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Is the power consumption of an amplifier proportional to the output volume, or the signal strength increase?

In other words, if I turn the amp volume down by half, and then turn the input line level up double (so the output volume stays the same), what happens to the power consumption of the amplifier? How does this change among common amp designs?

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  • \$\begingroup\$ You ask for a general rule which applies to most or all amplifiers but the truth is that it depends. A class A amplifier has a more-or-less constant power consumption, a class AB amp. will consume more for higher volumes. Signal strenght increase means little, just increasing the voltage does not mean the power also increases. Current is also a factor. \$\endgroup\$ – Bimpelrekkie Aug 3 '17 at 10:07
  • \$\begingroup\$ @Bimpelrekkie: hence the last sentence... \$\endgroup\$ – naught101 Aug 3 '17 at 10:12
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The power consumption of an amplifier tends to have no dependence on the gain, or input signal level. It may have a dependence on the power output. It will always have some static consumption.

In a class A amplifier, there is a high static power consumption, and virtually no dependence on the output power. Power efficiency improves from very poor at low output to poor at higher output.

In a class B amplifier, the static consumption is low, and the power supply current input increases with the load current. This means that the power efficiency is low at low output powers, and improves with higher output power.

In a class D amplifier, the static consumption is low, and the power supply power input increases with the power output at more or less constant power efficiency.

This assumes we are talking about a 'power amplifier'. Small signal amplifiers tend to be class A, though many opamps have class AB or B outputs, which are covered as above.

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  • \$\begingroup\$ @JorenVaes I look forward to your answer, with its survey of various types of small signal amplifier. Are you going to include parametric, VFB and CFB? \$\endgroup\$ – Neil_UK Aug 3 '17 at 14:38
  • \$\begingroup\$ I did not mean to suggest this was a bad answer - rather, through a question state that there are also other amplifiers. I am sorry if it didn't come across in that way. \$\endgroup\$ – Joren Vaes Aug 3 '17 at 14:41
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    \$\begingroup\$ I take your point that I haven't covered everything. My initial thought was that as the OP was obviously talking about audio power amps ('volume' etc), I would not take the trouble to cover small signal, and that you were welcome to and add your own answer if you thought it important. However, then I realised I'd already covered class A, which covers most small signal amplifiers. \$\endgroup\$ – Neil_UK Aug 3 '17 at 14:57
  • \$\begingroup\$ This answers the question I was getting at - audio power amps. Sorry for being vague. @JorenVaes, feel free to add more relevant detail if you think it's worth it. \$\endgroup\$ – naught101 Aug 4 '17 at 0:54
  • \$\begingroup\$ @Neil_UK: I'm really surprised that you say that there is no dependence on input gain. Is it actually just that changes in input gain are negligible, compared to changes in output power? (because power is additive, rather than multiplicative?) \$\endgroup\$ – naught101 Aug 4 '17 at 0:56

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