Boss Audio Systems BV8.5GA User Manual download pdf (Page 62)

^HCKERING

-3 PHONES

FREQUENCY

RESPONSE

FREQUENCY

(Hz)

The

published

specifications

the

Model

OA -3 phones

indicate that they

are quite efficient, requiring

only 0.1

volt

across their

nominal 15

-ohm im-

pedance to

produce

100

-dB

sound

pressure level (SPL)

1000

Hz. The

maximum

rated

input power

is 0.2

watt /channel,

but the

series

resistors

built

into

all amplifier

headphone

out-

put

circuits will provide

adequate

pro-

tection

even with

a high

-powered

am-

plifier.

The

distortion is

specified at 0.5

percent for

110

-dB

SPL output.

The

Pickering

Model

-3

head-

phones

are

supplied

with

an adapter

plug to

permit them to

be used with

small transistor radios

and cassette

decks. Retail price

of the phones

$39.95.

Laboratory Measurements.

tested

the frequency response

the

phones

on a

Koss

-designed coupler,

which is

a slightly

modified

version

of an accepted standard

earphone

coupler. The measured frequency re-

sponse of any headphone

closely

connected

with the dimensions

the

coupler

(or

artificial ear) on

which it is

mounted

so that it is virtually impossi-

ble to compare data

on different cou-

pler designs,

especially at the higher

frequencies. However,

one can obtain

a reasonably valid picture

of the

headphone's overall response

even

though the

specific peaks

and

dips

the response curve may

be as much

property

of the coupler

as of the

phone

(and would certainly

be still dif-

ferent through the

ears of any indi-

vidual

wearing the phones).

The measured frequency response

was relatively uniform from

100

Hz to

11,000

Hz, with a total variation

of only

±5

dB over

that range. The output fell

rather quickly at

frequencies

beyond

11,000

and at a smooth

6 d B /octave

low frequencies. The response was

measured

with

volt applied to the

phones,

producing an average

120

-dB

SPL over

the

measurement range,

which happens

to agree exactly with

the

published specifications.

Normally,

one would

expect distor-

tion to

be a function

frequency,

but

the

published

specifications

do not

specify the test

frequency. We

mea-

sured the

distortion

several

fre-

quencies

between

200

and

1000 Hz,

where the

output

was

both

strong and

smooth, at a

110

-dB

SPL.

The

distor-

tion

was typically between 1.2

and

1.6

percent

and

was

principally

second

harmonic. In view of the high

SPL

used, this

amount of distortion cannot

be considered

serious,

even if it does

slightly exceed

the

published

rating.

The

electrical impedance

the

phones was an

almost

constant 20

ohms across the audio range. It gently

rose to

ohms at

150

and under-

went a slight drop

16 -ohm

minimum

at 20

Hz.

Comment.

Since our

first

experience

with open

-air

phones

several

years

ago, we have

enjoyed

their

special

qualities -which eliminate many

the

objections voiced

about headphone

listening (heavy

weight

and pressure

exerted, inability to

hear desired

out-

side

sounds, such as

the ring

of a

tele-

phone, etc.). The Model

-3

phones

embody

all

the virtues

of a good

open -air headphone

and have

smooth response and the

ability to

handle

high

volume

levels without

ob-

jectionable distortion.

The vinyl-

coated ear cushions make

the Pickering phones

little

less

"open"

to outside noises than

are

some other

types that feature

simple

foam pads. In

fact, in

a quiet

room,one

might

almost

believe that the phones

are

the isolating type.

The low impedance and

high

effi-

ciency of the phones

makes

them ex-

ceptionally well

suited

for

use with

small radio

receivers

and

tape

record-

ers.

CIRCLE

NO.

READER SERVICE CARD

DRAKE

MODEL

SPR -4

COMMUNICATIONS

RECEIVER

(A Hirsch -Houck

Labs Report)

Versatile solid

-state

receiver

for

shortwave,

amateur

radio,

THE Drake Model

SPR -4

solid -

state

communications

receiver

covers any

bands (each

100

-kHz

wide)

between

150

kHz and 30

MHz.

Thus,

it offers the user

reception

international SW, amateur

radio,

and

CB radio transmissions,

among

others.

The receiver measures 121/4

in. deep

103/4

in. wide by 51/2 in. high (31.1

27.3x14

cm) and

weighs 18 Ib

(8.2

kg).

Its built

-in 5 -in. by 3 -in (12.7

7.6

-cm)

speaker

is mounted on the left side of

the metal receiver

cabinet.

The

price

the

Model

SPR -4 receiver is

$579.

Various

crystal kits are available for

the

amateur

radio, marine,

aeronauti-

cal,

MARS, Citizens Radio,

and other

special-

interest bands.

The

external

optional speaker,

100

-kHz calibrator,

and

noise

blanker are priced

at,

re-

spectively, $22,

$20,

and

$65.

Technical

Details.

The

receiver

employs a double- conversion super-

heterodyne design. Its first i -f of 5645

kHz is

obtained by the mixed product

of a crystal oscillator

selected by the

band switch, while a stable per -

meability -tuned oscillator

(PTO)

takes

care of

tlié' 4955 -kHz to 5466 -kHz

range. The PTO alone is used for the

lowest- frequency band.

Following the first mixer,

selectivity

provided

by a crystal

filter,

and

the

signal

goes without further

amplifica-

tion to the

second

mixer. Here,

it is

converted by

crystal

oscillator

(with

switched

crystals for

USB and

LSB

re-

ception)

to 50 kHz. Then

four -pole

filter and a tunable

-notch filter

provide

additional selectivity. After

two

stages of i -f amplification, the

signal

goes

to a product

detector

for

POPULAR ELECTRONICS

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