GOODSOUND!GoodSound! "How To" Archives

Published May 1, 2008

 

How to Recognize Amplifier Clipping and What to Do About It

In "How to Translate Speaker Sensitivity Ratings Into Real-World Requirements," published in February, I mentioned that amplifier clipping can lead speakers -- mostly tweeters -- to self-destruct. This time, I talk more about clipping -- what it is, what causes it, how to recognize it, and what to do about it.

When an amplifier is overdriven -- i.e., asked to output more power than it can reliably produce -- the waveform of the resulting sinewave is flattened at the top and bottom; in other words, it’s clipped. The clipping may be the same at the top and bottom, or it may be asymmetrical, meaning that the top is more clipped than the bottom, or vice versa. In either case, clipping is bad because it introduces direct current (DC) into the signal path, with possibly disastrous results.

There are several possible causes of clipping, some of which are found in solid-state amps, some only in tube designs.

The most common cause of clipping in solid-state amplifiers is when the amp is asked to produce an output voltage greater than its supply voltage. A cause of clipping in both solid-state and tube amps is that the power supply’s capacitors -- a reservoir of energy for sudden current drain -- become overtaxed and begin to electrically "collapse."

Tube amps can suffer from different causes of clipping: either the output tubes can’t transfer enough electrons from the cathode to the plate, or the output transformers can’t handle the amount of power applied to them and their core (windings and plates) and thus go into what’s called "saturation."


Normal sinewave


Clipped sinewave

Implications of clipping

Glossary of Terms

As its name implies, a waveform is a wave-like image that represents an electrical signal. It shows the changes in signal amplitude (i.e., magnitude) over a certain amount of time. There are an infinite number of possible waveforms, but the most commonly used in audio are the sinewave and squarewave. The latter, with its squared-off waves, is typically used to illustrate the performance of digital sources, while the sinewave has a rounded shape (illustrations: wikimedia.org).

Direct current (DC) is electricity that flows in only one direction. Alternating current (AC) switches between negative and positive from 50 to 60 times per second (Hertz), depending on your national electrical grid. The classic AC waveform is expressed as a sinewave (see above).

Capacitors are passive electrical devices that store electrical current and are thus analogous to the fuel tank of a car, ensuring that a steady amount of current is available to the circuits downstream. Their storage capacity is measured in Farads, most commonly microfarads (F or uF).

Collapse in this context means that the circuitry downstream of the capacitors becomes starved of current because the capacitors are not charging and discharging fast enough to meet the demand.

The anode (or plate) of a vacuum tube (or other electrical device) is the terminal at which current flows into the device. The cathode is the terminal where current flows out of the device.

A core is the heavy, ferrous-metal heart of a transformer. A standard step-up (increases voltage) or step-down (decreases voltage) transformer has two cores: the primary (or input, connected to the mains electricity) and the secondary (output). Transformers come in a variety of shapes, their names generally descriptive of the core’s shape: the "E" core, "I" core, or "toroidal" core. Cores are often made up of a sandwich of thin metal plates. Metal windings, typically copper, are wound around the transformer’s cores, the length of the windings determining the electrical characteristics of the transformer (for example, the primary winding’s length may be for 120VAC input and the secondary for 25VAC output). Saturation occurs when a transformer’s magnetic core can’t absorb a stronger magnetic field -- like a sponge that can hold no more water. Saturation can cause the transformer to rapidly overheat and greatly affects its performance.

A voice-coil is part of a loudspeaker’s motor assembly and consists of a length of wire coiled around a thin collar, which sits at the center of the speaker driver’s magnet assembly. Electrical current passed through a voice-coil creates a magnetic field. The induced magnetic field acts with the driver’s magnet to move the loudspeaker cone, which pushes air to create soundwaves. If too much electrical current is passed through the voice-coil -- in other words, if the voice-coil is "overdriven" -- it can be pushed beyond its safe operating range, or compliance.

...Colin Smith

Obviously, continuously driving an amplifier past its own limits won’t do it any good, and in all likelihood will damage it. In addition, clipping will more than likely damage your speakers’ drivers. In fact, although it sounds counterintuitive, it’s far easier to damage a pair of speakers with an underpowered amplifier that’s frequently driven into clipping than with an amplifier that can easily deliver far too much power. Tweeters are particularly susceptible, because clipping often creates strong harmonics, some of them beyond the range of hearing, that can force the tweeter’s voice-coil past its regular compliance, or make it overheat to the point that it simply melts. You want to avoid amplifier clipping at all costs.

How to recognize clipping

Most people don’t have test instruments in their listening room, and so must rely on their ears to tell them of any problems. Luckily, if you know what to listen for, you can hear when an amplifier clips. (You can’t always hear clipping, but if you do, do something about it.) For example, odd sounds emanating from your speakers, especially in passages of rapidly changing highs, say in violin trills or some guitar solos, can become blurred or indistinct, and the sound will more than likely become very edgy, hard, and distorted.

Another symptom of clipping is that the dynamic range becomes constrained -- all the sounds can sound as if they’re being played at the same volume. This is because the amp has run out of steam and simply can’t play any louder, no matter how high you turn the volume control. When that happens, even if the amplifier isn’t clipping, it’s probably close to it.

What to do about clipping

The best cure for clipping is prevention. If you can hear that your amplifier is clipping, turn it down immediately. If you find that your amplifier is frequently driven into clipping, it might be time to buy a more powerful amp (assuming you want to keep the same speakers) or more sensitive speakers (if you want to keep the same amp).

When you’re buying an amplifier and speakers, first make sure that the amp can deliver enough power to the speakers to play music at the volume you want. Obviously, you might not know exactly how much power you need until you get home, but you can do some work ahead of time to know approximately how much power you’ll need. (See "How to Translate Speaker Sensitivity Ratings Into Real-World Requirements" for a quick lesson on how to do the math.)

Also, be aware of your listening habits and what they might mean in terms of amplifier power requirements. If you like your music at arena levels (loud!), you’ll need far more power than someone who likes to listen at the level of a whisper. In addition, be aware that many speaker manufacturers suggest minimum and maximum power inputs, or amplification levels, for their products. Pay attention to those suggestions, particularly the minimum-power requirements, as that’s where the problems of speaker-damaging clipping will most likely occur. My final suggestion is that you always err on the side of too much rather than too little power. Remember, it’s easier to damage a pair of speakers with a low-powered amplifier driven into clipping than with one that can easily deliver an abundance of clean, undistorted power.

...Thom Moon


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