I know a similar question title has been asked, but I believe that does not answer my question (and I couldn't think of a better way to phrase the question).
I'm a bit confused by how exactly an amp can overload a speaker, and vice versa.
Many guitar amplifier speakers are 8Ω impedance.
If I understand correctly, the output amplifier (should) outputs a fixed voltage signal output regardless of what load is placed on it. If this step is wrong, please correct me.
So if there is a fixed voltage signal, (say, +-15V, i.e. 30V of swing) and if the speakers' impedance is ~8Ω (I understand it will vary with frequency but say it is around this figure), then how is the wattage varying with different amp combos even though the impedance is roughly the same? Is it that the voltage increases with higher wattage amplifier/speaker combinations.
For example a 10W combo with an 8Ω speaker vs a 100W amplifier connected to a 4 speaker cabinet wired for 8Ω impedance (parallel connect 2 series pairs of 8Ω speakers), the 100W is obviously louder. Is it that the output voltages of the 100W amplifier is more? Else how can you get the wattage to increase if you are keeping voltage and impedance constant?
What would happen if you connected the 10W amplifier directly to a 4 speaker cabinet? Would it overload the amplifier? Or just play it quieter? Theoretically, if the voltage is the same and the impedance is still 8ohms, the wattage should be the same, i.e. 10W through the 100W rated speakers.
If so, is this true then: when we say 10W 8ohm speaker, we mean its able to handle maximum peak voltages of (P=V^2/R, V=sqrt(PR)) ~9V. Whereas for a 100W 8ohm speaker, it is able to handle peak voltages of ~28V?
In what situation can you harm a speaker? By connecting a too powerful an amplifier to it? But then isn't that something that is recommended by many people? (Amplifier output of atleast 2x the speaker rating). If so then the voltage output of an amplifier is not fixed? It varies according to which speaker is connected to it? (even though the impedance is the same?)
In what situation can you harm an amplifier? By connecting too high a wattage speaker to it? Then why do I see so many people posting videos on youtube of 1/2W guitar amplifier builds connected to big wattage rating 4 speaker stack speakers or atleast even 2 speaker combinations?
Best Answer
You have many questions but I think you can understand it better with a single explanation. See that there are many myths around this subject. But it is also a matter of analog electronics.
Speakers are a Z load in your circuit that may vary its impedance in terms of frequency. Note that a speaker main goal is to maintain a stable and almost constant impedance in the frequency range that it was build to work on. This impedance is almost equal to the coil resistance. So, when your speaker is working in a system well designed, your Z load can be viewed as a almost pure resistive load (8, 6 or 4 ohms in most cases).
With that said, we should have ways to supply power to the speaker so it can reproduce sound waves. Note that the magnetic part of the speaker is directly related to the current that pass through it. So we can say that the speaker is a kind of resistive load that deals with current variations to produce sound (simple way to understand). So the way we can vary current in a resistive load is by swinging a voltage across it.
If you connect a speaker or a simple resistor into a amplifier's output and also plug an oscilloscope probe across the load, you will see the voltage variations just as your music is varying (sound waves). It's not a constant voltage in the output. Otherwise you cannot produce sound waves since you need current variations to produce magnetic variations and forces by Lorentz formula.
Besides that, wattage is the power consumed by your system. Instantaneous power is calculated by P = UI or P = ZI². So the greater the current passing through your speaker, more power it will dissipate (and also more power consumption since part of it will be transformed to sound waves).
Also, you have to consider the volume control. Those examples you gave can only be applied if your amplifiers are always working at full amplification (0 dB). This way, a more powerful amplifier should produce higher voltages in the output compared to a less powerful amplifier (both in 0dB). Since instantaneous power is also calculated by P = U²/Z, then you cannot increase power with voltage and impedance being the same.
When you make connections (amplifier + speaker) you should care about some details:
Amplifier power output: it will tell you how much power it can deliver to your speaker in a determined impedance. This is the maximum power it can produce. Note that if you turn it on with 20% of volume, it will not be delivering its full power. Note also that even in 0dB it probably wont be producing full power all the time because music varies its amplitude waves so you should calculate the average power by the integral of all the signal.
Amplifier minimum impedance: This will tell you what is the lowest impedance you can connect to its output. It does not matter if you connect higher impedances there. You will just not be able to obtain too loud sound in your speaker system. Generally speaking, when connecting higher impedance speakers you can have a cleaner sound (less distortion) but a lower sound volume. In the other hand, if you want a louder system, you should connect the lowest impedance allowed but you probably will have more distortion. Note that what may harm any part of your system is excess heat. And heat is produced by Joule effect which relates to power directly. So, it's also possible to connect lower impedances than allowed since you do not increase volume more than a certain point. This way, even with lower impedances you produce same power as a higher impedance in full volume. You can see that by connecting a 2Ohms speaker to a 4Ohms-minimum amplifier but in a very low volume. It will work and it will not harm anything.
Speaker impedance: as already said, its the nominal impedance that a manufacturer try to reach and maintain stable in the frequency range that the speaker is designed to work.
Speaker power: this is the highest power the speaker is built to tolerate. Of course there are always questions about the ways people use to measure that and indeed there are misconceptions about the terms like RMS POWER. A common way to do that is to connect the speaker to some signal which has a AVERAGE power P and see if it can tolerate that for a long period of time. The greatest P value you can reach doing that is your nominal average power (again, it's a simple way to explain).
So if you are connecting a speaker to an amplifier, you should watch those variables to see if you will harm anything. Generally, you can harm a speaker when connecting a too powerful amplifier to it. Let's say you have a 300 W/8 ohms speaker and you connect a 800 W/8 ohms amplifier. As I said before, it also depends on the volume dial. Whenever this system is at low volume, nothing will harm. But when you reach a specific point of volume that the average power in the output will go over 300W, you probably will start to harm your speaker. People also sometimes say that a very powerful speaker could harm a non-powerful amplifier. Or that a non-powerful amplifier cannot drive a powerful speaker. What happens is that you can have now a 20W/4 ohms amplifier with a 800W/4 ohms speaker. Note that you can connect them and it will work normally. This will be just like connecting a more powerful amplifier with low volume to it. The problems are: you will probably want to reach full volume to have some sound. THIS could harm your amplifier since full volume many times means more than 0dB (plus distortion). The excess heat in the amplifier may damage its output. Another common problem is that this distortion at full volume may damage your speaker. This happens because the speaker is build to work in movement. Many speakers have holes to dissipate heat and obtain air flow to refrigerate. Whenever distortion occurs, the mobile part of the speaker may stop moving for a little while. It starts to overheat the coil.
In short, any combination of amplifier and speaker should be possible. You just have to take care of the volume. If you don't want any possible trouble, get a amplifier which is a little less powerful than your speaker in the same impedance, and never exceed something like 70%~80% of the volume control. If your volume dial has a dB scale, try using in 0dB at most.
I hope this has cleared your questions. Sorry for bad English.