What AC resistance has to do with the combination of amplifiers and speakers

Most people know the number with the Ω. Professionals also know that impedance is resistance and that we should pay attention to this with speakers and amplifiers – here you can find out what else you should know about audio technology.

The basics: Impedance, measured in ohms, is the resistance of a circuit or device to alternating current (AC)

Such an AC circuit can consist of two interconnected audio devices. For example, a speaker and an amplifier that transmit audio signals. All other things being equal, more power (watts) flows through a low-impedance speaker than through a high-impedance one. This also puts more strain on the amplifier to produce that power. If the impedance is too low, the amplifier cannot handle the power and problems are bound to occur. Most modern electronic audio devices have high input impedances, allowing them to be driven with very low output power. This is one of the reasons why high-quality audio equipment can now be built much more cheaply.

The impedance specification on the speaker chassis is an average value

ITime and again we see the impedance as an ohm specification in product tables. What many people forget: A loudspeaker's impedance changes with the frequency of playback. For example, at 41 hertz (the lowest note on a standard bass guitar), a speaker's impedance might be 10 ohms. At 20,000 hertz (the high end of a violin's frequency range), the impedance of the same speaker can be as low as 3 ohms. Good audio engineers equalise the impedance of loudspeakers so flexibly that the speaker has an even sound over the entire frequency range. It's like gently sanding a piece of wood in the right places to remove bumps and sharp edges: The engineer uses appropriate circuitry to smooth out the high-impedance areas.

What can the amplifier handle and what does impedance have to do with it?

For example, before you buy a 4 ohm speaker, make sure your amplifier or receiver can handle the low resistance. The easiest way: The amp's data sheet gives power ratings for both 8 and 4 ohms. Most solid amplifiers without a built-in preampflifer can handle 4 ohm speakers. At low volume, even rather weak amplifiers work for low-impedance speakers. Unless you turn up the amplifier, it usually doesn't have enough power to supply the speakers. Then there is also the risk of material damage.

Some amplifiers have an impedance switch.

Use this to switch between different ohm settings. The problem with using this switch is that the impedance isn't a linear adjustment, it's a curve. Using an impedance switch to match your speakers limits what you can do with your amplifier. Leave the impedance at the highest setting and buy speakers that match your system's impedance settings. This is the only way to achieve the best performance.

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Many amplifiers can supply slightly more current than their ratings, but if this continues for too long, the amplifier may overheat, shut down, or fail completely. Speakers tend to be a very complex load. Even a typical 8 ohm speaker can exhibit an impedance curve where the load drops below half of 4 ohms at certain frequencies. Most amplifiers can handle that. But if you connect two of these speakers in parallel to an amplifier with a power rating of 8 ohms, the equipment can definitely be damaged. If you load an amplifier with a load impedance that is too low, the load will draw more current than the amplifier is designed for. This can have different causes. Precisely what happens depends on the amplifier.

Many older amplifiers try to dissipate the excessive current load. The result: blown transistors. With tube amplifiers, the extra heat can cause the plates inside the tubes to warp. This can cause a short circuit. Other amplifiers have fuses that protect them from damage. Some models can switch to a current limiting state, which usually leads to distortion, but at least protects the hardware.

The speaker impedance changes the power output of the amplifier

Depending on the impedance of your speakers, your amplifier's output can be almost half or double its capacity. Let's take a 200 watt RMS amplifier with 8 ohms. That 200 watts RMS is directly related to the 8 ohms. Halving the speaker impedance doubles the amplifier power (at 4 ohms it would be 400 watts). Doubling the speaker impedance halves the output power (at 16 ohms it would be 100 watts). The lower the speaker impedance, the greater the current coming from the amplifier, so the higher the power. However, we are assuming perfect conditions here, which are never given in audio reality.

Amplifiers cannot maintain the theoretical output levels (with half the impedance).

That's because the power supply of most amplifiers cannot sustain maximum performance when driving low-impedance speakers. They do not reach the theoretical values. The lower impedance speakers increase the power output, but the maximum power output does not double when the impedance is halved. This can be important for planning: Don't plan on doubling the power with half the resistance. It is also not necessarily advisable to drive your hi-fi amplifier with 4 ohms. This is because you may then be operating your amplifier at or beyond its design limits. The cheaper the amp, the closer you are probably to the limit of what the power supply can handle. It is better to use 6 or 8 ohm speakers that the amplifier can comfortably drive without reaching full capacity.


Speaker DIY kit

„It's a board with a couple of speakers in it. Nothing else. I am always amazed at what you can "do" in the bass with such a simple construction." (Klang+Ton 3/21)

A common way to change speaker impedance: Add another speaker.

Add the speaker either in series or in parallel with the existing speaker. As a result, the output power of the amplifier is divided differently – the speakers share the power. Although most modern amplifiers switch off themselves or reduce the output when overloaded, you should never rely on this. Always plan the power requirements conservatively. And, as a reminder: Doubling the power of the amplifier increases the volume by only about 23%. To double the volume, you need about ten times the power. If you need a maximum level from your speakers, pay attention to the sensitivity specified in the technical data (also known as sensitivity or characteristic sound pressure). Using a speaker with a sensitivity of 90 dB (1W/1m) compared to another speaker with a sensitivity of 87 dB (1W/1m) is equivalent to doubling the amplifier power for the speaker. Never use a speaker or speakers below the minimum impedance for which the amplifier is rated. If you hear distortion, it's a sign that a bigger problem is coming – turn the volume down, get rid of the distortion and consider rebuilding the system.

What do DI boxes have to do with impedance?

In situations where you want to connect a high-impedance output to a low-impedance input, such as when recording an instrument directly, you should use a DI (direct injection) box. A DI box converts the low-impedance signal into a high-impedance one and balances (grounds) the signal. You can then connect a high-impedance input to a microphone preamp, for example.

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Headergraphik: Adobe Stock | surasak

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