I'm currently working on setting up an amplifier circuit for driving an 8Ohm speaker, however, I'm having issues with the 386.
I'm producing my sound via a DAC, producing a voltage swing of between 0 and 3.3v. Please see the schematic below. (I thought fritzing would be the quickest way to put one together; I now wish I'd used a proper schematic editor but ah well).
I do intend to run the resulting circuit from a 12v supply, which should be within specs; however, the volume I'm getting out of it is pretty low. Currently, I'm supplying it with a 9v supply, and even with the input grounded, it's pulling 400mA.
I've swapped the chip out, with the same result.
Disconnecting the speaker brings current draw down to about 5mA (as expected), but why is it drawing so much current when there is no real signal?
Upon investigation, I found that the volume (set with the potentiometer) seems to have very little effect on the current draw of the amp at all.
With further investigation, I've found that adding a DC block cap to the output, brings the current draw down very low, but as does the volume and voltage swing across the speaker.
Any suggestions? There must be something fundamental I'm missing here.
You need a capacitor between the outupt and the speaker. See the [u]datasheet[/u].
You are putting constant DC current through the speaker...
AC audio signals swing positive and negative. In order to get a negative voltage out of an amplifier that has no negative power supply, the output is biased at half the supply voltage. Then, the output can swing relatively negative to around zero volts. A capacitor blocks this "DC bias" from the speaker, and the voltage to the speaker goes truly negative (relative to ground).
With further investigation, I've found that adding a DC block cap to the output, brings the current draw down very low, but as does the volume and voltage swing across the speaker.
You might need a bigger capacitor depending on the frequencies you want to reproduce. If you want to cover the full frequency range down to 20Hz you'll need something like 5000uF. If you don't need the deepest bass, you can get by with 1000uF.
I found a 2200uF cap which seems to have done the trick fine, although I'll probably have a play and see what I can get away with for the sake of space optimisation (I intend to take this to a PCB in time); I don't require particularly amazing quality.
The only question I still have is why the charge capacity of the capacitor affects the response at different frequencies? I was under the impression that any cap blocked DC?
Capacitors have an equivalent resistance to AC current that is frequency dependent, i.e. R = 1/(2PIf*C),
so you need a large capacitance to pass low frequency currents.
