WATTS PER CHANNEL.The maximum continuous power output in watts that the amplifier will produce, before distortion or "clipping", with both channels driven into a standard 8-ohm load over a specified frequency range (usually 20 Hz - 20 kHz), and at a specified level of distortion (THD) .

THD AT RATED POWER.Total harmonic distortion (THD) is a measure of the percentage of undesirable frequencies and harmonics added to the music signal ( by the amplifier) which weren't present in the incoming signal. THD levels of 0.01% to 0.09% or less are common in modern transistor amplifiers (the lower the figure, the better) , (tubes produce far higher distortions). Any figure of 0.3% THD or less would be inaudible on musical source material.

IM AT RATED POWER.Intermodulation distortion (IM) results when the interaction of two tones within an amplifier produces spurious distortion components that are the sums and differences of multiples of the input frequencies. IM distortion is much harsher to the ear than THD; look for levels of IM of 0.09 or less (the lower the better).;

POWER BANDWIDTH.An indication of an amplifier's ability, at full output power, to reproduce uniformly all the frequencies across the audible bandwidth from 20 Hz to 20,000 Hz. To be useful, this spec must be qualified by +/- "x" dB; 20Hz - 20,000Hz +/- 0.5dB would be excellent; 20Hz - 20,000Hz +/- 1dB, very good.

SIGNAL-TO-NOISE RATIO(S/N).The proportion of hum and internally generated circuit noise (hiss) compared with the maximum output of the amplifier. Power amplifiers are typically quieter than phono preamps, and carry excellent S/N figures of 90dB to100dB or more. Any figure greater than 80dB is inaudible on musical source material.

MINIMUM RECOMENDED LOAD IMPEDANCE.The lowest loudspeaker load impedance in ohms, which the amplifier can safely drive without incurring protection circuit shutdown, instability, or oscillation. A spec of 4 ohms is typical, but some new amplifiers can safely drive loads of 2 ohms or less.

DYNAMIC HEADROOM.Many amplifiers, depending of the relative "looseness" or regulation of their power supplies, are able to produce considerably more than their rated power output on brief, transient music signals lasting a fraction of second, Dynamic headroom, specified in dB, is a measure of an amplifier's short-term ability to deliver power to the speaker far excess of its rated output. An amplifier with a rated power output of 50 watts per channel and 3 dB of dynamic headroom would be able to deliver 100 watts to each speaker on a short-term basis without clipping or distortion.

SENSITIVITY.The input voltage needed by the amplifier (from the preamp or CD player) to reach its full rated output. Measured in volts (V) , or millivolts (mV) 1V or less is typical; usually a preamp or CD output between 0.5V and 2 Volts is compatible with the vast majority of amplifiers.

FEEDBACK. Typically, loop negative feedback is used to lower distortion and reduce output impedance. Because the feedback voltage is proportional to the ratio between the internal impedance of the amplifier and that of the speaker, changes of the speaker's impedance can cause changes in the feedback level-which affects the sound.

BRIDGING TO MONO.Certain amplifiers, depending on their particular design, can be bridged to mono operation, thereby doubling or even tripling the power output, usually by means of a rear-panel switch or adaptor. For example, a 100 watt-per channel stereo amp, bridged to mono, might typically deliver between 200 and 300 watts (mono) to an 8 ohm load. This is an attractive way to gradually upgrade to higher power without a large initial cash outlay.

POWER OUTPUT METERS.Some type of power output indicator - an LED that flashes, a peak-reading meter or bar graph indicator - can be useful for indicating when an amplifier reaches its output limits with consequent distortion or clipping of the music signal. A red LED that flashes at maximum power output (sometimes called a "clipping indicator") is usually sufficient. A "VU" meter or bargraph indicator , of course, is even more informative about the power output and the demands of the music and speaker.

CLASS OF OUTPUT.Class A amplifiers produce extremely linear replicas of the input signal, are inefficient, capable of extremely low distortion, run hot, are large and bulky and tend to be fairly low powered at a given price level. Class B Amplifiers employ output transistors that alternately turn on and off to amplify the positive and negative cycles of the total waveform. They are very efficient, run cool, but unless the output devices are very well-matched, can introduce a discontinuity called "switching distortion" to the output signal. Class AB amplifiers combine the advantages of both class A and B amps with none of the liabilities of either; they avoid switching distortions while achieving almost the same efficiency as a Class B amplifier. By far the most popular design, Class AB amps are often called by different proprietary names - "Dynamic Super Class A", "Hyperbolic Conversion Amplification", "Non - Switching" , "X-Balanced" and so forth. In Class D amplifiers, high frequency pulses carry the audio information. In Class G amplifier, low -power transistors handle the amplification most of the time, with high-power transistors switching only during peak signal demand. In Class H amplifier, two different supply voltages are switched in as required, depending on the size of the signal being amplified (Carver and Soundcraftsmen). A variation of Class H , Class "H+", is used by Technics in its line of receivers.


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