Amplifier output power specification.


Which figures belong in the real World? Geoff Kremer of English Valve Amplifiers Ltd explodes the myths.


I have always been aware of the plethora of figures banded by amplifier manufacturers relating to power output, but Geoff Kremer, proprietor and the brains behind English Valve Amplifiers, reminded me of the significance to choosing the correct one.  As he is the amplifier technical guru I could never aspire to, I asked if he would pen an article that would help clarify matters for us lesser mortals. I had an idea that Geoff's direct, no nonsense approach would ensure a concise, focused, no nonsense article. And so it came to pass…

The output power rating myth.

So you have an amplifier rated at 200 Watts per channel, 400 Watts combined. But can it genuinely deliver this power to the loudspeakers. The manufacturer’s specification sheet says so, or does it?
Convergent Audio Technology (USA) JL3 monoblock amplifiers. A genuine 200 watt Class A design which utilizes 16 power and 3 driver valves on each unit. Winter only listening is adviseable.

Convergent Audio Technology (USA) JL3 monoblock amplifiers. A genuine 200 watt Class A design which utilizes 16 power and 3 driver valves on each unit. Winter only listening is adviseable.

Take a look for the figure for the mains power consumption given on the amplifier's spec sheet or on it's rear panel. Suppose, for example, the amplifier information plate indicates that it consumes a maximum of 150 Watts mains power. So how can this miracle machine deliver a total of 400 Watts, when it only takes 150 Watts of energy itself? In simple terms, the power output can never exceed the power in. Perhaps the manufacturer has found a way of discovering the Holy Grail of thermodynamics? How you can conjure energy from nothing.

In fact, the exaggerated figure given boils down to how the output power is given. If the power consumption and the power output were measured and annotated in the same way, then that would indeed be a more accurate and meaningful measure of the amplifier's capability. The output power would be related, and proportional, to the input power. But some manufactures cheat! Are we surprised? Of course not.

The truest and most valuable measurement that ought to be given is the amount of usable continuous power available. This is the power available to actually drive the speakers to produce sound. It used to be called “Music Power” but now it's called “Continuous RMS power” or RMS, continuous.

Often, manufacturers make the output power greater than it actually is by quoting the Peak Power capability. This is the absolute maximum amount of power an amplifier can produce in very short bursts. This may only last a millisecond or two before the power supply runs out of steam, or the output devices blow up. The peak power quoted is often double the value of the continuous power figure, so in reality, the elevated 200 Watts of claimed power is now down to just 100 Watts.

Now, let's look at the RMS (Root Mean Squared) value. This is the average value of the sinusoidal signal all amplifiers develop to drive the speakers. Because the signal is made up of peaks and troughs, called a sine wave, we have to take the mean or average value of the peaks and troughs. This is something like 30% below the combined maximum value of the peaks and troughs, the Peak to Peak value. The RMS is the average or mean value of the Peak to Peak value, and takes into account the spaces between the sine waves where the power delivery is almost zero. It's the RMS value that really matters, because it's that which does the work.

So now our output power is down by a further 30%, to just 70 Watts RMS of continuous power. This is the truepower available to drive your speakers. The Real World calculation is now 70 Watts per channel x 2 which equals 140 Watts combined. The mains power used by the amplifier is 150 Watts, so this no longer breaks any rules. The 10 Watts surplus is due to power supply and other losses within the circuitry, so in reality the amplifier produces 70 Watts of Music Power, or if you must, RMS continuous, per channel. A much more believable figure.

The following tip may help you with getting an idea of the real world power of an amplifier:

If the amplifier power consumption is lower (and especially if it's something like half) than the stated combined output power, divide the output power by 4, and you probably won't be far wrong from correct the output music power.

 Geoff Kremer.

With many thanks to Geoff Kremer of the English Valve Amplifier company for his contribution. Geoff has been offering his expertise professionally for the servicing and repair of valve amplifiers of all types, as well as manufacturing his own designs.

English Valve Amplifiers LTD,

31 Turkey Road,
East Sussex,
TN39 5HB.
Office telephone: +44(0)1424 216245
Mobile  +44(0)7590386898