HEADPHONE AMPLIFIER
AM/FM TUNER
The | Headphone | Amplifier | utilizes | a separate special purpose operational | amplifier | to | drive the | high | ||||||||
current demands | of headphone | operation. | This | provides | the | extra power | necessary | for | driving | difficult | ||||||
headphone | loads, | and leaves | the | main | signal | path uncompromised for signals to | power | amplifiers. | ||||||||
The | Headphone jack is equipped with a sensing | switch | that | automatically turns off | the | Line | Output | when | ||||||||
headphones | are | plugged | in. |
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Antenna signal input to the FM Tuner section is via either balanced 300 ohm
The | FM front end features | FET’s | for | exceptional rejection of spurious noises | and the | reduction | |||||||||
of IM distortion in the presence of | high level | signals. | Tuning is | accomplished | via a | voltage | controlled | ||||||||
varactor diode, driven by a | digitally | synthesized | Phase | Lock Loop | system. | Twin | ceramic filters | in the IF | |||||||
stage | have | flat | group delay characteristics, | improving | selectivity, | phase | linearity, | and | lowering | distortion. | |||||
The | audio | output | features | multiplex | filters | for | removing | the | 19 | kHz | pilot | tone. |
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ADDITIONAL INFORMATION
GENERAL
TROUBLE-
SHOOTING
HINTS
GROUND LOOPS
Operation of the 945 Preamp/Tuner is straightforward, and few operational problems, should be encountered after a thorough reading and understanding of the owner’s manual. The Functional Block Diagram provides an excellent “picture” of the signal flow, and should be referred to along with the descriptive text.
If all controls are in the intended position and the Ready light is green, check all system power
connections, | interconnecting | and | speaker cables, | and | fuses. | The 945 | does | contain one internal power | ||||||||||
fuse, but this fuse should | not | generally | blow | unless | a | malfunction | has | occurred. | This | fuse | should be | |||||||
replaced only | by aqualified | technician, and | only | with the | exact type | and | rating | of | fuse | originally | supplied. | |||||||
If | this fuse | is | replaced and | blows again | within | a short | time, | disconnect | all | power | immediately | and | return | |||||
for | service. |
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If all controls, fuses, cables, etc. seem to be functioning properly, a process of
Ground loops are characterized by a low level hum or buzz in the system. Loops are caused by a voltage potential difference between two points in a ground circuit, and aggravated when multiple paths for a given circuit exist.
Mounting components to a rack with metallic rails may introduce ground loops between associated equipment, because the rails can introduce a second ground path. The extent of this problem will depend on the grounding arrangements of associated equipment. Ground loops can occur in
If | ground | loops occur, and | any other | component | in | the | system has a | three wire grounded power cord, | ||||||
the | first step | should | be to use a ground | adaptor | (with | the | ground tab | or | wire | of the | adaptor notconnected) | |||
on the power cord plug of the preamplifier. DO NOT cut | off the grounding pin on the plug! It may be | |||||||||||||
necessary | to | use | additional | adaptors | on | other | grounded | components | if | more | than | two components are |
earth grounded. (In other words, only one earth ground per system should exist.) Another potential
source of multiple earth grounds | is from coaxial antenna or | cable service feeds for FM or video sources, |
which usually are (and should | be) earth grounded. The | ground adaptor(s) should cure this grounding |
problem as well. |
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