How can an underpowered amp damage speakers???

[mandlebrot]

Hi

What HiFi magazine keeps stating that an underpowered amp is more likely to cause speaker damage than an amp which is over powerful and boy is this confusing me.

The way I see it an over powerful amp could damage speakers e.g. 500w 8 Ohm per channel amp wired to 50w 8 Ohm rated speakers and whacking the volume up. I could also see an under powered amp being either strained or damaged trying to drive speakers of a higher rating e.g. a 50w 8 Ohm amp driving or trying to drive speakers rated for 200w 8 Ohm or even the harder 4 Ohm.

How could an amp of say 50w 8 Ohm damage speakers rated at 200w 8 Ohm? Surely the amp would just struggle to drive them, not drive them at all or be damaged itself trying to drive them.

I find this a little confusing has What HiFi have never explained or tried to justify there reasons for this so often repeated statement, well if they have I've never seen it.

So could someone take the time to give a brief explanation of what the magazine is trying to say before my head explodes?

 

[MikeK]

The underpowered amp can damage speakers by "clipping".

The signal going to the speakers from the amp is AC (Alternating Current).
(the voltage is alternating too, but they have to call it something ).
This means that to play say a 100Hz tone, the amp will alternate the output voltage (and hence current) between say +12V and -12V (or +1.5A and -1.5A) 100 times a second (the actual voltage/current level depends on the volume you select and the impedance of the speaker, which I've assumed to be 8ohms in the above example).

Now, a powerful amp may be able to deliver a maximum of +/-100V across a speaker, or +/-12.5A into it, where a low powered amp can only deliver +/-40V or +/-5A.
What happens if the low powered amp tries to deliver +/-80V or +/-10A?
The tips of the voltage or current wave are simply "cut-off" or "clipped" - for the duration of the "clip", the voltage/current is steady, which means it's DC (Direct Current - ie not alternating). This is very bad for speakers, especially tweeters (you may think the series capacitor in the crossover would protect the tweeter from DC - it won't in this case, as essentially what you have is pulsed DC, which a capacitor will pass).

You may think that the manufacturers would only allow the volume knob to go round far enough so that the maximum ouput voltage could only be +/-40V, and hence max current would only be +/-5A into an 8ohm load. They may well setup like this, using a dummy 8ohm load (but the dummy load is a constant impedance)
In reality they can't be so precise, as a speaker is a complex load. The 8ohms impedance quoted is a nominal value - it's quite common for this to vary between 4-16ohms at varying frequencies throughout the audio range (and can rise to 40ohms or more low down, due to the resonance of the bass driver). Forgetting the resonace, this means that at +/-40V output, the current will be +/-5A for the time the speaker is at 8ohms, but will rise (or attempt to rise) to 10A when the speaker is at 4ohms - the amp can't deliver it, so it's clipped.
Following the above example, it becomes clear that when the speaker is actually at 4ohms, the amp will start to clip the current at just +/-20V (as this is the point where max current delivery is reached).
Using RMS figures, this is at just 1/4 of the rated output power.
This is also why low powered amps generally can't tolerate low impedance (nom. 4ohm) speakers. It's also why many manufacturers will attempt to give a guide for suitable powered amps, of say 40-100W. The lower figure is their estimate of the lowest powered amp you should use in order to avoid the problems above in average usage, while the upper figure is to avoid the problems you stated yourself - ie there is an upper limit as well, beyond which you'll damage your speakers.
However, the guide is just that - due to the way that some manufacturers quote their power figures. It's possible to take an amplifier and quote it's output at anything between 20W and 400W or more, and still technically not be lying!
(I believe that in the US, (not sure if it's just certain states though) they have now legislated on this issue, forcing manufacturers to quote their figures the same way - no idea if anyone actually checks them though).
One of the more common "tricks" at the moment though is to quote say 6x100W for a multichannel receiver. This doesn't necessarily actually mean all six channels driven simultaneously though - do that and the output can fall to 6x35W in some cases.


Imagine a sine wave, and simply slice off the tips - you'll have what's beginning to look like a square wave - the flat bits now at the tops and bottoms of the wave are the DC component.
DC can burn the voice coil out (if long enough in duration) - and can cause it (and hence the cone) to move too far, which can cause mechanical damage to the speakers motor assembly.


Sorry it's a long post - but you did ask

HTH

 

[uncle eric]

Originally posted by MikeK
One of the more common "tricks" at the moment though is to quote say 6x100W for a multichannel receiver. This doesn't necessarily actually mean all six channels driven simultaneously though - do that and the output can fall to 6x35W in some cases.

As in the case of my newly departed Denon AVCA1SE.
This is quoted at 170 watts per channel when the true firewall figure with all channels driven is around the 130-140watts mark.
As per Mikes explanation this amp drove my Nucleus Gallo's (rated at 100 watts) at reference level comfortably and without any damage to the speakers. Try the same levels with an amp pushing out 30 watts per channel and you will end up with Kentucky Fried Gallo's.

 

[buns]

I had never really considered this before. What hifi is ver good at quoting things, as a 10 year old reader i just believed it!

Well explained Mike.

Ad

 

[mjn]

yep....saved a lot of other people writing it aswell!!

 

[mandlebrot]

Thanks Mike thats explained it all and please don't be sorry for the long post, I would rather have the full explanation and only asked for a brief explanation inorder to get a fast reply because it was bugging me. You did reply fast and also managed a very detailed explanation and my brain can rest easy now so thanks again for that.

 

[Xyberbat]

Mike:
that's the best explanation of clipping that I have ever read ;

Congrats, . . . and thanks.
F

 

[MikeK]

Thanks

I should point out though that the post I made just explains the principle - in reality the current isn't actually clipped without the voltage (and vice versa) - ie if one is clipped, the other is, Ohms Law and all that.

 

[AaronC]

This was a great explanation of this. Thanks MikeK

My setup is a 2x100w amp (2x200w Max) 4 0r 8 ohm make and model QTX QA400
And Speakers Wharfdale Kinetic 15's, 250w 1000w Max. 4 or 8 ohm

(thats the info available - I understand as such it only from the above post)

Is this match up at much risk of underpowering do people think?

Thanks a bunch for any advice.
AaronC

 

[kbfern]

Congratulations this must be the record for resurrecting an old thread, nearly 11 years.

 

[TheBlueFalcon]

Like a phoenix rising from the ashes.

 

[AaronC]

Heh heh, I know right? But its just such good info. It may save speaker coils everywhere.

 

[TheBlueFalcon]

I doubt it. Smell that? Yes, that's another one biting the dust!!

I love the smeel of burnt out speaker coils in the morning.

 

[AaronC]

Noooo

But seriously, is pushing 200w into 250w speakers a major or minor danger do you reckon?

 

[TheBlueFalcon]

If it's a clean 200w and the speakers can handle it, then why not?

But if you start smelling a delicious crispy smell while pumping that 200w into your speakers, then it's probably not your tailpipe bruning, it'll most likely be your voice coils.

Shake & Bake!!

 

[KelvinS1965]

I doubt it. Smell that? Yes, that's another one biting the dust!!

I love the smeel of burnt out speaker coils in the morning.

Now I know why you wanted to swap your DXD-12012's for a BK sub.

 

[TheBlueFalcon]

Now I know why you wanted to swap your DXD-12012's for a BK sub.

Yup. No matter how hard I push these KK's, I'm just not getting any after smell. It's disconcerting. I can't even spell "smell" right, it's that bad.

 

[Cliff]

Yes! A record in reviving an old thread without a doubt. I started reading the posts and saw Uncle Eric and then looked at the date.

It is a real pity is was revived because the first long explanation is wrong!

Nothing to do with DC burning out tweeters. This is blocked by the hi pass capacitor in the crossover network.

The real reason is that when you clip a sine wave or program material and turn it to something like a square wave you generate a lots of powerful high frequency harmonics which reach the tweeter and burn it out!

 

[steverobertsbbc]

I was just about to add that very comment, Cliff!

 

[Jez2k]

Another bump again I'm afraid.

My ageing Yamaha DSP-AX620 specs say, per speaker, 90w RMS minimum output, 115w maximum output (presumably RMS, though it doesn't say). I need to replace the front speakers I've been using for the past 13 years (they are in fact old large Sony speakers from the early 1980's I inherited, rated at 30w "nominal", 50w "music", 8ohms - no mention of RMS as I think it predates that). I never had any noticeable distortion with them, its just that the sound of the left speaker has faded considerably.

In looking at suitable speakers for my amp and doing a LOT of research on the subject of speaker-amp matching, I was surprised to learn just how low-powered they need to be.

From what I can gather, the following "cheat sheet" I put together is pretty accurate, based on the principle that speakers need an amp with 50% to 100% more RMS output.

200watt RMS = amp with 300 - 400watts RMS
100watt RMS = amp with 150 - 200watts RMS
80watt RMS = amp with 120 - 160watts RMS
75watt RMS = amp with 112 - 150wats RMS
70watt RMS = amp with 105 - 140watts RMS
60watt RMS = amp with 90 - 120watts RMS
50watt RMS = amp with 75 - 100watts RMS

Now what I don't know is which of the two ouput values mentioned in the manual (90w min, 115w max), I should pay attention to. If its 90w, then using the above table would mean the most powerful speaker I can use is 60w. If its the 115w I should take note of, then a 75w speaker is the highest I can go for.

I'm also limited on space. My old speakers are narrow, tall and long - about the same height as my 32" TV. I only have about 24cm width of space either side. Most of the speakers I've looked at are too wide. The only 100w (50w RMS) speakers that can fit, are quite small - about 12cm wide and only 18cm tall. Very curious indeed, considering how big my amplifier is. I bought that amp back in 2002 after I had previously helped a friend set his identical one up, with some very large speakers and an equally large sub. Can't help thinking they must have burnt out long ago.

 

[dante01]

Speakers are commonly rated from the minimum power required to the maximum power they can handle, 15 - 70 watts for example indicates that the speaker needs at least 15 watts in order to power it and can handle up to 70 watts of power before it becomes damaged. This is termed the powwr handling. Speakers are not rated with a target wattage, but rather rated with a range that covers the power they need up to the power they can handle. Speakers are powered by the amp and wattage relates to that power. The wattage is not an indication of volume. When people do refer to speaker power then the wattage they refer to is usually the maximum power that speaker can handle befor the speaker is damaged. If your receiver can output 115 watts @ 8 ohm (if the speakers have an impedance of 8ohm?) then you'd ideally want speakers capable of handling more power than the amp can output or you risk blowing the speakers. Don't pair speakers with a lower power handling rating than the amplifier's power output. Us an amp that meets the speaker's minimum power requirements, but doesn't exceed the speaker's max handling. Note that the actually power output will depend upon the impedance of the speakers in question.

 

[Jez2k]

The amp is rated at 8 ohms. I'm not looking to replace it, just need new speakers. So what would you say is the lowest and highest rated speaker I can buy, (in RMS)?

Would a 100w RMS speaker be fine?

 

[dante01]

An amplifier supplies power so cannot impose resistance. Impedance is the resistance imposed by the speaker so only speakers have an impedance on the amp and an impedance rating. The 8ohm in reference to the amp is an indication of the available power if an 8 ohm load is imposed upon the amp.

 

[dante01]

I'd suggest you look at 8ohm speakers with a power handling range of 20 - 150 watts

 

[Jez2k]

Thanks!