Danger: Low Power

It’s a common occurrence, unfortunately: high-frequency components of loudspeaker systems damaged because they’re being fed by power amplifiers rated at lower (rather than higher) power output than recommended.

This can be difficult to understand. Let’s look at how it happens and then present some preventative solutions.

Too Little, Too Much
Not all musical notes are created equal. There is much more power in the lower registers of music than in the midrange and treble regions. If we examine the accompanying graph, we can see that the energy content of treble frequencies is typically 10 to 20 dB less than bass and midrange frequencies.

Therefore, even if we allow for 10-dB peaks in high frequency program material, which is common, the high frequency driver of a system will be called upon to handle only about one-tenth the power that the low and mid frequency components must sustain.

This natural distribution of musical energy works to our advantage. It means, for example, that a loudspeaker system capable of handling 100 watts should have a high frequency unit capable of handling 10 watts. Thus, if the high frequency unit is designed to handle 20 watts of power (characteristic of many JBL systems), we are building a 100-percent safety factor into the high frequency unit.

The result is that the capabilities of the components of a loudspeaker system parallel the natural energy distribution of music.

The Nature Of Amplifier Power
The power output specification of an amplifier is not absolute. Under certain operating conditions, such as when the volume control is set too high or when the input signal is too great, the amplifier can exceed its published output. The power output of an amplifier is rated with reference to a given level of total harmonic distortion (THD).

If required to produce more power, the amplifier will do so, but at considerably greater distortion levels. For example, an amplifier rated at 10 watts (20 to 20,000 Hz into an B-ohm load) at no more than 0.5 percent THD could be overdriven to produce 20 watts of output power to the loudspeakers.

Under these same adverse conditions, an amplifier rated at 20 watts could deliver 40 watts to the loudspeakers; a 35-watt amplifier could deliver 70 watts and a 50 watt amplifier could be overdriven to deliver 100 watts. This distorted output could very well be in the treble region, as we shall soon see.

Here’s The Killer
The additional power generated by overdriving the amplifier is rich in harmonics (distortion). These harmonics can be particularly dangerous to high frequency drivers. Harmonics are higher frequency multiples of the original signal; therefore, the high frequency component of a loudspeaker system must bear the brunt of the distortion-even though the original signal may have been generated by a bass guitar.

What It Looks Like On A ‘Scope
When a sine wave test signal (a signal consisting of a fundamental frequency without overtones or harmonics) is displayed on the screen of an oscilloscope, its top and bottom extremes will exhibit normally rounded contours.

Average output power is one-half the peak output power. When an amplifier is overdriven, the contours are ”clipped” off, producing a near square wave, having flat areas at the top and bottom limits, in which the average power approaches the peak power. When this occurs, up to twice the amplifier’s rated output can be delivered to the high frequency driver, which may not be capable of handling the abnormal load.

A higher powered amplifier, however, can generate the required power levels without clipping, allowing the loudspeaker system to receive program material containing a normal distribution of energy levels. Under these conditions, damage to the high frequency driver is most unlikely.

There are no hard and fast rules. Very few amplifiers have meters that are capable of accurately indicating when an amplifier is being overdriven to the point that it could damage loudspeakers. Even the volume control position is not a clue- half rotation often produces considerably more or less than 50% of an amplifier’s power. There are no absolutes. We wish there were.