When choosing an amplifier and speakers, it’s critically important that you understand three simple rules to avoid damaging your equipment!
Download this Speaker Specifications Guide that will help you understand the most important specs to look for when buying a speaker.
The first rule is to know the power handling capacity of your speakers.
The peak power capacity describes the maximum wattage that a speaker’s voice coil can handle in a very short burst. This is often a much bigger and more impressive number than the continuous power capacity, which describes the power level a speaker can handle over an extended period of time.
Because the peak power capacity is much more impressive, it is often the one that is boasted on the speaker’s box and marketing material. Just be sure to know the difference before making any calculations. All of the calculations in this post will be based on the speaker’s continuous power handling capacity.
Keep in mind that the power capacity of a speaker alone doesn’t tell the full story about how loud that speaker will get. A speaker’s maximum SPL output is determined by both its power handling capacity and its sensitivity, which describes how much SPL (or sound pressure level) can be expected at a given power input from the amplifier. This is often shown as the output level with a 1 watt input level, measured from 1 meter away.
If you know how loud a speaker is at 1 watt and you know how many watts it can safely handle, then you can determine how loud the speaker can potentially be. The next video will help you with running that calculation. There’s a link to that video in the description below.
Another thing to consider is the impedance of the speaker or speaker circuit (which is measured in ohms). Amplifier power varies depending on the impedance of the speakers you connect to it.
In theory, halving the impedance doubles the power output and doubling the impedance halves the power output. This would mean that an amplifier rated to provide 100 watts to an 8 ohm speaker circuit would provide 200 watts to a 4 ohm speaker circuit or 400 watts to a 2 ohm speaker circuit. But this is all theoretical!
In practice, you will rarely achieve these theoretical figures due to the design limitations of the amplifier. Within the specifications of an amplifier, you will see that it is able to provide more power at half the impedance, but rarely a full doubling of the power.
Ohm’s Law tells us that more current will be required at a lower impedance in order to provide the same amount of voltage.
It may be helpful to visualize this with a water pipe analogy. Think of voltage as the water pressure, current as the amount of water flowing through the pipe, and resistance as the diameter of the pipe. If the diameter of the pipe is increased, water can flow more easily (similar to more current flowing when the impedance of the speaker circuit is decreased). But this also means more water (or more current) is required to maintain the same water pressure (or voltage). Eventually, the source may be incapable of supplying enough water (or current) if the resistance, or impedance, goes below a certain level.
So, the second rule is to ensure that the amplifier is rated to provide the desired power output at the specified impedance.
The specifications of your amp probably show a range of impedances (usually between 4 and 16 ohms) and the power output that can be facilitated at each impedance.
This rule is simple – don’t ask the amplifier to go beyond its specified power output rating. Just because an amp can provide 100 watts at 8 ohms doesn’t mean it will provide 200 watts at 4 ohms, even though the theory might suggest that it can… It’s possible that the amplifier won’t be able to manage the extra current and heat that comes along with providing so much power through such a low-impedance speaker circuit.
The third rule is somewhat of a balancing act between the power handling capacity of your speakers and the power output rating of your amplifier. An amplifier that is too powerful or not powerful enough can cause damage to the speakers or even the amplifier itself.
This may be counterintuitive, but you’re probably more likely to cause damage with an underpowered amplifier than you are with an overpowered amplifier.
Notice that I said “YOU are more likely to cause damage”. Yes. The damage will ultimately be the result of improper gain staging in either case. No matter which amplifier and speaker combination you choose, it’s up to you as the person operating the system to ensure that the speakers and amplifier operate within their specifications and that limiters are in place that will prevent excessive signal level from reaching the amplifier or speakers.
With that said, I’ll share with you a general guideline for choosing an amplifier for maximum fidelity and performance: “Choose an amplifier that is capable of providing between 2- and 4-times the continuous power rating of the speaker.” For example, if my 8-ohm speaker has a continuous power capacity of 100 watts, I will choose an amplifier rated to provide at least 200 watts to 8 ohms.
You may hear this and think, “Won’t that cause damage to my speakers?!”. If you follow this guideline AND you allow the amplifier to operate at its max power output, then yes – your speakers will not be capable of handling the power from the amp, the voice coil will become excessively hot and your speakers will be destroyed.
The trick is to not turn the amplifier to 100%. Instead, run the amplifier at a level that doesn’t exceed the speaker’s continuous power capacity for an extended period of time.
Now you might be asking yourself if there is a benefit to this extra power. Why not just use an amplifier with a power rating that is equal to the continuous power capacity of the speaker?
The benefit of this additional power is increased headroom in the system. Headroom (in this case) refers to the difference between the average operating level of the amplifier and the maximum level it can deliver without excessive distortion.
You certainly can use an amp that is only capable of providing the speaker’s continuous power capacity. But what often happens in this scenario is that you’ll feel like your speakers are capable of producing more sound than you’re hearing, so you’ll increase the level of the signal going into the amplifier. At a certain point, the signal at the amplifier’s input is driven into clipping which results in a square wave signal that can cause damage to the speaker.
It’s much better to build in plenty of headroom so that you don’t have the tendency to turn the signal up beyond its limits. With extra headroom, the speakers will perform to their full potential without a risk of clipping the signal at the amplifier’s input. The additional headroom will also allow the system to reproduce transient peaks and dynamics in the signal much more easily and cleanly.
Remember – a doubling of power only equates to about a 3 dB increase, so for a full 6dB of headroom you’ll want to double the power again, using an amplifier that is CAPABLE of providing 4 times the continuous power rating of the speaker.