Matching speakers to an amplifier

You’d be forgiven for thinking that matching speakers to an amplifier is hardly rocket science. If you want to listen to music at a modest level in a limited space, you just combine small speakers with a compact amplifier. Conversely, if you want to play heavy metal or dance music at stadium-level volumes or you have a liking for Wagner operas played on the heroic scale, the only solution is a massive amplifier delivering hundreds of watts and the largest speakers you can get through the door of your listening room. Oh, and very understanding neighbours.

The problem is that, while either of those setups will deliver the desired results, this is a hit-and-miss approach to matching an amp to speakers: a little more thought will give you a better sound and will keep your options open for future system improvements. And that means delving deeper into how amps and speakers interact, and how you can optimise the way they work together.

What amplifier should I buy? Understanding speaker and amplifier compatibility

This guide will go light on the physics lessons, but will cover the significant factors, including speaker impedance and sensitivity, the real meaning of those power specifications and the physical limits of amplifier and speakers – and how they work together to produce the best possible sound. It’s not just a matter of matching up the numbers: it’s a little more complex than that.

Speaker impedance – and how less can mean you need more

At first glance, impedance seems to be simple: just about every speaker design quotes an impedance figure, commonly between 4 and 8 ohms, while amplifier specifications will give an output figure, such as 50 W into 8 ohms, and 80 W into 4 ohms. Easy, right? You choose a speaker with a lower impedance figure, and your amplifier will deliver more power, meaning the combination will go louder – except it’s not quite like that.

For a start, you’ll usually see the word ‘nominal’ in front of the speaker’s impedance figure, and sometimes a lower ‘minimum impedance’ figure. All speaker impedances vary with frequency, affected by the physical and electrical design of the speaker in question, and while some speaker designers and engineers work hard to keep the impedance as constant as possible while playing music, all designs do have a constantly changing impedance – it’s just that, in the best designs, the variation is kept as small as possible, meaning the speaker presents a relatively consistent load to the amplifier.

All the same, it’s easy to assume that a lower impedance will allow an amplifier to deliver more power, and that this is a good thing. Not so: a speaker with extreme dips in its impedance, or a very low nominal impedance, will place greater demands on the amplifier driving it. In very simple terms, it allows too much energy to flow from the amplifier, which the amp can struggle to deliver. To make a simple bathroom analogy, the water flows down the bath’s drain (the amp’s output section) much faster than the tap (the power supply) can replenish the bathwater, and eventually the bath runs dry… or the amplifier runs out of energy.

Conventional mains-powered amps use a transformer to convert the electricity from the wall socket followed by a bank of capacitors, which act as a reservoir for the energy to allow it to be delivered smoothly to the amplifier’s circuitry – in the same way as a water reservoir smooths out fluctuating demand.

If the demands of the speakers – and your volume setting – means the speakers are drawing power faster than the amplifier’s power supply can deliver it, then the amp’s output devices, which finally drive the speaker can be starved of power and can react by limiting their operation, literally clipping the peaks and troughs of the waveform being delivered. This is heard as distortion in the form of a hardening of the sound at high levels and can be an effective way of damaging your speakers – unless the amplifier’s protection circuits cut the output when they sense this is happening. For that reason, it’s best to be aware of the impedance range an amplifier is designed to handle and avoid pushing it too hard.

Speaker sensitivity

Every speaker on the market will quote a sensitivity figure, for example 86 dB/W/m. This indicates the sound pressure level the speaker will deliver for an input of one watt (or 2.83 V), when measured at a distance of one metre from the speaker. So, it’s tempting to jump to the conclusion that the higher the sensitivity, the louder the speaker will go for a given amplifier power – which, although theoretically true, is again not quite that simple.

True, a speaker with 89 dB/W/m will be louder for the same amp power – at least at that one metre distance – but there are factors at play. For example, you need a doubling of amplifier power to achieve a 3 dB gain in level, while a doubling of perceived loudness requires a 10dB increase in level, and thus 10x the amplifier power. What’s more, every time you double the distance from the speaker to your ear, you lose 6 dB. The result is that, at a typical listening distance, and with that nominal single watt of input, an 86 dB/W/m speaker will be delivering 68 dB, with the more sensitive design producing 71 dB. Given that normal conversation is around 60 dB, you may well consider either of those levels is more than sufficient.

Power handling – and what it means for your amp

If you think throwing ever greater amounts of amplifier power will enable you to reach stadium/nightclub levels of sound, it's time for another reality check: all speakers have their limits, which will be expressed as a ‘max SPL’ figure. Defined by the physical ability of the drive units in the speaker to shift air, this means that however much power you feed into the speaker, if it quotes a maximum SPL of 118 dB then that’s all you’re ever going to get (although the ability of a speaker such as this to play louder without distortion is also a factor here).

Mind you, that’s going to be sufficient for almost any listener: at that level, you’re well into the "jet taking off" zone, and you probably should be using ear defenders. Around 80-90 dB is a much more realistic "very loud" domestic listening level.

With a speaker rated at 88 dB/W/m sensitivity, the raw figures suggest that it will require around 1,000 W to reach that max SPL of 118 dB, at least at one metre, or about 88 dB at a more common 5 m listening distance. And yet use it with an amplifier with power output meters even vaguely accurate, rather than just a decorative element, and you’ll note that at common listening levels the needles are hardly moving, suggesting that only a few watts are required to fill the room with music – so how does this whole matching amp to speakers thing work?

Well, while only a few watts of continuous power will be needed to deliver music to speakers of reasonable sensitivity and generate levels from the comfortable to the truly exciting, music isn’t all at one level – it would be rather dull if it was – but rather has a dynamic range from the quietest moments to great loud crescendos. So much of that amplifier power is there to handle the dynamics of the music, from the big swings of level to the microdynamics of, say, a guitar string being struck and the sound then decaying, or the crack of a drummer’s rim-shot. This dynamic ability is what gives music its speed and attack and allows rhythms to be reproduced accurately.

So how much power do you need? Well, when looking at specifications, the important figure to note is a speaker’s minimum requirement, not the maximum. On paper, although a speaker like the DALI KORE may be able to handle up to 1,500 W, in reality it can handle even more power (provided you can find an amplifier able to deliver such output cleanly). However, the KORE’s minimum power requirement is just 50 W, which probably rules out most single-ended valve amplifiers that deliver very low power. But, it means you don’t need huge amplifiers to drive it. Mind you, it does get a lot more fun when driven by amplifiers with a healthier dose of power…

Why avoid very-low-powered amps? Well, if you ask the amp to drive the speaker at very high levels, you may get close to that clipping already discussed, which is neither good for the sound nor, ultimately, the speaker or amp. You’re much less likely to damage a speaker with an amp at, or even above, the quoted maximum for the speaker, but causing problems with too little power is unfortunately much easier to do.

Beware, too, amplifiers quoting their output as ‘dynamic power’ or ‘peak music power’: this practice, used as a selling tactic to allow tiny music centres to claim outputs of 1,000 W or more, is thankfully much less common now than it used to be. Such figures used to be achieved by quoting the output with much more distortion allowed in the signal from the amplifier (distortion isn’t good for speakers).

Think of it in car terms: while the maximum speed quoted for a vehicle may be impressive, you’re unlikely to drive at 150 mph (240 km/h) for extended periods, and doing so wouldn’t be too great for the car, its tyres or indeed fuel efficiency. Better by far to look at the acceleration and torque figures to get a better idea of usable performance.

Bigger speakers, bigger amps?

It would seem logical that the bigger your speakers, the more powerful the amp you need – after all, the amplifier will need to drive "more speaker". But in fact, the design of larger speakers means they tend to be more sensitive than smaller two-way bookshelf models, not least because larger drivers are able to move more air for a given input.

How to match speakers to amps – a checklist

• Look at the specifications of speakers and amplifiers: the amp should at least meet the minimum power requirements of the speakers at their nominal impedance.

• Look out for a minimum impedance figure on the speaker specifications – a speaker with a very reactive impedance, e.g. one with nominal 8 ohm impedance with a 2 ohm minimum (to give an extreme example) will present a much more demanding load to an amplifier.

• Check the amplifier specifications to see how it responds to different impedances: the ideal would be a doubling of power as impedance halves, indicating a "stiff" power supply able to cope with the changing load presented by the speaker.

• Bear in mind, too, that newer Class D amplifiers have become much more low-impedance stable, in other words they perform much better with speakers with demanding – or difficult – impedance characteristics.

• Think about how and where you want to enjoy your music: if you want to play gentle, relaxing music in a small space, compact mini-monitor-style speakers driven by a modest amplifier of 50 W or so will be more than adequate, whereas delivering realistic concert-hall levels in a huge room is best done with larger speakers and more amplifier power.

• Don’t go crazy with the amplifier power: achieving high power output from an amplifier while maintaining sound means more money is needed for better audio circuitry, especially in the power amplifier and power supply sections. You will probably do better with a more modest amplifier rather than chasing the wattage numbers, as modern speakers are designed to be amplifier-friendly and easier to drive.

• Price vs. performance: A rough guide is to invest 50% of the speaker cost on amplification: you’ll get better sound with an average amp driving good speakers, rather than going overboard with the amplification driving average speakers.

• Upgrade your speakers first, then choose amplification to suit them, rather than vice versa.

Stereo vs surround sound amps

In general, the guidelines for matching speakers with surround sound amps are much the same as when using stereo amplifiers, with a couple of caveats. One is that companies marketing surround amps and receivers tend to be a little more creative with their quoted power outputs, so an amp quoted at, say, 200 W per channel may do so on the improbable basis of having just one channel driven, while the actual figure with all channels working will be much lower.

After all, a mainstream stereo amp would have to work hard to achieve that kind of output, so it makes sense that delivering high outputs from a single transformer and power supply arrangement within a receiver offering seven or more channels of output is asking a lot of the design. Again, make sure the multichannel amplification at least meets the minimum power requirements of your speakers when all channels are being driven.

Remember, too, that if you’re the kind of home cinema enthusiast who lives for big, bombastic action movies, the chances are you’ll enjoy playing them at higher levels than you do music, so you should opt for a big, powerful amplifier and speakers with high sensitivity.

Using one or more subwoofers in such a system will take some pressure off the receiver’s amplifier stages: with active subwoofer bass, it’s possible to reduce the bass content going to your main speakers, meaning neither the receiver nor the speakers need to work so hard to handle all those room-shaking explosions.

As an aside, it’s worth considering a surround amp or receiver with pre-outs for all its channels, or at least the front left and right – that way you can always add more powerful amplifiers to those channels if required, or even run a separate stereo amplifier for your music connected to the main front speakers: many modern stereo amps offer a ‘home theatre bypass’ or similar for just this purpose. Connect your music sources to the stereo amp – this will also give you the purest signal path, then switch on the surround amplifier and engage the stereo amp’s bypass when you want to watch with surround sound: the stereo amp will then just be acting as the front channel power amplification.

Also look for a surround amp with the ability to bi-amp your speakers with unused channels of amplification: this won’t give you more power, but it will enhance clarity and definition. For more information, check out our guide to bi-amping your speakers.

Preamp/power amp or integrated amp?

While most consumers opt for an integrated amplifier – which combines the preamp section for input selection and volume control with the power amplification responsible for driving the speaker – many high-end enthusiasts choose separate preamplifier and power amplifier units.

The immediate advantage is that the delicate low-level signals in the preamp are kept away from any interference from the higher voltages in the power amp(s), and that each unit has its own power supply dedicated to its task; going forward, it’s easier to upgrade your amplification by adding bigger power amps, adding extra amp channels for bi-amplification, or even an improved preamp for more inputs, added functions or simply better sound quality. Look out for a more in-depth look at pre, power and integrated amplifiers in a future article.