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. For more details on this see the article Understanding Speaker Sensitivity.

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.

If you have a question, please read the FAQs before submitting your question.

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Jay (Kenya)

hello Geoff
I have a speaker with a dual voice coil and I want to interconnect the two coils. how do I go about it to give me maximum bass, the amplifier is 200watts and each coil has 8 ohms. am i supposed to connect them in series or in parallel?

Pate

Hi geoff,

I have a 100 watts @ 8ohms amplifier how much power it will produce on running a 3 pieces of 8 ohms speaker

Thanks

christian smith (los angeles, ca usa)

Hi Geoff, I am in the process of replacing our current amps and was wondering the following for running 4x 4ohm subs: I plan on running 2x 2 channel amps, rated at 4000W per channel at 4ohms (Crown iT8000). Each amp channel dedicated to one sub. However a friend of mine is running the same 4 subs daisy chained off of a bridged 2 channel amp rated at 18000W (Powersoft k20). Should I be concerned of my friends setup at the total impedance is 1 ohm (4/4ohms) and the k20 bridged rating is at 4 ohms? Thanks in advance for… Read more »

Henry (Australia)

Hi Geoff,
I have an 8ohm cab (single speaker),
A cab with 3 8ohm speakers in it.
Overall rating 1400rms.
Do I see 2 ohm impedance speakers when daisy chained? With an overall rating of 1400rms?
600rms 18in sub box and dual 15 200rms drivers 400rms tweet box
And does that mean I can ‘ideally’ run a 2500w @ 2ohm head into that daisy?
Any help is appreciated and not necessarily required 🙂
Henry

Garbanotas (Romania)

Hi,

If I connect two 250 watts speakers to a 300 watts channel, would this mean that every speaker will get only 150 watts and not more?

Garbanotas

Zachary Pitts (Flower Mound)

I am running 2 dual 2 ohm subs at .5 at the amp and I run at a fairly loud volume i have yet to hear distortion luckily but I am running it in one channel. Would it be better/safer to put each sub in one channel at 1 ohm or can I leave it

The subs are 300rms
1 ohm stable amp 600rms
In series at half ohm
All sundown equipment in a ported box

Alex Santos

Thanks Geoff I sent you a message months ago. I have a 6 Ohm stereo amp rated at 20W per speaker. I connected 4 speakers, two are 4 Ohm and the remaining 2 are 6 Ohm. I used your calculator, https://geoffthegreygeek.com/multiple-speakers-share-power/ and total load impedance is 5 Ohm for the 40 Watts. So far it sounds great. No distortion but then again I never play loud music. The volume is always pretty low as the speakers are only a foot or so away from me. I read the article on this page a few times and feel comfortable. Thank you… Read more »

Jason (London)

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… Read more »

Chris (Utrecht, Netherlands)

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) (-)… Read more »

AMIN.RZ (shiraz-iran)

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