Power output of an Icon Audio MB150 valve amplifier reads 135W at a 1% distortion limit.


Power, measured in Watts, tells us how loud an amplifier can go. A 100 Watt amplifier goes louder than a 20 Watt amplifier for example. However, it does not go five times louder, much less in truth (+7dBW), and with a sensitive, modern floor standing loudspeaker 20 Watts will go very loud, loud enough for most homes. That’s because it gives +13dBW more power than 1 Watt and just 1 Watt gives 93dB at one metre from a stereo pair of 90dB sensitive floorstanders. That amounts to 106dB (93+13) sound pressure level at full output just 1m away which is extremely loud. In a room (i.e. reverberant field) this will decrease around -4dB for every doubling of distance, so expect 98dB at 4m (12ft) from the loudspeakers – and that is very loud. A 40 Watt amplifier adds 3dB to this and an 80 Watt amplifier 6dB, so suit less sensitive loudspeakers and bigger rooms

Power is no arbiter of quality either. High power solid-state amplifiers generally use a common and well understood circuit topology and are much alike in sound, no matter how much their designers tweak them and eulogise over the outcome. Low power designs tend to deviate more from the norm, Class A working being one example and these can offer better sound quality.

Valve amplifiers using popular 1950s audio power valves like the KT88 deliver 40W maximum, considered to be a lot of power in its day. This is plenty enough to play very loud with modern floor standers of 88dBSPL  (sound pressure level, from one watt) or higher sensitivity. Higher powers are available from parallel output pairs but this makes for a bulky amplifier.



Amplifier output power is measured with the amplifier connected to custom built, high power resistors that act as loads. We have two load sets, one heat sink mounted and fan cooled, one open air convection cooled using high working temperature resistors. Both use one 200W, 8 Ohm resistor on each channel to present an 8 Ohm load and two in parallel to present a 4 Ohm load, doubling the thermal capacity.

Although the load resistors are large, mounted on heat sinks and have fan cooling, in practice amplifiers do not have the thermal capacity to be run at full power into them for much more than a minute or so, and this would not be a realistic representation of real life usage. Our loads have a high power rating only to ensure there is sufficient capacity to handle 500 and 1000 Watt amplifiers.


Resistive loads for power measurement use four 8 Ohm, 200 Watt resistors.

The load resistors are custom built to our specification. They use zero hysteresis (iron free) wire to avoid high frequency distortion from magnetic effects. This ensures our high frequency  (10kHz) distortion measurements are representative of the amplifier under test, and are not influenced by the load.

No test equipment earth connections are made to the loads; they are fully balanced. Balanced input test equipment is connected directly, unbalanced through an balanced-to-unbalanced buffer. This is especially important with balanced bridge amplifiers, and digital amplifiers running a large d.c. output offset, like the 40V found on a B&O Icepower and Rotel RB1072.


Balanced to unbalanced buffer amplifier.

We use the most commonly adopted and broadly understood measurement of power, output of a sine wave at 1kHz into 8 Ohm load and 4 Ohm loads. The voltage into each is measured and the power calculated. Power at 10kHz and 40Hz is checked, important with valve amplifiers as this is affected by output transformer quality.


Waveform 'clipping', where peaks are chopped off. Power is measured just before this happens.

The voltage limit is that just before ‘visual clip’ on an oscilloscope (around 1% max). With low / no feedback valve amplifiers where overload is progressive we measure 1% and 3% thd limits, and quote output for 3% level.

Full power measurement is made quickly, within 15 seconds or so, to avoid overheating.

Power is measured by a Rohde & Schwarz UPL and HP8903B test set, both of which have strengths in this test. The R&S UPL has great resolution and provides a frequency domain spectrum analysis; the HP8903B is a ‘classic’ analyser that provides a time domain view of the distortion residual, making crossover distortion, with its higher order components, visibly obvious on an oscilloscope. Both have balanced inputs, so there is no connection to ground.

When an amplifier’s current detection protection circuits operate to limit output time exposure into 4 Ohms, 0.2-0.5mS sine wave bursts at 1kHz are used to find the overload voltage limit.

Mains voltage is monitored and runs at 230V-240V in London, where a nominal of 230V is declared. If an amplifier falls below its quoted specification a high current Variac is used to set supply volts precisely. When this is necessary the amplifier is usually over-specified. Most amplifiers, especially those from Japan, easily meet their quoted power figures. A few specialist designs are ‘optimistic’ in their specs. and their manufacturers generally question our measurement techniques. We invite manufacturers to attend measurement of their products to give us a critique of our measurement methods and equipment if they feel there is error, but to date the few that have done so have found no criticism to make.

A majority of modern amplifiers use solid-state (bipolar transistors, FETs, etc) and feedback to set bandwidth, linearity etc. Most clip (overload) symmetrically and suddenly, so their overload point is precise.

Valve amplifiers with little or no feedback overload slowly. Their overload threshold is not obvious as waveform ‘clipping’ on an oscilloscope. In this case we quote power as output achieved for 3% distortion, a figure commonly used in the past. Although this sounds like a lot of distortion against the 0.2% or so a transistor amplifier produces just below clip, it comprises low order harmonics, predominantly second harmonic, and audibly such overload is barely perceptible.

Today’s solid-state amplifiers have no trouble delivering full output across the audio band. Valve amplifiers are constrained by the quality of their output transformers which can magnetically overload (saturate) earlier at low frequencies than in the midband, and also induce slewing at high frequencies as falling impedance of the primary load draws excessive anode current. To account for these common problems we measure power at 40Hz and 10kHz, using a 3% distortion limit. This was a significant limitation in old valve amplifiers like the Quad II, that had comparatively small output transformers. Much of its ‘soft’ sound is attributable to this (but the Quad II had great strengths too, of course). Good modern valve amplifiers, from Quad or EAR for example, can deliver full output at 40Hz and 10kHz without difficulty and they have a commensurately more muscular sound.



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