If you replace a 4 ohm speaker with an 8 ohm speaker can the amplifier blow the speaker?

A1: The output impedance of a power amplifier is always less than 0.1 ohms. There are no 4 ohm amplifiers on the market!

A2: I actually just bought a new car radio/CD player that specifically shows 4 ohm speakers on the sticker. It is designed to work optimally with 4 ohm speakers. If you use 8 ohm instead, it will work, but you will not get the full power output that is specified. The amp can only kick out so much voltage and so much amperage - if it is designed for 4 ohm speakers, and 200 watts (stereo, so 100 watts per speaker), it can supply 20 volts (P = V^2 / R). If you used 8 ohm speakers to this amplifier, each speaker would only get 50 watts powered at full output voltage.

Alternatively, if it is designed for 8 ohm speakers and 100 watts per speaker, it can provide 40 volts. If you used 4 ohm speakers instead, each speaker could be given 200 watts at 40 volts, resulting in severe overheating and damage to the amplifier.