Balanced And Unbalanced Inputs, Levels; Input Level Controls; Grounding; Decibel Explanation - Bose Entero 440 Installation & Operation Manual

10. Audio Input Connections

10.1 Balanced and unbalanced Inputs, Levels

10.2 Input Level Controls

10.3 Grounding

10.4 Decibel Explanation

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The amplifier can accept unbalanced or balanced lines. For optimum performance and with longer cable
runs, use balanced lines whenever possible. The driving device should be equipped with a balanced
output. With short cables inside one rack the issue is not that critical.
A balanced connection provides excellent noise immunity from interference and ground loops. A balanced
audio connection is accomplished by using a differential pair of signals, isolating them from ground.
While these connections provide great noise immunity, care must be used when connecting balanced and
unbalanced equipment together.
All models are equipped with rotary level controls for each channel at the rear side of the amplifier. The
level control knobs are digital rotary dip controllers with 16 dedicated steps. The values are indicated on
the rear panel.
The chassis ground of the amplifier is connected with the ground of the AC power cord (the non-fused
earthed protective conductor). As soon as several devices are connected with each other in a signal
chain, a ground loop is created. Ground loops are caused when current flows from the analog ground
plane of one piece of equipment to the ground plane of another. A compensating current travels between
the different devices on the shields of the audio cables. This is mostly causing hum-problems.
Using a proper cabling, ground loops should be avoided.
NEVER isolate the ground of the AC power cord (the non-fused earthed protective conductor) to
eliminate hum-problems. This is a violation of the law and dangerous !
A decibel is a logarithmic scale commonly used to express differences in signal levels. It is useful in audio
because it can express a wide dynamic range with relatively small numbers (or a small movement on a
meter), and it more closely matches how we perceive sound.
The measurement quoted in dB describes the ratio between the quantity of two levels, the level being
measured and a reference. The absolute quantity of the signal is not relevant. This means that decibels
are always comparing one quantity to another. For example, when we measure gain in dB, we are
comparing the output level to the input level.
To describe an absolute value, the reference point must be known. There are different reference points
defined.
dBV represents the level compared to 1 Volt RMS. 0dBV = 1 Volt with no reference to impedance.
dBu represents the level compared to 0,775 Volt RMS on an unloaded, open circuit.
dBm represents the power level compared to 1 mWatt. This is a level compared to 0,775 Volt RMS across
a 600 Ohm load impedance.
1dBV equals +2.2dBu, +4dBu equal 1.23 Volt RMS, the reference level of -10dBV is the equivalent to a
level of -7.8dBu.
Headroom is a measure (usually in dB) of how much higher the peaks of a signal can be compared to the
nominal level without clipping. That is, it compares the peak level (in volts RMS) to the nominal level (in
volts RMS). The difference between the two (in dB) is the headroom.