care that much about current gain. You only need enough current gain to drive your ohmic load to the voltages you want.

Instead of looking at current gain, look at the power rating to determine if you have enough current to drive the speaker load, to the levels you want. That LM386 can drive under 1W, so if you a 4 ohm load, about the highest peak to peak voltage you could drive it would be about 5.6V, or 2 Vrms. You'd really want a higher impedance load for a tiny amp like the LN386 by the way.

You need current and voltage gain to drive a speaker from a line level signal. The LM386N-4 has both of these. It has an internal voltage gain of 20, so it's not an open loop operational amplifier, it has an internal feedback network to limit gain to 20. There are gain pins, so you can raise the gain if you want.

You need to figure out what your input signal is. Since it's not specified, it's probably just line level: https://en.wikipedia.org/wiki/Line_level

Speakers don't exactly have a minimum voltage by the way. They will work from millivolts up to tens of volts, it's more of a question of volume.

[Smoke44]

Thanks a lot, Earthwormjim, for your kind reply and guiding me in details and giving me the nice source. I never thought like you. I got my point. I just want to say wow!

Many many thanks again!.