Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
High Pass or Rumble filter (HPF)
The input stage in the PHAB is fed by a 1:7
transformer. When a capacitor is included in series
with the transformer primary (it is usually short-
circuited out of circuit), the capacitor forms a
half-section fil ter with the primary inductance of
the transformer primary; resulting in a sharp cut-
off, high-pass filter. The response of the PHAB
with and without the rumble fil ter is illustrated
(right).
PHI circuit description
The system diagram for the Phædrus Audio PHI-DI Box is shown. In effect, before the signal
from the instrument is applied to the transformer, it is buffered by a dual cathode-follower
circuit. The first stage runs at very low current and is biased by a separately derived bias supply.
This ensures a high and consistent input impedance. The second stage is DC-coupled to the first
and runs in a higher quiescent current; consistent with having sufficient drive to overcome the
excitation current in the transformer at low frequencies.
Loudspeaker input
In this configuration, it is necessary to attenuate the loudspeaker drive signal before presenting
to the DI circuitry and this is exactly what is done when the "LOUDSPEAKER/INSTRUMENT"
switch is moved to the "LOUDSPEAKER" position. The attenuation is -32dB. In the Phædrus Audio
PHI - DI unit, this feature is enhanced by the inclusion within the unit of a loudspeaker-emulation
filter. The general response of this fil ter was illustrated above. The LF roll-off is accomplished
with a single-pole, RC response. The upper skirt of the passband is implemented by means of a
single-terminated LC filter.
