What is impedance in audio?

Buyers of hi-fi components often come across the concept of impedance, while audiophiles deal with its optimization in their systems. Here I will explain what is impedance in audio and what you need to know when you pair the amplifier and speakers.

When assembling audio systems, great attention is paid to pairing amplifiers and speakers, more precisely how they match in tone, how they emphasize each other’s virtues, or, conversely, compensate for possible shortcomings, to the extent that audiophiles have long determined winning combinations of brands. with few exceptions and almost no rehearsals, they guarantee synergy. However, whether you are assembling a new system or want to change the amplifier or speakers, the most important thing is to make sure that these two links of the audio chain fit in with their electrical characteristics – impedance and power.

What is speaker impedance and why does it matter?

When the speakers are connected to an amplifier, it sees them at their outputs as a load called impedance. It works as follows: the lower the impedance, the more power the speakers consume, and the higher the load for the amplifier, and vice versa.

This load together consists of three quantities: resistance, inductance, and capacitance of the speakers, but since together they build the resistance that the amplifier has to overcome, they are calculated as one quantity – impedance – and expressed in ohms ((). In addition to representing the load for the amplifier, the impedance is not constant but also depends on the frequency, so at some frequencies, it can jump multiple times or, conversely, decrease.

Therefore, the nominal impedance is mentioned as a characteristic of the speaker, as its typical value, which is measured in a standardized way. In order to match the amplifiers well, the speakers are produced with a standardized nominal impedance that changes over time: at the time of the tube, the usual speaker impedance was 16 ohms so that the tubes could produce and deliver enough power. With the advent of transistor amplifiers capable of delivering more power at a lower cost than tubes, the standard impedance has dropped to four ohms, while in the last three decades, the standard has risen to eight ohms, although six-ohm speakers are not uncommon.

On many speakers you will find the specified and minimum impedance – this information is important because it shows how much the speakers will load the amplifier. When asked if an eight-ohm speaker is easier to start than a four-ohm speaker, the answer would be: mostly yes. However, ease is affected by other parameters, which we will talk about on another occasion.

What is amplifier impedance?

Now let’s look at an amplifier that has the specified power and impedance in its specification, and almost always on its back. What do these two data say? This is not about the impedance of the amplifier (which has an output impedance, but we will not deal with that here), but about the ability of the amplifier to deliver the declared power to the speakers of the specified impedance.

For example, an amplifier declared at 100 watts at 8 ohms can theoretically deliver 100 watts of power to speakers of this impedance and is equipped with an appropriately dimensioned transformer, power supply, and output stage for the task. Many amplifiers are designed to work with eight and four-ohm speakers, which is stated in their specification: in the latter case, the declared power is 50-100% higher. With such amplifiers, both speaker impedances are designed and safe to use.

However, if a lower impedance is not specified, which is the case with cheaper devices, there is every chance that the amplifier is not provided for that or that the combination will not be optimal, and in that case, it is better to look for another amplifier or other speakers. Six-ohm speakers are also allowed at 4/8 ohms.

Therefore, the first and most important rule is: the impedance of the speakers should be in accordance with the specification of the amplifier.

How to match the impedance of two pairs of speakers connected to an amplifier?

The next dilemma that often arises is how to match the impedances of two pairs of speakers connected to an amplifier. Many amplifiers have this option, which is convenient for sounding separated or large rooms. In this case, the rule of impedance matching is somewhat different, and here’s why: if we peek into the amplifier, we will see that both speaker pairs are connected in parallel to the same output stage since they are separated only by the switch for selecting A, B or A + B speaker pairs ( on some devices they are marked as MAIN and REMOTE). As long as pair A or B is in the individual function, it is sufficient to satisfy the first rule of impedance matching.

However, when both pairs are turned on at the same time, the amplifier sees them as one load, only with halved impedance, as the Ohm rule says (the calculation is somewhat more complex, but with the identical speaker pairs it is stated here).

For example, when two pairs of eight-ohm speakers work together, the amplifier will see them as a single four-ohm load. Therefore, if the amplifier is declared at four ohms, this will be a good combination. On the contrary, if two pairs of four-ohm speakers are connected, the moment they are turned on together, the amplifier will see them as two ohms, which will not suit him. Since the impedance at certain frequencies drops significantly, this means that at certain moments this amplifier will work at less than one ohm, which is certainly risky.

As a consequence, stronger heating may occur due to higher power consumption and distortion at louder levels due to the weakness of the amplifier; in the extreme case, at very loud levels, the amplifier or speakers may burn out due to the strong direct current flowing directly through the speakers, which supplies power to the output stage of the amplifier.

Often this information is listed next to the speaker terminals, but sometimes it is not clear enough. “Min. impedance” means the lowest nominal impedance that the amplifier can withstand, while the higher will not harm. Two impedances can also be read, for example, eight and 16 ohms. The rule of thumb is that lower impedance always applies to individual pairs of speakers, while they must be higher impedance if you are going to listen to them at the same time.

Finally, there are cases when different pairs are combined, for example, four and eight ohms or six and eight ohms, which together give a lower impedance. Although audiophiles may advise you to plug in this combination, be careful when listening loudly so as not to overload the amplifier.

Impedance in multichannel systems

When multi-channel audio is purchased as a set or with the seller’s recommendation, there should be no worries about impedance. However, by changing the receiver, to one or all speakers, the owner inevitably returns to the topic of impedance. In surround sound systems, each channel has a separate amplifier, as in stereo amplifiers, the left, and right channels.

However, all channels draw current from a single power supply that is dimensioned to deliver the declared power to the impedance speakers specified in the specification. Since there are more speakers, the eventual choice of a lower impedance of one pair or the whole set would significantly increase the total consumption of the receiver. In order to maintain the sound quality, but also to save the receiver from burning out, it is best to respect the specification of the receiver.

Impedance in headphones

Stereo headphones work on the same principle as speakers, so they also have an impedance that ranges from a wide range of 16 to as much as 600 ohms. The headphone output on the amplifiers is usually connected in parallel to the speakers (with certain adjustments), but since they are a small consumers of high impedance, there are usually no pairing headaches here.

With headphones, however, impedance has to do with the type of source and the sound quality. If you use a portable audio source, it is best to choose headphones with an impedance of 16-32 ohms. For audiophiles, connecting expensive headphones also means matching the source, which is usually a ready-made or special headphone amplifier.

In this case, follow the recommendations of the manufacturer or constructor. Or, if the output impedance is specified in the specification, in practice the impedance of the headphones should be at least 10 times higher.