Amplifier ratings, how many watts did you say?

The Amplifier power ratings issue, that’s right, its an issue. Some manufacturers claim huge power ratings and are very misleading but here’s some info on how to read these ‘impressive’ ratings.

One of the first questions clients ask about an amp is “How many watts is it?”

Though power is one of the most important features of an amp (or receiver), an amp’s watts per channel is one of the industry’s most misleading specifications and it’s misused by many manufactures to make their amps look good on paper compared to the rest.… the more watts on paper the “better”.   Watch out for ratings into 4 or 6 ohms, at 1 kHz, at 1% distortion (or even 10%!!! like most “all in one” systems) or “per channel” or worst of all…a peak/max/pmpo output. (not rms)… this is why.

4 ohms vs 8 ohms?

Specified power ratings into 4 ohm speakers give the highest rating sometimes twice the 8 ohm rating. The problem is these amps can seldom drive these 4 ohm speakers.  The reason is that according to Ohm’s law (Volt = Current x Resistance) when the resistance goes down the current has to go up to balance Ohm’s equation. More current means more heat and that will trip the amp into protection mode very easily. Therefore it is very important to look for amps with current driven technology rather than voltage driven technology if your speakers are 4 ohm. 4 Ohms speakers are very popular nowadays because they give greater sound output levels for the same amount of power. This is important for home theatre where sound levels are generally above normal listening levels for music. Worst of all, the amp has to cope not only with 2 speakers but 5 or even 7 speakers which puts a lot of strain on an amp’s power supply.

1 kHz vs 20hz-20khz?

Many brands only specify the power rating at 1 kHz.  Frequencies at and above this are the easiest to amplify and thus that will give the highest rating. Its much more demanding if it has to drive the whole frequency range from of 20 Hz to 20 kHz the normal range for audible sound. The most difficult is the low bass freq from 20 Hz to 150 Hz. This is why adding a powered sub to take care of these low frequencies takes a lot of stress from the amp leaving it to amplify the rest of the signals at greater efficiency.

1% vs 0.1% THD?

The amplifiers watts rating should state what level of distortion is being mesured at.  It is very easy for humans to hear 1 % THD (total harmonic distortion) but 0.1 % THD is inaudible so the power rating must be specified at this level of distortion or less… Typically an amp that can deliver 50w at 10% distortion will only give 10w at 0.1% distortion. A lot of car audio amplifiers are measured at 10% distortion and this practice now appears in some Home theatre amplifiers on the market today.

Per channel vs All Channels?

The highest wattage rating is achieved when the amplifier & power supply only has to drive one channel… many Brands state 100W “per channel” but watch out, that can mean only one channel is driven. When measured, most brands fell to 50% their power ratings if they drive 5 or more channels at the same time.

Peak power vs RMS?

The biggest misuse is some use “peak” power that is the power it can deliver for short burst of time… but that is a far cry from the real issues of how long the amp can sustain this power… the RMS (root mean square power) or simply continuous power is the only true measurement of any amps power.

So if your are looking for true power specs, don’t just believe the salesman “this amps 140W where as that amps only 60W”  look at the ONLY INDUSTRY ACCEPTED STANDARD ….which must state it in the following manner

“100W RMS into 8 ohms from 20Hz to 20KHz at no more than 0.1% THD with 2 (or more) channels driven”

Don’t trust any power rating if it is not specified in this manner…. they are trying to hide something.