Below is a discussion of amplifiers, what they do, what manufacturers claim that they do, and what does and does not matter when choosing an amplifier. Following this discussion is a list of important parameters, tolerances and definitions including power , to take into consideration when purchasing an amplifier.
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"This amplifier sounds super smooth, powerful, and yet produces velvetine subtle details bathed in an atmosphere
of shimmering translucent scintillation."

The above prose is characteristic of many purported reviews of Audio amplifiers. It is pure hyperbole that feeds on the kind of marketing-created subjectivism that infects all too much advertising about audio, and a good many other products. Remember, science tells the tale best, at least in electronics.

The plain fact of the matter, is that the aim of every high fidelity amplifier designer is to create a product that has no effect whatsoever on the signal fed into it, other than to increase its amplitude and prepare it to be sent to a speaker.
Most of the subjective commentaries on amplifier performance that refer to such qualities as "warmth," or "precision" are either the reviewer's impressions of the music, speakers, or listening environment, or evidence of great deficiencies in the design of the amplifier.

Amplitude
In physics and electronics, amplitude is literally the measurable high value of a periodically varying quantity. In lay terms, it is the strength of a signal or sound without regard to its content. Amplitude measurements of audio signals generally refer to the signal voltage, which is only one component of what determines power (watts), or the ability to do work. Thus, it is important to understand that amplitude alone does not singly determine power (or loudness in audio), but does affect it. In the physical world the amplitude of a sound is measured in decibels of SPL (sound pressure level), which again does not wholly define overall sound power or intensity, only the sound level at one point in time, and one frequency of the audio spectrum.

Amplifiers are devices or circuits that increase the amplitude of the signal fed into them. Any additional changes in the nature of signal are a form of distortion. A popular engineering aim, is to construct amplifiers as, " a straight wire with gain." Qualities present in the output that were not in the input are to be avoided. In the perfect amplifier, the best thing that could be said of it is that, "It works."

Impedance
Most consumer amplifiers are made to conform to standard expectations for input and output impedances. Most inputs accept impedance matches from 47k to 100k ohms, and outputs are rated for 4 ohms (most automotive speakers), with high power systems capable of 1 ohm loads or smaller. Most home and pro speakers and amps are rated at 8 ohm loads. There are a few units that have different ratings to accommodate special purpose applications, but we need not consider those here.

Distortion
Modern designs and devices inside most amplifiers allow distortion levels that at 1 percent or less, are essentially imperceptible by a normal human being when the unit is played at the prescribed parameters and loads. Thus, Total Harmonic Distortion (THD) and Intermodulation (IM) distortions have effectively been reduced beyond the threshold of human perception when played within expected levels. Keep in mind that there are plenty of other opportunities for distortion to creep into the audio program at many other points in the chain of reproduction. Microphones, speakers, and their operating environments are the most common sources of serious distortion.

Power
But, the real trick for most consumers is to figure out how much power is required for the specific speaker and application involved.

Loudspeakers have a complex inductive reactance associated with the voice coil and a resistive loading factor that in combination are known as impedance. This impedance varies considerably with frequency. But the standard procedure is to select a nominal value for the purpose of computing the power calculation. For car amplifiers, this is usually four ohms, but with higher power systems this can be much lower. The standard formula multiplies the voltage by the current, or the voltage squared divided by the impedance. The product is the amplifier's power potential in watts.

Note that amplifiers, like light bulbs only consume watts when they are actually creating sound in speakers. For this reason, no matter what the capacity of the amplifier and woofer, normal room volume will, on average, only need about 5 to 10 watts for sound thought to be loud. The rest of the extra ten to thousand watts are used only in creating deep loud bass.

An amplifier's power is dependent on the voltage applied to it and the current flow it can sustain without breakdown or overheating in the presence of a normal load. Moreover, these functions must be performed without incurring any form of perceived distortion. Fortunately, modern electronics makes it possible to meet these requirements without great expense. For more on this critical subject, see - Power

Clipping Distortion
However the worst form of amplifier malfunction occurs when users try to make them perform outside of their capacity. The usual symptom is an item called clipping distortion. This is present when either the amplitude of the signal being passed is too large, or the impedance load is too heavy for the rated output capacity. In practical terms, this is the equivalent of increasing the volume level to the maximum on a very loud sound, or attaching 10 woofers in parallel on each output terminal. This means that the most important specification to be observed when choosing an amplifier, is to make certain that both the usage and the intended load will not cause the amplifier to go beyond its clipping point.

Peak Power
Because many people prefer to listen to something other than constant-tone sine waves, program dynamics play an important role in that they vary the load on an amplifier from moment to moment. Consequently, most amplifiers are tolerant of the occasional demand for peak power that goes beyond its continuous power rating, and in many cases this tolerance can be sustained for very small periods (microseconds) of time so as to pass sudden burst energies without objectionable distortion or stress. With bass amplifiers, you will need to use a capacitor to enable the power supply to keep up, and prevent damaging clipping.

Probably the best practical advice that can be given to the prospective amplifier buyer is to select speakers appropriate for the usage and space you intend, and then choose an amplifier that can meet those requirements without stress.

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A LIST OF IMPORTANT PARAMETERS, TOLERANCES AND DEFINITIONS TO TAKE INTO CONSIDERATION WHEN PURCHASING AN AMPLIFIER
However the majority of amplifiers in today's market will not list all or even most of these figures.

DISTORTION TOLERANCES
The minimum distortion specifications that will permit an amplifier to meet high fidelity standards are:

Total Harmonic Distortion (THD)
At normative load (4 ohms) the THD should be less than 1 percent. (The lower the number, the less significance it has. 004% is no better in practical terms, than .1%)

Intermodulation Distortion (IM)
The IM should be less than 2.5 percent ( the lower the better, but usually not listed in most current specifications.)

Other Important Measurements And Features

Power Bandwidth
The frequencies for which the full rated power output of the unit can be applied: at least 20 to 20,000 hertz, the range of human hearing. More is not necessarily better.

Signal to Noise ratio (SN)
The ratio of inherent noise produced by the internal components of the amplifier to the average program material: Min 85 dB (although in a car environment which is inherently noisy, 55dB would be OK. The higher the number, the-slightly-better). Howsever, if you have perceptible noise associated with the engine operation, check here

Sensitivity Rating
The sensitivity rating should be at least 230Mv @47k ohms for full output.

Power Output Ratings
Use the RMS or continuous power ratings instead of the much higher and much less reliable so-called peak or MAX output specification. In the absence of an RMS rating, it can be safely estimated that RMS power is at most 50 percent of the peak power specification. Remember that to double the loudness in perceived sound, the amplifier must produce 10 the amount of electrical power in watts. Also, a sound in a car that is subjectively judged to be really loud can be produced by as little as 10 watts. One twice as loud would have to use 50 watts, and twice as loud as that, would require 500 watts. See our section on Power Ratings .

Bridged Power
This is an arrangement of the amplifier circuit where-in the separate built-in stereo amplifiers are each given one phase of the unified mono input signal, which is then recombined to yield an output that is usually at least twice as high as the individual stereo outputs. Or, 50 watts stereo would be converted to 100 watts mono.

Built-In Crossover
There are arrangements on some amplifiers that provide a special switch allowing the amplifier to respond only to the bass (LPF), or the treble (HPF), or the full spectrum of the signal. This allows the amp to feed only the appropriate drivers without an external crossover in the speakers. It also means that either you will depend on the receiver for the full range source signals, or you'll have to have more than one amplifier.

Bass Boost Circuit
A switched circuit that increases the bass response by anywhere from 8 to 20 dB. Be careful though, as this can quickly produce clipping distortion in some systems.

Mosfet Output Circuitry
Popular Output devices that produce less heat and greater efficiency in class AB circuits than older bipolar output transistors. These are now fairly common in most amplifiers.

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