Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
The adoption of balanced signals, and especially audio transformers should ensure that ground-
loops (or earth-loops) should NEVER occur. Certainly the Phædrus Audio PHI, by being both
double-insulated and incorporating an earth-free, transformer-balanced output stage can never
create a ground-loop in a correctly wired studio. However, connection to an earthed (or grounded)
guitar amplifier can cause a circulating current should pin 1 of the output XLR be earthed
(grounded) elsewhere in the installation.
This is the role of the GROUND-LIFT switch. It interrupts the connection of pin 1 of the output
XLR to the chassis of the PHI and to the sleeve and screen of the guitar wiring. Adjust the
position of this switch to eliminate any ground-loop “hums”.
Please note, as explained in the Chapter 3, that, because the Phædrus Audio products
are double-insulated, they CAN NEVER BE RELIED UPON PROVIDE A PROTECTIVE
OR SAFETY EARTH (GROUND). This advice obtains IRRESPECTIVE OF THE
SETTINGS OF ANY "EARTH OR GROU ND LIFT" SWITCHES.
Buzz?
The PHI DI-Box, like a lot of modern audio gear, is designed to operate from a double-insulated power supply. If the rest
of the equipment in your recording set-up is double-insulated too (for instance, you might be using the PHI with a laptop
computer and USB interface), it is possible for your entire audio installation to operate without a mains earth. This
represents no problems from a safety point of view. However it can cause the unbalanced, high-impedance input of the PHI
to be very susceptible to electrosta tically induced mains-interference. The tell-tale signs here are a high level audio “buzz”
rather than a “hum” on the outpu t signal, and the phenomenon that the instrument (particularly electric- and bass-guitar) is
very “live” so that simply touching the strings may worsen or improve the buzz.
In this situation, it is recommended tha t pin 1 of the output connector of the PHI be connected to mains-earth (ground) or
a technical-earth, and the “LIFT” switch be left in the down position.
Chapter 6 - Circuit descriptions
PHAB & PHAME
The Phædrus Audio PHAB and the PHAME preamplifiers share roughly the same preamplifier
circuit which is illustrated in general form below. The differences mainly lie in the input circuitry
and attenuator controls. Both devices operate with a constant-gain amplifier and attenuate the
signal reaching the amplifier for level control. In the case of the PHAB, the valve preamplifier has
a fixed gain of +40dB (with a switch available to raise this to approxi46dB). The amplifier
is preceded by a 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 amplifier and reducing the overall unit gain to unity.
In the case of the Phædrus Audio PHAME, the preamplifier does not incorporate an input,
balance-to-unbalance transformer. Instead the instrument feeds to grid of the EF86 pentode via a
continuous attenuator (pot') to ensure a very high input impedance. This is vital to ensure the best
tone from all electric instruments, and especially electric and bass guitars. The fixed gain of the
amplifier is +24dB (>30dB with the high-gain switch depressed).
