Parasound A 52 Manuel d'utilisateur télécharger pdf (Page 16)

TECHNICALLY SPEAKING

Audio Trigger Sensitivity Adjustment

The Audio Sensitivity Control sets the threshold of the

audio trigger signal. You can adjust this level from a

maximum sensitivity of 50 mV (fully counterclockwise)

to a minimum sensitivity of 250 mV (fully clockwise).

If you set this control to 50 mV, the A 52 might be

falsely triggered on by non-musical or noisy signals that

can appear in the system, such as when you switch

preamp inputs at high volume levels. If you set this

control to 250 mV, the A 52 might not turn on during

quiet musical passages. The detented position (click

stop) at 12 o’clock corresponds to 100 mV. It’s a good

starting point and will be suitable for most systems.

Balanced and Unbalanced Lines

Recording and broadcast studios use balanced con-

nections exclusively because of their inherent ability

to reject noise and hum, thus assuring the best

sound. Certain high quality preamplifiers and surround

controllers built for residential use utilize balanced

connections with XLR jacks for the same reasons.

All Parasound Halo series power amplifiers have

balanced inputs with XLR jacks so you can take full

advantage of their inherent noise reduction capability

and superior sound quality.

Unbalanced connections with RCA jacks are found on

all home audio equipment. RCA jacks and two-conduc-

tor wires are less costly than the additional circuitry,

higher priced XLR connectors and three-conductor

wiring required for balanced connections.

In an unbalanced line, the positive audio signal appears

at the center pin of the RCA jack and the negative

signal on the outer shield wire, which also functions as

the ground connection. Unbalanced interconnect cables

are vulnerable to hum from an AC line, or other noise,

such as RF (Radio Frequency), which can be reproduced

through your loudspeakers. Since the unbalanced line’s

ground also carries the audio signal, there is no way

for the connected amplifier or preamplifier to distinguish

between the audio signals you want and unwanted

noise emanating from external sources.

Balanced lines are superior because they utilize sepa-

rate conductors for audio and ground: two inner

conductors carry the positive and negative audio

signal, and a third outer wire connects the grounds

and also shields the two signal conductors. When

the positive and negative signals appear at the com-

ponent receiving the signal they are equal, but 180

degrees out of phase with each other with respect to

ground. To send and receive balanced signals requires

special differential circuitry.

A differential input circuit amplifies only the difference

between the positive and negative signals. For exam-

ple, when a 1 Volt signal arrives at a balanced input

stage, the differential input “sees” a positive 1 Volt

minus a negative 1 Volt, or 2 Volts total. External hum

and noise that somehow gets into a balanced line is

common to both its positive and negative conductors

with respect to ground. Therefore, it is canceled or

rejected by the differential input circuit.

This phenomenon of rejecting noise signals common

to both positive and negative conductors is called

common mode rejection. Differential inputs are speci-

fied according to how well they reject signals com-

mon to both conductors. This is measured in dB and

is called the common mode rejection ratio or CMRR.

Bare Speaker Wire Ends

If you plan to use connections with bare wire ends,

use a wire stripper to remove just enough insulation

to expose a

⁄2” (13 mm) length of bare wire. You can

insert the stripped wire into the hole that goes side-

ways through the terminal’s metal post. Before insert-

ing the wire, twist its bare strands to prevent any of

the strands from making contact across the two

speaker terminals. If you have a soldering iron, you

can “tin” (apply a small amount of molten solder) to

each stripped bare wire to prevent it from unraveling,

fraying and oxidizing.

Choosing Interconnect Cables and Speaker Wires

We are often asked to recommend specific brands

of interconnect cables and speaker wires. It’s true

that with some amplifiers, sound quality will vary

greatly according to interconnect cables and speaker

wires. However, Parasound amplifiers use robust

circuitry that sounds superb regardless of intercon-

nects and speaker wires. Therefore, we feel that

choosing a brand of cable for Parasound amplifiers

is purely a matter of personal taste.

Ground Loops - Eliminating Hum and Buzz

Audible hum and buzzing noises in a system are usu-

ally related to issues with the component grounds.

Ground (sometimes called common) is a point of

reference for voltages in virtually all audio and video

components. Ground is supposed to remain at zero

volts while the audio signal swings positive (voltage

above ground) and negative (voltage below ground).

If ground isn’t at zero, there can be an audible 60 Hz

hum (or 50 Hz hum in regions with 50 Hz AC). The

harmonics of these frequencies (120 Hz, 240 Hz, 480

Hz or 100 Hz, 200 Hz, 400 Hz) may add buzz in addi-

tion to the hum.

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Parasound A 52 Manuel d'utilisateur Page 16

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