if I were to change the output transistors to darlingtons, would this make the amp louder? I assume that it will increase current gain.
A darlington transistor has more saturation voltage loss than an ordinary transistor so the amplifier will have
less output power.
The typical saturation voltage loss of a TIP31 ordinary transistor is 0.8V and is 1.4V for a TIP110 darlington transistor. The PNP losses are similar so with darlingtons you typically
lose a total of 1.2V which is a lot when the supply is only 12V.
More current gain does not make an amp louder. It simply allows the design to be changed so the driver transistor conducts less current so it is cooler.
I am familiar with bridging audio amplifiers, you can do that with the LM386 and get twice the output power.
Not good enough.
A little LM386 has a small amount of output current. When two are bridged then the current is trying to be doubled which is too high for the little amps so the output power is doubled instead of quadrupled like with more powerful amps.
One LM386 produces 450mW into 8 ohms at clipping with a 9V supply. Its heating is 0.5W.
Two bridged LM386 amps produce 545mW which is only slightly louder than 450mW. Each one heats with 1W. So kiss the battery good-bye.
I just used it with a dual polarity power supply..... It got two times louder, The two R4s were burning hot, along with Q7 and Q3.
Your hearing's sensitivity is logarithmic so you need 10 times the power to be twice as loud. You didn't get 10 times the power.
Twice the supply voltage doubles the voltage and doubles the current in most parts so of course the resistors get 4 times hotter.
Each R4 dissipates 0.34W when the supply is plus and minus 12V. Q3 dissipates 0.68W which is more than its max allowed heating.
Q7 should not get hot because it has a low voltage across it.
The fuse protects the power supply from catching on fire. The speaker can still easily burn out and the amplifier can easily overheat without blowing the fuse. When a transistor overheats then it melts inside and is destroyed. No hole.