Q: Can an audio amplifier be damaged by running it without speakers connected?
A: No. See Amplifier Damage | Sound & Vision
Q: Can you plug in two sets of speakers into the same stereo output terminal.
A: If you connect them in parallel (both connected to the terminal on the stereo) you will reduce the impedance (e.g. 2 8 ohm speakers will result in 4 ohms), so the amp may be overloaded. Connecting in series will give 16 ohms and the amp may not have enough power to drive them.
See Is it safe to connect two pairs of speakers to one stereo system? | answers.yahoo.com
Speaker Selector Switch Summary - Geoff the Grey Geek
How to wire Four Speakers to One Amplifier - Geoff the Grey Geek
Some very budget speaker selector switches have no impedance protection what-so-ever. They are just a switch turning each speaker on or off. These are not recommended.
Series Resistor: this is used on lower cost speaker selector switches. It normally means there is a resistor (4-6 ohms) wired in series with the speakers. This gives the total circuit a minimum resistance which will protect the amplifier.
Impedance Matching Transformer: this is used on higher power and higher cost speaker selector switches. An impedance matching transformer inside the speaker selector multiples the impedance of each speaker which effectively keeps the total impedance around the same as any of the individual speakers (providing the speakers are the same impedance as each other).
Amazon.com: in wall volume control for speakers: Electronics
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last updated 1 Aug 2014