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Output current and protection
A commonly asked question about a power amplifier concerns its maximum output current.There is some
good reason behind this question.There is always some trade off between output power, 'specmanship',
protection and cost in a power amplifier.
Output power specification (into 8 Ohms) can be increased simply by increasing the amplifier power supply
rails slightly.But what then happens with a 4 Ohm load ? Can the amplifier now supply the extra current a 4
Ohm load needs or will the power supplies collapse,or will the amplifier need to protect itself?
Short circuit protection can be achieved simply,but at the cost of accidental operation when the
loudspeaker load is reactive (almost always) rather than purely resistive (almost never).
Output current and ruggedness can be increased with the cost of paralleled output transistors,but can the
driver circuitry cope?
Hence the interest in output current.However the implication that 'more is better' is true only up to a very
modest and calculable limit.An electrical load,be it a resistor,capacitor, inductor,heater, a loudspeaker or a
hairdryer takes an amount of current dependent on the voltage applied to its terminals,and its operating
condition.Although it is not always entirely obvious how much current a load will take with a non-
sinusoidal signal (like music) its maximum possible value is easy to calculate,and to monitor in real
operation.If an amplifier can provide this amount of current into the specified load while keeping safe and
stable and not suddenly dropping its output voltage,it is enough.The load cannot draw any more.The
current draw is the result of the voltage applied.
If an amplifier protects itself by 'current limiting' it will drop its output voltage if too much current is
demanded.This is a bad thing for an audio amplifier, but not uncommon.If the current limiting never
operates under real conditions it is both a harmless and a good way to protect an audio amplifier.The
problem can then be to know if the protection ever triggers or not.This nagging doubt,though often ill-
founded,has resulted in some concern (and boasts) about amplifier output current.
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