where to add gain and volume controls to LM386 guitar amp circuit?

Question

Im trying to make a super simple guitar amp.

heres the circuit using an LM386 operational amplifier:

What exactly would affect gain but not volume, and volume but not gain? In the example circuit on the LM386 datasheet theres a 10k potentiometer at the input, but online some people called this a volume control and some a gain control. Im confused. For a gain control I was thinking of adding a variable resistor or potentiometer between pins 1 and 8, instead of R3 in my circuit, because thats how gain seems its supposed to be regulated on the LM386. But will this also increase the volume? Does a bigger voltage swing of the signal mean a higher volume of the sound? I want to be able to get the distortion and clipping of high gain but at a low volume.

Answer 1

The LM386 is one of the major choices in building small 9V powered practice amps. I do believe it was the pignose was the first one out there with it. It used the encapsulated version of this chip that Velleman Kits sells to their diy market. Overall, it makes a great clean practice amp.

Now with your questions about gain staging, you would would use the negative feedback pins (1&8 ) along with the resistors that is added on the input pins to achieve the total gain. its not necessary to vary the negative feedback for your application because of the lead length that will occur doing that might give you poor results. I would just set this chip at a reasonable amplification and leave it at that.

As far as making a practice amp with distortion, I would put a two more op amp stages before the lm386, so I could vary amplitude as well have a pre-buffer and a makeup amp stage, so I could sandwich some diode distortion there between those two op amps. Alternatively, you could use a distortion pedal that would have the same circuit.

Answer 2

If your are talking about the "loudness" of the audio signal coming out of your speakers then gain and volume are the same.
Simplified: gain is a technical term how much a signal is amplified. Thus a gain of 10x means the output signal is ten times larger then the input signal. A gain of 0.1 means output signal is ten times smaller then the input signal. The more 'gain' the 'louder' the music.
If your are dealing with music it is normal to have the same 'gain' but in order to control the volume, you somewhere in the system make the signal amplitude variable between 0 and max. (As your hearing is logarithmic you use a logarithmic variable resistor). With the volume control you make a variable 'gain' between 0 and 1.
Lets say your amplifier has a total 'gain' of 10.000. What you do with the variable resistor is you make the input signal smaller. If we make it 1000 times smaller. That is a gain of 1/1000. the 'gain' of the whole system is 1/1000 * 10.000 = 10. If you turn your volume completely down the gain is 0. That gives 0 * 10.000 is a gain of 0. (No amplification which should be silence but you will still hear noise. Let's ignore that). If you turn the volume control to max the 'gain' of the system is 1 * 10.000 is 10.000.
The answer to your question is that you can NOT change the volume and not the gain or vice versa.

I want to be able to get the distortion and clipping of high gain but at a low volume.

Straightforward hard 'clipping' is simple using .e.g. a set of diodes but I assume you want to keep the 'volume' the same and variable the amount of clipping. In that case I suggest you look for a good circuit using the www. because it is a bit beyond the scope of 'stackexchange' to make a design for you.

Answer 3

Note that minimum gain for LM386 is \$ v_{out} / v_{in} = 20 \$. You can raise the gain by making R3 a variable resistor. It is paralleled with an internal resistance of 1350 ohms. If R3 is infinite, gain is 20. In this condition, you have maximum feedback, lowest distortion.

If your guitar input signal is large enough to badly overload the output by clipping tops & bottoms of peaks, you'd want to add the 10K variable resistor at the pin 2,3 input.
If your guitar input signal is very small, the 10K input variable resistor isn't used, and you'd increase gain with the R3 resistor.

Your pin 7 bypass capacitor could be much larger than 0.1uf. (greater than 50uf perhaps).