The circuit-level "why" of the loudspeaker / amplifier “POPP” sound
The loud pop you hear when turning on a power amp is most likely caused by DC blocking capacitors somewhere in the signal chain.

Suppose you have to connect two stages together, for example the input stage and the pre amp stage of your amplifier. For design reasons, the output of the input stage has an output DC voltage of 1 V, while the input of the pre requires -2 V DC to operate correctly. The numbers I am giving are just random, but they help proving the point.

To connect these two stages, you usually use a (fairly large) capacitor. The largest it is, the better, because it introduces a zero in the origin and a pole at a frequency that is proportional to the inverse of C, so basically it is an high pass filter, and for audio you want that pole below maybe 20 Hz or something.

This big cap needs to be charged when you power up your device. In our example, the input stage will source current to set its output to 1 V, while the pre will sink current to set its input to -2 V. Assuming that the input stage output is more powerful, which probably is the case, the voltage on one side of the cap goes to +1 V almost immediataly after power up, so the other side goes to +1 V as well. 

This sudden voltage spike is the pop you hear.

A picture is worth a thousand words though. I have devised this stupid simple circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

If you run a time domain simulation for 100 ms, this is what you get:


the orange trace is the voltage I have called Vcright, and it is the voltage that is propagated down the signal chain. See the spike? I can almost hear the POP :)

To solve this issue there are several approaches. The easiest one is to have relays just before the power output, when the amp is powered on the relays are open, and they are closed after a few seconds. Another way is to limit somehow the bias currents when the equipment is turned on, in order to smooth out the spike you see.