Amplifier power is measured in “Watts” at a specific output impedance in “Ohms”. Note that this is a measure of the electrical power created by the amplifier and not the loudness of the amplifier (that depends on the efficiency of the amplifier and speakers used).
There are several different standards for measuring amplifier power but the most relevant one is the “RMS power”. This gives an average power rating across the entire frequency range and is the most reliable specification to use when comparing the power of amplifiers.
A typical power specification for an amplifier would be -
350 Watts RMS into 4 ohms
The “350 Watts RMS” is the output power of the amplifier and “4 ohms” is the impedance of the speakers required for that power. Note the same amplifier will usually have different power ratings with different impedance loads e.g.
RMS power is a universal specification in musical instrument amplifiers. However, if a specification does not say “RMS” do not assume that this is implied. Unscrupulous manufacturers will sometimes use other power measurement standards (e.g. “peak power” or “music power”) to give their products a seemingly higher power rating.
You should only compare amplifier powers if the “RMS power” is quoted.
In theory if you half the impedance, say from 8 ohms to 4 ohms, you should get double the wattage, but this never happens, usually because the power supply wont cope with with the extra demand.
e.g.Scenario 1: You run your 500 watt/8 ohm SS amp with a 28v output into an 8 ohm speaker. It will deliver 100 watts. Add a second 8 ohm speaker, leaving all the settings the same. The voltage swing will still be 28 volts, the current will double into the halved impedance load, and you will get 200 watts.
Scenario 2: Run your 500 watt amp at full power into an 8 ohm cab. The output swing will be 63 volts. Add a second 8 ohm cab. You won't get 63 volts, because the amp's power supply isn't capable of delivering maximum voltage into all impedance loads. It generally drops by a factor of .7 for each halving of the load impedance, so instead of 1,000 watts into 4 ohms you might get 700.
So while you will get a doubling of power with a halved impedance load if the amp is well below full power you won't get it at full power.
Power is not the limiting factor of how loud an amp will push a speaker. A speaker's output is determined by voltage swing, not power consumed. The entire concept of going to as low an impedance load as one can to 'get all the watts out' of an amp is intrinsically flawed. The main result of so doing is to cause the amp to generate higher heat levels, shortening component life.
There is resistance in the connections between the amp and the speakers, including internal resistance in the valves/transistors themselves. The power lost as heat in these components is proportional to the square of the current (P=I²R), so if you double the current, the losses are quadrupled. This is also the reason why we have high-voltage power lines: it keeps the current low, meaning less power loss in the lines.
Speakers are rated by their power rating in “Watts” and their impedance in “Ohms”.
The power rating in Watts tells you how much amplifier power the speaker can handle without damage. It does not tell you how loud the speaker will be, as this depends of the efficiency of the speaker and the speaker enclosure. As with amplifier ratings, the power should be quoted in “Watts RMS” to be meaningful.
The impedance is expressed in Ohms and is a measure of the resistance to an audio signal delivered from an amplifier. The lower the impedance, the more power is required from the amplifier to drive the speaker. Impedance is similar to resistance but the two are not the same – impedance is measured over a wide range of frequencies, whereas resistance is not frequency dependent.
When matching a speaker to an amplifier you should obtain the RMS power ratings and relevant impedance values of both.
The exact power rating required for the speakers depends on the speaker design, but as a general rule of thumb it is advisable to use speakers that can handle more power than the amplifier can produce – around 50% more is a good starting point (see NOTE below).
This means that for an amplifier rated at 400 Watts RMS it is advisable to use speakers rated at 600 Watts or more. This will give you a lot of clean headroom (i.e. the speakers will not distort when the amplifier is operating near full power) and allows a safety margin to prevent peaks in the amplifier signal from exceeding the power capability of the speakers.
IMPORTANT NOTE: it is quite possible to damage high powered speakers by driving them with a lower powered amplifier that is being driven at full volume. Although this seems counter-intuitive the physics of sound make this possible. For this reason, you should avoid driving your amplifier at full volume to the point where is starts to distort badly (some mild “musical” distortion is acceptable). If you cannot get enough clean volume from your amplifier you need a more powerful amplifier.
When selecting the impedance of a speaker you should consult the user manual for the amplifier to determine the minimum safe impedance for that amplifier. If the amplifier has a valve (tube) power section then this is critical.
Non-valve amplifiers will automatically adjust to match speaker impedances down to a minimum value (usually 4 ohms but sometimes as low as 2 ohms). As you reduce the impedance the output power available will increase.
Valve amplifiers often use a manual selector switch for selecting the output impedance. If your amplifier uses such a switch then you must ensure that it is set correctly to match your speaker impedance or expensive damage will occur to the amplifier.
You can use more than one speaker cabinet with an amplifier to obtain more volume or a greater spread of sound. If your amplifier has two speaker output connections then simply plug one cabinet into each.
Alternatively, you may find that your speaker cabinet has a “link” connector as well as an input connector, which allows the speaker cabinets to be “daisy-chained” together.
When you use more than one speaker cabinet, the total impedance presented to the amplifier will change. If you use either of the above techniques to connect multiple speaker cabinets then the speakers will most likely be connected in parallel, since most professionally produced equipment uses parallel connections for this purpose (check the user manual if in doubt).
Although possible, it is not often practical to combine more than two speaker cabinets to one amplifier in this way - the resulting total impedance will often be too low for the amplifier.
To calculate the total impedance of parallel speakers use the formula –
1/Rt = 1/R1 + 1/R2 + 1/R3 + etc.
Where Rt is the total impedance and R1, R2, R3 are the impedances of each speaker cabinet.
Generally it is better (but not essential) to use speaker cabinets that have the same impedance (e.g. all 8 ohm cabinets). Here are some examples of parallel speaker cab combination impedances
Always check the minimum impedance allowed for your amplifier. Often this will be quoted at 4 ohms; therefore you should ensure that combining speaker cabinets would not result in an impedance less than this (e.g. two 4 ohm cabinets in parallel will result in 2 ohms which could damage an amplifier with a minimum impedance rating of 4 ohms).
** Be careful with speaker loads less than 4 ohms. Only high-end professional amplifiers will be able to run continuously and safely into these low speaker loads.
It is possible to connect speakers in series rather than parallel, but this is usually done when installing multiple speakers inside a single speaker cabinet rather than by combining individual speaker cabinets.
To calculate the total impedance of speakers in series, use the formula
Rt = R1 + R2 + R3 …etc.
Finally, here is a link to a handy impedence calculator: http://www.duncanamps.com/technical/impedance.html
The loudness of an amplifier and speaker combination is very difficult to predict from specifications alone. The power rating in Watts of each can give a very approximate guide to the loudness but cannot be used to compare set-ups with very similar powers.
Perceived loudness is not linear therefore you have to output increasing amounts of power to change the loudness the louder you go. As a general guide you will need 10 times the output power to achieve twice the loudness. This means that, all other things being equal, there would be disappointingly little difference between the loudness of a 350-Watt amplifier and a 500-Watt amplifier.
The efficiency of the speaker cabinet is a much greater indicator of loudness, and a specification exists for this. Sound Pressure Level (spl) measured in decibels (dB) at a specific distance (usually 1 metre) from the speaker is a measure of how much volume a cabinet will produce. A very efficient speaker cabinet will sound considerably louder than a poorly designed cabinet with the same amplifier setup.
Increasing the number of speakers in a setup will almost always result in more volume for two reasons. Firstly the total speaker impedance will usually be reduced, resulting in more power from the amplifier. Secondly, more speaker units mean more air being moved which results in more volume (badly designed speakers can, however, negate this effect).
