Is an audio power amplifier (not a preamp), commonly used in the high-end home sound systems of audiophiles, a voltage-controlled voltage-source?
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\$\begingroup\$ An audio power amplifier must not oscillate, despite 10:1 variations in impedance and in phase shifts. Thus the power supplies have unusual requirements, compared to opamps driving resistive loads. And the magnetic fields of the power-cords may be adequate to couple into low-level cables from turntables, and upset the waveform precision, or just cause oscillation. \$\endgroup\$– analogsystemsrfCommented Sep 21, 2018 at 4:36
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3 Answers
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Short answer: Yes, a voltage-controlled voltage source.
The most-expected load is a low-impedance dynamic speaker in the 2-ohm to 16-ohm ballpark. An amplifier having a very low Thevenin equivalent output resistance is helpful to provide good damping. An ideal voltage source fits that profile.
A voltage-controlled voltage source might be specified as having a voltage gain, but you do not often see a spec like this for an audio amplifier. Of more interest is how many watts can be delivered to a load resistance before distortion sets in. The typical distortion type will be clipping of top peak and bottom peak of a sine wave.
The input side of an amplifier might have 10k - 100k input resistance: a voltage across this resistance is amplified and reproduced at the output. So a little current does flow from the driving source.
Many audio amplifiers are AC-coupled, so that voltage gain does not extend to DC. Some audio amplifiers have DC-coupled output stages.
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Yes, it is a voltage controlled voltage source, but not ideal. An input voltage of a few volts and a few micro-amps is amplified by about 20 to 30 times to drive a speaker of 2 to 8 ohms typical.
This requires the amplifier to source much more current with a rise in voltage, until the sound level the user wanted is reached, or the amplifier or speaker is overloaded. A quality amplifier will sense this and shut down.
A close-to-ideal amplifier has a very small output impedance much less than 1 ohm, so it has a firm grip on the motion of the speakers, especially the woofers which draw the most current. The power supply may have capacitors well above 20,000uF to support high current surges. There is a reason why high-performance amplifiers may be wired to the speakers with short runs of twisted 8 gauge Monster Cable. Also large diodes may be used to protect the outputs from back-emf from the woofers trying to return to a center position.
The frequency range of a high-performance amplifier is often way below and above the human range of hearing, so capacitors may be used for input and outputs to block DC offset voltages, but some of the best amplifiers use a servo-loop IC to cancel out any DC at the amplifier outputs.
This avoids using capacitors which tend to cause phase shifting at very low frequencies, hence the term "muddy" bass. However this could be the fault of the woofer enclosure as well.
Undersized speaker wire such as zip cord can also cause muddy and weak bass due to the resistance of the wire.
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No, it is not that simple. For example, a VCVS, like any ideal voltage source, provides the same voltage regardless of the load current. A real audio amplifier doesn't do that. Likewise, an ideal VCVS doesn't draw any current from the controlling voltage but a real amplifier does draw current from the input.
The input to an amplifier is generally modeled as an impedance to ground. The output is best modeled using a Thevenin equivalent circuit.
answered Sep 20, 2018 at 20:42
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1\$\begingroup\$ Regardless of its actual output impedance (which is usually a tiny fraction of an ohm), the output of an audio amplifier is essentially a voltage source. \$\endgroup\$ Commented Sep 20, 2018 at 23:59