Elenco Electronics MO-1251 Instrukcja Użytkownika Pobierz pdf (Strona 65)

100

fiers

will

deliver only about 50%

more power into 4 ohms than

ohms, due to power -supply

cur-

rent

limitations

or the intervention

of their

protective

circuits. Each

time the load resistance is halved

(causing

the

power output to be

doubled), the amplifier's power

supply is called upon to deliver

twice as much current to the out-

put devices at a given

input

signal.

Obviously, that can create prob-

lems. Under conditions of exces-

sive current drain, the power

supply's

voltage level

sags. As the

power

-supply voltage falls,

does

the maximum

output -power

capability of the amplifier.

Any

at-

tempt to drive the amplifier

beyond

its

lowered power- output

capability will result in clipping

distortion.

To recapitulate,

the sequence

goes this way: A very low value

speaker

impedance

causes exces-

sive current to be drawn

-via

the

output transistors -from the

am-

plifier's

power -supply. The exces-

sive

drain,

in turn,

causes the

power -supply voltage to fall. The

drop

in power

-supply

voltage

re-

duces the signal

level

that can

handled

the

amplifier.

When

that limit is

exceeded,

waveform

clipping results. The

audibility of

clipping

depends both on

its

se-

verity

and its

duration. If only the

tips of the highest

peak

waveforms

are truncated,

then the clipping

may pass

unnoticed.

However, in-

sofar as the music waveforms

be-

gin to resemble

square waves, the

problem

becomes very audible.

Overloads such as I've

de-

scribed

are

likely

to have addi-

tional consequences. The

ampli-

fier may run hot

enough

to trigger

its thermal -protection

circuit, it

may

blow

fuses,

or -worst case -

it may

burn

out

its

output tran-

sistors

or the tweeters in the con-

nected speaker

systems. Some of

the early circuits that were

de-

signed

to protect the output de-

vices from

excessive

current

tended

to be complex and unrelia-

ble and, worst

of all,

would some-

times

audibly

misbehave. How-

ever,

with

today's improved output

devices, simpler

and

more reliable

protection

circuits (sometimes

consisting

only of speaker

-line

power -supply

fuses) provide

the

required

protection.

Current thinking

Given all

the

above,

it's

easy

see

why many

amplifier

manufac-

turers have recently begun to fea-

ture "current

reserve"

as an

important

specification.

Even con-

ventional

8- ohm

-rated speaker

systems can

fall

as low as

to 3

ohms at some test

frequencies,

musical material

containing

those

frequencies. If the

current

reserves of an amplifier are

not

sufficient to sustain

its wattage

output

into low impedances, the

amplifier

momentarily runs out of

power -and frequently right at the

time

when it is musically needed

the

most!

I'm not

too embarrassed

that it

took

many years to appreci-

ate

the importance of high- current

capability

in amplifiers;

most

the Japanese manufacturers had

no more insight into

the

matter

than I did. The reason, I suspect,

had to do

with

the

impedance

characteristics of

Japanese speak-

ers. In the 1970's, you still

found

Japanese amplifiers that overhe-

ated or shut off if you tried to drive

American

-made 4 -ohm

speakers

with

them. And

it was

only five or

six

years

ago,

if memory serves,

that an engineer from a major Japa-

nese manufacturer asked me

for

reference material that explained

dynamic headroom and current

capability.

One final note: If you check the

current EIA amplifier -test standard

you will find no mention

out-

put-

current capability

either the

primary

or secondary ratings.

the

time we were working

on the

standard, none of us thought

about output current as

an impor-

tant

factor.

Today, there still does

not appear to be a universally

agreed -upon method of rating am-

plifier-

current capability.

there-

fore assume that output- current

ratings

such as 20, 30 or 50 am-

peres usually refer to an amplifier's

short -term

capability,

not

its

continuous rating. But in

any

case,

I'm

encouraged by the fact that

output- current ratings

continue

be played up

in press releases

and

spec sheets. I

believe

that a large

output- current capability can

make a significant

contribution

sound quality, given today's speak-

ers and the way they are

designed

and used. R -E

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