Hafler 945 manual Audio Line Level Input Selector System, Switching Line Amplifier, Muting System

CIRCUIT DESCRIPTION

REMOTE RECEIVER and

CONTROL LOGIC

AUDIO LINE

LEVEL INPUT

SELECTOR

SYSTEM

JFET BUFFER AND RECORD OUTPUT DRIVER

VIDEO

SWITCHING

TAPE MONlTOR

SWITCHING

LINE AMPLIFIER

TONE CONTROL SYSTEM

MUTING SYSTEM

The

Remote

Receiver

circuitry

decodes

the

infrared

pulses

received

from

the

Remote

Control

via

the

front

panel

Infrared

Receiver. This decoded information is sent to

the

Control

Logic,

which

controls all

internal

signal

routing and Front Panel indicators. The Control

Logic

also

receives

input

commands

from

the

Front Panel buttons.

The

Control

Logic

creates digital codes fed to a CMOS-FET

electronic

selector

switch.

This

arrangement,

besides allowing convenient remote operation, 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

audio

Selector

System

is

followed by

a

very

high

input-impedance

JFET

buffer

pair

to

establish

negligible

CMOS

switch loading

(for

low

distortion)

and

to

provide

a

low-impedance

source

for

the

Tape

Outputs and the Volume and Balance controls.

The

Control

Logic

creates digital codes fed to a CMOS-FET

electronic

selector

switch.

These

inputs

are

selected simultaneously with the corresponding audio

inputs.

The

selected

video

input

is

fed

through

separate

wide

bandwidth

+6

dB amplifiers

for each

of

the

Video

Monitor

and

Video

Record

Outputs.

Only

Video

1

and

Video

2

signals

are

available

at

both

the

Video

Monitor

and Record

Outputs.

Video

3

is blocked

from

the Video Record Output to prevent

feedback

from

occurring.

The

Tape

Monitor

buttons drive CMOS-FETelectronic switches

to break the line input path

to

insert

tape

input

signals.

This

switching

arrangement

allows

optimum

signal

routing

by

placing the

switches

close

to

the

Tape

Input

jacks

and

the

Input

Selector

System.

The 945 Line Amplifier is a Class-A JFET design employing a differential input stage, driving a

complementary high gain output stage,	symmetrically	driven	for balanced slew-rate and	low	harmonic
distortion. This topology uses only four	active devices	per	channel and extremely short	loop	feedback,
yielding high bandwidth and excellent	stability.

The feedback loop contains two user selectable paths: one conventional path for flat response, and another path allowing insertion of a tone control network. This arrangement allows maximally flat response and the shortest, cleanest signal path when tone control action is not required.

The output of the Line Amplifier is capacitor coupled to the output of the preamplifier.

The

Tone

In/Out

button

allows complete removal

of

the

tone controls from the signal

path.

This switching

is

accomplished

with

the same

type

of CMOS-FET electronic switch used for the

Input

Selector.

This

switching

arrangement

eliminates

signal

degradation

due

to mechanical switch contacts, and allows

the

electronic

switch

to

be

located

very close to

the

Line

Amplifier circuitry.

The Bass Control is of a moving inflection, variable turnover type, and the Treble Control is of a shelving, variable turnover type. These types of tone controls offer the best “tools” for modifying tonal balance without introducing unwanted side effects. See Operation Section for further information.

The Muting System monitors the power supply voltage. During the power-up cycle, the Audio and Headphone Outputs are muted until the regulated voltages reach a predetermined level, and then remain muted for several seconds to allow all circuitry to stabilize. During power-down, or in cases of excessively low AC line voltage, the Audio and Headphone Outputs are instantly muted to avoid extraneous noises.

The muting is accomplished by grounding the Line Output jacks and disconnecting the output of the Line Amplifier, using a relay with gold contacts.

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