Max Output
Attenuation
7.0 V RMS
-0.64 dB
9.0 V RMS
-2.83 dB
11.0 V RMS
-4.57 dB
13.0 V RMS
-6.02 dB
Table 14.1: Examples of minimum at-
tenuation required to externally con-
vert the Maximum Output Levels from a
source device to 6.5 V RMS at the input
of the BeoLab 90.
6
8
10
12
14
16
18
20
22
24
−12
−11
−10
−9
−8
−7
−6
−5
−4
−3
−2
−1
0
Maximum Output Voltage (V rms)
Attenuation (dB)
Figure 14.7: The minimum attenuation
required to apply to a source with a
given Maximum Output level to reduce
it to 6.5 V rms in order to prevent clip-
ping the BeoLab 90 analogue inputs.
14.5
Cable recommendations
There are innumerable beliefs and
opinions, both founded and unfounded,
regarding cables used for connecting
audio devices. The following is a small
set of recommendations that are based
on common practices for wiring
professional audio systems such as are
found in recording and mastering
studios. Decisions regarding the
specific the brand or construction of
the cables used for connecting BeoLab
90 are left to the reader’s preferences.
14.5.1
Analogue cables
In order to ensure that the noise floor
of analogue sources is as low as
possible, the following guidelines are
recommended:
Use cables with good shielding (or
screening) to reduce RF (Radio
Frequency) interference on the audio
signals from external sources.
When connecting the XLR input to the
XLR output of a source device, use
twisted pair (preferably bonded-pair)
or “starquad” cables to ensure the best
possible matching of low-frequency
magnetic interference noise at the
differential input. This will ensure the
highest possible common mode
rejection and lowest noise floor. Note,
however, that the use of starquad
cable construction generally has a
higher inherent capacitance than a
twisted pair cable of the same length
and will therefore have a higher loss of
high-frequency signals over longer
cable runs.
Avoid ground loops when connecting
audio devices to each other.
In order to reduce magnetic inductance
of interference (typically 50 Hz or 60
Hz “hum”) from power cables on the
audio inputs, it is also good practice to
physically separate signal cables and
mains cables as much as possible. In
cases where these cables must cross
each other, it is recommended that
they cross at a 90
◦
angle.
For a thorough guide to installation of
high-end audio equipment, “Audio
Systems Design and Installation” by
Philip Giddings is highly recommended.
Although this is book intended for
installation of audio devices in
recording and mastering studios, the
practices and recommendations
detailed therein are also applicable to
consumer-level audio equipment.
14.5.2
Optical cables
It is recommended that high-quality
optical cables are used for the BeoLab
90, particularly for longer cable runs.
This is due to the fact that there is
attenuation (dimming or loss of light
intensity) of the optical signal on the
plastic or glass fibre in the cable. This
attenuation is proportional (in dB) to
the length of the cable. Therefore, in
order to ensure that the optical
receiver on the BeoLab 90 has an
adequate signal at its input, the light
attenuation on the cable should be
minimised either by using short cables
or high-quality optical fibre.
Traditionally, many people have
claimed that optical digital signals are
less reliable than electrical connections
(such as the AES/EBU and S/PDIF
protocols) due to higher levels of jitter
caused by the limitations of the rise
and fall time of the LED in the
transmitter. The BeoLab 90 uses a
very-high-quality sampling rate
converter at its input for all digital
signals which attenuates the jitter of
incoming sources, thereby reducing
this concern considerably.
14.5.3
S/PDIF cables
When connecting a source to BeoLab
90’s S/PDIF input, it is recommended
that a cable with a 75
Ω
impedance is
used. This will ensure that there are no
reflections of the signal on the cable
which may increase the level of jitter at
the input of the BeoLab 90. Note that
this recommendation is particularly
true for longer cable runs. It should,
however, be stated that the sampling
rate converter at the digital inputs of
the BeoLab 90 is very effective at
attenuating jitter artefacts caused
either by the signal source or problems
in the cabling.
14.6
AC mains cables
It is highly recommended that an
additional device used to filter the AC
power from the mains (sometimes
called an “audiophile mains filter” or
“power purifier”, for example)
not
be
used with the BeoLab 90. This is
because the internal power supply of
the BeoLab 90 has a custom-designed
filter that reduces noise on its AC
mains input. This filter has been
optimised for the time-variant current
demands of the BeoLab 90, making a
generic external filter redundant (at
best) or detrimental (at worst) to the
performance of the loudspeaker.
Similarly, it is unnecessary to use a
so-called “exotic” or “audiophile”
mains cable for the BeoLab 90.
48
