Phaedrus Audio PHAB-8 User manual

Ver. 1.0 - © Phædrus Audio Ltd. 2017 All rights reserved.
User Manual for
PHAB-S
Two channel TUBE preamp & stereo SHUFFLER
This manual is also available as a pdf download, Go to: www.phaedrus-audio.com

Ver. 1.0 - © Phædrus Audio Ltd. 2017 All rights reserved.
Chapter 1 - Background
The new PHÆDRUS AUDIO PHAB-S is a 2RU, mains-powered unit. It is a recreation of two
preamplifiers from the famous 1960s console used by The Beatles. Also included is the stereo
shuffler which was an integral part of these Stereosonic consoles.
Chapter 2 - Warranty and Service
Please refer to for the latest information and advice about warranty and service at:
http://www.phaedrus-audio.com/phaedrus_t&cs.htm
Chapter 3 - Safety
General
Before using any piece of equipment manufactured by Phædrus Audio, be sure carefully to
read the applicable items of these operating instructions and the safety suggestions. Keep
them for future reference. Follow the warnings indicated on the unit, as well as in the
operating instructions.
User Access & Servicing
Phædrus Audio equipment employs thermionic valve (vacuum-
tube) technology and employs hazardous voltages for the high-
tension supplies. THE USER SHOULD NOT ATTEMPT TO SERVICE
THE UNIT. ALL SERVICING SHOULD BE REFERRED TO QUALIFIED
SERVICE PERSONNEL OR FACTORY ONLY.

Ver. 1.0 - © Phædrus Audio Ltd. 2017 All rights reserved.
Phædrus Audio products should NEVER be connected to the external power supply or in any
other way energised when the case is opened and/or the circuit board is accessible. Always
replace mains fuse with a similar type (3 Amp).
General Safety Instructions
Do not operate Phædrus Audio equipment near any source of water or in excessively
moist environments.
Keep your Phædrus Audio equipment away from babies, children and pets.
Do not let objects do not fall, or liquids be spilled, into the enclosure.
Situate the Phædrus Audio equipment away from heat sources or other equipment
that produce heat.
Ensure Phædrus Audio equipment has adequate ventilation. Improper ventilation will
cause overheating, and can damage the equipment.
When cleaning Phædrus Audio equipment, remove all connections to the unit;
including power and gently wipe with a clean lint-free cloth; if necessary, gently
moistened with lukewarm or distilled water. Use a dry lint-free cloth to remove any
remaining moisture. NEVER use aerosol sprays, solvents, or abrasives on Phædrus
Audio equipment.
Phædrus Audio equipment should be serviced by qualified service personnel or
returned to Phædrus Audio Ltd. when: an object (or objects) have fallen into the
enclosure; or liquid has fallen into, or been spilled into the unit; or the unit has been
exposed to rain or high humidity; or the unit does not operate normally or exhibits a
marked change in performance; or the unit has been dropped, or the enclosure has
been damaged.
Chapter 4 - Instructions for use
Phædrus Audio’s products employ valves (vacuum tubes) in the audio path. Valves (vacuum
tubes) and their associated circuitry need time to reach an electronic equilibrium before they
will operate at optimal specifications. Please therefore allow these products to warm up for, at
least, five minutes before using them in your signal chain. To prolong the life of your valves
(tubes), it is recommended that you turn off these units when not in use.
Application and connections
The signal processing architecture of the PHAB-S is best understood by studying the block
diagram as illustrated below.

Ver. 1.0 - © Phædrus Audio Ltd. 2017 All rights reserved.
Architecture of the PHAB-S
Connecting up
Rear panel view (for levels and impedances see Specification)
Mains: Three core with earth on IEC connector.
oFor operation with a studio technical-earth, circuit 0V may be disconnected
from mains earth by removing the wire link between the two binding posts on
the rear chassis. The black terminal may then be connected to technical earth.
The chassis is always connected to earth via the mains cord. The green terminal
is hard-wired to the mains earth pin as illustrated below.

Ver. 1.0 - © Phædrus Audio Ltd. 2017 All rights reserved.
Never disconnect the mains earth
from this equipment.
Inputs: accept balanced or unbalanced signals on XLRs or ¼ inch jacks (concentric
connectors.)
Outputs: +4dBu nominal fully balanced signals on XLR and balanced/unbalanced signals
on ¼ inch jacks at -2dBu nominal. Outputs may be used concurrently with no
interaction between pro and semi-pro outputs.
Input stage controls
The input stage controls consist of: the
DI switch (right/ side channel only); the
input attenuation controls (both
channels); the phantom power switches;
the DRIVE rotary switches; and the pad
toggle switches.
The DI switch routes the signal on the
front-panel jack to the preamplifiers.
This control is only present on the
right/side channel.
The input attenuation controls are
precision, stepped attenuators maximum attenuation (at anti-clockwise end-stop) is 40dB.
The phantom power switches route 48V to the microphone inputs. Please observe the usual
precautions when selecting phantom power.
DRIVE control
The standard configuration of the Type 47 amplifier module when used as a microphone
preamp' was with 40dB gain and with no HPF engaged. The ability to change the gain, or to
select the HPF in the original Type 47 amplifier was the duty of technical staff. Indeed,
selection of the HPF involved powering down the console, waiting one minute whilst the killer
HT voltage subsided, withdrawing the amplifier module and removing an internal link. And
then refitting it all again. Hardly a user control then!
HPF
Nevertheless, when recording with a cardioid, or figure-of-eight microphone close to the
talent, it is helpful to be able to attenuate the excess of low frequency information caused by

Ver. 1.0 - © Phædrus Audio Ltd. 2017 All rights reserved.
the proximity-effect. Thus, the first position on the DRIVE control permits simple selection of
the HPF into the circuit: a luxury not easily afforded to the engineers working with The
Beatles!
GAIN and +
The ability to select the higher gain regime of +46dB was included in the design of the Type 47
amplifier because the standard module played various duties in the console and, in some
instances, this meant driving lower loads or making up for more loss in the mixing electronics.
Thus, when the Type 47 amplifier was operating at 46dB, it was not to give more gain in the
sense we understand today, it was to work the amplifier harder to overcome more loss or to
drive a lower load. This is the essence of the DRIVE control in the PHAB-S. In positions 3 and 4
of the DRIVE control, more gain is liberated from the amplifier, but the amplifier is also double,
and then triple, terminated.
Thus, the DRIVE control needs to be used in conjunction with the input attenuation control.
When GAIN is selected, you will notice a small loss in volume caused by the extra load, this
needs to be overcome by advancing the INPUT ATTEN' control. You will notice a "hardening" of
the tone when operating the preamp in this way. The final position, exaggerates this effect.
Still more gain is required to overcome the extra load and you will notice that the tube
amplifier really starts to "show the strain".
Even more distorted effects may be obtained by reducing the master fader and driving the
preamplifier even harder when GAIN/+ are selected.
Pad switch
The Pad switch introduces 20dB attenuation on the primary side of the input transformer.
Stereo Shuffling
(For information and history on Stereo Shuffling go here: http://www.phaedrus-
audio.com/intro_to_shuphlers.htm)
The PHAB-S incorporates the Stereosonic shuffler invented to correct for various shortcomings
in conventional two-channel stereo. It is simply selected by switching the STEREO SHUFFLER
control downwards.
As with the SHUFFLER in the original 1960s mixer, the correction circuit is entirely passive and
therefore has some static insertion loss. That’s to say that the signal level drops by about 2dB
with the SHUFFLER in circuit. Compensate for this loss by advancing the master faders by one
click when the SHUFFLER switch is operated.
Note that stereo shuffling is not appropriate when the preamplifier is being used as a two
channel device, for example for a microphone and a guitar.

Ver. 1.0 - © Phædrus Audio Ltd. 2017 All rights reserved.
Master faders
The master faders are equivalent to the channel faders on a mixer. The inclusion of these
controls allows you to, for example, to reduce the signal level at the master fader point, so
that you can drive the channel harder.
Signal levels should be set using the blue-red overload indicator controls. The default setting
for these controls is 12-o’clock.
Operating levels & overload indicator
To prevent inadvertent clipping, the PHAB-S contains a clip detect circuit which turns the,
normally blue, LEDs red when the audio level is 3dB below peak. This indicator should only be
allowed to flash red occasionally in operation. The PHAB-S operates at the standard recording
industry level of +4dBu = 0VU.
Chapter 6 - Circuit description
The Phædrus Audio PHAB-S microphone preamplifier incorporates the front-end of two
channels of the famous Stereosonic consoles, in which the modular-amplifier is combined with
a rotary attenuator, switchable pad, high-pass filter and gain-switching circuitry.
The amplifier block is known as a Type.47 and the circuit of this device is illustrated below.

Ver. 1.0 - © Phædrus Audio Ltd. 2017 All rights reserved.
In this 1959 tube-amplifier design a EF86 pentode valve is followed by an ECC88 dual triode
"strapped" as a single, high-mu tube. The PHAB-S incorporates two examples of these
amplifiers; one for each channel.

Ver. 1.0 - © Phædrus Audio Ltd. 2017 All rights reserved.
Specifications
Electrical connections
Input:
Unbalanced: 3 pin XLR Connector Balanced: 3 pin XLR Connector
Pin 1 – Ground Pin 1 – Ground
Pin 2 – (+) Signal Pin 2 – (+) Signal
Pin 3 – Connect to Pin 1 Pin 3 – (-) Signal
Output:
Unbalanced: 3 pin XLR Connector Balanced: 3 pin XLR Connector
Pin 1 – Ground Pin 1 – Ground
Pin 2 – (+) Signal Pin 2 – (+) Signal
Pin 3 – Leave o/c Pin 3 – (-) Signal
PHAB-S Specification
Input: Balanced, transformer coupled
Output: Balanced on XLR, balanced/unbalanced RTS jack
Gain (maximum): 53dB
Preamp attenuator: 40dB range in 4dB steps
Pad: -20dB (30dB selectable via an internal link)
Frequency response: ±1dB 40Hz and 15 kHz (HPF out of circuit)
Crosstalk (L & R mode): better than -60dB at 10kHz
EIN: 400nV RMS, -126dBu; max gain, input terminated in 200R, unweighted reading (BW 300Hz – 20kHz)
Maximum output level (XLR): +24dBu @ 1kHz. Clip LED indicates RED at 3dB below clipping level
Maximum output level (Jack): +18dBu @ 1kHz un-balanced. Clip LED indicates RED at 3dB below clipping level
Power supply: 230 AC or 110V AC (specify at time of order)
Power consumption: 20 Watts
Phantom Supply: +48V DC, up to 5mA/channel
Phaedrus Audio Ltd. reserves the right to alter these specifications without notice.

Ver. 1.0 - © Phædrus Audio Ltd. 2017 All rights reserved.
Declaration of Conformity
The Manufacturer of the Products covered by this Declaration is
Phædrus Audio Ltd.
head office address
The directives covered by this declaration are:
89/336/EEC Electromagnetic Compatibility directive
73/23/EEC Low Voltage Equipment directive
The products covered by this declaration are:
Phædrus Audio PHAB-S
The basis on which conformity is being declared:
The manufacturer hereby declares that the products identified above comply with the
protection requirements of the EMC directive and with the principal elements of the safety
objectives of the Low Voltage Equipment directive, and that the following standards have been
applied:
IEC INTERNATIONAL STANDARD 60065 - Audio, video and similar electronic apparatus –
Safety requirements
The technical documentation required to demonstrate that the products meet the
requirements of the Low Voltage Equipment directive has been compiled and is available for
inspection by the relevant enforcement authorities. The CE mark was first applied in 2011.
Signed: Richard Brice . Phaedrus Audio
Date: July 2017