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How Much Amplifier Power Do I Really Need?




- How much Power can my speakers handle? -

Check your speaker's Specifications for Nominal Impedance; it will be 2, 4, 8 or 16 Ohms (usually "8 Ohms" and not accurate; most speakers are closer to 4 Ohms than 8, regardless of what the manufacturer claims; get the Impedance Modulus, and you'll know for sure). Next, look for the Continuous Power Handling, Continuous Power Rating, IEC Power Rating, or Power Capacity.

Suppose the impedance of your speaker is 4 Ohms, and its Continuous Power Handling is 100 W. If you're playing light dance music, the amplifier's 4 Ohm power should be 1.6 x 100 W or 160 W continuous per channel. To handle heavy metal/grunge, the amplifier's 4 Ohm power should be 2.5 x 100W or 250W continuous (RMS) per channel. If you use much more power, you are likely to damage the speaker by forcing the speaker cone to its limits. If you use much less power, you may damage the speaker (and amplifier) from clipping the amplifier (an unstable condition, which neither the amplifier or speaker was designed for). The Rule-Of-Thumb is 1.6 to 2.5 times the speaker's continuous power rating. Keep in mind, though, that more speakers are damaged by too little power rather than too much. If you hear your amplifier distorting, turn the volume down immediately to avoid damage.




- Power vs Application -

Loudspeakers with High Sensitivity need less power than loudspeakers with Low Sensitivity.




- Calculator -

AllegroPowerCalculator determines the amplifier power required to achieve the desired SPL at the listening distance.

To use
AllegroPowerCalculator , you need to know the loudspeaker Sensitivity, listener Distance in Meters (1 meter = 40 inches) measured from your nose to the front of the speaker, and the desired Sound Pressure Level (SPL). For Classical music, 85 db be a reasonable average level, with 100 db reserved for the loudest peaks.



- Sensitivity -

The Sensitivity spec can be found in the loudspeaker's Data Sheet (typically 3-4 db optimistic). Bigger speakers generally have higher sensitivity than smaller speakers, and high-frequency drivers generally have higher sensitivity than low-frequency drivers.




- Peak Headroom -

Because music has transient peaks that are 6 to 25 db above the average level, the power amplifier needs to produce enough power to handle those peaks without distortion.

For example, if you need 100 Watts continuous power to achieve the desired average SPL, you need 1000 Watts to handle 10 db peaks, 3000 Watts to handle 15 db peaks, and 10,000 Watts to handle 20 db peaks. (flea-power SET-lovers take note).




- Listener Distance from Speaker -

This be the distance from the front of the speaker to your ears measured in Meters (1 meter = 40 in). If you must guess at this measurement, guess high, not low.




- Desired SPL -

Listed below are typical sound pressure levels (SPLs) for various types of music measured with meter set to C-weighting and Slow Response. You might want your system to be at least 10 db above the background noise level to achieve a good signal-to-noise ratio.

New Age:  60-70 db.
Folk:  75-90 db.
Jazz:  80-95 db.
Pop:  90-95 db.
Classical:  75-100 db.
Rock:  95-110 db.
Heavy Metal:  110 db.




- Considerations -

The calculations used here apply to anechoic (or outdoor) measurements. When speakers are placed in a room, the room reverberation will increase the SPL by 6 db or so.

Suppose you need to supply 200 Watts for peaks, and your speaker's Continuous Power Handling is 50 Watts. A speaker's Peak Power Handling is typically 4 times its Continuous Power Handling, so the speaker can probably handle 200 Watts peak. That means you can use a 200 Watt amplifier to drive that speaker, as long as you use that power for peaks and not continuously.

If your system uses an active crossover and a separate amplifier per channel for each driver, then apply the
AllegroPowerCalculator to each driver. If you have a typical 2-way system, determine the power separately for the low-frequency driver(s) and high-frequency driver(s), which should produce the same SPL at the same distance. Horn-loaded drivers tend to have way higher sensitivity than subwoofers, so the horns need far less power (typically 1 Watt) to produce the same SPL as the subs (150-1000 Watts).



*AllegroPowerCalculator


Measured Distance from Listener to Speaker
Meters
Desired Sound Pressure at Listener
db SPL
Speaker Sensitivity Rating (1w/1m) + 5db
db
Desired Amplifier Headroom (3 db min)
db


Amplifier POWER Needed
Watts


AllegroPowerCalculator provides the required electrical Power (Watts) from the amplifier to produce a desired Sound Pressure Level (SPL) at a given distance (in Meters) with an amount of Headroom to keep the amplifier out of clipping.

EXAMPLE:
The distance from your ears to each spkr is 3.5 Meters.  The highest Sound Pressure Level measures 100 db SPL.  The speaker has a Sensitivity "rating" of 87 db.  Taking into account typical Ratings (-), Room Gain (+) and 2 spkrs (+),
ADD 5 db to the Sensitivity ( 87 db + 5 db = 92 db ).  With recommended amplifier Headroom of 4 db, you should choose a quality amplifier that can deliver 200 Watts to each of the two speakers.



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AllegroSound
−  Established 1973  −
"love what you listen with"
Los Angeles, California USA
Rick@AllegroSound.com

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AllegroSound
−  Established 1973  −
"love what you listen with"
Los Angeles, California USA
Rick@AllegroSound.com
telephone 323.960.5014

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