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 How to calculate the gain of the speaker?
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Poning
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2 Posts

Posted - Mar 08 2017 :  04:24:16 AM  Show Profile  Reply with Quote
Hi,

I have been playing with LM386 audio amplifier IC and I was wondering how much output power it can give me. For example, I have tried using it with 8ohm 0.5W speaker and it seems to work. But I don't suppose I can drive 100+W speakers with it ( To be honest, I think that because chip looks so tiny and fragile)

I am trying to figure out how big a speaker I can make it work with this chip.

I have also another question about gain.According to the datasheet #65288;http://www.kynix.com/uploadfiles/pdf2286/LM386M-1.pdf#65289;,I can set the chip to give me 26dB to 46dB gain compared to input voltage. I assume that computer or phone jack gives me around -/+ 5V signal (I approximated this from couple of things that I saw on the internet). 5V signal with 8ohm resistance uses 3W of power. This makes me think that standard phone or computer output is more than enough for my speaker.

On the other hand, when I connect this speaker to my phone, I get very little volume. I guess I am making a mistake somewhere. How should I be calculating the gain I need for a specific speaker?

Edited by - Poning on Mar 08 2017 04:25:09 AM

audioguru
Nobel Prize Winner

Canada
4213 Posts

Posted - Mar 16 2017 :  9:05:37 PM  Show Profile  Reply with Quote
The datasheet of the LM386 and every other amplifier shows exactly how much power it produces with different supply voltages. For example with a 6V supply its output power when it is barely clipping is only 0.2W and with a 9V supply it is only 0.45W into an 8 ohm speaker. The datasheet shows more power when the output has severe 10% clipping distortion but the distortion graph shows when the distortion begins to climb when it is clipping.

The speaker rating is not its sensitivity, it is the maximum power it will survive. A 0.5W speaker fed 0.5W might produce the same loudness as a 100W speaker fed 0.5W. Of course an LM386 can drive a 100W speaker with its 0.45W.

The gain is how sensitive the amplifier is. 0.45W into 8 ohms is produced by a signal that is 1.9V RMS. A consumer line level from a phone or computer is typically 0.15V RMS so an amplifier with a gain of 1.9V/0.15V= 12.7 will just make it but a gain of 40 will allow extra gain for faint sounds to be heard, then the volume control can be turned down a little for normal levels.

A line level signal that is 5V RMS has an extremely high level. 5V RMS in an 8 ohm speaker produces (5V squared)/8 ohms= 3.13W. +/-5V is 10V peak to peak which is 3.54V RMS.

Your phone and computer are designed to drive an external amplifier with an input impedance of about 10 thousand ohms and a gain of about 40 times, not an 8 ohm speaker.
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