Wattage

bill,

the importance of wattage really depends on the speakers you're trying to drive with that wattage and the volume you want to achieve - the more difficult a load the speakers are to drive, the more power (wattage) needed to drive them and make them achieve a given volume.

the relative "heaviness" of the load presented to amplifiers and receivers by speakers is captured by the "sensitivity" rating you'll often see in their specifications - it looks something like "89db / 1w / 1m", whih means that the speaker will produce a volume of 89db with 1 watt of power when the listener is 1 meter away from the speaker. (sometimes it says something like "89db / 2.83 volts / 1 m" which, depending on the impedance of the speakers, can translate into more than 1 watt...)

the rule of thumb after that is that every 3db of volume requires twice as much power to produce, so:

89db - 1w
92db - 2w
95db - 4w
98db - 8w
101db - 16w
103db - 32w
106db - 64w
109db - 128w
112db - 256w

and so on.

then there's the impedance of the speaker, rated in "ohms"; the lower the impedance, the more power the speaker draws from the amplifier or receiver. so make sure you check on the impedance of the speaker and the amount of power the amplifier or receiver is capable of putting into that impedance. for instance, at 4 ohms, the amplifier is required to put out TWICE the wattage it puts into the same speaker at 8 ohms, and not all amplifiers are capable of sustaining that sort of output.

of course, the rated impedance of speakers is usually an average or "nominal" rating - the actual impedance at any point in time will fall along a wide range of numbers, depending on the frequency of the sound it is trying to reproduce, with the lower frequencies typically having the lowest impedances.

these numbers also vary with the distance from the speaker to the listening position, although i forget the exact relation - something like -3db per 1m away, but someone else will be able to correct me on this.

so. how much wattage matters depends on how much volume matters to you and the sensitivity and impedance of the speakers you intend to drive with the that wattage, and how far away you will probably be from the speakers when you're listening to them.

wattage also matters when it comes to hurting your speakers - conventional wisdom says that the most damage you can do to your speakers is done by putting TOO MUCH wattage into speakers that aren't capable of handling it.

this is not necessarily true - by far a more common source of damage to speakers is done by giving them TOO LITTLE power; what happens, basically, is that the amplifier reaches its maximum power rating, at which point it starts "clipping" the tops off of the signals it sends to the speaker. these clipped signals are incredibly difficult for speakers to reproduce, and can fry drivers (especially tweeters) in no time at all.

anyway. some stuff to think about.

i'm sure others will fill in the gaps and probably paint a clearer picture...

- jd

I would only add to the excellent explanation John has given you by saying that the original specs you quoted may also be an issue. Not every manufacturer lists it's power ratings in the same way. So it is entirely possible that a given 75Wx6 receiver is the equal of or even superior to a given 100Wx6 receiver.

What you want to look for is how the power is described:

Continuous power, 20Hz-20kHz, all channels driven (or at least more than 1-2) with a given Ohm rating (usually 8)
will make for a more powerful unit in the real world.

If you do not see the unit's power described in these terms then the original rating is optimistic at best and will usually fall precipitously as more channels are involved.

Another thing to look for is whether the receiver has any impedance limitation on the back panel. If the unit is insufficient for driving 4 Ohm speakers it will usually be listed there due to product liability. Some other receivers might have an impedance switch which will allow for 4 Ohm use but usually at a cost of current flow limitation. Effectively lowering wattage output can reduce dynamics. Many multi-channel receivers nowadays are sporting these switches. I take this as a sign of weakness in the power supply and try to avoid them like the plague.

Bill,

In general it is accepted that a 1 dB difference is the smallest difference that can be heard, although this is quite arbitrary (i.e. the dB is not defined that way; it is a mathematical thing). Anyway, the difference in dB from 75 w to 100 w is:

10 * log ( 100/75 ) = 1.2 dB

So you will probably be able to just tell the difference, but it will be slight.

Of much more importance to the overall loudness of the system is the sensitivity of your speakers. Very sensitive (efficient) speakers can be rated at anywhere from about 96 to 105 dB or more (1 Watt @ 1 metre). Such speakers may include Cerwin Vega, Klipsch, and others. Very inefficient speakers may be rated at 86 dB or less. These require an enormous amount of power to make sound. The average speaker is around rated at 89 to 91 dB sensitivity. Note that a 10 dB difference requires 10 times the power (this is the mathematical definition of the dB). So a 98 db efficient speaker will play as loud with merely 10 watts as a 88 dB efficient speaker will play with 100 watts. Feed the efficient speaker with 50 watts, and you will be experiencing loudness that the other speaker can hardly dream of recreating, and would require 500 watts of clean power to do if it could.

Wow! Great responses guys.

Let me pull-back to some concepts.

In your fathers 2-channel stereo world, power was king. The speakers were HUGE with big woofers and you wanted them to fill every corner of the NEXT room with continous sound.

A modern HT system is .. very different. You surround yourself with 5 speakers, all focused in on a couch or some chairs. You are NOT trying to fill the corners with sound.

You often have a self-powered sub in a corner which takes a large power load away from your receiver. And the speakers - well they are not all pumping sound all the time. The center speaker runs 100% of the time, the L/R run about 40% of the time and the rears only get used maby 20%.

(Hint: Disconnect your front 3 speakers and fire up a favorite movie and sit in the room reading a book. You may be startled when the rear speakers decide to make noise).

So you dont need TONS of power for a HT system. More power is better but it's no longer the most important think to look for in a receiver.

You can always hedge your bets by getting a receiver that has pre-outs. This allows you to add an external amp later on.

Just remember that power is only part of the equation. Let's put it this way, you have a 12 volt battery in your car. If you were to put 8 AA batteries together, would they start your car? Well, 8*1.5=12 volts. Why wouldn't it start your car? Because of the amperage. This is where the power comes from to run your speakers. You can have a 200 watt per channel receiver that could conceivably be much worse than a 60 watt per channel. You have to take into account the power supply, capacitors, transformer, etc. The best well to tell how good the amplifier section is is to look at what it can do into different ohms. Hope this helps.