The LM386 is a power amplifier designed to drive a speaker. The LM324 is a quad operational amplifier.
You would not use the LM386 as an operational amplifier. You would not use an LM324 to drive a speaker.
Each opamp in an LM324 is capable of high ampliciation, it is typically unnecessary to cascade them all to get the needed gain. In fact, if you need to do that you are getting into the area where an instrumentation amplifier would be a better choice.
Lets look at use of an LM324 as a microphone preamplifier, not designed to driver a speaker.
The LM324 falls to unity gain at 1.2MHz. This number is usually stated as GBP or gain bandwith product, because the gain times the frequency cannot exceed this number. So we have gain 1 at 1.2Mhz which means we can have a gain of 10 at 120KHz or 100 at 12 KHz.
At the upper end of the audio spectrum, the gain would be 1.2M / 20K or 60.
Typical output of an electret microphone is 5mV. So a gain of 60 would get us to 300mV. Another gain of 60 would be 18V. Nominal audio line level is about 1Vp-p. So a gain of 200 is required to get from an electret microphone to line level. You cannot achieve this across the whole audio spectrum with 1 of the opamps in an LM324, but you can easily with 2, each having a gain of about 14.
You could take that output of that and feed it into an LM386 with a volume control to drive a speaker from the microphone.
That said, the LM324 is not the best choice for audio. TL084 would be a better choice, though it would require dual supplies.
Note also, that if you use an LM324 and single supply, you would want to use an inverting amplifier and tie the + input to Vcc / 2 and use input and output coupling capacitors to block the DC levels.
Hope this helps.
Bob
