Prism Sound Orpheus Operation Manual page 56

Prism Sound Orpheus Operation Manual Revision 1.05
emissions (airborne radio interference) can also be a problem, but it is less common that this will
have such a serious effect on audio quality.
Conducted power-line interference can cause problems in analogue equipment within the installation
if its own power supply allows the transients to pass through to the audio circuits. However,
conducted ground interference can be even worse since, if the ground connection of the analogue
equipment is modulated by switching interference, there is nothing that the designer of the equipment
can do to combat it.
How much any conducted ground interference affects audio quality depends on many factors, mostly
to do with how the various analogue boxes in the system are interconnected and grounded. Where
possible, high-level balanced connections should be used, just as in the case of hum-loops as
discussed in the previous section.
Where ground-potential variations are caused by switching power supplies, the effect can be more
difficult to resolve, since the signals can occur at more noticeable frequencies: although the supplies
usually switch at frequencies too high to hear, the frequency is often modulated by variations in the
load current over time, resulting in a continuous modem-like chirping in which can be heard particular
events such as computer screen updates, disk activity etc.). Another problem is that even heavy
ground cabling may not reduce the effect of the interference, since high-frequency currents may not
see much less resistance in a thick conductor than a thin one.
How do the equipment manufacturers get away with this? Surely there are stringent regulations
covering conducted and radiated emissions? Well that's true, but the level of emissions which can
result in audible degradation of low-level, unbalanced audio interconnections are well below
legislation levels. Unfortunately, computer power supplies (and especially the switching wall-warts
and line-warts which power notebook computers and other small items) are amongst the worst
offenders.
Vinyl decks
Orpheus is equipped with an RIAA de-emphasis filter to allow direct connection of a vinyl deck, as
described in the Analogue inputs section. Since vinyl decks usually have a low-level, unbalanced
output it is important to minimise interference as discussed above when connection a vinyl deck.
Since most magnetic cartridges require a higher input impedance than that of the Orpheus
microphone preamplifier input, it is usually best to connect a vinyl deck to the instrument inputs using
a pair of phono-to-mono-jack cables. The instrument gain controls can then be set to an appropriate
level for the particular cartridge. The 1MR input impedance of the instrument inputs will work
satisfactorily with most magnetic phono cartridges (which are 'moving magnet' types), but with some
cartridges, improved frequency response and noise levels can be achieved by fitting the cartridge's
required load resistance (usually 22kR or 47kR) across the instrument input terminals; this is best
achieved by soldering it inside the jack. Moving coil cartridges have a lower output level and require a
lower preamplifier input impedance. These are best connected to Orpheus' mic inputs, or may
require a dedicated preamplifier.
Most vinyl decks have a ground wire separate from the audio connectors. Connection of this wire for
lowest hum is often a matter of trial and error. Ideally this should be connected to Orpheus' analogue
signal ground (the outer of the instrument input jacks, or pin 1 of the mic input XLRs). Since no
dedicated terminal exists on Orpheus, it is usually easiest to connect the wire to the outer of one of
the deck's unbalanced output connectors. In some situations, a direct connection to local mains
ground may work better.
In summary
· Use good-quality cables with reputable connectors;
· Use balanced connections where possible; if you must use unbalanced connections, keep them
short;
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