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 (µF 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 |
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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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