Home Hi-Fi Amplifiers How many watts do you really need?

How many watts do you really need?

How many watts do you really need?

“These speakers are 500 watts…” We still sometimes see it online. If that were really true, the energy crisis would be solved immediately, since then a speaker could generate power from nothing. What is meant by such specifications, however, is either the peak load a speaker can handle or the power a speaker needs to function properly: the nominal load capacity, so to speak. And that in turn says something about the amplifier you can combine…. Let’s take a look at that.

By now you know that amplifiers come in countless shapes and sizes. Preamplifiers – we are going to discuss those later – that control the volume, power amplifiers that power the speakers and an integrated model that incorporates both in one box: handy!

How much power?

We’re now going to talk about how much power you now need to decently play music. That’s less than you might think! But it does depend on a few factors.

Let’s say one thing: more power is not always better. What does make a difference, is stable power and some power in reserve. That is usually audible in control or grip on a speaker. But not every speaker will sound better as a result. Some speakers like it when they get more room to play. That’s partly design and partly taste of course…. So do keep that in mind and above all: listen and experiment for yourself.

Now let’s get to the point: how many watts do you really need?

That depends on the speaker and the room you are playing in. A speaker with an extremely high efficiency – say over 100dB / watt / meter – in a small room needs less power than a speaker with a very low efficiency: say 84dB / watt / meter.

This specification: efficiency, is crucial. It says how much sound pressure a speaker can generate at one meter distance with one watt input. So the speaker with 100 dB / watt / meter generates 100 dB of sound pressure at one meter distance with one watt of power. The other only 84 dB. that’s 16 dB difference! (And dB is a logarithmic scale!)

Impedance dips

Besides the efficiency of a loudspeaker – and the size of the room in which a loudspeaker stands – there is another important variable: impedance. And the electrical phase associated with it.

The impedance – or complex resistance – of a loudspeaker is crucial to its power output. As described earlier in the series on loudspeakers, a speaker forms an electrical circuit with the amplifier. It is, in effect, one entity. Current will flow only when there is a closed circuit AND there is a certain resistance. This resistance is, in this case, the impedance of the speaker.

This impedance is not constant! In fact, the impedance varies quite a bit. What we look at at Alpha Audio is mainly the nominal impedance and the minimum impedance. The peaks are not very important because an amplifier can handle them just fine. A tube amplifier may present some coloration, but a transistor amp doesn’t.

However, dips in impedance are annoying: then much more power is demanded and the stability of the amplifier plays a major role. Dips below 2 Ohms are an issue. Few amplifiers can handle them well.

Below three easy to drive speakers. There are however difficult models…

Now there are not many speakers that dip below 2 Ohms, but they are out there. For example, the Bowers & Wilkins 804 D2. That one dips to 1.3 Ohms. Right: 1.3 Ohms … that’s practically a short circuit. The 702 S2 is also pretty notorious, as it combines a 4 Ohm ‘dip’ with -48 degrees of phase rotation at this dip. As a result, although the impedance is not very low, an amplifier has to supply nearly infinite power due to the -48 degree phase. In short: these are not easy speakers to drive properly. There are more examples.

Power scaling

Let’s give an example of how an amplifier must scale to deliver the desired power at low impedances. Suppose a Class AB amplifier delivers 100 watts of power at 8 ohms. That needs to double to 200 watts at 4 Ohms. We see this is done with the better amplifiers. But at 2 Ohms, that should go to 400 watts if the power supply can scale all the way.

Few amplifiers are designed for this, because this means that for a 100 watt stereo power amplifier there should be at least an 800 VA power supply in it and the buffer should also be scaled. In many cases this is overkill and costs serious money. In short: many manufacturers choose not to do this. With the consequence that these amplifiers do not work well with difficult speakers.

Very loud … right?

100 watts into a speaker that has an efficiency of about 88dB results in huge sound pressure, right? Yes… that would come out to about 108dB. Not very pleasant.

Still, it is useful to have this “headroom”, since sometimes an amplifier does need that peak power to reproduce, say, a loud bass kick. Or a singer’s outburst. It may also be necessary to compensate for dips in impedance, as explained above.

But to be honest, it is not a prerequisite for a system to perform properly. And frankly, we at Alpha Audio never look at the power output of an amplifier. On a decent speaker with realistic efficiency and no nasty impedance dips, it’s not really necessary either.

For example: the 25 watt Pass Labs INT25A played fine on the Focal Sopra No1. And it did that in an acoustically damped 50m2 room. Although it couldn’t go ultra-loud; it played stable and extremely pleasant. In a regular living room, it would work just fine.

Rounding up

How much power do you really need? We say: less than you think… We will not – and cannot – give any values, because it depends on the speaker and the room you play in. But the fact is: almost nobody needs 1000 watts. A decently designed speaker placed in a medium-sized room has in many cases enough with a few watts. Add some peaks and with 50 watts of stable power you will often be fine.

Of course, there are exceptions. So always consult the manual and follow the advice of the manufacturer – and dealer! And also trust your own ears … does it sound good, then it’s probably just fine. Does it not sound good … then by all means try a few other combinations.