Matching Amplifier To Speakers Guide


Basically: Amplifiers (often also called amps) amplify an audio signal so that the desired level is emitted from the loudspeakers. The audio signal usually comes from a mixing console in which it has been pre-processed or from an input device such as a CD player or computer.

An amplifier converts power into an audio signal. The audio signal used for control should only be larger but not changed otherwise.

The reason for this is the complexity of the loudspeaker. A loudspeaker (similar to a dynamic microphone) consists of a magnet, voice coil and membrane. The voice coil is mounted in the magnetic field and is deflected when the current of the amplifier flows through it. The diaphragm, which is firmly connected to the voice coil, then moves the air and generates the sound pressure. Due to the weights of diaphragm and coil and the electrical resistance of the voice coil, a lot of power is needed to generate high sound pressures. In addition, the mass inertia and interactions of the loudspeaker with the air ensure that the impedance (= frequency-dependent resistance) of the loudspeaker is not constant over the frequency range.

The nominal impedance of a loudspeaker is typically 4 or 8 ohms. At 8 ohm impedance (R), an amplifier must generate a voltage (U) of 8 volts for a current (I) to flow from one ampere (U=R*I), which corresponds to a power (P) of 8 watts (P=U*I).

Due to the above mentioned effects the impedance is unfortunately not constant, it can range between 4 – 20 Ohm with an 8 Ohm loudspeaker. Amplifiers must therefore be equipped with ample reserves to cope with these fluctuations.

Output power of an amplifier

The output power of the amplifier determines, among other things, the achievable volume. The sensitivity of the loudspeaker used, expressed in dB/watt at a distance of 1 meter, also plays an important role, but more about this elsewhere. An amplifier with 200 W at an 8 Ohm loudspeaker sends 40 Volt voltage to the loudspeaker, resulting in an output current of 5 Ampere (I = U/R). Conversely, 40 volts multiplied by 5 amps produce the output power of 200 watts (P = U*I).

If we now want to double the deflection of the diaphragm, we also have to double the current, i.e. from 5 to 10 amps. Since the impedance of the loudspeaker is still 8 ohms, we have to apply double the voltage, i.e. 80 volts. If we now calculate the output power of the amplifier, we come to 80 V * 10 A = 800 W, a quadruplication! It is therefore not surprising that large public address systems require amplifiers, some of which have an output of several kW.

Power limits of an amplifier

The power that an amplifier can deliver is limited. For example, the voltage that an amplifier can deliver can be as high as the voltage that the power supply can deliver. If an attempt is made to exceed this power, the signal is cut off (where amplifiers nowadays have protection circuits that prevent it from being exceeded). This so-called “clipping” leads to the typically distorted sound of an overdriven power amplifier. Power supplies with higher voltages and powers are technically no problem, but increase the costs and weight of the power amplifiers. At some point, the maximum power, which can e.g. be delivered by a socket (230V/16A), comes into play.

The minimum permissible impedance is another important parameter of the amplifier. It should be less than or equal to the impedance of the connected speakers. The lower the impedance of the connected loudspeakers, the higher the current and therefore the power that has to be supplied by the amplifier. You should therefore pay special attention when connecting several loudspeakers to one amplifier channel. If the loudspeakers are connected in parallel, the impedance is considerably reduced. Two 8 Ohm loudspeakers in parallel correspond to 4 Ohm impedance, 4x 8 Ohm in parallel correspond to 2 Ohm total impedance, which is often the limit that an amplifier can achieve. As we already know, the impedance of an 8 Ohm loudspeaker can vary between 4 and 20 Ohm depending on the frequency, 4x 4 Ohm in parallel then correspond to 1 Ohm total impedance! If the total impedance is too low, too much current flows in the output and the amplifier overheats and (hopefully) switches off.


It is common to think that the more you power, the more volume. Watt (W) refers more to what a loudspeaker can withstand and how much power an amplifier is able to deliver. Power is usually indicated in amplifier specifications as continuous output power or RMS and dynamic power (or peak power).

The most important thing to keep in mind is the continuous power. This one tells us how powerful an amplifier is. For example, “50 W in continuous power at 8 ohms” means that the amp gives 50 W to an 8 ohms loudspeaker. The dynamic power or peak power measures the maximum power that the amplifier can take out in some circumstances (which usually last milliseconds) when a song requires it.

You’ll find that some loudspeaker manufacturers simply indicate the recommended power within a range and specify neither continuous nor dynamic. In these cases see that the continuous power of the amplifier falls within the recommended power range of the box.

Altavoces de columna


This parameter is only found in the specifications of one loudspeaker. It means what volume level you hear one meter away from the loudspeaker when you amplify it with 1 W of power.

For example, a loudspeaker with a sensitivity of 88dB will give a volume just at that level if you are one metre away from the loudspeaker and amplify it with 1W.

To give you an idea of what decibels measure (db), a normal conversation is usually between 30 and 50 dB, a vacuum cleaner about 65 dB, a street with a lot of traffic can reach 75 dB, a police siren, 90 dB, and within a nightclub we can reach 110 dB.

This doesn’t really tell you anything, but it’s a way of indicating how loud a speaker sounds. At the same distance and at the same amplification power, a loudspeaker with low sensitivity (e.g. 85 dB) will sound lower than a more sensitive loudspeaker (e.g. 88 dB) in the same room and configuration. What you have to keep in mind is that a loudspeaker, the more sensitive it is, the less amplification power you need to make it sound like the volume you like.

For practical purposes, we can divide the sensitivity of a loudspeaker into three ranges:

  • Low Sensitivity: Less than 85 dB.
  • Normal sensitivity: 85 dB to 88 dB.
  • Very sensitive: More than 88 dB.

When you have a loudspeaker that is not very sensitive, the power of the amplifier should be close to the maximum recommended power of the loudspeaker. If you put an amplifier whose power does not exceed much of the recommended minimum, you will always have to have the amplifier at a very high volume to sound.

On the other hand, if you have a very sensitive speaker, you can connect it to an amplifier whose continuous power is close to the recommended minimum. It doesn’t matter if you put in more amplification as long as it’s within the recommended range, but you’ll always have to work with the amplifier at a low volume or you won’t be able to withstand as much sound pressure.

Another thing you might want to know about sensitivity: You have to double the amplification power to increase the sound pressure level (SPL) of a loudspeaker. For example, a loudspeaker with 88dB sensitivity needs 1 W to give that dB at 1 meter distance. Well, 2 W would give 91 dB and 4W, 94dB. And the volume drops 6dB every time you double the listening distance to the speaker.

Sensibilidad de altavoz

Example: Find a set of amplifier speakers that go well together.

Imagine you want to pair a Bowers & Wilkins 707 S2 with the integrated Rotel A12 amplifier.

We looked at the B&W specifications:

  • Impedance: 4 to 8 ohms.
  • Sensitivity: 84 dB.
  • Recommended power: Between 30 and 100W continuous at 8 ohms.

Then the Rotel:

  • Impedance: Minimum 4 ohms.
  • Power per channel: 60 W at 8 ohms.

Let’s look at the impedances. The speakers work from 4 to 8 ohms and the amplifier needs speakers with minimum 4 ohms. OK. [CHUCKLES]

We classify the boxes by their sensitivity: 84 dB. They’re not very sensitive.

We see what power they recommend to move the speakers: between 30 and 100 W.

As the boxes are not very sensitive, we need an amplifier that is close to the maximum of the recommended power. The Rotel outputs 60 W, and 60 is slightly less than half the range 30 – 100. It would not be a recommended amplifier.

We would need an amplifier between 80 to 100 W continuous, for example the Rotel A14.

Rotel y Bowers & WIlkins

How do I know which is the ideal pair of amplifier – speakers?

There is no perfect synergy between amplifier and speakers. It is true that, over the years, and by listening, one can point out ideal couples. If you’re looking for an amplifier for your enclosure, speakers for your amp, or both, don’t worry too much.

The important thing, as in everything, is to choose a good product that fits the impedances and powers as we explained above. If so, the team will sound good. Another thing is that you like one sound more than another… but it will always be subjective.

One recommendation we can give you is to choose amplifiers and speakers from the same brand or group of companies. They are usually very well studied and sometimes you can even buy them in packs.

But there will always be other factors that will affect the final sound: the size of your room, the size of the speakers, their placement, etc.

Parte trasera de un amplificador integrado


You’ll see the impedance in both the amplifier and speaker specifications. It measures the electrical resistance of your components and is measured in ohms (Ω) and is an important part of determining the synergy between your speakers and your amp.

The loudspeakers are normally between 4 and 8 ohms and the amplifiers normally give a range of between 4 and 16 ohms. If the impedance of your speakers falls within the amplifier’s range, we’re doing fine.

Please note, however, that in the specifications of the amplifiers indicate the output power according to impedance. For example, an amplifier can give 80 W per channel in 8 ohms and 4 ohms, but power peaks of 80 W in 8 ohms and 150 W in 4 ohms.

Higher impedance speakers can usually be connected to an amp but not the other way around. For example, don’t connect 4 ohm speakers to an 8 ohm amplifier. Anyway, the most common thing nowadays is to find amplifiers and loudspeakers with a wide range of impedances, so what we’ve just told you doesn’t usually need to be looked at very much. If one of your components is quite old, you might have to look at it.

There is a very important parameter when connecting loudspeakers to an amplifier, which is called “impedance“, and which is measured in Ohms.

It could be said that impedance is the resistance provided by an electrical audio signal when passing to a particular device (speaker) from an audio source.

We must always choose the minimum impedance values supported by our audio source (amplifier), as this way the signal would arrive with more force. There must always be a match between the impedance of the amplifier and the impedance of the speakers. The higher the ohms, the lower the output power of the speakers.

If the impedance of the speakers you put in the example is from 4Ω, 6Ω and 8Ω and the amplifier only sends from 6Ω, that (6Ω) should be the correct impedance.

The basic idea is that the loudspeakers that we connect to the amplifier must be within the Ohms range that the amplifier supports. If what we connect “has less Ohms”, we can charge our amplifier, especially if it is a tube amplifier.

Generally amplifiers need to have connected speakers that have an impedance of at least 4 Ohm (this is a reference value, which depends on each amp). The range of impedances allowed is usually indicated on the back, where the cables connecting the amplifier and loudspeaker are located. Several outputs may appear for 4, 8 or 16 Ohm (these are the most common values), or a single output indicating the allowed impedances.

The value indicated on the back of the amplifier is the total impedance of what we connect. This means that we can connect more than one loudspeaker, as long as we respect the indicated values.

When more than one speaker is connected, the total impedance depends on the impedance of each speaker, but also on how the speakers are connected together. It is not the same to connect the speakers in parallel or in series.

It’s common to connect the same speakers, all of which have the same impedance.

Let’s see some examples of what happens when you connect several speakers:

– Two 8 Ohm speakers in series. In a series connection the total impedance is calculated simply by adding the impedance values of each of the loudspeakers. So the total impedance will be 8 + 8 = 16 Ohm

– Two 8 Ohm speakers in parallel. In a parallel connection things change. We have to use the following formula, in which Zt is the total impedance, and Z1 and Z2 the impedances of each of the speakers: (1/Zt) = (1/Z1) + (1/Z2). In practice, if the speakers have the same impedance, the calculation is greatly simplified: the total impedance will be half the impedance of one of the speakers. That is, for two 8 Ohm loudspeakers in parallel the total impedance will be 8 / 2 = 4 Ohm

Connecting a 4-speaker display

Now let’s see how the speakers of 4-speaker displays are usually connected to each other. I’m going to assume the 4 speakers are 8 Ohms.

– Two speakers will be connected to each other in parallel. The total impedance of these two loudspeakers will be 8 / 2 = 4 Ohms

– The other two loudspeakers will be connected to each other, also in parallel. The total impedance of these two speakers will also be 8 / 2 = 4 Ohms

– We then have two sets of two speakers, in which the speakers of each set are connected to each other in parallel. Well, now what you do is connect those two sets to each other in series. The result will then be 4 + 4 = 8 Ohm

Using this connection, all four speakers will have the same total impedance as a single speaker, so the amp doesn’t find out.

Anyway, don’t worry, you don’t have to do weird things with the tinner. This is an internal connection within the display, and it is common to have switches and connectors on the outside of the display to choose between different display configurations.

Finally, a couple of important considerations:

1.- In the tube amps you have to be very careful with this, and you should never turn on the amp without a speaker connected.

2.- The cables that connect amp and speaker are not guitar cables even if they look like them. A much higher current (amps) circulates through the speaker cable than through the guitar cable, and for that reason it has to have a larger section. In other words, the copper wire that goes through the inside of a speaker cable is considerably thicker than in a guitar cable.

If we use a guitar cable to connect amp and speaker, it’s very likely that we’ll load the cable first and consequently the amplifier.

Well, you always have to be aware that when we talk about impedance we are talking about energy sending sensitivities. An amplifier can be forced to send more power than it supports if we connect a few speakers 2Ω and the amp works from 4Ω, for example.

There are amplifiers that have switches (or software control) to choose different output impedances that change the way energy is managed by limiting the current output. This avoids problems in demands that cannot be assumed by the amplifier source (audible distortion, clipping…).

The amplifiers of certain power or high range do not usually have these systems because they do not require that level of self-protection and are able to deliver enough current without risk.

Normally amplifiers are assigned an impedance X, which usually corresponds to the minimum allowed. If you connect speakers from 8Ω to an amp from 4Ω it should support that power demand without problems. The problem for the amp would be to connect speakers from 4Ω to this one, being the minimum impedance supported from 8Ω.

A loudspeaker working at 100W to 8Ω will work at 200W to 4Ω and at 50W to 16Ω (always respecting your limits).

It is always necessary to use the same impedance in the transmitter as in the receiver. This way you get the most out of the signal and therefore the background noise will be less. The lower the impedance, the greater the sensitivity. This is especially important if we pull long distances of cable where background noise can arise. For a ‘home’ use this point does not affect so much.

The lower the impedance the lower the consumption! This is a point to keep in mind

The important thing is that the amp doesn’t work more ohm than the speakers (you could burn the amp). Otherwise, there wouldn’t be much of a problem.



Please enter your comment!
Please enter your name here