Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
AN AMPLIFIER. NOT SO DOING WILL DAMAGE YOUR Phædrus Audio PHI. FU RTHERMORE
YOU MUST OBSERVE THE WARNING GIVEN BELOW, AS FAILURE TO DO THIS COULD
DAMAMGE YOUR AMPLIFIER.
When the Phædrus Audio PHI is used in LOUDSPEAKER mode, it is important to observe the correct polarity of the signal
fed from the amplifier to the PHI. To avoid short-circuiting the output of the amplifier, the earthy side of the loudspeaker
signal MUST be connected to the sleeve of the input jack. In addition, as a precaution, ALWAYS operate with the Ground
lift switch in the “LIFT” position when operating the PHI in this way.
Furthermore, the Phædrus Audio PHI is NOT a dummy load and a loudspeaker MUST be attached to the THRU output so as
to provide the correct load for the power amplifier. Valve amplifiers especially may be damaged without the correct
speaker-load connected.
Flat/ Cabinet switch
The problem with the signal from the amplifier is that it often contains many high frequencies
which are not reproduced by instrumental loudspeaker cabinet. The resul t is a “fizzy”, over-bright
signal; very different from that heard from the
loudspeaker. The reason for this is that the
loudspeaker and cabinet act together to form an
asymmetrical band-pass fil ter; steeply attenuating
the
treble above about 4kHz, and gently rolling off the
bass.
The signal from the amplifier may therefore be
very
greatly improved by the incorporation of an
electrical filter which reproduces the electro acoustic band-pass of the loudspeaker cabinet. To
this end, the Phædrus Audio PHI - DI unit, is enhanced by the inclusion within the unit of a
loudspeaker-emulation filter which imprints the response of a typical, closed-back, guitar cab'
onto the signal. The general response of this fil ter is illustrated above. Essentially the response
falls away gently in the bass, peaks in the mid-treble and falls away rapidly above about 4kHz as
shown in the figure.
Ground (earth) lift
A ground-loop (earth-loop) forms when an unbalanced, audio signal flows in a circuit in which the
signal-return (usually the cable braid), provides a current path in parallel with the installation
earth (ground). Small potentials in the installation earth (ground) wiring, drive a circulating current
in the braid of the audio cable. The resul t is a “hum” vol tage superimposed upon the audio signal.
