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How Much Power Is Enough?

Paul Wilson looks at the relationship of the power requirements of an amplifier to a speaker…

For those who may remember the TV show “Home Improvement,” Tim Allen, the title character, loved “more power.” In fact, it was a central theme to the show. Tim souped up everything, from dishwashers to lawnmowers, to, you just about name it. More power was always welcomed. Many audiophiles feel the same way. 

AR-PowerSmallScaleVersion.jpgOne of the more notable differences between high performance audio and mid fi audio is the amount of power available to reproduce music. A mid fi, 7-channel, 700 watt receiver (100 watts per channel or “WPC,” at 100% rated output being totally unrealistic – 20 to 30 WPC is more likely) simply lacks the “oomph” to do much in the way of accurately recreating the recording of dynamic range, or the difference in decibel (dB) output between the softest and loudest sounds. The relationship of how far past zero the volume knob is turned and how loud music out of the speakers sounds is not a one to one ratio. As most audiophiles certainly know, each increase in amplitude of 3dB requires the amplifier wattage to double. Depending on the power capability of the amp and efficiency of the speakers, this may possibly make for some interesting listening sessions. In fact, it is the primary reason why it is so important to have a proper amp / speaker match – because one is highly dependent upon the other. 

When I was considering buying the KEF Blades, I read a review where the reviewer started out driving them with a 200WPC stereo amp and noted the highs didn’t sound right. In fact, he thought what he was hearing was clipping. When he changed to a set of 560 watt mono blocks, the issue was suddenly resolved. On my Blades, I drive them with a 400 WPC  at 4Wstereo amp with an intermittent ceiling of 500 WPC and I have never had any feeling like I needed to “soup” things up at all. Would a 12 watt SET tube amp perform the same? I cannot say as I have never tried such an amp in my system but my inclination is that combination of amp to speaker would be less than ideal – particularly in those instances where large scale dynamics were required. 

So how much power is actually needed in the average system? 

AR-DynamicRange.jpgNot surprisingly, this is a difficult question to answer. There are many variables involved in the determination of matching an amplifier to a set of speakers. For instance, what type of speakers are being used – small bookshelf or large floor standing? What is the speaker design – dynamic, planar, electrostatic or something else? What is the impedance rating of the speaker, 8Wor 4W? What is the damping factor of the amp? Damping factor being the ability of an amp to control the movement of the drivers in the speakers -especially important in dynamic speakers where the drivers are pistonic and the forward and backward movement of the drivers are moving air and producing sound. There are also separate conditions for electrostatic and planar speakers, each having their own unique list of “do’s and don’ts.” 

When matching the amp to the speaker it is also a useful idea to think ahead. If small bookshelf speakers are currently being used and large floorstanding speakers are planned, then matching the amp to the floorstanding speakers might be best. Just be sure to not overdrive the bookshelf speakers and blow a driver or crossover network. 

AR-Clipping.jpgHaving too little power can be as problematic as having too much power. It can cause amps to overload, or go into “clipping” and may be very harmful to both the amp and speakers. And the actual amount of power is also, at least somewhat, guided by how loud the listener likes to play music. A listening level of 70 dB taxes an amp far less than a listening level of 95dB – not to mention being far better for your long term hearing. 

Driving a set of speakers with a lower sensitivity, say 88dB at 1W would need 2W to produce 91dB, 4W to produce 94dB and for each additional amplitude increase of 3 dB, the wattage must again double. This rating is generally based on an output measured at 1 meter and does not take dynamic peaks into consideration. To accurately recreate the musical power of a wide swing in dynamic range, like a flute solo to a full symphony for instance, a sudden input of more power will probably be required. For speakers with higher sensitivity, perhaps 94dB, less power is required. The difference is that to produce a quick musical peak of 109dB, the speaker with the lower sensitivity will need 128W of power, while the speaker rated at 94dB would only need 32W to produce the same swing in decibel output – again, measured at 1 meter. 

AR-Impedance.jpgAnother critical factor is the operational impedance of the amp. Amp impedance is the resistance the speakers create for the amp to accurately provide sufficient power. Generally speaking, as the load, or impedance of the speakers is cut in half, the output power of the amp usually doubles. So an amp rated at 200W @ 8 Ohms will double to 400W @ 4 Ohms. Check the amplifier manufacturer’s technical information to ensure this is the case with the amp being considered or currently in use. A set of speakers rated at 4 Ohms may require an amplifier with a higher output power rating to ensure musical peaks are faithfully recreated. To be sure this actually applies, it is always best to demo the amp / speaker combination in question to ensure sonic quality. 

Amplifiers and speakers. Probably the most important match in an audio system. While this article barely scratches the surface, it is important to remember the amp and speakers are linked together by more than a cable. They are interdependent upon each other and the success of any system effectively starts where the signal chain ends – the amp and the speakers. 

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