Amplifiers - how small?
The other question we are frequently asked is about the minimum recommended power for an amplifier. This question is a little more complex that the 'too big?' question as I'll try and explain here. When I say smaller amplifier or bigger amplifier I don't mean the size of the case, I mean only the power output rating of the amp. The case size is irrelevant: the circuitry may only occupy a small part of the inside. You cannot judge the power rating of an amplifier by the size of the case! Marketing people are smart enough to disguise small amplifier circuits in big cases! You have to check the specification. It will say something like "Power rating .... 75W into 8 ohms continuously per channel".
You ask: "If I look inside a small amplifier or a big amplifier what difference would I expect to see?"
My answer: The circuit may be exactly the same. But the bigger amplifier has more of a reservoir of power compared with the small amplifier.
You ask: "What do you mean that the bigger amplifier has 'more of a reservoir of power'?
My answer: I mean that the bigger amplifier has a bigger transformer and bigger PSU capacitors so that it can draw more power from the mains and store that inside the circuit ready to deliver to the speakers according to how you adjust the volume control.
You ask: "So if the speakers are not connected, or if the volume control is at zero, no power is being drawn from the reservoir so that It doesn't matter if the reservoir is small or large. Right?
My answer: Correct. The only (technical) advantage of a big amplifier is when you are playing loud. At low volumes, there are no (technical) advantages to having a big power amplifier. If you are not actually drawing from the power reservoir then it doesn't matter how big the reservoir actually is. It's like the reservoir from which you draw water for your home. If you don't turn on your water taps then it's irrelevant how much water there is in the reservoir - it could even by completely dry.
You ask: "How much power is needed to play music at home?"
My answer: It depends upon how you set the volume control, the type of music, and how loud you like to listen, and how good your hearing is.
You ask: "OK, but typically, how much power is needed to play at a moderate listening level?"
My answer: Surprisingly little. To make a medium-loud sound on classical music (or jazz) you only need about 5W -10W or so.
You ask: "Really? Only about 10W? But why would I need an amplifier of more power than 10W then?"
My answer: Because a 10W will give you no reserve power because it has no power reservoir within the case from which you can draw immediate power to handle loud peaks. It will have small capacitors to store the power and/or a small transformer and/or it will waste much power as useless heat. So the available power reserve for the speakers will be very small.
You ask: "So it's a good idea to have some reserve capacity available on demand for those sudden peaks in the music, or where I chose to briefly play an exciting musical piece louder than normal?"
My answer: Yes it it always a good idea to have some reserve power. But how much? If 10W will fill a room with sound, then 50W or 100W should give plenty or reserve power. Anything more than about 100W is just a luxury - a power reserve that you will never dare command from the amplifier.
You ask: "Never dare? What do you mean?
My answer: I meant that even though you have a massive reserve available in a big amp, commanded by your operation of the volume control, you have to ask yourself can the speaker handle so much power?
You ask: "But I am always careful and responsible when adjusting the volume. I have neighbours and they will not tolerate too much volume from my hi-fi system. So I will never strain the speakers."
My answer: That's good. No matter how big the amplifier, if you are careful then you won't strain the speakers.
You ask: We're talking here about how little power I need. You said 10W is typically all that's necessary to make a reasonably loud sound. Is there any advantage to using a small amplifier? Maybe I should buy a small amp? Or a big one? I can't decide.
My answer: As I've said, let's pick a minimum power rating of (say) 25W. It could be 15W or 30W, something in that range would provide some reserve for louder than normal use. At the other end, perhaps about 150W represents all the power you would ever sensibly need before you risk damaging your hearing and if you are careless with setting the volume, your speakers too.
You ask: "So if I pick an amplifier in the 20W - 150W range I should be able to make great music at home?"
My answer: Yes; the lower powered amplifier will fill the room with sound but will not have much reserve power. The 150W amplifier will have a lot of reserve which you must command carefully. You must respect the fact that the speakers can't handle 150W continuously.
To be continued ...
Alan A. Shaw
Harbeth Audio UK