Boss Audio Systems BV8.5GA User Manual download pdf (Page 24)

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Bandwidth.

excessive

amplifier

power is

sometimes

a problem, exces-

sive amplifier frequency

response

(bandwidth)

can be worse.

Now

and

again

a speaker manufacturer

will

refer nostalgically

to the

carefree days

when

amplifiers

had

output trans-

formers,

those wonderful

low -pass fil-

ters that

blocked

dc and never

let

any-

thing

much higher

in frequency

than

the

ear could hear (and

sometimes not

even

that much)

get through

the

speakers.

Today

almost seems

if many

transistorized

amplifiers have

power

responses

from

dc to

practically infin-

ity.

Occasionally

a defective

unit is

found

with

much

ultrasonic

oscilla-

tion running

around inside that

al-

most

violates

the FCC's

regulations

illegal

r -f radiation.

Ultrasonic

stuff,

particularly

when it's

a constant,

steady -state

signal, is

extremely hard

tweeters.

some

high

frequency

the

mass

of any tweeter

will prevent

it -or

least

parts

it -from

moving

in response

to the

signal. This means

that

the entire input

is converted

into

heat;

being unable to move,

the voice

coil doesn't

even

have

the

benefit of

the

circulating

currents of cooling

air

that

are set up to some extent during

normal

operation. Recently, I heard

a case in which

an amplifier

so af-

flicted, when

hooked up to

speakers of

extremely high power -handling

capa-

bility, wiped

out all the tweeters

before

the music

even had

a chance to

start.

The

preventive measure

you should

take

here -and

I've

been observing it

ever

since the big, wide

-bandwidth

amplifiers

started to arrive -is to

con-

nect

up the

oscilloscope

before you

connect

up the

speakers. This

will en-

able

one

check for

ultrasonic

out-

put,

and also to

inspect

any low -

frequency

turn -on

pulses. It's

best

have the rest

of the

system hooked

too,

since certain

combinations of

components

seem occasionally

able

set up oscillations that

other com-

binations avoid.

Most

woofers today

are quite

sturdy,

physically

and electronically. You've

got to be in

a position to deliver really

brutal

amounts of

power

to them

be-

fore

they'll

sustain

much

damage.

Still, this is

a possibility that must

considered. I have

not heard lately

any

woofers

being hurt

by record

warps

and such

perturbations,

al-

though

the amount

of subsonic

energy

they can generate

some-

times

startling. But

amplifier mis-

behavior is another

subject.

Any

decent

amplifier

should be

sta-

ble within the

limits

of its

power

output capabilities,

and even beyond

them.

And

yet stories

persist

about

certain

designs that

are believed

produce

horrendous,

speaker -des-

troying pulses

of subsonic

energy

when

overdriven, presented

with

unfortunate

type

of speaker load,

otherwise

mistreated.

True

enough,

some early

amplifiers

were

subsonic -

ally

unstable, and probably

the re-

membrance

them

feeds the

fires

suspicion about modern

units. Fur-

thermore,

there

are undoubtedly

numerous

cases of

amplifier

defects

that have

caused

strange,

unpredicta-

ble signals to

appear

at the

outputs.

But

from

what I've

been

able to learn,

such misbehavior

can clearly

attributed to

any of the

modern, popu-

lar models.

Speakers are returned

their

manufacturers

daily

with

the

woofer

voice

coils

torn

out

by the

roots,

among

other sorts

of mayhem.

But it's

rarely possible

to tell whether

the blame lies

with

the

amplifier

the

user.

Needless

to say, an amplifier

would

have to be a big one to destroy a

woofer in

such

dramatic fashion.

Cautionary

notes

are always

order

with high

-power

amplifiers.

Whether

suspects trouble or

not,

it's proba-

bly a good idea for

any owner of one

spend an evening

listening

with the

speaker grilles off, just to familiarize

himself with

the degrees of excursion

the woofer cones routinely

through.

Another

point worth

discussing

the matter

of dc

voltages

(or dc "off-

sets") that may appear

the

output

capacitors. In

a paper presented

sev-

eral years

ago,

Kerry

Gaulder,

who

has

served

design and engineering

capacities

with

several

major

com-

panies, treated this subject at

length.

amplifier

with dc

offset

will, of

course, produce a constant current

through the

woofer voice coil. But this

source

of voice -coil heating

is rarely

sufficient

magnitude to concern any-

one.

What

problematic, according

to Gaulder,

is the

possibility of seri-

ous woofer -cone offset. In other

words, the dc current displaces the

voice coil in the

magnetic

gap (either

forward

or back, depending on polar-

ity),

so that

it is

approaching the limits

of its excursion

one direction, even

when it's not reproducing

any sound

and is presumably at

rest.

If this theoretical possibility

true,

acoustic -suspension

woofers

might

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