A helpful guide for what to look for when shopping for an amplifier.
When you’re shopping for amplifiers of any kind, one of the key features you’re probably looking at is the rated wattage it can put out. In this post we’ll go over some of the ways certain manufacturers fudge these numbers and what to look out for so you can be more sure you’re getting what you think you are.
First, let’s look at how power ratings are supposed to be calculated so that the ways people sometimes fudge the numbers will make more sense.
In 1974 the FTC mandated some guidelines for amplifier manufacturers. In a nutshell, the amp’s rated power had to be given with these things in mind:
- The power rating should be displayed in terms of RMS, the power the amp can sustain for a prolonged period of time (and not simply short bursts), generally a sustained note played at 1KHz frequency.
- “Sustained power” must be an amount the amp can hold for at least 5 minutes straight.
- The RMS power rating must be something the amplifier can sustain with all channels driven. If it’s a 2-channel amp, it means both L/R channels driven at that power simultaneously. (Though it gets a little weird when talking about A/V receivers with 5-7 channels.)
An example of a power spec given for an amplifier that stands a good chance of being legit might be written like: “100 watts RMS per channel, both channels driven, <0.05% THD.”
That would tell you that the amp can properly generate 100 watts in a sustained manner to 2 channels simultaneously, and generates less than 0.1% total harmonic distortion while doing it. That’s promising for a stereo amplifier.
Things to look for in amplifier specs that might be suspect:
If the power given isn’t specifically stated as RMS. This is a big one, because it’s possible in a testing environment in unrealistically ideal conditions for an amplifier to read at a significantly higher power level than would be typical for music or movies. For instance, a 50W amp might be able to spike at 300W in certain conditions, but that wouldn’t be typical or something you should count on, and would be very misleading for the manufacturer to say it was a 300W amp.
Of course seeing “RMS” next to a power rating isn’t a guarantee it’s legit, but if the manufacturer is willing to make that claim there’s a higher chance it’s true. (Otherwise it’s a blatant lie, and they know that.)
If the THD isn’t explicitly expressed alongside the maximum RMS power rating. One example of this I’ve seen is where an amp company said “300W amplifier, with a THD lower than 1%.” At a glance that sounds good, but the fine print made it clear their 1% THD measurement was at 1W, and the amp actually generated closer to 10% THD when fully driven.
Whoa, that’s a big difference!
But notice they didn’t say, “Capable of 300W RMS with less than 1% distortion.” They gave a power rating, and then made a separate and disconnected claim about THD.
When they don’t state the power is with all channels driven. Like I mentioned above, it’s easier for an amp to put 100W into only one of multiple channels for heat reasons etc., but that wouldn’t necessarily mean the amp could properly push that power to all of its channels at once.
When the “peak” power rating is substantially higher than the RMS rating. Generally you should only pay attention to the RMS power anyway to avoid clipping and damaging your speakers. While an extra 50%-100% might make sense for a “peak” power rating, ratings that go way above that are likely hyping the numbers to sell products.
For instance, a 200W RMS amp with 350W peak power makes sense. But a 200W RMS amp with 1000W peak power is probably BS, and they are likely using the 1000W part to get buyers excited.
Look at fuse ratings or the amp’s power supply to ensure it jives with stated power output.
An amplifier cannot output more power to speakers than it’s taking in through the car battery or AC wall outlet. Also, note that all amps have varying efficiency ratings given which class they are. Class A/B amps tend to be 50-60% efficient, for instance, and Class D amps are closer to 80-85% efficient typically.
60% efficiency would mean that of the total power the amp pulls from an AC outlet (or battery), 60% of that is directed at the speakers. The rest is dissipated as heat. So if it’s sending 300W to the speakers, it would mean that amp is actually drawing 40% more to do that, or 420W.
Here’s where that comes into play.
One example I’ve seen where looking at power supply specs revealed how much an amplifier’s numbers were fudged is when it stated info like this:
- Class D 300W amplifier
- Power supply: 32V @ 5 amps
Wattage is a calculation of voltage x amperage. With the above specs, it means this amp is actually pulling in 160W (32×5). Since a Class D amp typically has an efficiency rating of 80%, that would mean its actual RMS capability would likely be more like 128W.
How is a sustained 300W possible if the power supply is only capable of roughly half that in the first place?
There’s a similar philosophy at play with an amplifier’s fuse rating.
The fuse in any electrical circuit is meant to stop the flow of power if the current goes over a safe level, given the application. Obviously the intention then is that the amplifier should never actually draw that much current in real usage.
One example of this gone awry is an amp claiming to be 1000W RMS, but also stating its fuse was intended for 110V @ 5 amps. 110V makes sense since it’s your typical AC outlet, but since wattage is volts x amperage, that would mean we can’t safely exceed 550W without blowing that fuse.
How is that amp supposed to put out 1000W?
Technically you *can* run more power than a fuse is rated for through it for a second or two without blowing it, so theoretically you might be able to draw 1000W for a couple seconds. But it also wouldn’t be true RMS power then since sustaining that power would blow the fuse.
