Author Topic: Implications of underpowered amp on speakers  (Read 531 times)

Offline kinosfronimos

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Implications of underpowered amp on speakers
« on: February 15, 2020, 09:00:15 AM »
So preaume an amp has a rated power output of 30W to speakers of 6Ω @ 1kHz (and the amp can handle 6-16Ω speakers).
What would happen if I tried to drive speakers with a recommended power requirement of 40 - 150 W at 6Ω?
Would the speakers play fine, but not particularly loudly?
Or would it tax and overheat the amp?
Or any other damage/issues?
Thanks!
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Offline Michon

Re: Implications of underpowered amp on speakers
« Reply #1 on: February 15, 2020, 10:31:38 AM »
The loudspeakers will most likely play fine.

"Recommended power requirement" differs slightly from "power handling". "Recommended power" allows, but doesn't necessitate, for marketing woo woo to enter the conversation so, for now let's rather talk about "power handling", i.e. the maximum input power that a loudspeaker is capable of receiving without being caused damage.

Even though the amplifier has an output capability of 30W and the loudspeakers have a handling capability of 40W, or more, you may not necessarily be using the full 30W of the amplifier or be required to put 40W of power into your loudspeakers to achieve your desired volume level.

The efficiency of the loudspeakers are an important element to consider here.

If you have a pair of loudspeakers that can handle 40W and they have an efficiency of 95dB SPL/1W/1M (meaning for 1W of power fed into the loudspeakers they will radiate 95dB SPL of sound at a distance of 1 meter in front of the loudspeaker) you could use a 3W amplifier to achieve a comfortable listening level in a small to medium sized room.

If you have a pair of loudspeakers that can handle 40W and they have an efficiency of 80dB SPL/1W/1M (meaning for 1W of power fed into the loudspeakers they will radiate 82dB SPL of sound at a distance of 1 meter in front of the loudspeaker) you could need a 30W amplifier to achieve a comfortable listening level in a small to medium sized room.

What you want to avoid is using a low power amplifier that isn't capable of delivering the output power you desire to drive your loudspeakers to the volume you want then turning up the input and output volume controls on the amplifier to such a point where it distorts, because when your amplifier distorts it could cause damage to your loudspeakers (even if the loudspeakers are capable of handling many times more power than the amplifier is capable of delivering).


To return to your example:

Presuming that the loudspeakers you mentioned, with a recommended power requirement of 40 - 150 W at 6Ω, have a fairly average efficiency of around 85dB SPL/1W/1M in order to achieve a comfortable listening level, which for me is around 85dB SPL at listening position, an amplifier with with a rated power output of 30W to speakers of 6Ω should work fine and not cause damage to the loudspeakers. For safety you could, while there is an input signal (like music) fed into the amplifier, turn up the volume on the amplifier slowly, while listening for distortion emanating from the loudspeakers to be sure that your amplifier is capable of delivering the power needed to your loudspeakers, without causing them damage.

The variable which isn't clearly defined in this example is the "recommended power requirement" of the loudspeakers. The recommendation is a guideline from a manufacturer which holds the loudspeakers efficiency and average desired volume levels in room in consideration, but what I consider to be a more useful specifications for loudspeaker power handling are: 1. power handling, stated in terms of continuous power (alternatively RMS or AES ratings), and 2. peak power power handling.
« Last Edit: February 15, 2020, 10:51:12 AM by Michon »

Offline kinosfronimos

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Re: Implications of underpowered amp on speakers
« Reply #2 on: February 15, 2020, 11:26:56 AM »
Thanks for the detailed explanation!
So the above speakers are a 90dB sensitivity at 1W at 1m. Would the above amp drive those fairly loudly enough in a medium sized room?
Also, in your explanation, I don't understand how the speakers could distort, if they can't be driven hard enough by the underpowered amp?
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Offline Michon

Re: Implications of underpowered amp on speakers
« Reply #3 on: February 15, 2020, 12:24:39 PM »
You're welcome.

If the speakers are 90dB/1W/1M a 30 Watt amplifier should have sufficient power to drive them to a comfortable listening level.

I use a pair of loudspeakers that are 95dB/1W/1M with a 3W amplifier to achieve satisfactory listening levels. Having headroom (more power delivery capability than what is needed) is not a bad thing though.


Also, in your explanation, I don't understand how the speakers could distort, if they can't be driven hard enough by the underpowered amp?

In my explanation when I mentioned "distortion emanating from the loudspeakers" I meant distortion present within the amplifier circuit caused either by a user attempting to get more output power than the amplifier is capable of delivering, or one of the various stages of the amplifier being distorted due to receiving a signal that is higher than it was designed to receive, and thus clipping (or distorting the output signal) which the loudspeaker then receives and reproduces.

It is possible for an amplifier to distort while not necessarily causing the loudspeaker to distort (i.e. subjecting it to mechanical excursion in excess of what it is capable of performing). In this instance you will hear the distortion of the amplifier being played through the loudspeakers, even though the loudspeakers themselves aren't mechanically distorting, as the loudspeakers are only following or reproducing the distorted signal that they are receiving from the amplifier. When the amplifier's distortion is sufficiently high it can cause damage to the loudspeaker though, depending on the loudspeaker.

A guitar player for instance may on purpose cause any of the various stages of their amplifier to distort (which will colour the timbre of the sound in a desired manner), but then feed that distorted signal into a loudspeaker that can handle both the power output and the distortion being produced. But it is also possible to send a 100W output, void of clipping distortion, to a loudspeaker that is only capable of handling 20W and thus causing the loudspeaker itself to distort and soon malfunction.
« Last Edit: February 15, 2020, 12:48:09 PM by Michon »

Offline kinosfronimos

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Re: Implications of underpowered amp on speakers
« Reply #4 on: February 15, 2020, 02:58:56 PM »
Ah! Okay that's, I think I have a better understanding now...

So of I understand correctly: the amplifier's own distortion (cos the underpowered lower wattage amp is trying too hard to drive higher wattage speakers) will mess the speakers - as although not mechanically driving the speakers too far back and forth, rather the disharmony of the white noise (amp's distortion) the speakers are playing will damage them?

Considering my initial scenario above, this wouldn't happen with a 30W amp trying to drive 40-150W speakers with a sensitivity of 90dB per watt at 1m?
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Offline Tzs503gp

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Re: Implications of underpowered amp on speakers
« Reply #5 on: February 16, 2020, 12:26:34 PM »
I you are the only one going to use (control) the system, and you have an ear for distortion, and you listen at fairly low level, then go ahead. If not, then donít do it. In any scenario where music is played loudly, this setup is a no go.

Offline chrisc

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Re: Implications of underpowered amp on speakers
« Reply #6 on: February 16, 2020, 12:30:48 PM »
If you play it too loud and it distorts, the first thing to go will be the tweeters
Music is the shorthand of emotion

Offline Michon

Re: Implications of underpowered amp on speakers
« Reply #7 on: February 16, 2020, 03:33:33 PM »
So of I understand correctly: the amplifier's own distortion (cos the underpowered lower wattage amp is trying too hard to drive higher wattage speakers) will mess the speakers - as although not mechanically driving the speakers too far back and forth, rather the disharmony of the white noise (amp's distortion) the speakers are playing will damage them?

Yes, what happens when an amplifier distorts is that the smooth round peaks of the music (a collection of sine waves) are clipped off and gradually the waves become more square. This produces higher order harmonics, other than what is present in the program material being played, and when the clipping is sufficiently excessive the loudspeaker will be damaged in the process of attempting to recreate or follow the square wave that it is being subjected to.

Though you understand it correctly, technically the wave that is formed when an amplifier is clipping is not called white noise. White noise is defined as sine waves at each frequency in the audio band played at the same level at the same time. They do not inherently cause damage to loudspeakers and you can listen to what white noise sounds like here: http://onlinetonegenerator.com/noise.html. You can also listen pink noise and brown noise there.

Considering my initial scenario above, this wouldn't happen with a 30W amp trying to drive 40-150W speakers with a sensitivity of 90dB per watt at 1m?

An amplifier with a maximum output power of 30W is sufficiently powerful, for my desired in room SPL level, to drive a pair of loudspeakers with an efficiency of 90dB/1W/1m (even though the loudspeakers are capable of handling more power than 30W). I haven't had a pair of speakers that my strongest amplifier (a push pull EL84 circuit with around 14W of output power) wasn't capable of driving sufficiently loud for my tastes.



When you push more power into a loudspeaker than it is capable of handling either the loudspeaker will exceed its Xmax (maximum physical excursion) and thus have its suspension system (the surround and spider) damaged or the voiceoil will overhead and the wires coiled around the former will melt. In exceptional cases (for instance 300W RMS applied to a high efficiency transducer which has low power handling and a small Xmax) the voicecoil of the loudspeaker can pop out of the voicecoil gap and the suspension can be torn.
« Last Edit: February 16, 2020, 03:41:06 PM by Michon »

Offline Vaughan

Re: Implications of underpowered amp on speakers
« Reply #8 on: February 18, 2020, 02:19:51 PM »
Quote from: kinosfronimos
So of I understand correctly: the amplifier's own distortion (cos the underpowered lower wattage amp is trying too hard to drive higher wattage speakers) will mess the speakers - as although not mechanically driving the speakers too far back and forth, rather the disharmony of the white noise (amp's distortion) the speakers are playing will damage them?

Distortion by itself won't necessarily damage anything. Your amp could clip momentarily on brief peaks and your speakers could survive just fine.

Amp clipping can be benign and it can be masked by complex music so you may not know your amp is even clipping, but it can be more insidious and potentially lead to speaker damage in extreme situations. I think we can all agree though that clipping should be avoided where possible.

The significance of clipping is that it provides a mechanism whereby the amplifier can deliver more power than was expected given the power rating of the amp. For example an amp rated at, say, 200 W/ch into 8Ω measured with a sine signal at the threshold of clipping can put out much more than that when it is driven into heavy clipping.

In the end, whether clean or clipped, too much power (IMO) is what usually does the damage.

Offline kinosfronimos

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Re: Implications of underpowered amp on speakers
« Reply #9 on: February 18, 2020, 02:54:00 PM »
Thanks for all the replies, I think I have a better understanding: both over powered and underpowered amps will cause speaker damage for different reasons.

If as in my initial instance, an underpowered amp-CD player combo output to 3.5mm jack, and into 2RCA of an adequately powered AVR, there would be no harm of mismatched amp, as the AVR would be doing the work of powering the large speakers (just not the decoding)
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Offline Tzs503gp

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Re: Implications of underpowered amp on speakers
« Reply #10 on: February 18, 2020, 03:23:22 PM »
.....both over powered and underpowered amps will cause speaker damage for different reasons.


No. Both overpowered and underpowered amps COULD cause speaker damage. Always for the SAME reason:

Playing too loudly

Offline Michon

Re: Implications of underpowered amp on speakers
« Reply #11 on: February 18, 2020, 03:51:41 PM »
The concern with clipping (as produced by underpowered amplifiers which are overdriven), as mentioned earlier here, is that square waves contain more energy than sine waves of the same peak amplitude. The current of square waves is larger for longer periods of time than that of sine waves. Square waves have the same energy as DC. An AC sine wave has the energy of its RMS value, whereas square waves have the energy of their peak value, which is 140% greater than the peak value of sine waves. Energy is the square of the peak value, so essentially it is two times greater for a square wave than it is for a sine wave.

Accordingly a technically correct description is that subjecting your loudspeakers to more energy than they are designed to handle will damage them. For example if you drop a loudspeaker driver and the basket cracks it was subjected to more energy (or force, more appropriately in this example) than it could handle while maintaining its structural integrity. In the same sense subjecting the conductors in the voicecoil former to more energy than they were designed to handle will cause them to be damaged.

Sine wave:


Square wave: Thinking of square waves as alternating DC could be a useful mnemonic device.

Online Ampdog

Re: Implications of underpowered amp on speakers
« Reply #12 on: Today at 06:33:24 PM »
Kinosfronimos,

In order not to cause too much 'upset/deviation' on your thread, I have started  a new thread 'Amplifier clipping causing loudspeaker demise ...' elsewhere in 'General', which strictly could have belonged here.

You and other members reading here might intere4st yourself in that!
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)