Installation manual

MYTHS, MAGIC AND FACTS
Amplifiers: What are they and what do they do?
ALL amplifiers are basically power supplies! This will shock almost all who read this.
Audio amplifiers are really a valve between the power supply and the speaker. This valve
is “instructed” by the incoming audio signal to allow a given amount of the voltage and
current from the power supply to reach the loudspeaker dependent solely on the
amplitude and polarity of the incoming signal. This blows away the pre conceptions of all
who read this of what amplifiers really are. Remember there are two main components in
any amplifier, a power supply which is the sole source of power/energy and the
“amplifier” which channels the power from the power supply by virtue of “commands”
given by the driving audio signal. With this said let’s see what’s cookin’ inside our black
boxes.
So with these facts established, our class A, class B or class D amplifier channels
POWER SUPPLY energy to the speaker but waste a lot or a little energy performing this
feat (depending on the class of amplifier used).
The task of the amplifier is quite simple really. Take the input signal, make it larger in
terms of voltage and supply enough current from the power supply to drive the
loudspeaker. Remember the speaker’s impedance is typically less than 8 ohms and
greater than 1 ohm.
The task of increasing the voltage and current levels to that sufficient to drive a typical
speaker is not difficult at all. The challenging part is how well the amplifier can do this.
The frequency response and phase must be unaltered, and distortion must not be added
to the signal. This of course is impossible to do perfectly but in the past 40 years we have
come a long way and there are many fine amplifiers.
Amplifiers can be of two types, a voltage source or a current source. 99.99% of audio
amplifiers are voltage sources. In other words the amplifier tries to hold its voltage output
constant (assuming a steady signal being a sine wave) no matter what the load
impedance. The ONLY way an amplifier can achieve this is to have zero output
impedance (ie an infinite damping factor) and zero losses in the output stages. The first is
somewhat achievable by introducing positive feedback which in theory will give the
amplifier a NEGATIVE output impedance. Zero losses in the output stages are impossible
to overcome. The prefect output transistor/MOSFET/tube has not yet been invented and
never will be!
A current source amplifier tries to keep the current through the load constant no matter the
load/speaker impedance. Some simple examples on the following page.
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