as a percentage, THD refers to what fraction of the output is not exactly
like a larger mirror image of the original. For example, 1% THD would
suggest that 1 part out of every 100 contained amplification errors. Obviously,
a larger amount of errors would raise the THD and the unlistenability
of the sound signal.
For most listeners,
3% THD is the threshold whereby distortion becomes noticable with most
types of music. At the 10% level hard clipping, which manifests itself
with audible cracks and screeches usually provokes the hand to fly up
and turn the knob down to something less offensive. Barely audible even
with test tones, (and who listens to test tones) is distortion in the
This is a toughie.
There are two primary modes or types of audio amplification--class A and
class AB. While I would love nothing better than to talk dB about these
two types for the next twenty years, the gist is that class A amps are
expensive, sound great, very inefficient, and impractical. And I love
to design and build them. Class AB amps are inexpensive, good sounding,
moderately efficient, and practical. Probably 99.9999% of all amps sold
today are based on a class AB design. We shall not make mention of the
dreaded economy class B amps. They cost pennies to make, sound rather
nauseating and have no business in audio reproduction.
Hop in a brand spankin'
new Ford Mustang Cobra. Feel the raw V8 power. Feel the pressure in the
small of your back when tromping on the accelerator. Fly around the curves
with all four tires screaming in protest as they approach their wipeout
limits. Raw, unrefined power, and a lot of it. Now, slip into a BMW M5
sedan. Enjoy the solidity and confidence at 150 mph as only a BMW can
deliver. Gracefully swoosh around turns without spilling your latte. Roll
down your window and snobbishly ask for Grey Poupon.
Class A amps deplete
your wallet like the M5 but once the sheer quality invades the senses,
anything less is suddenly slumming it. Class AB is by no means low rent.
Properly executed these designs can be very musical without breaking the
bank. However, all this drivel about amps does us no good if they aren't
connected to anything. Since the human ear can't hear the electrons in
amplifiers, hooking some wire from them to a device capable of moving
air molecules seems like the best course of action.
The final or "back
end" of a sound system is the actual reproducer itself. Something
must move air for the human ear to perceive and interpret sound. Existing
in two primary forms are driver elements, active (electrostatic) and passive.
Someday, a direct connection to the brain via a neural interface might
just be possible, but none are available as of yet.
element is usually found in large, costly speakers such as Quad or Martin-Logan.
These are speakers that require high voltage charge, courtesy of your
120V wall outlet, to operate. No, they do not have internal power amplifiers
within their boxes, think of it like a charge on a capacitor. The passive
element is the most common and prolific element. It consists of a driver
that needs no external power other than the amplifier signal. Passive
sub-woofer drivers, such as those found in Vandersteens are not covered
in this article.
Speakers have limits
as to how much juice can be put to their inductive coils before they physically
bang against their end stops. A loud crack is heard when this happens
and it's not recommended that anyone try this to any speaker you plan
to listen to in the near future. Rest assured that when I spy "30W"
stamped on the back of a driver, that is an absolute limit (within certain
tolerances of course).