I am looking at making a simple audio amplifier as part of a school project im doing. One thing that it must do is have adjustable volume so this circuit seems like it will do the trick. To adjust the gain of the LM 386 a resistor can be added in series with the 10uF capacitor between pins 1 and 8 so naturally, I would think to add a variable resistor there to control the volume. From the schematic, however, it seems that there is a potentiometer connected to pin 3. I am wondering if this is used to adjust the volume or if it is for something else. To achieve my desired effect should I attach a variable resistor in series with the capacitor or is there a better way? I am not sure if this photo is referring to the potentiometer in the schematic already or if it saying to add one in series with the capacitor.
3 Answers
Interesting question, and I see where the confusion comes from.
There are things that need to be kept separated from each other, i.e what the chip itself does in the circuit, and what the circuit around the chip does, and what the whole circuit does.
You are correct that pins 7 and 8 on the chip are used to set the gain of the chip.
However those pins are not supposed to be used for volume control, they are only supposed to be used to set the chip to a fixed gain of voltage amplification. When those pins are disconnected, gain is 20x, and when shorted with the capacitor, the gain is 200x, and a resistor in series can be used to select a gain between 20x and 200x.
It may not be obvious, but for a volume control, that is quite unusable range, considering that the audio amplitude at the input must be first attennuated to very low levels so that the whole 20x to 200x gain range can be used, and still you could not turn the volume down to 0.
Also for chip stability, it is best not to have long wiring on those pins, or a scratchy potentiometer. It is best to have absolute minimum wiring and components on those pins, and the default gain setting of 20x by just leaving them completely unconnected will usually be just fine, unless the audio signal is really low in amplitude.
The potentiometer between the audio input and chip input is the correct place for volume control. The potentiometer can be used to provide the chip input with any percentage between 0% and 100% of the actual signal that needs to be amplified. And then the chip amplifies it, for example by the default gain of 20x.
The method shown in the schematic is normal.
Changing the gain by changing the resistance between pins 1 and 8 has three serious drawbacks (as do any schemes that change the feedback, in any amplifier):
- It doesn't change the gain that much. Human hearing has a tremendous dynamic range, you really want a volume control that goes all the way down to zero (pro tip: use an audio taper pot, if you can find one).
- It can change the frequency response of the amplifier. Granted, the LM386 isn't the world's best amplifier, so you'll be changing it from mediocre to differently mediocre, but still.
- Your resistor would need to be floating above ground, and any stray signals that coupled into it would couple into your amplifier. Having a pot that's grounded on one end is just cleaner, electrically.
The 10K potentiometer with the wiper connected to pin 3 of the LM386 is the normal way to connect a volume control.
