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Suppose we have a powerful sound amplifier, which is set at 90% of it's power, and nothing is being driven into its inputs.

Is it using more power at this high volume (and producing heat, etc.)?

Is it bad to an leave amplifier at a high volume even if nothing is playing?

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    \$\begingroup\$ Depends on the class of amplifier.Class A yes, others no. \$\endgroup\$
    – user16324
    Dec 5, 2014 at 22:10
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    \$\begingroup\$ The volume control doesn't 'set the power'. The output power available is always the same, determined only by the rail voltage, and the OPT if there is one, or SOA protection circuitry if that exists. The volume control just determines how much signal goes in. \$\endgroup\$
    – user207421
    Dec 5, 2014 at 22:56
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    \$\begingroup\$ The title of the question is ambiguous. Would not be an unloaded amplifier, one that that had no load, in that it is not connected to speakers. Rather than as the question states an amplifier without an input connected. \$\endgroup\$
    – Dan D.
    Dec 6, 2014 at 8:11
  • \$\begingroup\$ If you're referring to load correctly, i.e. a speaker, most valve amplifiers shoudl not be operated without one. \$\endgroup\$
    – user207421
    Dec 6, 2014 at 22:19

4 Answers 4

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It is using power and producing heat, but probably not as much as if it was driving the outputs to full power. The amount of power will depend on the type of amplifier. A Class D amplifier may use very little power when quiescent, a Class A amplifier will use about as much power idling as when it is producing full volume output. Most consumer electronics will use Class D or Class AB amplifier for the output stage, so it won't use much power compared to when it's driving speakers to full volume. Class A for a power amplifier is mostly the province of audiophiles and audiophools.

I don't think it typically makes much difference where you leave the volume control- that's an attenuator that's back of where the heavy lifting gets done. I guess if you're plugging and unplugging input cables you might damage your speakers from the loud hum and buzz that occurs if the volume is cranked up to 11.

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The attenuator, or "volume control" on a power amp is just that: an attenuator. It goes before the amp itself, which has a fixed voltage gain. So there's no difference at all to the amp if you drive it with 1Vrms and -6dB of attenuation or 0.5Vrms and 0dB of attenuation. The amp itself gets 0.5Vrms in both cases and so that's what gets amplified by the amount fixed by the manufacturer.

Output power is then determined by the speakers' impedance and the actual output voltage, which is why the spec is higher at 4 ohms than at 8 ohms. Watts = Volts^2 / Ohms. You don't actually get twice the max. power with half the impedance because the amp's internal power supply will sag a bit (the Volts^2 part). This doesn't affect the sound, just reduces your headroom compared to the ideal math as reflected in the spec.

When I calibrate a sound system, I set all of the amps and whatever processing is between them and the sound board so that the entire chain from the board to each amp clips at the same time as the board is turned up. There's no point in going louder than that because something will clip at that level anyway and the board will still say it's okay. If you have multiple amps that work together, calibrate them all individually as above, then turn all but one down for balance. This gives me maximum volume with minimum noise compared to that volume, and it lets the board tell me when the amps are about to clip.

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It will use marginally more power because any pre-attenuator noise will be amplified. Marginal being the operative word.

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"Is it bad to an leave amplifier at a high volume even if nothing is playing?"

I do this all the time and have never had any problems - as long as any source you have plugged into the amp is muted or turned down, ie 'earthed'.

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