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Is volume knob position proportional to power output?

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Old October 6th 06, 12:06 PM posted to rec.audio.tech,rec.audio.opinion,rec.audio.misc
John Phillips
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Default Is volume knob position proportional to power output?

On 2006-10-05, eponymous cowherd > wrote:
> I am thinking about buying a new amp and would like to know how much
> power I need. My current amp is rated at 25 watts and I never put the
> volume knob past the 1/3 point and I never have problems playing music
> LOUD enough. Is the power usage proportional to the volume knob
> position, am I always using less than 12 watts?

Depending on what you want, a better way forward may be to:

- select the amplifier on its ability to drive your speakers; and

- adjust each source input level (if possible, if necessary) to produce
similar loudness from all sources at the same, comfortable setting of
the volume control.

Selecting the amplifier - speaker power handling.

In general if your speakers are rated as handling between X and Y Watts
then that is a good indication of the sort of power amplifier you need.
For example mine are rated 20 - 180 Watts power handling. I use a 140
Watt amplifier (I would normally use something close to the top end of
the range or even a little higher - others may not agree).

Selecting the amplifier - speaker sensitivity and loudness.

You can also look at the sensitivity of the speakers (sensitivity is
rated at X dB SPL at 1 metre for 1 nominal Watt, or X dB/1m/2.83Vrms
which is the same).

If your speakers are sensitive they can probably be driven to a loud
enough level by a 12 watt amplifier to handle the desired *peak* volume
without the amplifier clipping. Peak levels can be well above average
sound levels in classical music - less so in modern popular music these
days. Clipping is to be avoided if possible (in most cases, anyway).

Specifically, sensitive speakers (say 94 dB/1m/1W) may well be fine with
a 12 Watt amplifier. That combination can (in theory) produce about
105 dB SPL peaks and that's about as loud as being in the front row in
an orchestral concert. It isn't as loud as being in the front row in
a rock concert. But your neighbours might complain at some level.

Insensitive speakers (say 84 dB/1m/1W rating) would produce just 95
dB SPL *peak* (not average) from a 12 W amplifier before clipping.
That would be enough for quiet *average* level listening but no more.
A 40 Watt amplfier would be needed to get to 100 dB SPL peak in this case.
That's much better. I would personally not regard 40 W as having enough
margin in this case but that's a personal choice.

My speakers are in between at 88 dB/1m/1W. A 140 W amplifier will not
clip until the sound level is over 109 dB SPL (in theory). That's VERY
LOUD (in the home, anyway) but it gives me a lot of margin at normal
listening levels so I don't think my amplifier will ever go into

Selecting the amplifier - speaker impedance.

Regardless of the above you may need to have a "better" amplifier of any
power if the speaker impedance is low (say 4 ohm rated) instead of high
(say 8 ohm rated). That's a matter of seeing if the amplifier is rated
to drive the impedance of your particular speakers.

Even so some 4 ohm speakers are easier to drive than other 4-ohm speakers
(ditto for all other impedances) so it's a matter of getting specific
advice and/or of listening to the specific amp-speaker combination.
(FYI, my speakers are 8 ohms and a fairly easy 8 ohms at that.)

John Phillips

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