1.50
Operation Manual
© 2013 Prism Media Products Ltd
Revision 1.00Prism Sound Lyra
connected to mains safety-ground, transients can also be conducted down the ground connection.
Radiated 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
Lyra 2 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 Lyra 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 Lyra's 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 Lyra's analogue
signal ground (the outer of the instrument input jacks, or pin 1 of the mic input XLRs). Since no
dedicated terminal exists on Lyra, 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;
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