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Guitar Speaker Power Handling (Part 2)

Speaker power ratings are commonly misunderstood by guitar players. How can you make sure to choose guitar speakers to last a lifetime?

Wednesday, 27. July 2011 - by Kurt Prange

In Part 1, we examined the basics of how speakers work and how their power rating is determined. Now, let’s look at things from the perspective of the guitar amplifier so that we can choose guitar speakers that will last a lifetime.

Guitar Amplifier Power Output Ratings

The power output rating of a guitar amp is mostly a ballpark figure for what it can put out. Amp specifications commonly list power output in a form similar to the following:

Power Output: 50W into 8Ω at 5% THD

This type of power output rating is obtained by using a sine wave from a signal generator (usually 1 kHz) as the input signal. The 5% THD (total harmonic distortion) figure means that the sine wave was able to generate 50W of power output with relatively low distortion (near the threshold of clipping or overdrive). THD measurements were one of the first conventions used to objectively compare the fidelity of audio amplifiers.

Guitar amps are unconventional audio amplifiers. While most audio amplifiers are designed to keep distortion as low as possible, guitar amplification has evolved to where overdrive distortion is usually a requirement. For example, the Marshall® JCM800 2203 is a 100W tube amp that has a highly regarded overdrive sound. The owner’s manual lists the power output as follows:

Typical power at clipping, measured at 1kHz, average distortion 4% 115 watts RMS into 4, 8, 16 ohms. Typical output power at 10% distortion 170 watts into 4 ohms.

This example shows that for many guitar amplifiers, the power output rating (100W in this case) is not a maximum power output rating, but more of a ballpark clean power output specification.

RMS and Overdrive Distortion

RMS (root mean square) is a kind of average value that can be used to compare the power dissipation from different signals on equal terms. For example, a 20 VDC power supply dissipates the same amount of heat across an 8 ohm resistor as a sine wave with an RMS value of 20 VAC.

Guitar amp output ratings are usually based on a sine wave at low distortion, but if the volume is turned up further or a gain boosting effect is used, the sine wave becomes more overdriven and can approach the shape of a square wave. The RMS value of a square wave is equal to its amplitude, while the RMS value of a sine wave is equal to its amplitude divided by the square root of two.

Plugging the RMS values into the equation for power shows that a square wave dissipates twice as much power across the same load as a sine wave with the same amplitude.

This simplified overdrive distortion model illustrates how the 100 watt Marshall® amp which puts out 115 watts at 4% THD could put out an additional 50 watts at 10% THD.

Tube vs. Solid State Outputs

Many tube guitar amps use output transformers with secondary taps connected to an impedance switch allowing for the same power output when connected to 4, 8 or 16 ohm load impedances. Solid state amps do not use output transformers and do not have the same power output when connected to different load impedances.

For tube outputs, it’s important to match the load impedance to the amp’s output impedance. For solid state outputs, it’s important to use a load that is greater than or equal to the rated minimum load impedance and to know the amp’s power output at that load. For example, the Fender M-80 is a solid state amp rated for 69 W(RMS) at 5% THD into 8 ohms and 94 W(RMS) at 5% THD into 4 ohms (the minimum load impedance).

With solid-state amps, overdrive distortion generated by the power-amp is not generally considered musically pleasing, so most people will not exceed the amp’s low THD power rating. Tube power amps, on the other hand, are often played well beyond their low THD rating.

Amps with Multiple Speakers

When an amp uses multiple speakers the output power is divided between them. The nominal impedance of each speaker should be the same value so that power is distributed equally and so that the output impedance of the amplifier can be matched.

Choosing Guitar Speakers to Last a Lifetime

There is no standard method used by all amp manufacturers when selecting an appropriate speaker power rating. If you want to choose a speaker to last a lifetime, you’ll want to choose a speaker that can handle the maximum amount of preamp and power amp overdrive distortion that can possibly be put into it and safely avoid exceeding the speaker’s thermal limits. In the case of single speaker setups, this means choosing a speaker rated for at least twice the rated output power of the amp. For multiple speakers, choose twice the rated power that would be distributed to it.

You might decide to go with a lower power rating because you know that you’ll never be cranked at full power and love the sound of a lower power rated speaker. In the same way you may choose a speaker with a much higher power rating because of the way it sounds.

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