Electronic – Question about audio amplifier circuit

Soft limiting can be a really simple way of preserving the gain for smaller signals whilst limiting (with some measured distortion) the output volume for bigger signals. Feeding an input to a PA stage can be intercepted with back-to-back diodes so that for small signals the diodes are barely conducting whilst higher amplitudes get limited to about 1.4 Vp-p.

Another way is to use an AGC (automatic gain control) amplifier such as this one: -

Q1 is non-conducting when small signals are fed into the circuit. This means that Q2 (P channel JFET acting like a variable resistor) is biased into pinch-off by R8 and, doesn't attenuate the incoming signal (to ground) via R2. As the signal gets bigger Q1 starts to conduct and this drags the gate voltage on Q2 down towards ground turning on the JFET and attenuating the input signal. C4 acts to smooth Q1's effect and give more of a DC control signal to Q2 based on what the op-amp's output amplitude is.

The LM386's internal circuit is similar to an opamp, but optimized for high power audio. To get the best out if it you should follow the recommended circuit shown in the datasheet.

Your circuit actually does work, but not very well. The main problem is that you don't have a capacitor on the output, which is required to block DC voltage from getting to the speaker (without it the speaker could draw a high quiescent current, which will reduce its linear range and might burn it out).

The input may also be a problem. The audio signal should be AC coupled to it via a capacitor, to stop DC voltage from upsetting the bias point of the amp. It's possible that your input source already includes a blocking capacitor, but unless you know exactly what circuit is being used you should not rely on it.

The LM386 already has internal feedback resistors to set the gain, so using external resistors is redundant. The DC gain is set to 20 in order to keep output voltage offset and drift low. You can increase the audio gain by putting a lower resistance between pins 1 and 8, but it must be AC coupled via a capacitor to avoid increasing the DC gain.