Hafler 0915P Circuit Description, OUT 1, OUT 2 AND OUTPUT OFF SWITCH HEADPHONE, Phono Section

OUT 1, OUT 2 AND OUTPUT OFF

SWITCH

HEADPHONE

OUTPUT

Activating this switch (UP position) turns off both Out

1 and Out

2.

It

does not

affect

the

headphone

output.

This control is useful for turning off

the

signal to the

power

amplifier(s) during

headphone

listening,

or

for

general system

muting

purposes.

When

activated,

the output switch

disconnects

the

line

amplifier

from

the output

jacks,

and grounds

the

jacks

to prevent

possible system noise

due

to

open

amplifier

inputs.

Out

1

and

Out

2 are

connected

in

parallel,

to

provide

two

equal

outputs to amplifiers. This feature is

useful

for

bi-amplified

speaker

systems,

or

for

running

two

amplifier/speaker

systems.

A common system “malfunction” can often be traced to inadvertently leaving the Output Off Switch engaged. In case of no sound from the speakers, check that the Output Switch is de-activated (DOWN position).

The headphone receptacle is a standard 1/4” stereo phone jack, with the tip of the plug as the left channel, the ring the right channel, and the barrel ground.

The headphone driver circuit is separate from the main line amplifier, and therefore, driving

headphones will not cause distortion on the		main	outputs.
Unless	simultaneously operating loudspeakers	during	headphone	use, the	Output	Off	Switch	should
be used to disable the outputs. Remember to		reduce	the Volume	Control	setting before		turning	on the
outputs again, since a normal listening level		for headphones may correspond to				a surprisingly		high
volume	for loudspeakers.

CIRCUIT DESCRIPTION

PHONO SECTION

(When Installed)

LINE LEVEL INPUT SELECTOR SYSTEM

Each Phono channel consists of six

low-noise

JFET's

connected

in

a

complementary-symmetry,

non-

differential

configuration.

Shorted-input

moving

coil

equivalent-input

noise

measures

about

-137

dB

(140

nV)

wide band.

Only

two

stages

are

used

to

yield an

open-RIAA-loop gain of

about

110

dB in

moving coil

mode

and

90

dB

in moving

magnet mode. RIAA

loop closure

yields

58.5

dB and

38.5

dB

respectively,

at

1 kHz.

The

first

stage employs

an

active

load

technique in which the

load

JFET’s

are

connected

as

gyrators,

acting

as

if they were very large inductors. This arrangement

allows

very

high

gain

at low

frequencies

and

a

natural

roll-off

of

high

frequencies

in

the

amplifier

loop,

before

the

RIAA

negative

feedback

loop

is

closed.

DC operating point stability is obtained

by

long

time

constant

DC

negative

feedback

applied

from

output

to

intermediate

points

in

the

gain

structure.

AC

gain

is set by the RIAA

network

feeding

back

output

signals to

the sources

of the input JFET pair.

Switching

to

moving

coil

(MC)

from

moving

magnet (MM)

results

in

three

operational

changes: 1)

open-loop

gain is increased by 20 dB,

2) closed-loop gain is increased

by

20

dB, and

3) user

adjustable

cartridge

loading

is

changed

from

capacitive

to

resistive.

The phono outputs are capacitively connected to the high level Input Selector System.

The front panel rotary input Selector Switch creates digital codes fed to a CMOS-FET electronic switch. This arrangement eliminates signal degradation due to mechanical switch contacts, and allows the electronic switches to be located close to the rear panel inputs for minimum signal path lengths and reduced interchannel crosstalk. The output of the Selector System is grounded between positions of the rotary knob to minimize switching noise.

-lO-