Elenco Electronics MO-1251 Manual de usuario descargar pdf (Pagina 64)

70

AUDIO

UPDATE

The importance

of

amplifier output current.

MANY YEARS AGO, I WAS INVITED TO

attend a record company's press

conference, given

to introduce

a

new

"blockbuster" four

-channel

classical recording. To ensure that

there

would

be adequate sound -

pressure

level in the very large re-

cording studio where the record

was being

demonstrated, the engi-

neer

in

charge had set up four

Acoustic Research LST

speaker

systems

facing inward

from the

corners of an

imaginary

25- by 25-

foot square

in

the middle of the

studio floor. Those speakers were

to be driven by a pair of Phase Lin-

ear

700

amplifiers, thus providing

350

watts

per speaker channel. The

engineer

in

charge of the setup

was

an old

friend

of

mine

and

he

invited me to come

early

to help

balance the system.

When I arrived, my

engineer

friend was

cursing to

himself

be-

cause the amplifiers,

for

some rea-

son, were distorting badly on

musical peaks. I was mystified,

be-

cause,

in

my experience, the pair

of LST's I happened to have

at

home certainly didn't seem to be

so

low in

efficiency as to cause

clipping

in

a 350 -watt amplifier.

My

friend subsequently

solved

the

problem by replacing the power

amps

with

another brand (I

think

they

were Crowns)

borrowed

from

a

local dealer -and the press con-

ference

went

on as scheduled

without further technical hitches.

I

forget whether I

didn't

get a

chance

to

ask

about the amplifier

problem, or whether

I

simply

didn't understand

my friend's an-

swer.

In

any case,

I filed the matter

20

4

o

LARRY KLEIN,

AUDIO

EDITOR

WM,

IMPEDANCE

CURVE OF

A

TYPICAL "8 -OHM"

SPEAKER SYSTEM

10

11111111111

20

loo

1000

FREQUENCY

-HERTZ

FIG.

1

away

in my head in the "unsolved

audio

mystery" category-where

it

remained until

many years later. It

was

during

a discussion about

the

strange

reactive loads presented

by some speaker

systems that ev-

erything

suddenly

fell into place.

As

I mentally reconstructed

the

problem, it wasn't that

the

original

amplifiers

weren't capable of

put-

ting out their

rated power

into

standard

8 -ohm

loads; they just

couldn't deliver

their

power into

the

very low impedance

reactive

load characteristics

of the

AR LST

speaker system.

(As

I recall, the

LST

system

had

both

a tapped

in-

ductor

and a

very large capacitor at

its input terminals).

As

the

curve

in

Fig. 1 shows,

the

impedance of a

typical

"8 -ohm" speaker

system

can

vary

widely with frequency.

10000

20000

Speaker

impedance

and power

The best of today's better solid -

state amplifiers

approximate a

constant -voltage

source. That

means that

fora

given

input signal,

the

amplifier

will, within reason,

put out a constant

voltage

across

the

speaker

terminals-whatever

the load. If, for example, there are

20

volts

at the amplifier's

speaker

terminals,

the relevant part of

Ohm's

Law (watts

=

volts2 /speaker

impedance) tells us that

there are

50

watts

being

fed to an 8 -ohm

speaker.

Theoretically,

if

a

4 -ohm

speaker

were

connected,

the

same

20

volts

across

it would pro-

duce

100 watts.

However, very

few

amplifiers

double their

maximum output

power

when switched

from an 8-

ohm to

a 4 -ohm load.

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