Speaker Impedance Changes Amplifier Power

Speaker impedance changes amplifier power output. In fact, your amplifier power could be nearly half or double its capacity – depending on the impedance of your speakers. But how much should this concern you?

Impedance is measured in ohms. The Omega symbol (Ω) is used for shorthand.

Amplifier Output Power

Let’s say we have an amplifier. The specifications might say the output power is 100 watts RMS at 8 ohms.

Notice the power output (100 watts) is at a specified load (8 ohms). This is telling us that with an 8 ohm speaker, the maximum output power will be 100 watts.

An Ideal Amplifier

If our sample amplifier were an ideal amplifier, then we can also calculate¹ that:

  • With a 4 ohm speaker, the maximum output power will be 200 watts.
  • With a 16 ohm speaker, the maximum output power will be 50 watts.

The above shows that for an ideal amp, halving the impedance doubles the power output. Doubling the impedance halves the power output.

Halving speaker impedance doubles amplifier power.

Doubling speaker impedance halves output power.

An ideal amplifier is an amplifier which is theoretically perfect. Of course, such an amplifier does not exist, but they are useful when explaining how speaker impedance changes amplifier power.

You can use the following calculator to determine the possible power output with different impedance speakers. Simply type in the power rating of your amplifier and the specified impedance (ohms) – Eg, 80 watts @ 6 Ω.

¹All these calculations use standard electrical formulas which you can read more about in the articles What is Electrical Power, and The Dreaded Ohms Law.

In summary, in an ideal amplifier, the current from the amplifier will depend on the speaker impedance (ohms). The lower the speaker impedance (in ohms) the greater the current that can be drawn from the amp, which means the greater the power.

Real World Amplifiers

The above calculations work well for an ideal amplifier, and help show how speaker impedance changes amplifier power output.

In reality, amplifiers cannot maintain the theoretical output levels as calculated above. This is because the power supply on most amplifiers cannot maintain the maximum power when driving the lower impedance speakers.

In a real amplifier, the above principles still hold but the theoretical values will not be achieved. The power output will be increased with lower impedance speakers, but the maximum power output will not be doubled when the impedance is halved.

As an example of a real world amplifier, let’s look at the specifications of a popular PA Amplifier purchased at Amazon through this site, the Crown XLS1000.

XLS1000 specs - speaker impedance changes amplifier power

This shows that for this amplifier, with both (dual) channels used at the same time, the maximum power output of the amplifier changes as the speaker impedance changes:

  • With an 8 ohm speaker, the maximum output power will be 215 watts.
  • With a 4 ohm speaker, the maximum output power will be 350 watts.
  • With a 2 ohm speaker, the maximum output power will be 550 watts.

This example shows that in a real world amplifier, the principle of “speaker impedance changes amplifier power output” is true, just not as much as in an ideal amplifier.

Please note: this amplifier is designed to work with a speaker impedance as low as 2 ohms. Most HiFi amps are only designed to work with a speaker impedance above 4 ohms.

So What?

So what should you do with this marvelous knowledge? If 4 ohm speakers gives you nearly double the power of 8 ohm speakers, should you only use 4 ohm speakers?

Answer: Yes, and No.

4 ohm speakers are used widely in the car audio industry, as they want to squeeze every bit of power capable from a fixed voltage (~12-14 volts from a car battery). They also mostly design and build their amplifiers to cope with 4 ohms and often 2 ohm loads.

However, it may not be wise to run your Hifi amp flat out at 4 ohms. The reason being, it may mean you are running your amp at or beyond its design limits. The cheaper the amp, the closer you are likely to be at the limits of the power the power supply can cope with. Better to use 6 ohm or 8 ohm speakers, and let your amp comfortably drive them without reaching full capacity. This is similar to a car: better not to constantly drive with the motor at full revs. Interestingly, most Hifi speakers are 6Ω or 8Ω.

A common method of changing speaker impedance is by adding another speaker, either in series or in parallel with the existing speaker. While this will change the output power of the amp, the speakers will share that power. For more details see How Multiple Speakers Share Power.

Most modern amplifiers will, if they are overloaded,  either turn themselves off or reduce the output to protect themselves. However, it is wise not to rely on this self-preservation circuitry, best to design your system conservatively.

Keep in mind that all this is describing the maximum power output of an amplifier. If you don’t run your amp anywhere near full volume, then all this is fairly much irrelevant.

Also keep in mind double power is only around 23% louder to our ears. For a better explanation of this statement see the article on Double Amplifier Power does not Double the Volume. 

If you need maximum level from your speakers, pay attention to the sensitivity in the specifications. Using a speaker with a sensitivity of 90dB (1W/1m) compared to another speaker rated at 87dB (1W/1m) is the same as doubling the amplifier power driving the speaker.

While speaker impedance changes amplifier power output, it is not a major consideration for most users. It only becomes relevant when running your amplifier at full power, and then it is best not to run it too close to its design limits.

Never use a speaker (or speakers) below the minimum impedance the amplifier is designed for. If you hear any distortion, it is an indication that major trouble is just around the corner – turn the volume down, eliminate the distortion and consider a redesign of the system.

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7 COMMENTS

  1. Hi Geoff, I need a bit of help please can you advise me? I am a lover of vintage hi-fi and have no love for the digital sound. I have purchased a new (second hand) Sony STR-V555ES amp and I have a question in regards to speaker ohms to amp impedance. I want to put two sets of 8 ohm speakers on my amp (4 speakers) and I have the setting switch on the amp A+B speakers it’s my chosen speaker set up. However I have a switch at the back of the amp that can allow me to switch it from 4 to 8 ohms. Reading the amps manual it states “Be sure to connect front speakers with nominal impedance of 8 ohms or higher if you want to select both sets (A+B)of front speakers. In this case set the impedance selector to 4 ohms. Do not set the speaker selector to A+B if you connect speakers with a nominal impedance of 4 to 8 ohms to either FRONT A or B jacks” First of my questions is what the hell does that mean? And secondly although the manual states that I should switch the amp to 4 ohms in the A+B setting…. I have read online many times that I should ignore this and stay at 8 ohms always. Is this correct? I’m very nervous to do this wrong as my new amp is well know for going into “protector mode” if it’s not happy. Should this happen I’ve learnt it’s a bugger to get it out of this mode should it be triggered. Please can you advise me? Please keep it as simple as you can for me I’m rusty with this type of thing. Many thanks.

    • Hi Jason,

      I think you basically have two questions:

      1) What is the A+B and 4 ohms/8 ohms speakers all about?

      Basically your amplifier (and most Hifi amps) are designed to work with a speaker impedance of 4-16 ohms. The important bit is 4 ohms. If you go lower than this, then the amp needs to produce more power than it can, so it will shut down, blow a fuse or make smoke.

      So, it wants a total load of more than 4 ohms. One 8 ohm speaker is fine. Two eight ohm speakers in parallel (A+B) gives a total impedance of 4 ohms, which is also OK – as in your case.

      When the manual says “Do not set the speaker selector to A+B if you connect speakers with a nominal impedance of 4 to 8 ohms to either FRONT A or B jacks” it is saying that you cannot use the A+B setting if any one set of front speakers is below 8 ohms. This is because if you do, the total load impedance will be below 4 ohms which the amp wont like.

      Hopefully that is sort of clear enough, which leads us to your next question:

      2) What about the 4 ohms/8 ohms switch on the back of the amp?

      This is a little more tricky. I had to download the service manual and study the schematic diagrams to sort this one out.

      Earlier I said most amps are designed to work with a total load impedance of 4 ohms or more. However your amp is optimised to work with a total load impedance of 8 ohms. The specs of the amp tell us this without actually saying so. If you look at specs for a 8 ohm load, it has an output capacity of 120 watts. Interestingly, the output with a 4 ohm load is only 100 watts. I say interestingly, because normally an amp should produce more power with a lower load impedance. So what is going on? Basically, your amp is designed for a total load impedance of 8 ohms. If you connect a total load of 4 ohms, it will try to produce more power than it can provide.

      To overcome this under-capacity, they provided a switch which effectively lowers the maximum power capability. This sounds counter-intuitive, so let me try to explain. If you connect a 4 ohm load with the 4/8 ohms switch set to 8 ohms, you would be trying to draw close to 200 watts from the amp (at full power). This would trip the protection circuity. Flicking the switch to 4 ohms, reduces the maximum power to 100 watts, which will not trip the protection circuit. Therefore you can use a 4 ohm speaker load.

      Technically, the switch lowers the voltage inside the amp, which reduces the maximum power.

      Let me try to summarise. The amp will cope with a total load impedance or 4 or 8 ohms. If using a 8 ohm total impedance, then the maximum power produced will be 120 watts with the 4/8 ohms switch set to 8 ohms. If using a 4 ohm total impedance (as you are with two speakers connected), then you need to set the switch to 4 ohms to limit the maximum power of the amplifier to 100 watts, which it will cope with.

      I suggest many people say not to use the 4 ohm setting, as it reduces the maximum power, and most people are fixed on getting the maximum possible. All this is only applicable if you run the amp somewhere near full power. Personally my lounge room speakers run around 5 watts of power for normal listening levels.

      Designing an amp that can’t cope at full power into a 4 ohm load is an interesting design concept, and strangely one not mentioned as a feature in the advertising. I have seen the switch on a number of amps, but this is the first time I’ve looked at the schematic diagram to sort out what it actually does.

      Hopefully my explanation is simple enough. It is difficult to explain a technical concept without getting a bit technical.

      Geoff

  2. Hi Geoff,

    “Sure” amplifier: 15 watts output and 6 ohms empidance.
    I connected 4 ohms and 2 ohms speakers in series.
    It’s sounds good, specially the 4 ohms speakers, but yet there is a problem.
    The 2 ohms speakers volume is approximately the double of the 4 ohms speakers volume.
    I wish to decrease only the output level of 2 ohms speakers to get (approximately) the same output level as the 4 ohms speakers.
    Is that possible?
    If it’s possible, what can i use for it and how can i connect it?

    amp output (+) ———> (+) (4 ohms) (-) ———> (+) (2 ohms) (-) ———> (-) amp output

    Thanks in advance,

    Chris

    • Hi Chris,

      You have connected the speakers up the best way to achieve a total impedance of 6 ohms.

      Normally, with speakers in series, the lower impedance speaker draws less power. However if it is the opposite in your case, then I can only suggest that the sensitivity of the 2 ohms speaker is greater than the 4 ohm speaker. This means it is more efficient, or will produce a louder sound for the same input power. As this is a characteristic of the speaker, there is not much that can be down, especially as the speakers are in series.

      So I’m sorry, I can’t think of an easy/cheap solution.

      A different approach would be to use a speaker selector switch with volume controls. While this would wire the speakers in parallel, the multiplier set to x2 would double the impedance (and half the power) to each speaker. However it would give you individual volume control over each set of speakers. Not a pretty (or cheap) solution, but it would work.

      Geoff

  3. hi,
    I have 1 amplifier kenwood hqr 8400 & 8400 is 4 channel-
    I have 4 speakers: kenwood hqr 7100
    Kenwood hqr 8400 at 4 ohm 60 watt, at 2 ohm 90 watt, at bridge 180watt 4ohm. One speaker 7100 is 150rms at 3ohm!!!!!!!!!!
    In your opinion 8400 kenwood can 90% 7100 kenwood at 1 Channel supported…
    if is possible
    Explain-tanks very much

    • Hi Amin,
      The HQR 8400 amplifier is designed to run 4 speakers, as long as they are above 2 ohms each. Since your 7100 speakers are 3 ohms, that will be good. Each amplifier channel can drive one speaker each – no problems.

      Geoff

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