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Measurable Performance Characteristics
A detailed description of the more important (from a sound perspective) of the various amplifier parameters is given later in this article, but a brief description is warranted first. Items marked with a * are problem areas, and the effect should be minimised wherever possible. The parameters that should normally be measured (although for those marked # this is rare indeed) are as follows:
* Important parameter
# Rarely measured
Input Sensitivity : The signal level required to obtain full power at the amplifier's output. This is determined by the gain and power rating of the amp. A 10W amplifier requires far less gain than a 200W amplifier to obtain full power for the same input voltage. It would be useful if all amplifiers had the same gain regardless of power, but this is not the case. Sensitivities vary widely, ranging from about 500mV up to 1.5V or more.
Total Harmonic Distortion (THD) * : This is a measure of the amount of distortion (modification) of the input signal, which adds additional signal frequencies to the output that are not present in the input signal. THD is commonly measured as a percentage, and can range from 0.001% to 0.1% for typical hi-fi amplifiers. A theoretically perfect amplifier contributes no distortion.
Transient Intermodulation Distortion (TIM) * : Also sometimes called slew induced distortion, this is a form of distortion said to occur when the input signal changes so fast that the output cannot keep up with it. When this happens, feedback ceases to be effective, since the output signal is delayed too long. This remains somewhat contentious, and most modern amplifiers are quite capable of handling the normal programme amplitude and frequency range without difficulty.
Crossover Distortion *# : A form of distortion caused by the power output devices in a push-pull amplifier operating in Class-AB. This occurs in valve and solid state designs, and is caused by one device switching off as the other takes over for its half of the waveform. There are some designs that claim to eliminate this distortion by never turning off the power devices, but in reality, only Class-A amplifiers have zero crossover distortion. This is generally measured as a part of the THD of an amplifier, and becomes worse as power is reduced from the maximum.
Frequency Response * : The amount of frequency versus amplitude distortion in an amplifier. A perfect amplifier will amplify all signals equally, regardless of frequency. Realistically, an amplifier needs a response of about 5Hz to 50kHz to ensure that all audible signals are catered for with minimal modification.
Phase Response : This indicates the amount of time that the input signal is delayed before reaching the output, based on the signal frequency. Variations in absolute phase are not audible in an amplifier system, but are generally considered undesirable by the hi-fi press. Since it is not difficult to ensure phase linearity, this is not generally a design issue except with valve amplifiers.
Output Power : This is most commonly measured into a non-inductive resistive load. This is not done to improve the figures or disguise any possible shortcomings, but to ensure that measurements are accurate and repeatable. Power should only ever be quoted as "RMS", which although is not strictly correct, is accepted in the industry, and may be measured into 8 Ohms, or other impedances that the amplifier is capable of driving.
Output Current # : Not often measured, but sometimes quoted by manufacturers, this represents the maximum current the amplifier can supply into any load. It is rare that any amplifier will be called upon to deliver any current greater than about 3 to 5 times the maximum that the nominal speaker impedance would allow for the amplifier's supply voltage. Greater variations may be possible with some speaker designs, but (IMO) this represents a flaw in the design of the loudspeaker.
Power Bandwidth : This is usually taken as the maximum frequency at which the amplifier can produce 1/2 of its rated output power (this is the -3dB frequency). A 100W amplifier that can produce 50W at 50kHz will be deemed as having a 50kHz power bandwidth.
Slew Rate # : Closely related to power bandwidth, the slew rate is the maximum rate of change (measured in Volts per microsecond) of the amplifier output. The higher the amplifier power, the higher the slew rate must be to obtain the same power bandwidth.
Open Loop Bandwidth # : The bandwidth of the amplifier with no AC feedback applied. Very few amplifiers will have an open loop bandwidth greater than a few kilo-Hertz, but valve amps and some solid state designs have a comparatively high open loop bandwidth.
Open Loop Gain # : Rarely quoted except for DIY amps (and few of them as well), this is the gain of the amplifier without any AC signal feedback. It is not really a helpful parameter for most people, but can be used to determine the ...
Open Loop Distortion #* : The THD of the amplifier with no feedback applied. This should be as low as possible, but realistically will usually be quite high by normal standards. The open loop distortion is reduced by an amount approximately equal to the feedback ratio.
Open Loop Output Impedance # : The output impedance of the amplifier with no AC feedback applied. This may range from a few Ohms to 10 or more Ohms, depending on the design of the amplifier. Valve amplifiers will normally have an open loop output impedance of 0.7 of the designed speaker impedance.
Feedback Ratio # : How much of the open loop gain is fed back to the amplifier's input to obtain the sensitivity figure quoted for the amp. For example if an amplifier has an open loop gain of 100dB, and a gain of 20dB, then the feedback ratio is 80dB. The application of feedback will
Increase bandwidth
Reduce phase shift
Reduce distortion
Reduce output impedance
Output Impedance * : This is the actual output impedance of the amplifier, and has no bearing on the amount of current that can be supplied by the output stage. Valve amplifiers usually have a relatively high output impedance (typically 1 to 6 Ohms), while solid state amps will normally have an output impedance of a fraction of an Ohm. By use of feedback, it is possible to increase output impedance (> 200 Ohms is quite easy), or it can be made negative. Negative impedance has been tried by many designers (including the author), but has never gained popularity - possibly because most speakers react very poorly to negative impedances and tend to sound awful.
Every amplifier design on the planet has the same set of constraints, and will exhibit all of the above problems to some degree. The only exception is a Class-A amplifier, which does not have crossover distortion, but is still limited by all other parameters.
The difficulty is determining just how much of any of the problem items is tolerable, and under what conditions. For example, there are many single ended triode valve designs which have very high distortion figures (comparatively speaking), high output impedance and low output current capability. There are many audio enthusiasts who claim that these sound superior to all other amplifiers, so does this mean that the parameters where they perform badly (or at least not as well as other amps) can be considered unimportant? Not at all!
If a conventional (i.e. not Class-A) solid state amplifier gave similar figures, it would be considered terrible, and would undoubtedly sound dreadful.