How an amp can get underpowered?

[PENG]

What I don't understand is why this happens for speakers rated with more watts, compared to the amp rated watts, instead of for those rated with less watts.

That's where @TLS Guy "more complex...." comes in. He's a doc, not an EE, but he read a lot over many years, you can keep reading and will soon understand more than many audiophiles too.

You are at a good starting point so I would suggest when you do your online Googling/learning, try to focus on reading about voltage and current, instead of watts, then at least you won't get confused by many poster's including TLSGuy's last post about "apparent" power.

On that, basically, understand that a speaker that has the following specifications first, before trying to understand how it's "rated with more watts, compared to the amp rated watts, instead of for those rated with less watts...." that you are now trying to understand:

MAXIMUM OUTPUT
110 dB
That most likely means listening at 1 meter from the speaker, you may get roughly 110 dB of sound pressure level, without hearing unacceptable distortions (or you might, actually, it depends...)

AMPLIFIER REQUIREMENTS
15 - 180W
If you want to know what this mean, you need to ask the manufacturer directly, you may or may not get a reasonably accurate answer, as again, in depends... Manufacturers such as SVS, KEF, B&W, Revel probably might, as long as you can get pass their first level customer support.

A good guess of what that 15 - 180 W mean is probably, that depending on your listening habit (how loud, and the types of music genre etc., and distance, you can get away with an amplifier rated 15 W average into 4 ohms, but 180 W would probably be more appropriate for this speaker, in terms of distortions and longevity.

NOMINAL IMPEDANCE
4 Ω (min. 3.2 Ω)
That is a relatively more meaningful specifications, many speaker manufacturers might just tell you 4, 6, or 8 ohms period, within giving info on the minimum, and/or whether it is a "nominal" value, leaving you to guess.

SENSITIVITY (2.83V/1m)
87 dB
This is better than the format 87 dB/W/1m. It tells you that if your amplifier outputs 2.83 Vrms to the speaker, the speaker will produce 87 dB at 1 m.

Some specifications use the format dB/W/1m, in that case it would be 87 dB/W/m, the problem with that is, as I mentioned many times, it is not a good idea to use the term Watt for speakers, that is basically a voltage sensitive device, so we really don't know how much watt is dissipated in, or consumed by a speaker.

When people say the speaker takes 100 W, we really don't know what they are talking about, other than they probably mean if the amplifier applies the same voltage to a pure resistor of 8, 4, or whatever the spec says the speaker's impedance is, then the amp would dissipate 100 W in the resistor. I really have no idea, not exactly anyway about the meaning of 100 W speaker. Those who claimed they do, be careful of their rationale or ask for link to the information source such as lab measurements etc.

For now, it may suffice to say, an amplifier driving an electromagnetic transducer (speaker), will deliver current to the speaker, thereby dissipate power in the speaker, but also dissipate power in the amplifier, mainly in the output stages, especially the output devices (such as power transistors), so there really isn't much point in talking about speaker wattage, but that's the way it is, and 99.999% of the consumers are not engineers or scientists, let alone electrical engineers, so it may the better way to begin with anyway, whether I agree or not.

The example speaker specifications I used above is from one of KEF's R series, that's why the information is quite detailed.

I hope it at least give you an idea, without getting too technical, why some of us are saying it can get more complex than you think if you really want to satisfy your curiosity and understand more about the answers given to your apparently simple questions.

 

[Eppie]

The thing I understand is that an underpowered amp would send such a signal to the speakers that their tweeter would break. I understand the signal is clipped hence the tweeters will spend more time playing something very stressful for them, enough long to break them. It's as if I would have to keep in my arms a heavy recipient, open at the up-side, filled with some poison which can harm me if I touch it; I might be able to keep it few seconds but left enough time in my arms I'll eventually shake and drop it hence pouring the poison on me.

What I don't understand is why this happens for speakers rated with more watts, compared to the amp rated watts, instead of for those rated with less watts.

Speakers are designed to take alternating current. A 1kHz tone, for example would produce your typical sine wave with +X volts above a reference of 0 and -X volts below. Power equals voltage times current (P=VI) but here we have voltage that is varying so power over time is expressed as an average. Manufacturers would often use RMS power (root means square) but that is not accurate due to speakers being a reactive load that varies with frequency (as explained previously). It's only meaningful at a given frequency but I don't want to get on too much of a tangent.

The point being, that while the peak voltage to the speaker may be +/- X volts, the average voltage applied is smaller and determines how much power the speaker voice coil must dissipate over time. Much of this is dissipated as heat, which is why manufacturers started using ferro fluid in tweeters and take heat dissipation into account when designing woofer voice coils and magnets, etc. When an amplifier goes into clipping, the top of the sine wave gets flattened because it cannot exceed a specific voltage. That flat part of the sine wave is direct current (DC) as the voltage is basically fixed at the clipping point. Direct current applied to a voice coil causes heat to build up quickly, which eventually melts or burns off the insulation on the copper wire and causes the winding to short. The shorted winding in turn becomes a 0 ohm resistive load to the amplifier which can cause the amp to fail (or at minimum trigger any protection circuitry).

This heat build up can be severe enough that you can damage a speaker even if its rated power handling is higher than the rated power output of the amplifier. You can use an amplifier that exceeds the rated power handling of the speaker because the output will be a clean alternating signal with an average power the speaker can handle. If the amp is rated below the speaker power handling but you drive it hard into distortion, the resulting DC can burn out the voice coils.

Those with good hearing will notice the resulting distortion in the sound and not drive the amp into clipping for too long. I have met plenty of people, though, that do not notice it or recognize what it is. I used to work as a repair tech and did some car audio installations and I lost count of the number meat heads that would crank the volume to max and then come back complaining that the speakers were blown.

 

[TLS Guy]

I’m curious about how an amp could get underpowered and thus be able to change de amp volume without breaking something; it seems for the moment that in order to determine whether the volume is too high only hearing the speakers would tell me that something is wrong.

That is absolutely correct. Listening is your best guide, and also how much the amp heats up.

I have to say though that generally overdriving amps, will usually blow the amp more often than the speakers, unless they are small speakers.

It is often mentioned that overdriving amps tends to blow the tweeters. This is true, but I don't think it is as common as people make out. In my experience woofers and mids blw more often, as those are the power bands they are handling.

However overdriving does produce high frequency harmonics of the fundamentals and so with serious odd harmonics of the fundamental frequencies, as F3 and F5 principally. Certainly this can cause tweeters to blow. However I don't think this problem is as common and often stated, but I have no definite data about that.
I suppose this is also program dependent. The vile rocks bands and instruments purposely add boat loads of distortion to their instruments, with things like "fuzz" controls on guitars. I don't play that sort of music or program, so my experience may not be typical. But I think it reasonable to suppose that if you add amp distortion to those rackets, there could be a problem.

Generally natural instruments do not have huge energy in tweeter range. The one exception is the pipe organ which can produce huge high frequency energy for prolonged periods. This is particularly true of J.S. Bach's registrations.

The only tweeter I think I blew form overdriving was playing a Bach organ work at pretty high volume. I blew the coaxial tweeter in my center. That was a tricky repair, but done successfully. I modified the order of the crossover so the tweeter rolled off 18 db per octave and not 12 db per octave. It has not blown since, despite pushing it hard.

That is another benefit of designing and building your own speakers.

I will say though, more amp power is a good thing in my view. This tube amp craze, and single ended tube amps, make absolutely no sense to me. I use plenty of power, and doubt I have clipped an amp in my rig. I am pretty sure that unless you use a receiver in two channel mode, you have a lot less power than you think you do.

 

[PENG]

I used to set my pc at 100% volume and control the AVR output with the remote but got tired of messing with batteries for said remote. Now I set the amp (in my case an old Denon AVR) that's hooked up to my pc to less as a form of protection on some speakers I hook up to it, and control the output from the volume + - on the pc keyboard instead.

I also control the subs and dsp from the desktop as well. I have noticed no audible difference doing it this way as far as sound quality is concerned after having done it both ways. This way, I can sound level and control everything on the fly from one place. This helps me adjust without volume normalization having to be included in some software like when playing CDs from the same system. I have always used EQ/tone controls on the fly since I started listening to music on my own.

It is a matter of preference, though technically speaking it would make a difference in terms of SNR, that likely has no audible effect in most cases.

 

[PENG]

View attachment 66041

Hypothetical or not, this helps a lot.

In this case, you basically have two volume control, one being the HTPC's windows, or MacOS controlled "Volume", and the other is the volume knob on the integrated amplifier.

You obviously know that an integrated amplifier is an amplifier that has both preamp and power amp in one box.

 

[davidscott]

NAD used to have a "soft clipping" feature on their amps and receivers. It worked to limit the power sent to the speakers but also limited the maximum power you got out of the amp. Since the NADs also were a high current design with extra headroom, I usually ran them without the soft clipping.

 

[Eppie]

NAD used to have a "soft clipping" feature on their amps and receivers. It worked to limit the power sent to the speakers but also limited the maximum power you got out of the amp. Since the NADs also were a high current design with extra headroom, I usually ran them without the soft clipping.

That's partly why I mentioned hard clipping. The soft clipping feature does not flatten the top of the sine wave. I believe it is more of a current limiting feature that limits power without affecting the sine wave adversely.