Amplifier Power - understanding basic parameters
Selecting the right amplifier
The procedure for determining the nominal power of a loudspeaker set, described in the article SPEAKER POWER AND MAX SPL, is an excellent starting point to answer the question: What power amplifier will safely drive this set to its full potential in most applications? In the previous article, the primary focus was on the Crest Factor parameter, while in this one, I will concentrate on power, although the amplifier has several other important parameters that affect its quality and proper operation. However, as Jeremy Clarkson says, POWER speaks most to the imagination.
It may seem that choosing a weaker amplifier will provide an additional safety margin to prevent speaker damage. It might play a bit quieter but safer, right? After all, if the speaker is rated at 300W, using a 200W amplifier should be a reasonable solution? Unfortunately, it doesn't work that way. As you learned from the previously mentioned article, an extremely important parameter, which we rarely find in specifications, is the Crest Factor of the signal that will power the amplifier. Music and any sounds are signals that take the form of voltage in the electrical domain. The Crest Factor is the difference between the root mean square (RMS) value of the voltage and its peaks. It is a parameter that we almost never know in our daily sound reinforcement and studio work! (perhaps with the exception of signals generated by synthesizers, such as Sine Wave). Each type of music has a different Crest Factor – classical music can have a 20dB difference between peak and average values, whereas heavy metal or highly compressed dense pop can have up to 6dB difference. It is generally accepted that entertainment music has a CF of about 10-12dB. This is important when choosing an amplifier for a speaker set.
Manufacturers usually specify amplifier power based on a sine wave signal at a frequency of 1000Hz. This is a demanding, dense signal with a Crest Factor (CF) of 3dB – the worst possible scenario for both the amplifier and the speaker (perhaps except for square signals). So why do manufacturers use this signal to determine power? It's due to history, habits, and because it simply looks better on the specification sheet. After all, you’re more likely to buy a 1000W amplifier (with power specified at CF=3dB, even though it's quite possible the amplifier will never encounter such a signal in its life) than one that can deliver "only" 500W (with power specified at CF=6dB). Not knowing that both amplifiers are the same, which one would you choose? Of course, the 1000W one. After all, more is better!
From the speaker's perspective, however, a speaker powered by too weak an amplifier can be damaged more quickly than one driven by an appropriately powerful amplifier, using the same input signal.
Since the test signal used to determine the power of professional amplifiers is often a signal with a CF=3dB, an amplifier that generates 1000W at 8 Ohms with such a signal will deliver half that power, i.e., 500W, with a CF=6dB signal. For normal music with a typical Crest Factor of 10dB, it will deliver 200W! This means that your 1000W amplifier, when playing a typical music program, will provide 200W of power to the speaker – yes, that’s exactly what it means! Considering the varied dynamic nature of most music programs, where the difference between the average RMS value and the signal peaks is usually around 10-12dB, it is customary to specify the Continuous Power of the speaker (twice the Nominal Power) as recommended to fully utilize the thermal and mechanical capabilities of the driver without the amplifier clipping.
It is important to remember what has been described above: If the amplifier has too low RMS power, users tend to "search for volume" from a system that cannot provide it. Turning up the volume (e.g., on the mixer) at some point mainly generates distortion and noise. A surprising conclusion and practice to remember: It is easier to damage a speaker with too weak an amplifier than with one that is too powerful!
Can You Turn Down the Amplifier to Protect the Speaker?
Unfortunately, many people are unaware that an amplifier is only fully muted when the knob is turned to 0 or minus infinity (depending on how the manufacturer labels it). In any other position (unless a voltage limiter is enabled, which higher-end professional amplifiers possess), the amplifier can still produce full power according to the AMPLIFIER GAIN setting (more on this later). For the amplifier to do this, it only needs to receive a sufficiently high input voltage.
- TO REMEMBER: The VOLUME knob (amplifier volume) DOES NOT REDUCE ITS POWER but merely attenuates it.
What Are the Consequences?
Despite "turning down by half, let's say to 12 o'clock," it is still entirely possible that your sound source-mixer-processor chain can generate enough voltage for the amplifier to reach maximum power or even exceed it (which happens in the case of clipping). If the signal generated by these sources is set too high (in the red!), it can exceed the capabilities of your amplifier's inputs, sending it a signal close to a square wave. Is this healthy for the equipment? NO. Does your speaker like it? NO. This is exactly the scenario described above where the crest factor approaches values between 3dB and 0dB. In such a situation, you could burn a 1000W speaker with a 100W amplifier – it’s enough to "fry the coil" long enough with nearly constant current with a very low Crest Factor (CF of constant current is "1" or "0dB" – two different notations for the same value).
- CONCLUSION: The amplifier's volume knob should always be turned to maximum during operation, and system volume should be controlled on devices before the amplifier. This can be a mixer and a DSP processor or just the mixer if you don't have a processor. This ensures better control over the signal and is part of the "correct signal gain structure of devices in the chain" (a topic for another article).
Amplifier gain is one of the fundamental properties of any amplifier. Its purpose is to amplify the voltage input to achieve a higher voltage at the speaker terminals. Voltage is expressed in volts (V), and AMP GAIN indicates by how much this voltage will increase after passing through the amplifier.
Gain can be expressed as a multiplier or in decibels (dB). For example, an amplifier with a 40x multiplier (forty times) will generate 40V from a 1V input. 40x is the same as 32dB. Conversely, a value half as much, i.e., 20x, equals 26dB.
Not every amplifier manufacturer allows this parameter to be changed. In consumer devices, it is usually fixed and sometimes not even specified in the documentation.
You might already know that in the electrical world, However, I don’t want to leave you without an answer to the question of what gain an amplifier should have. In professional audio, it will be between 26dB and 38dB. Likely, setting it somewhere in the middle of this range will yield a good effect, provided the manufacturer even allows changing this parameter.
As you can see, delving deeper into device parameters makes settings more complicated, and the most common answer is: "it depends." That’s why at FAVO, we focus on offering you complete solutions where you don’t have to worry about this. Each element of the audio system is matched and appropriately set to achieve the best possible effect.
- Sine wave and music signals
As mentioned earlier, amplifier power is typically specified for a 1000Hz sine wave signal. The Crest Factor (CF) of a sine wave is 3dB (or a ratio of 1.41). The voltage we have in our sockets is 230V, with a frequency of 50Hz, and it is also a sine wave, so its CF is also 3dB (1.41).
Assuming an amplifier has 1000W power assumes it receives a sine wave input. This situation mostly occurs in laboratories where amplifiers are tested. Even in the case of musical bands using sounds generated by synthesizers that resemble sine waves, these sounds are just a small part of the overall music. Usually, music does not resemble a sine wave; it is complex, dynamically variable, with loud and quiet moments, very short but loud impulses (e.g., bass drum or snare hits), and periods of complete silence. This leads to the average CF for music being considered around 10-12dB. This means a 12dB difference between the effective (RMS - root mean square) value of the signal and its peaks. The difference in the effective value of an audio signal between -3dB (sine) and -12dB (music) is 4x less.
- Power consumption comparison
Below you can see graphs of the measurement of the time of delivering maximum power of modern Class D amplifier designs measured under different loads and for different signals (source: Production-partner.de)
Many technological and construction factors contribute to this, such as efficiency, heat generation, and the ability to dissipate it externally. The truth is that every amplifier will generate as much power as its topology and transistor cooling system allow.
The time in which the best amplifiers can deliver their full nominal power with a sinusoidal signal is counted in seconds rather than minutes. And a heater? It can boil water for many hours.
So, how much power will an amplifier draw from the mains during a concert or performance? Since a music signal is on average 4 times less dense than the current powering a concrete mixer or heater, and amplifiers reduce their power over time, it is generally accepted that this value is about 1/8 of the nominal power of a Class D amplifier. 1000W/8=125W, and that is what we can assume as the power draw from the mains for such an amplifier.
I think these are excellent topics you can discuss with your family at a Sunday barbecue. It promises to be quite an exciting conversation :)