Koss Totem Mani-2 Manuel d'utilisateur télécharger pdf (Page 20)

Feedback

Nu ts&Bolts

hat is the impedance of

your loudspeakers? Is

it 8 ohms? Or 4 ohms?

Perhaps you know that

it is not just a single number, but what

difference does it make anyway?

At the very least you are no doubt

aware that a speaker with a very low

impedance can present a problem for an

ampliﬁer, and potentially can damage it.

Think about the fact that short-circuit-

ing an ampliﬁer output can either break

it, or blow a fuse, or trigger a protection

circuit. The lower the impedance of

a loudspeaker, the closer it comes to

being a short circuit. Some ampliﬁers

can drive a load of 2 Ω or even 1 Ω, but

most will not. (The Greek letter Omega

is of course the symbol for resistance).

In any case, low impedance may not be

your only worry.

With this issue, UHF intends to begin

publishing impedance curves for loud-

speakers reviewed, and for that reason

I have been asked to explain speaker

impedance, and also to suggest a simple

manner of measuring a speaker's imped-

ance. “Simple” in this case means using a

minimum of specially-purchased equip-

ment, though in day-to-day operation it

is less simple than using a purpose-built

instrument that can spit out a complete

impedance graph in a few seconds. Yes,

impedance measurements result in a

graph, not just the single ﬁgure usually

found in loudspeaker literature, but let

me begin with some basic concepts.

What is impedance?

If a loudspeaker were to be driven

by DC (direct current) we could speak

simply of its resistance. The speaker’s

internal wiring has a certain (low) resis-

tance, as does the ﬁne wire that makes up

each driver’s voice coil. However loud-

speakers are intended to be driven by AC

(alternating current), whose frequency of

alternation is that of the sound we are

attempting to reproduce. Thus we need

to take into account the speaker’s induc-

tance and capacitance. The voice coil is an

inductor, and the internal wiring may be

as well. Inductance can be thought of as

a resistance that is frequency-dependent,

with its ohm value rising as frequency

drops. Most crossover networks include

capacitors, which introduce capacitance.

A capacitor can also be thought of as a

frequency-dependent resistor, whose

ohm value rises with frequency. Since a

capacitor’s impedance characteristic is

exactly opposite to that of an inductor, it

is easy to see how capacitors and induc-

tors can be combined to make ﬁlters.

I shall add, without great elabora-

tion, that these are not the only factors

determining the impedance reﬂected

back to the ampliﬁer. For example, as

a woofer cone moves back and forth,

acting as a linear motor, it also acts as a

generator, actually generating a voltage

that is opposite to that coming from the

ampliﬁer. That this complicates things

is an understatement.

It must also be evident that, in a

speaker that combines resistance, induc-

tance and resistance, the total impedance

cannot be a single number, since it will

inevitably vary with frequency. This is

not typically taken into consideration by

designers of ampliﬁers, who test their

designs by loading them with an 8 ohm

resistor, possessing neither capacitance

nor inductance, and having a constant

impedance at all frequencies.

The ideal, and the practical

The closer a speaker is to a pure

resistance, the more confidence an

amplifier designer can have that his

product will behave in the customer’s

home exactly as it did on the test bench.

That said, few loudspeakers are very

much like resistors at all, and so in fact

ampliﬁers must be designed to operate

with impedances that are vastly different

from that ideal resistor. What is more,

the designer cannot know in advance the

characteristics of the speakers that will

be used with his product.

To see what he (and we) are up

against, let us look at the impedance

curve of a small two-way speaker, which

has a famous name I do not propose to

reveal. It is shown on the next page.

The curve has been drawn by a

technique I shall describe presently

(see Measuring Impedance on page 20).

Most speakers, I might add by way of

explanation, have a considerable peak in

impedance at the point of resonance of

the woofer and cabinet. The one I have

arbitrarily selected has only a small rise,

centred around 100 Hz, which would

be the practical lower limit of its bass

response.

The manufacturer’s nominal imped-

ance rating is 4 Ω, but you need only

glance at the curve to see that it deviates

from that rating quite considerably. It

dips to about 3 Ω at 16 Hz, which should

present little problem for an ampliﬁer

designed the least bit competently.

Speaker Impedance

by Paul Bergman

18 ULTRA HIGH FIDELITY Magazine

1 2 ... 15 16 17 18 19 20 21 22 23 24 25 ... 83 84

Commentaires sur ces manuels

Pas de commentaire

Koss Totem Mani-2 Manuel d'utilisateur Page 20

Commentaires sur ces manuels