How does ohm's law apply to the amount of speakers that can be supported for a power amplifier (PA)?

Question

I read that alot of Power Amplifier (PA) has 8 Ohms circuitry. And that supports well for 2 speakers. Then when you add 2 more, it drives the amplifier to work harder because it reduce down to 4 ohm which is dangerous for most PA. Why is that?

Answer 1

In the days of vacuum tube amps, the speakers were driven by output transformers and the impedance level was important to get maximum power transfer. Modern transistor amplifiers use transistors to directly feed voltage into the speaker, and they will drive a wide range of loads. Where ohm's law comes in, the current into the speaker is the drive voltage divided by the speaker impedance:

I = V/R

The power dissipated in the speaker and in the amplifier is proportional to I^2. If R gets too low, the current can rise to excessive levels, dissipate too much power, and cause overheating of the amp output stages or the speaker voice coil.