Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
The Phædrus Audio PHAB tube preamplifier is built around a modular amplifier with a fixed gain of
+40dB (with a switch available to raise this to approxi46dB). This amplifier is preceded by
a rotary, switched attenuator control, offering attenuation of 0 to -30dB in steps of 6dB. A
further pad (operated by a front panel switch) is available, to increase the attenuation to -40dB;
thereby matching the gain of the modular amplifier and reducing the overall unit gain of the
equipment to unity. These controls (the input attenuator, pad and the gain switch) are used to set
the overall preamplification level of the unit and feed a suitable electrical level to your
downstream equipment. (see
Operating level
section for more information).
Normal operation should be to operate WITHOUT the -10dB pad engaged and with the
preamplifier gain set to 40dB. Only operate the pad switch when the signal from the microphone is
too great for attenuation via the rotary attenuator control: and only operate the +46dB gain
switch when the signal from the microphone is too low, even when the rotary attenuator is set to
0dB.
HPF (
rumble filter
)
By virtue of its very high quality input and output transformers and its minimalist, wideband, valve
circuitry, the pass-band of the Phædrus Audio PHAB preamplifier is extended in both the bass and
extra-high frequency ranges. Due to this, unwanted, very low frequencies, due to traffic or air-
conditioning "rumble" may be picked up by the microphones and amplified. The high-pass filter
(HPF) filters out these frequencies and prevents them from either, intermodulating with the
wanted signals with in the microphone preamp' itself, or, being fed to downstream equipment. This
filter section is engaged by depressing the HPF switch.
+48V "Phantom" supply
An internal phantom supply is provided to power the microphone connected to the input. This
supply is engaged by depressing the "+48V" switch on the front panel of the unit. Al though this
supply is designed to "ramp-up" slowly, it is NEVER a good idea to switch phantom power onto a
microphone on a channel with an open fader because it can create "pops and bangs" which can
damage electronic equipment, speakers and fray nerves! Always mute any following circuit before
switching on the phantom (+48V) supply. To avoid loud transients, always make sure phantom power
is off when connecting or disconnecting microphones.This supply is designed to support the
hungriest of microphones.
NOTE: Some microphones may be damaged by having phantom-power applied to them: ribbon microphones are especially
delicate in this regard. ALWAYS check what type of microphone is connected, and that it is suitable for phantom-powering
BEFORE operating the front-panel switch.
