How much power do you need? Well as many of you know I have been looking for a long time for a new amplifier. I have posted questions both here and elsewhere, as well as quizzing many shops who are trying to sell me the amplifiers. What has become apparent is how few people know how much power is needed. I am fed up with people telling me I should buy a Rotel as it has good current capability. To be brutally frank these people don't understand how amplifiers work or more to the point how the amplifier / speaker interactions work. So I decided to write on how to work out how you pick 'what you need' and then you can compare it with what you have. Many may then be surprized just how far 'from the mark' they are. This is the main reason for the differences people hear between amplifiers as distortions rise with output power as the amplifier approaches it's maximum output. I am not really talking about continuous output here as this is relatively low but more the very large peaks (that get distorted if you have too little power). If people want to see more info on this I can add it a later stage. So what matters: 1 Amplifier output (in Watts) 2 Speaker sensitivity (in dB for 1W input at 1 m) 3 The room or more importantly how far you sit from the speaker? 4 Do you use equilization? 5 Do you use a subwoofer? 6 How loud do you listen to music? Not once have I been asked 3 - 6 in the shop, and only about one time in two am I asked about point 2! A watt is a watt. This may sound daft but a watt from a SE Triode amplifier is EXACTLY THE SAME as a watt from a Krell or a Bryston or a Tag (substitute your favourite here). The ability to drive a low impedance may be different for valves and transistors but it not down to power per se but more the stiffness of the power supply used. Dont forget sensitive speakers are not always easy to drive. Kef Q series have always been real buggers to drive well due to low impedance swings in the bass region. Speakers are just 'dumb' devices and don't know what they are doing. Give them a watt and they produce an acoustic output according to their sensitivity but it is hard to correlate amplifier power in watts with acoustic output. We need to specify amplifier output in dBW so look at this table for a few common ones (and the odd extreme) 10W 10dBW 20W 13dBW 50W 17dBW 100W 20dBW 200W 23dBW 250W 24dBW 400W 26dBW 1000W 30dBW every doubling of power gives another 3 dB of acoustic output. Right for the important bit. A decent system should be able to produce 100dB of sound, an enthusiasts system really should be looking at 105dB and the anal retentive should aim for 110 dB )). This will allow any music to be reproduced comfortably. 9 This is why nominally the same quality amplifies (say Bryston SST range) sound different (better) with the increase in power. The amplifier is actually the same quality but it is not driven into the peak clipping range. Look inside the 120w, 300w and 600w Brystons and you will see the same quality components, only the power is different. Right, do you remember loudspeaker sensitivity being rate at 1m distance? Well I suspect that few of you will listen at more than 1m from your speakers and you therefore have to correct for this. In theory there is a 6 dB loss for each additional 1m from the speaker. In practice 5 dB is a better figure to use (as we dont live in anaechoic chambers). Right armed with all this information it is easy to calculate the peak capability of a system. Take the loudspeaker efficiency, add the dBW figure for the amplifier and deduce the additional loss from a more distant listening position. So for Speaker 87 dB/ 1m 100w amplifier (20dBW) 3m from speaker ((3m 1m ) * 5 dB =10dB we have 87 + 20 10 = 97 dB actually well below my minimum (WAF) 100dB specification quoted earlier and miles away from the anal retentive 110dB despite the fact we are talking some very ordinary everyday figures here. So in my case with 92dB speakers, at 3 m listening, wanting 110dB peak I need an amplifier of 110 92 + (2 *5) = 28dBW This equates to just over 500w, say 600w approx. Now my speakers can just about handle this sort of power but most cant. I also have one of the more efficient speakers available, for most people, this will be MUCH worse (ie even more power) and now you know why amplifiers sound different. A few things to consider. Using a sub woofer will greatly ease the situation as most of the power is required at low frequencies. Equilization may or may not effect power requirements as well. The louder you listen to music, the potentially the worse it sounds Finally if you bi amp your speaker you may be able to gain an additional 4 dB head room so this may actually be VERY beneficial, perhaps even more so than a nominally bigger amp? So the bottom line is I think a 600w amp is what people really should be aiming at for normal speakers / room. This is wide of the current mark. Why are we so wrong? Still so sure your amp is fine at driving your speaker?