Boss Audio Systems BV8.5GA User Manual download pdf (Page 22)

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Stereo

Scene

BLAZING

SPEAKERS

HAVE you

blown

out

any

good

loudspeakers

lately? The

ques-

tion is not frivolous,

because

the

fac-

tory return rates

for many high

-quality

speaker

systems, certified

safe

for

loud home listening,

are apparently on

the

rise. Manufacturers

have even

begun encouraging

the press to

stories about loudspeaker

fail-

ure,

presumably in the

hope

educat-

ing

the consumer

and forestalling

dis-

aster.

The

trouble

is,

such stories have

a way

growing

into

"pageants," to

use the word

one company

spokesman. Although most

speaker

blow -outs result from

a simple

causation -an

attempt on the

speaker's part to

absorb

electri-

cal energy than is

good for it -it

is next

to impossible

be both exhaustive

and concise

about all the ways

which this

can happen. There

are

I-

a 5

FREQUENCY IN

OCTAVES

Fig.

1. Sine -wave fundamental

with

two harmonics results

in a waveform approaching

square

wave,

which

ideally

would

have an infinite series

of odd -order

harmonids.

FUNDAMENTAL

THIRD

HARMONIC

FIFTH

HARMONIC

RESULTANT

By Ralph Hodges

more, it

seems,

than meet

the

eye

(figuratively)

or the ear (literally).

Big Amps

and

Small. One

cause of

speaker

failure is

obvious,

you'd

think:

the

proliferation

of super

-power

amp-

lifiers. And

yes, I'm

sure

there

are and

will continue

to be those

who

over-

power their

systems

into

occasional

attacks

of silence,

although the likeli-

hood

is that this

comes

about most

often through

accidental

signals

-a

loose

ground

connection,

severe

switching transient,

a dropped

tone

arm, or even (as

used to plague

rock

groups

when

setting

up)

a sudden

outbreak of

acoustic feedback

to the

microphones

when no

one can get to

the

controls in time to

save the

situa-

tion. Now

and

again someone does

destroy

a speaker by playing loud

music

through it. But

since

the

speaker almost

certainly exhibited

audible

signs of distress

before

suc-

cumbing, he usually

can't say that he

wasn't

warned.

Excessive

amplifier power

isn't

al-

ways to

blame, however.

From

the

latest

reports,

one

the

serious

and

growing problems

the blowing

out

speakers by

under -powered

ampli-

fiers. This

was first

brought to my

at-

tention

an article by Peter Mitchell,

who among

other distinctions

is the

president

of the

Boston

Audio

Society.

His

written

explanation was

so plausi-

ble in

theoretical

sense

that

I was

first

tempted to

think the

whole busi-

ness

was just that:

theoretical

possi-

bility that doesn't

occur much

in real

life.

A few phone

calls to manufactur-

ers

set

straight. It's not

a rarity.

Naturally,

the resulting

customer

un-

happiness

is worse,

as a rule, than

when

one of

the

super -power

af-

ficionados

does

his

direful deed. The

under -powered

customer, you

see,

thought

he was taking pains to

stay

within the

power limitations

specified

the

speaker

manufacturer,

and yet

still

got

into trouble.

This

is what

evidently takes

place.

amplifier

called upon to

operate at

its limits

much

of the time will

clip

fre-

quently,

squaring off the tops

and bot-

toms

of any waveforms

it can't pass.

According

waveform

analysis,

these

flat tops

and bottoms represent

odd -order harmonics

of a sine -wave

fundamental:

i.e.,

spurious high fre-

quencies

significant amplitude

(see

Figs. 1

and 2). These

harmonics

-and

some

them

can

be very high in

frequency

-get routed

to the tweeter,

which

has to cope

with them in

some

way. If

clipping is frequent,

the tweeter

has

almost continuous

input of dis-

tortion products

to handle. In

time its

relatively

fragile voice

coil, given no

chance

to cool off,

shorts, or opens

up,

and

that's it.

The

putative

problem here is that

there's more

(and more

frequent)

high- frequency

energy in

the clipping

distortion than

in most music,

and

therefore

energy than the

tweeter's

designer

anticipated

its hav-

ing to take.

course, you wouldn't

expect the

distortion

sound too

good. But

perhaps, if

it were

high

enough in frequency,

and if the

re-

cording had

typically

hard, wiry

top

end and

a continuous

background

high

-hat

cymbal, it wouldn't

espe-

cially

annoying. In

any

case, it evi-

dently

happens,

as the man

says,

and

audiophiles

are advised to

give a close

listen

to the high

frequencies

whenever

they run their

systems

near maximum

levels to make

sure

things

don't

sound

worse than

they

should.

This

is a particularly good

idea

with pop

music having heavily

com-

pressed

dynamics,

since a constant

level means

constant

clipping when

the

amplifier is

operating at its

design

limits.

FREQUENCY

OCTAVES

Fig. 2.

Part of the spectrum

for a perfect square wave.

POPULAR

ELECTRONICS

1 2 ... 17 18 19 20 21 22 23 24 25 26 27 ... 101 102

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