Parasound JC 1 Amplifier - Manual (Page 16)

TECHNICALLY SPEAKING 13
Audio Trigger Sensitivity Adjustment
The Auto Turn On 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 of 250
mV (fully clockwise). If you set this control to 50 mV,
the JC 1 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 JC 1 might not
turn on during quiet musical passages. The detented
position (click stop) at 3 o’clock is a good starting point
and will be suitable in most systems.
Balanced and Unbalanced Input Lines
Recording and broadcast studios use balanced
connections 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-
conductor 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
separate 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 component
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 example,
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
specified according to how well they reject signals
common to both conductors. This is measured in dB
and is called the common mode rejection ratio or
CMRR.
Choosing Interconnect Cables and Speaker Wire
We are often asked to recommend specific brands of
interconnect cables and speaker wire. 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 interconnects and
speaker wires. Therefore, we feel that choosing a brand
of cable for Parasound amplifiers is largely a matter of
personal taste.
Signal Polarity
The JC 1 does not invert polarity. Therefore, if you
know that your preamplifier or surround controller
inverts polarity, you may connect your speaker + and -
opposite of the above instructions to reverse polarity.
This applies to all speakers in your system driven by
amplifiers that don’t invert polarity. Contact the maker
of your preamplifier or surround controller to verify its
polarity if this is not mentioned in its owner’s manual.
Speaker Wire Length and Gauge (thickness)
When selecting speaker wire, follow these guidelines:
• Keep the length of your speaker wire as short as
possible
• Use the thickest wire practical. For lengths greater
than 50 feet, use speaker wire with an AWG (gauge) of
14 or lower. The smaller the AWG, the thicker the wire.
• Do not use speaker wire that is thinner than 16 AWG.
• Use the same length of speaker wire between each
JC 1 amplifier and speaker, regardless of the actual
distance between them.