Decoding Amp Power Specs

You're a newbie to this audio game, and are just putting together your first home theater. Or perhaps you're making your first major upgrade. You're pouring over the spec sheets, looking for the best AV receiver for the cheapest price.

Chances are, you're looking at the power output specs. That's fine, but do you know what they are telling you?

A fully specified 7-channel amp or AV receiver should include something like the following in its power ratings:

Power output (7-channels driven): 130Watts per channel (Wpc) into 8 ohms, continuous, 200Wpc into 4 ohms, continuous, from 20Hz to 20kHz at 0.05% THD (total harmonic distortion).
The power rating with just two-channels driven, if provided, will usually, but not always, be more than the power rating with all channels driven:
Power output (two-channels driven): 180Watts per channel (Wpc) into 8 ohms, continuous, 225Wpc into 4 ohms, continuous, from 20Hz to 20kHz at 0.05% THD (total harmonic distortion).
The actual numbers in these specifications aren't the point here; the contents are. The word continuous in the spec means that the amp can put out that much power into the specified load all day long, if needed, without overheating, shutdown, or damage. Some manufacturers call this RMS power instead.

Some specs will also list something called "Dynamic" power, "peak" power, or "IHF" power. This means that the amp should put out that much power for a short time, but not from breakfast to the late news. There is some validity to this since music and even soundtracks rarely demand maximum power for more than a brief period, sometimes just milliseconds. Still, run away very fast unless the continuous power rating is given too.

Let's take a look another real-world example (some of the numbers have been changed to protect the innocent!):

Surround: 125 Watts x 7 (20Hz-20kHz, 8 ohms, .07% THD)
Stereo: 125 Watts x 2 (20Hz-20kHz, 8 ohms, .07% THD)
This is a good specification for a $1,500 AV receiver. But note what is not specified: the power output into 4 ohms. That means that either the manufacturer doesn't recommend 4 ohm speakers, or the power output at 4 ohms isn't impressive enough to publish.

Manufacturers will often specify a minimum speaker impedance of 6 or even 8 ohms, or include an 8 ohm/4 ohm switch on the back panel. The latter limits the current (and power output) when 4 ohm speakers are used. Leaving out a 4 ohm rating is fairly common with receivers. If you speakers have a specified impedance of 4 ohms, look very closely at the power specs and be certain of your vendor's return policy if the AVR has problems driving your speakers.

Here's a second example, from the same manufacturer, but this time for a 5-channel receiver that sells for under $300:

Surround: 100 watts x 5 (8 ohms, 1 kHz. 1% THD)
Stereo: 100 watts x 2 (8 ohms, from 20Hz to 20kHz, 0.7% THD)
Note that the surround power rating is at 1kHz only. But at this price, you have to expect compromises.

We've also seen this sort of spec for an AV receiver:

100 Watts per channel
Front: 100W +100W
Center: 100W
Surrounds: 100W +100W
Back Surrounds: 100W+100W
If you've already figured out what's missing, collect your credit for the course. There's no specification for the load impedance, the frequencies over which the power output applies, or the distortion. And it's clear that the 100W spec only applies to two channels driven at the same time.

Now that we've made you totally paranoid, however, I'm going to suggest that much of this really matters a bit less than you might think. There's a school of thought that says you'll never—or almost never—have all the channels operating full throttle at once. Have you ever listened to the surrounds of a home theater system by themselves? Try it sometime. While they might be highly active, they are nowhere near as loud as the fronts, at least on soundtracks. Even with the Governator doing his best Terminator thing, the surrounds are likely only pulling down a few Watts each.

Nevertheless, you should give extra credit to manufacturers that provide a reasonably full, clear set of specs. Or at least the power output and THD, with all channels driven, across the full 20Hz-20kHz audible spectrum into 8 ohms. If it's an expensive receiver, you have a right to expect this. But if it's a bargain basement piece, be prepared to accept some compromise. Just know that that those surrounds will seldom be drawing much power. And the knowledge that a powered subwoofer will likely be tending to most of the chores below 80Hz, relieving the receiver of those power-sucking low frequencies, will also help you sleep better.

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