Amplifier Clipping Amplifier clipping (output overload distortion)

What clipping is

Clipping is waveform distortion that occurs when an amplifier is overdriven and attempts to deliver an output voltage or current beyond its maximum capability. The output cannot rise above the supply rails, so the tops and bottoms of the waveform are cut off. Wikipedia's entry notes two consequences: driving an amplifier into clipping "may cause it to output power in excess of its power rating," and in the frequency domain clipping "produces strong harmonics in the high-frequency range (as the clipped waveform comes closer to a square wave)." The extra high-frequency weighting, it adds, could make tweeter damage more likely than if the signal were not clipped.

There are two varieties. Hard clipping, typical of solid-state amplifiers, flattens the peak abruptly and generates odd harmonics, with the third harmonic standing out. Soft clipping, typical of tubes, rounds the knee gradually and leans on lower-order and even harmonics, which many find less objectionable. Both are distortion; the difference is the spectrum.

How clipping damages tweeters

A tweeter is a small voice coil that handles the top of the spectrum. In a normal movie or music signal, most of the power is at low frequencies and very little reaches the tweeter. A clipped waveform changes that distribution twice.

First, the average power rises. A sine wave's RMS value is its peak divided by 1.414. A square wave's RMS equals its peak. At the same peak voltage, a fully clipped sine carries twice the average power, a rise of 3 dB. The amplifier is delivering more heat to the speaker than the "clean" volume setting suggests. Second, the extra energy lands in the harmonics. A square wave contains odd harmonics at amplitudes of 1/3, 1/5, 1/7 and so on of the fundamental, and those harmonics keep going far above the original note.

Worked example, as arithmetic rather than a measurement. Take a 100 Hz bass note that a 50 W amplifier plays as a clean sine into 8 ohms. If it is driven so hard that it becomes a square wave at the same peak voltage (28.3 V), the average power doubles to about 100 W. Summing the Fourier components, about 2.0% of that power, roughly 2 W, sits in harmonics above 2 kHz, the region a typical crossover sends to a tweeter. The clean sine put essentially none there. Real clipping is partial and music is not a pure tone, so treat this as the upper end of the effect. Sustained power, not a single peak, is what heats a voice coil.

Total power and its spectrum are the mechanism: clipping raises the average power and moves part of it up to where the tweeter receives it, so a clipped amplifier can overheat a tweeter at a volume where a larger, unclipped amplifier would be safe.

How rated power relates to clipping

The FTC's amplifier rule gives rated power a definition tied to distortion. Under 16 CFR 432.3, rated power is "minimum sine wave continuous average power output, in watts, per channel," measured with all associated channels fully driven. Any power level from 250 mW to the rated power must be obtainable from 20 Hz to 20 kHz without exceeding 1.0% THD+N at 8 ohms after the signal has been applied at full rated power for at least five minutes. Testing starts with a one-hour preconditioning at one-eighth of rated power with a 1,000 Hz sine wave.

So a rated watt is a clean watt at 1.0% distortion, and the amplifier is not clipping hard at that point. Past it, THD climbs fast. Compare power figures only at the same distortion and load (see continuous vs dynamic power rating, the FTC amplifier rule and all-channels-driven).

Headroom: why bigger amplifiers protect speakers

A bigger amplifier clips later. Film soundtracks need it. Cinema calibration sets the reference electrical level 20 dB below 100% modulation (SMPTE RP 200:2012), so the system must be able to reproduce peaks 20 dB, or 100 times in power, above the average level. With 2 dB of dynamic headroom a 100 W receiver has about 160 W for peaks. The Home Cinema Guide notes that typical receiver headroom is around 20 to 50% over rated output, and its example of a 100 W receiver is 120 to 150 W at peaks (see maximum SPL and headroom).

The practical rule is to buy power for the peaks you intend to play and the sensitivity of your speakers: low-sensitivity speakers in a large room at high volume are the setup most likely to push a receiver into clipping.

How to recognize clipping

Signs, from simplest to most reliable. Audible: harsh, gritty or compressed sound on dynamic peaks that gets worse as you turn the volume up. Behavioral: the receiver shuts down or shows a protection message when pushed. Measured: an oscilloscope shows flattened tops, or a spectrum analyzer shows a rise in odd harmonics, with the third harmonic as the obvious outlier. Wikipedia describes the detection method for circuits: attenuate the output by the gain (for 10 dB of gain, attenuate by 10 dB) and compare it with the input; any difference is clipping.

Preventive steps: lower the master volume or the speaker trims, avoid pushing the sub and the mains at once, enable speaker size and crossover settings (bass management) to keep deep bass away from small speakers, and add amplification (pre-outs to an external amplifier) before an AVR's internal amplifiers run out of room.