Prism Sound Orpheus Operation Manual page 55

Prism Sound Orpheus Operation Manual Revision 1.05
Cable quality
Use of good-quality, heavy duty audio cables is recommended. For microphone use, quad-twisted
cables may give best results. Cables with heavy screens are recommended, especially for
unbalanced use. Owing to mechanical differences between connectors from different manufacturers,
it is advised to use cables with identifiable connectors from reputable manufacturers. This is
especially true for jacks, where unreliable tip connection can owing to the slightly non-conforming
shape of some manufacturers' parts. Neutrik connectors are used in Orpheus, and these are
recommended to ensure reliably-mating cables.
Balanced versus unbalanced connections
Where possible, balanced interconnections should be used, since the audio signal is represented as
a voltage difference between two dedicated conductors (neither of which is ground-coupled), which
are usually closely-twisted to ensure that any interference pickup is cancelled out. In unbalanced
connections, the signal is represented as a voltage difference between a single signal conductor and
an accompanying ground conductor. Where dynamic ground-potential differences exist between the
source equipment and the receiving equipment, this difference is effectively added to the unbalanced
audio signal.
This effect has long been familiar in audio systems as 'hum loops', where the variation in ground
potential occurred at line-frequency, and was developed by the flow of line-frequency currents to
linear power supplies. Hum loops were usually resolved by either steering the currents along
non-critical routes by re-arranging the topology of the system ground interconnections, or by
mass-interconnection the system grounds using heavy gauge cable so as to minimize the hum
voltage resulting from the current.
Obviously many items of analogue audio equipment only have unbalanced connections; this is
especially true of consumer equipment, which is often used for monitoring even in professional
studios. If you must use unbalanced connections, keep them as short as possible and use
good-quality cables with substantial screens. If you have a choice, keep the signal level as high as
possible on the interconnection, since this will make any interference proportionally less noticeable.
Instrument connections are often particularly vulnerable to hum and other interference, since they are
usually unbalanced and low-level, and frequently employ a long cable not selected for its
interference-immunity qualities. Also, the source impedance is usually high, making the connection
particularly vulnerable to interference.
Some digital audio and computer equipment with switched-mode power supplies can cause
particularly troublesome interference problems, especially for low-level, unbalanced signals. This is
discussed in the following section.
Interference
The increasing use of low-cost digital equipment and computers in the audio production process
results in various potential problems for the remaining analogue devices. It is well-known that the
hostile power and EMC environment inside a typical computer is likely to be the limiting factor
governing the audio quality of an internal analogue sound card. A solution to this is the use of external
'sound cards', such as Orpheus, with their own enclosures and power supplies allowing adequate
space, power and electromagnetic peace and quiet for the well-being of studio-quality analogue
circuits.
However, even the sound quality of external devices can be compromised by the proximity of some
types of digital equipment. Many low-cost switched-mode power supplies emit interference which can
compromise system audio quality even at a distance. The hostile mechanism is usually 'conducted
interference', wherein the high-speed switching action of the power converter results in voltage and
current transients being conducted back down their power cords. If the equipment is connected to
mains safety-ground, transients can also be conducted down the ground connection. Radiated
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