Join Date: Dec 2000
Location: Longmont, Colorado
Re: Question about ohms and speaker cabs.
Thanks for the cut and paste definition, it is a good general explanation, but not as relevant here. From the 2nd paragraph one might get the idea that a lower impedance = more power from an amp. On most solid state amps, the answer is yes, if you notice, most solid state power amps are rated something like, "75W per side @ 4 ohms, 55W per side at 8 ohms, 150W into 8 ohms in bridged mono". SS power amps feed the current directly into the speaker, some straight from the power transistor or IC, others through a transformer, but, it doesn't matter what load you plug in, it connects to the same place. So, a lower impedance = more power (unless you hook up too little impedance and it = smoke).
Tube power amps are different, they can't deal with different impedances, hence the need to set the impedance on the back of the amp. What this does is taps the output transformer at different spots so the power tubes always see the same load, and, consequently, are always putting out the same amount of power. A 50W tube amp puts out 50W into 16, 8, or 4 ohms, provided the impedance selector is set correctly. What is bad is to run a 4 ohm load with 8 or 16 selected. You can go the other way, i.e. a 16 ohm load with the selector on 4 or 8 and it shouldn't hurt the amp (try at your own risk, I won't be responsible) and theoretically it will make the amp work harder but reduce the output, although you probably won't be able to tell the difference though. The transformer taps are also why you have to reduce the selector by half when pulling 2 tubes out of a 4 tube amp, i.e. if you pull 2 tubes out of a 100W Marshall to run it at 50W and have an 8 ohm cabinet, the amp should be set to 4 ohms.
Hope this helps,