150W Krell - and why we specify minimum ratings
150W available to drive M40.1 and a first class amplifier brand reputation says to me that this is home setup is likely to deliver a great sonic experience with oodles of power reserve for those thrilling climaxes.
Originally Posted by Heretic
Consider this imperfect analogy: the (incandescent) light bulb lighting your room may well have a 150W rating. That will fill every corner of your room with bright light. Remove that bulb and fit a 15W (one tenth of the power) and what then? The room is miserably dark: there are deep shadows in the corners. You cannot even read. You are stumbling around in the gloom. Why would you want to live like that when you are advised that 50W is necessary to throw the minimum light into the environment?
I don't just dream-up the minimum amplifier requirements for the fun of it. I am aware that if I suggest a minimum amplifier power rating that's too high, those customers with smaller amps may be frightened off Harbeth and select another brand which emphasises higher efficiency. Conversely, I am aware that to produce a reasonable sound pressure level that gives a satisfying quasi-real life experience a certain amount of power is necessary. So the minimum power amp ratings I've listed in the brochure are really minimums bearing in mind that you should interpret the manufacturer's amplifier power outputs with a very large degree of scepticism. Outside the lab, many amps are unlikely to be able to deliver their rated power on music when driving real-world speakers.
What tube fans don't realise (and I've heard SHL5s driven by 300B 7W tubes and sounding good) is that when you connect a 'scope across the 7W amp's output terminals and look at the music waveform, it shows significant clipping. And that clipping may be present for 25%, 50% or even 100% of the time. That shouldn't be a surprise at all: 7W is a minuscule amount of power. So how is it that those users are satisfied with what they hear? Simple - the tube amp (and I stress this, the situation with a transistor amp is totally different) clips is a sonically benign way. In other words, it runs out of power in a way which does not immediately reveal itself to the human ear. It doesn't sound hard or coarse when clipping: it sounds fat and warm. And that's all part of the magic sonic experience of small (tube) amps.
So - rule of thumb: give yourself a fighting chance of high fidelity sound at home: have a sensible amount of amplifier power available at all times.
Alan A. Shaw
Harbeth Audio UK