How to tell if an amplifier is powerful enough to drive speakers??

Hi,


Ok, I have had a bit of trouble finding the information I need.. how do you tell if an audio amplifier has enough power to drive speakers? In other words, what is the relationship between watts/channel, efficiency in dB of the speakers, and the impedence of the speakers? Hell, what does the efficiency even mean?


Please help.


I didn't find the right solution from the Internet.


References:-
https://arstechnica.com/civis/viewtopic.php?t=983747
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Thanks!

Speakers do not have a minimum wattage rating and pretty much any amplifier should be able to drive a speaker and produce sound, even a less than 1 watt amplifier could be plugged into a 4x12 speaker cabinet and would be able to drive it (in fact you might be surprised at how loud it would be!).

Speaker efficiency is, as its name suggests, a rating of how efficient a speaker is. What the number in dB essentially means is that if you feed the speaker with a 1 watt signal the sound pressure level at 1 meter from the speaker will be whatever the rating is - so the higher the number the more efficient the speaker and the louder it will be with any given amplifier.

As for impedance, in simple terms this needs to match the output impedance of the amplifier. All amplifiers will have an output impedance rating in ohms (this may be a range e.g. 4 - 8 ohms or it may be a single figure e.g. 8 ohms) and the speaker or combination of speakers attached to the amplifier needs to have an impedance that matches this.

First off, it's worth saying that most numbers that describe speakers are approximations or, sometimes, fabrications. Two speakers with the same theoretical efficiency may have quite different actual loudness, depending on how or if it was measured. And the impedance changes with the frequency the speaker is trying to reproduce, so it's a best guess depending on what you're amplifying... Sorry to complicate the issue.

As @PaulD mentioned, almost any amplifier can drive almost any speaker. There are a number of 1/4 watt amplifiers out there including several by Lovepedal and the iconic Smokey. They're actually quite loud driving a 412 cabinet, or a super efficient speaker like an EVM-12. Vox put out the Nightfly Mini that uses a 12AU7 as the output tube, puts out 1.5-2w and is QUITE loud. As long as there's enough current to actually move the voice coil, you'll get some sound. Heck, there's a version of one Lovepedal fuzz that was famous for being able to drive a speaker cabinet and that wasn't even designed as an amplifier.

Tubes are designed to function best when the internal resistance of the tube and the resistance it's sending a signal to match (usually somewhere around 2500-4000 ohms). Most amplifiers have a transformer that "transforms" the 8 ohm impedance (for example) of your speaker to the 4000 that the tube needs. As the impedance (or resistance at a given frequency, which is what impedance means) moves farther away from that ideal number, the tube becomes less efficient and has higher distortion levels. Get far enough away (say 4-8 times higher or lower) and things like tubes and transformers start to burn up, plus you get lots of less pleasant distortion. So, for best clean tone, match impedance with what the amp expects. You can get more distortion and probably be safe at 1/2-2 times the rated impedance, although some amps, like vintage Marshalls, may not even tolerate that for long periods.