QuestionI bought a pair of bookshelf speakers (Sony SS-B 3000) i.e. 8ohms, max input power 120W. I am unable to decide on an amplifier for the speakers. I am willing to spend around 100 bucks for the amp. Usually I see amps with 2 channel rms 100W output.. would that work with my speakers?
AnswerYes. Will work just fine.
Amplifier power ratings are very precise, usually, and you can rely on them producing that amount of power into the rated load impedance. (At least if it is a real high fidelity product as they are measured according to known standards which all reputable manufacturers adhere unto.)
Speaker power ratings are very vague and almost meaningless other than to give you a broad idea of their true power rating. There are no acceptable and reliable power measurement standards for home type loudspeakers and hence it becomes more of an advertising department hype than a scientific rating.
You can connect any power amplifier to any loudspeaker so long as the impedance of the speaker is greater than the rated impedance of the amplifier. If the speaker is 4 ohms, for example, and the amp is rated at 8 amps the speakers could endanger the reliability of the amp over time and cause it to fail by overheating.
If the impedance of the speaker is higher than the rated impedance of the amp it will work just fine but you may not get all the power you wanted or expected.
If the output of the amp is greater than the (true) rating of the speaker then you must be careful with the volume control so as not to overpower the speakers.
If the output of the amp is less than the power rating of the speaker it merely means you may not have enough power to bring the speaker to its full sound level output.
One of the most important specifications of a speaker is its sensitivity rating. Many manufacturers do not publish this figure because they don't want you to know what it is! This specification gives a the amount of acoustic or sound power you can get out of the speaker with a given amplifier input. This test is conducted by placing a calibrated microphone in front of the speaker and applying a one watt electrical input signal at 1,000 Hz to the speaker. A typical reading could be any where from 85db to 105 db. Most home hi-fi speakers are in the 89db to 103db range. A 3 db increase represents twice power ratio. For example, if one speaker is rated at 91db and another is rated at 94db, the more sensitive one would produce the same sound pressure with 1/2 watt or half power.
Another way of looking at is if you wanted to double the sound pressure output so that is sounded twice as loud to the ear it takes, determined by subjective testing on humans, in increase of about 9db or 8 times the power. So, to double sound output of an amp/speaker unit that was a 10 watt amplifier you would need an 80 watt amp to give twice loudness. Hope this makes sense to you. Sorry to bore you with these details but it may be helpful for you in choosing an amplifier to go with your speaker.
I looked for the sensitivity of the Sony SS-B 3000 system but could not find it. Perhaps it is with the papers when you purchased them. Some Sony speakers have it on the ID plate at the back of the speaker.
If the sensitivity is over 93db the 100 watt amp will drive them very good. If their sensitivity is, say 89 or 90db, the amp may be a little on the low side.
Hope this helps, but don't hesitate to ask more questions.
C
PS: Just another side note. There is no such thing as rms watts. It is a misnomer that has been perpetuated by the industry and is mostly referred to by the advertising and marketing departments. The engineers back in the lab cringe when they see this printed because it is technically incorrect. RMS is only for current and voltage measurements and not power. The correct term in hi-fi amps is average continuous power.
