Thursday, 7 April 2011

How much power does an amplifier need

Its obvious from posts one sees on various forums that most people have no idea how much amplifier power is needed to reproduce modern recordings properly and neither do they understand the relationship between Decibels and Watts.
This is understandable given the nature of magazine reviews and because so many advertisers amplifiers simply don’t have enough headroom. TVs for example have about 12 WPC and many expensive so called specialist hi fi products are still well below 100WPC.
Most speakers are around 87-89 dB/W/M and this represents a loudish listening level with pink or white noise, Heavy and Death Metal or simply a recording of a Black and Decker drill. Continuous sound appears much louder than music.
Music is very different and a typical continuous listening level is more likely to be 80 dB, which means 1/4 of a Watt! The problem is that this 1/4 Watt is interspersed with enormous peaks, some requiring hundreds of Watts and if the amp can’t produce them, it’ll clip, and ruin sound quality. 
Speakers are not a fixed load either, at low frequencies impedance rises to 30 odd Ohms and in the mid, where all the musical energy is, it falls to about 5 (assuming 8 Ohms nominal impedance) and as frequency rises, so does impedance and much more so as the crossover is approached.
This means that an amplifier must produce more Volts when impedance is high and more current when it’s low as well as continuous flow of peaks that may run into the hundreds of Watts for the music. 
The solution, where high quality sound is required, is an amplifier that is capable of several hundred Watts per channel as are our ADM9Ts.
In simple terms, if 80 dB is an average listening level, the system needs a good 25 dB extra in reserve for the peaks and to cope with the varying impedance of the load. If higher volume levels than normal are required, say 90 dB, then the amount for headroom must be added to that. It all sounds pretty simple until you realise that doubling amplifier only gives you an extra 3 dB, which is why so many Watts are required to properly reproduce modern recordings.
To Sum Up
A typical average requirement is about 80dB, at this level various music instruments including drums, pianos, even human voice produce instantaneous peaks that may require hundreds of Watts and if an amplifier can’t produce them, it will distort or clip trying. Therefore an amplifier should be able to produce at least 25 dB more than an average listening level to avoid problems. This means approximately 400 times the power or 100 Watts. However many enthusiasts and professionals require far more because they may like to listen at live or at least higher levels from time to time. 
A concert grand piano produces 110 dB at a metre, which equates to 400 Watts and a Snare or Tom Tom 139 dB! Think about it.
In our opinion 100 WPC should be considered a minimum power requirement for average domestic listening levels and 250 WPC for serious enthusiasts and professionals and it is important to understand that these power requirements are recommended to improve sound quality, not necessarily to increase sound pressure level.
Interestingly AVI measured music (as have Crown the US amplifier manufacturer) many years ago to establish these parameters, we even appeared at a major hi fi show with a special device that showed precisely the requirement and we demonstrated it to most hi fi journalists of the day. Although they were obviously surprised, none ever mentioned this in a magazine and to our knowledge, no reviewer has ever used a Scope to see if an amplifier is clipping when he’s listening to it. Lamentable but true.
We found huge variations in the dynamic range of music, one track in particular that didn’t sound very loud required peaks that equated to 600 instantaneous Watts when the average listening level was only slightly above normal and the continuous average power requirement 600 milliWatts. In our opinion Watts are misleading where decibels are not. The enormous peaks that we’re describing in decibels are actually volt/amp peaks that average out to a fraction of a Watt.