Axiom Home Page Forums General Discussion Stereo Wattage Question

I read the FAQ on the website but it didn't quite go into the details as to what the wattage numbers mean and how they relate to impedance.

Say you have an amp that can provide 50 watts-per-channel at 8 ohms. This means that for a speaker with an impedance of 8 ohms, the amp can provide 50 watts of power (okay, I'm being redundat to make sure I understand things correctly).
From the faq, "low wattage amplifiers are the normal cause of speaker damage" . Just how much is "low wattage"? Say for example the M3, the max. amp power is 175Watts and it has 8 Ohms of impedanc - will an amp that can provide 50 watts of power be enough for that speaker? Can you ever have too much power?

If I understand it right, I think the reason people say that low wattage amps are one of the biggest causes for blown speakers is because of clipping. As long as you don’t take your amp to the clipping point, then you are ok, but if you turn it up to the point were it starts to clip, this is very hard on a speaker. This is because if you look at the output of the amp when it is clipping on a scope, you can see it cutting off the tops and bottoms of the waveform, and this means that the speaker is being “told” by the amp to move completely in one position, and move completely in another position right away without any intermittent steps.

Example: If we were to put a perfect signwave of lets say 1k into the amp, and look at the output of the amp when it is not clipping, it looks perfectly round on the tops and bottoms of the wave, thus the speaker will move in and out very smoothly…this is good. But if you look at the same 1k signwave when the amp is clipping, it flattens out the tops and bottoms of the signwave, making it look more and more like a square wave the further you take it up. Depending on the quality of the amp, clipping can get very bad, very fast.

When the output of the amp looks like a squarewave it is telling the speaker to instantly move form the in position to the out position….this is very hard for the speakers to do, and isn’t good on them. This is also a very unnatural sound…..

Therefore, people say don’t run low powered amps on speakers, because they will clip sooner then higher powered amps. You still have to watch out that you don’t give the speakers more wattage then they are rated for, but at lease you will not damage the speaker before the speaker gets to its maximum wattage…..whereas you easily could with a under-powered amp if your running it hard…

Well, that is as I understand it….I could be wrong….we’ll see what Ian says….

Low wattage amplifiers dont necesarily clip sooner than higher power amplifiers. Its the fact that the people using this low-power equipment turn the things up too loud and try to push the amplifier beyond its capabilities.

You will not damage a speaker with too low power as long as you are not clipping.

In fact, a low quality "high" power amplifier may damage your speakers sooner than a high quality lower power amplifier for one reason. Many manufacturers tend to over rate their amplifiers output and may actually be stating max output power using a clipped and distorted signal to attract buyers with the "Look how much power our amplifier makes" advertising.

The whole wattage and impedence issue is very much misunderstood by the general public.

I would write a post to summarize it, but I am lazy and I am sure Ian has probably done it here at some point.

The basic premise is that by pushing the amp beyond the design capabilites, you risk damaging any down stream device. Typically these are speakers, but are not necessarily limited to these. However, speakers are in the original question, so lets stick to them.

The mention of lower power amps in this context is as mentioned previously. People get heavy handed with the volume control, and drive the amp past the logical point that it can supply quality output. As Milzie mentioned, this can also happen with "other" amps. Typically mass market companies develop and design equipment that is rated very " enthusiastically". But because they are mass market aimed at price points, many design (flaws is such a harsh term...) trade offs have been made.

Specifications are typically subjective, and some manufacturers test their equipment in a lab. Amp output can be measured by driving a "dummy load" which is not indicative of changing impedance of a speaker. A speaker is a complex array of resistance, inductance and capacitance, and the mix changes with the frequency it is asked to reproduce.

But hey, we are now treading on Ian's area of expertise.....oh wise one, comments please?

Regards,

BBIBH

I must say the other posts on this topic are bang on. I am going to expand on this subject a bit, hopefully without getting too lengthy or complicated. If we start by just looking at this relationship in regards to its effect on the speakers it may help to simplify the equation. There are two ways to damage a speaker, one is pure overpowering and the other is clipping. Fortunately both of these will manifest as audible distortion prior to damaging the speakers. The key is to turn it down right away if distortion is detected. Overpowering of a speaker is a combination of frequency, time, and power.

Essentially the higher the frequency the speaker is asked to reproduce the better it will be at sustaining high amounts of power. This statement can be skewed if the tweeter x-over point is too low or too linear, as the lower frequencies the tweeter can sustain are much higher than the woofer. Therefore in designing a speaker it is important to monitor this, especially since there can be (not always) off axis advantages to keeping the tweeter x-over point low.

By there design conventional speaker motor assemblies are excellent at reproducing very high amounts of power for very short periods of time. This allows for the reproduction of music, or movies, with all of their varying levels without being limited to the short bursts of high power that are required, and all this assuming the designer has been diligent about the mechanical capabilities of the speaker.

Sustained high power will at some point, depending on the speaker, over heat and burn the voice coil or damage some mechanical part of the speaker component. Sustained high power tends to be where most speaker companies get their power ratings. The basic test is modified pink noise for 8 hours at a given power. Though it is nice to have a standard the reality is that unless the requirement for the short burst of power is also considered the rating can be somewhat meaningless in the real world. At Axiom we have two separate tests to encompass both of these considerations in order to come up with our ratings.

As for the amplifier the situation is somewhat the same. Amplifiers are rated at their ability to produce a stable input signal without clipping. Since the real world involves a myriad of various input signal levels this rating can be a little bit meaningless, though at least a standard benchmark. The amplifiers ability to produce high power for short periods of time without clipping can really boost the power available in the real world.

So to tie all this together and get back to the original question, unless you have reached the mechanical or heat maximum capability of the speaker, the only and most common way to damage a speaker is through amp clipping. This is when the amplifier is asked to produce a level of power beyond its capabilities. This capability can be reached quite quickly in amplifiers that have limited ability to produce bursts of high power for even short amounts of time. So there is no amplifier power which is too low for the speaker, but you may be quite limited in the overall volume your system can achieve without clipping. Relating this back to impedance does not change any of the above equation. The lower the impedance of the speaker the more it will demand from the amplifier. Some amplifiers can produce quite a bit more power when presented with a lower impedance. By continuously lowering the impedance though at some point it will get so low as to overheat the amplifier. Obvious 0 ohms would be a dead short to the amplifier. As to how low an impedance a given amp can sustain or how much extra power you will achieve by lowering the impedance of the speaker is a question that has to be answered by the amplifier manufacturer or by measuring this data from the amplifier in question.

I hate to say it but this is the short answer!


Ian Colquhoun
President & Chief Engineer

Thanks for the answer guys. Essentially, it's better to get a high-quality amp with lower power output than a lower-quality amp with a higher power output (at the same price)
By the way, what does "clipping" sound like?

I think we may have confused you. The answer to your questions as follows:

Clipping - clipping is the result of the amplifier power supply not able to supply the transistors enough voltage to continue to produce accurate output. The transistors become " Voltage Limited" and the result is distortion of the output waveform. Typically this is in the high frequency range, and will damage tweeters, which are designed to produce high frequencies of limited and instantaneous energy levels. Instantaneous means that if you keep it up, smoke will get in your eyes! Clipping produces sound that is edgy, terrible to listen to, and very bright with sharp harmonics.
This can also happen to tube amps, but they are designed with very heavy duty power supplies(because tubes by nature require this to operate), and the sound becomes "Compressed". This is a phenomena that some musicians like, and devices to mimic tube amp compression are made for them ("Fuzz Boxes")

Clipping can be dangerous to both the speakers and the amps output stages and power supplies.

So, to your question about purchasing....the answer is still dependant on how hard you drive the amp. A high quality, low power amp can, and will still clip. The high power mass market amp will also clip, but may not clip BEFORE the hi/lo amp would because it is producing poor quality sound that is not clipping. This is because the amp produces garbage sound because of design constraints, but it has not exceeded the power supplies capabilities. The one guarentee is that the actual sound quality and dynamics will be better with the hi/lo amp than the lo/hi amp.

You may need to evaluate the levels you listen to music. A good quality amp should have a rugged power supply, quality output stages, and enough power that you are not going to have to turn it to the maximum volume level to get suitable output.

Sorry for the long winded reply......

Regards,

BBIBH