Phaedrus Audio PHAB User manual

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
User Manual for
PHAB - tube microphone preamp'
PHAME - tube instrument preamp'
&
PHI - tube DI box
This manual is also available as a pdf do nload, Go to: .phaedrus-audio.com

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
Preface – About Phædrus Audio
Phædrus Audio as formed to design, manufacture and sell high-quality products for the
professional and semi-professional audio market. Phædrus Audio's founders remain inspired by the
music and the recording practices of the fifties and sixties, and are motivated to re-establish the
values of the great audio-technology legends of the past ith their ideals of transparency, hand-
built quality, and serviceability. Using modern manufacturing techniques and the benefits of
modern component technology, Phædrus Audio's aim is to reproduce the quality and character of
classic equipment but in a modern, highly reliable, and cost-effective ay.
Chapter 1 - Background
The Phædrus Audio PHAB, PHAME and PHI products came about because t o, recording musicians
anted to o n a "classic" console from the nineteen-sixties. Ideally a famous EMI, REDD, all valve
(vacuum tube) mixer used to record The Beatles.
But those desks make a collection of
hen's teeth look positively prosaic -
as ell as cheap! So, e set about
researching ith the idea of
producing a replica console.
In order to do this, first it as
necessary to have a design for the
amplifier modules around hich the
mixers of this vintage are organised.
In the famous EMI REDD mixers,
these amplifiers ere either the
German manufactured V72(S)
amplifiers or the, very rare, EMI
built, REDD.47 amplifiers.

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
V72 and REDD.47 modular amplifiers
Ironically, despite the fact that many more V72 amplifiers exist than do REDD.47s, the EF804
valves hich the German amplifiers use are no longer in current production and are becoming very
expensive: hereas the EF86 and ECC88 (E88CC) valves employed in the REDD amplifiers remain in
production and are idely available. So, a ne amplifier, designed from the common source of
Mullard's reference audio designs, and thereby sharing a common heritage ith the REDD
amplifiers, as developed. We called this amplifier the Phaedrus "PHILHARMONIC" (literally ,
music loving
). This PHILHARMONIC amplifier forms the heart of the Phædrus Audio LONDON
Mark II console. The PHILHARMONIC amplifier circuit is described in detail belo in section
Circuit descriptions .
Mullard published many reference designs which were used by EMI in developing the REDD
amplifiers. Phædrus Audio went back to these same references to develop the PHAB, PHAME
and PHI products.
The prototype London Console (Mark II)

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
PHAB
The Phædrus Audio PHAB microphone preamplifier is essentially the front-end of a single-channel
of the LONDON console, in hich PHILHARMONIC modular-amplifier is combined ith a rotary
attenuator, s itchable pad, high-pass filter and gain-s itching circuitry similar to that employed in
the famous sixties consoles.
PHAB microphone preamp’ and PHAME instrument preamp’
PHAME
The Phædrus Audio PHAME is a development of the PHAB in hich the very high input impedance
of the input pentode valve is exploited to provide a very high quality instrumental preamplifier.
PHI
The Phædrus Audio PHI DI-Box is a product designed for a
ide range of applications; from live use to the recording
studio. Partnered ith the PHAB it provides a solution for
almost any instrumental or vocal preamp' application.
Construction - or star earths (grounds) and
curly tracks
Given their vintage, the V72 amplifiers and the REDD.47 amplifiers ere built on a metal chassis,
ith the components hard ired onto tag-strips. Although Phædrus Audio equipment uses printed
circuit boards to ensure consistent performance and reliability, the equipment follo s "classic"
practices such as star earths and "natural contour" tracking, just like hook-up ire.
Every Phædrus Audio PHAB, PHAME and PHI product is hand assembled and individually tested. A
test pro-forma is provided ith every unit. Phædrus Audio offer comprehensive service for
products both inside and outside of their arranty period.
PHI DI-Box

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
Chapter 2 - Warranty and Service
Please register your purchase ith Phaedrus Audio at .phaedrus-audio/registration.htm . This
ill simplify service & repair should you need this service. Your name ill be placed on our mailing
list (unless other ise requested) for future updates and ne product announcements.
Service
If you experience a problem ith a Phædrus Audio Ltd. product, contact:
[e-mail]
TELEPHONE: [phone]
Skype ID: phaedrusaudio
We ill diagnose the problem remotely and advise you of the arranty status. If a repair or
replacement is required, e ill issue a Return Merchandise Authorization (RMA) number and tell
you here to send the unit to be repaired. You MUST have an RMA number before you return the
equipment to Phædrus Audio Ltd.'s support service. Be sure to rite RMA number on outside of
shipping box and to include your name, address, phone number, a copy of original sales invoice and a
detailed description of the problem. Phædrus Audio Ltd. ill not accept responsibility for loss or
damage in shipping or for equipment returned ithout valid paper ork and/or a valid RMA number.
Remember, arranty is void if product serial numbers have been removed or altered, or if the
product has been damaged by abuse, accident or unauthorized modification and/or repair (see
Phædrus Audio Ltd. Limited Warranty for details). There are no user serviceable parts inside.
PLEASE RETAIN YOUR SALES RECEIPT. IT IS YOUR PROOF OF PURCHASE COVERING YOUR
LIMITED WARRANTY. LIMITED WARRANTY IS VOID WITHOUT SUCH PROOF OF
PURCHASE.
Phædrus Audio Ltd.'s Limited Warranty
This limited arranty is valid only if you purchased the product from Phædrus Audio Ltd. of from a Phædrus Audio
authorized dealer in the country of purchase: a list of authorized dealers can be found on Phædrus Audio ebsite
.phaedrus-audio.com, or by contacting [e-mail]. Phædrus Audio Ltd. arrants tha t the equipment it
manufactures shall be free from defects in material and orkmanship for a period of one (1) year from the original date of
purchase; unless a longer minimum arranty period is mandated by applicable local la s. If equipment fails due to such
defects ithin this period, Phædrus Audio ill, at its option, repair or provide a replacement for the defective part or
product. Valves (vacuum tubes) are excluded from the one-year arranty, but are arranted for 90 days from day of
purchase. This arranty does not extend to any Phædrus Audio Ltd. product that has been damaged or rendered defective
as a result of: accident, misuse, or abuse; or by the use of parts not manufactured or supplied by Phædrus Audio Ltd.; or by
unauthorized modification or attempted repair to the product; or by acts of God/Nature (accident, fire, flood, etc) or any
other condition that is beyond the control of Phædrus Audio Ltd. There are no user serviceable parts inside. This limited
arranty is invalid if the factory-applied serial number has been altered or removed from the product. This limited
arranty is extended exclusively to the original buyer (customer of Phædrus Audio Ltd., or authorized retail dealer) and is
not transferable to anyone ho may subsequently purchase the product. No other person (retail dealer, etc.) shall be
entitled to give any arranty promise on behalf of Phædrus Audio Ltd. Phædrus Audio Ltd. makes no other arranties,
expressed or implied, of merchantability, fitness for a particular purpose or other ise. Phædrus Audio Ltd. liability is
limited to repair or replacement by Phædrus Audio Ltd., at its sole discretion and, in no event, ill Phædrus Audio Ltd. be
liable for any direct, indirect, special, incremental or consequential damages resulting from any defect in the product,
including lost profits, damage to property and, to the extent permitted by la , damage for personal injury, even if Phædrus
Audio Ltd. has been advised of the possibilities of such damages.

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
Shipping Charges
For any hard are defects experienced by the customer hile the product is under arranty, Phædrus Audio Ltd. ill incur
the shipping cost to the customer and the customer is responsible for the shipping costs to Phædrus Audio Ltd's
designated after-sales service office. For defective products that are out of arranty the customer is responsible for all
shipping costs to and from Phædrus Audio Ltd’s designated after-sales service office.
Extended warranty and out-of-warranty services
Various services are available from Phædrus Audio Ltd. These include repair services for
equipment once the arranty period has expired, and the ability to extend the arranty period.
These are:
PHAB-REP - Repair of a defective PHAB preamp', charged at a flat-rate
PHAME-REP - Repair of a defective PHAME preamp', charged at a flat-rate
PHI-REP - Repair of a defective PHI DI box, charged at a flat-rate
PHAB-MISEAJOUR - Mise à jour for PHAB preamp'*
PHAME-MISEAJOUR - Mise à jour for PHAME preamp'*
PHI-MISEAJOUR - Mise à jour for PHI DI box*
PHAE-EXTWARR - Extends standard arranty (see above) by a further year**
* A mise à jour service includes a complete inspection, re-valve (re-tube) and the implementation
of any engineering updates as ell as a re-test to performance specification.
** If extended arranty is ordered after the initial arranty has expired then the intervening
period is charged ithout exception.
Please contact your dealer or Phædrus Audio Ltd. for current prices.
Warranty service conditions are subject to change ithout notice. For the latest arranty terms and conditions and
additional information regarding Phædrus Audio Ltd. limited arranty, please see complete details online at
.phaedrus-audio.com .
Chapter 3 - Safety
General
Before using any piece of equipment manufactured by Phædrus Audio Ltd., be sure carefully to
read the applicable items of these operating instructions and the safety suggestions. Keep them
for future reference. Follo the arnings indicated on the unit, as ell as in the operating
instructions.
Selection of PSU
Suitable PSUs for the Phædrus Audio Ltd. PHAB and PHAME preamplifiers and the PHI DI-Box
are available as line items from Phædrus Audio.
They are:
PHAE-PSU(110) - 110V mains plugtop PSU: plug is North American type.
PHAE-PSU(220Euro) - 220V mains plugtop PSU; plug is European type
PHAE-PSU(220UK) - 220V mains plugtop PSU; plug is 3 pin 13 AMP, British type

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
A 12V AC supply is required for the Phædrus Audio PHAB, PHAME and PHI units. A direct current
(DC) supply is NOT suitable and ill damage the unit. If a supply is sourced else here than from
Phædrus Audio, it must have the follo ing specifications:
•12V AC, 12 Watt supply or greater (> or = 1000mA supply current)
•Must be suitable for connection to the appropriate mains voltage
•Must be Class-II, double-insulated
•Must have 30% regulation or better
•Must contain one-shot thermal fuses, resettable fuses, or Polys itches for over-current
protection.
UK and Euro units should comply ith EN 60950-1 and CE requirements and o ly PSUs with UL
a d CSA approvals should be provided on North American units. Units supplied to other countries
should be verified to comply ith the relevant standards hich obtain in those territories.
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.
The PHAB and PHAME preamplifiers and the PHI DI-Box should
NEVER be connected to the external po er supply or in any
other ay energised hen the case is opened and/or the circuit board is accessible.
PHI – Exposed valve (vacuum tube) safety
The Phædrus Audio PHI DI-Box has an exposed 12AU7 (ECC82) valve (tube). In order to ensure
safety, in all circumstances - even if the glass envelope of the valve (tube) ere smashed, the
exposed voltages ensure touch-current is limited to a safe value. Nevertheless keep this product
a ay from babies, children and pets. DISCONNECT THE EXTERNAL PSU AND ALL OTHER
CONNECTED EQUIPMENT WHEN REPLACING THE VALVE (TUBE).
Ground (earth) issues
Phædrus Audio Ltd.'s PHAB, PHAME and PHI products are all po ered by external (12V AC) po er
supplies hich should be double insulated, class-II types (see above). It should therefore be
appreciated that,
A CONNECTION TO EITHER THE PHAB, the PHAME of the PHI product CAN NEVER
BE RELIED UPON PROVIDE A PROTECTIVE OR SAFETY EARTH (GROUND). This
advice obtains IRRESPECTIVE OF THE SETTINGS OF ANY "EARTH OR GROUND
LIFT" SWITCHES .
General Safety Instructions
•Do not operate Phædrus Audio equipment near any source of ater or in excessively moist
environments.
•Keep your Phædrus Audio equipment a ay from babies, children and pets.
•Do not let objects do not fall, or liquids be spilled, into the enclosure.

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
•Situate the Phædrus Audio equipment a ay from heat sources or other equipment that
produce heat.
•Ensure Phædrus Audio equipment has adequate ventilation. Improper ventilation ill cause
overheating, and can damage the equipment.
•When cleaning Phædrus Audio equipment, remove all connections to the unit; including
po er and gently ipe ith a clean lint-free cloth; if necessary, gently moistened ith
luke arm or distilled ater. 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. hen: 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 - About Valves (Vacuum Tubes)
Valve (tube) types
The Phædrus Audio PHAB and PHAME preamplifiers
employ an EF86 pentode input valve (tube) and an ECC88,
or E88CC, valve (tube) as the output device. Positions for
the valves are very clearly marked on the PCB silkscreen
as indicated in the illustration. The valves (tubes) that
are installed in Phædrus Audio’s products during
assembly are selected to give the best possible
performance. We offer versions of all products ith NOS (rather than current production) valves.
But these must be chosen carefully, and be pre-screened. Replacing the supplied valves (tubes)
ith different, collectible, or “high-end” valves (tubes) types may not affect any improvement and
might cause deterioration of performance.
EF86
The EF86 is a lo noise pentode, originally manufactured by many different factories and
branded by Philips, Mullard, Telefunken, Valvo, and GEC, TESLA and Siemens/RFT. Many
NOS valves are still available. Equivalents include the CV2901 (the UK service designation
for the EF86) and the CV4085; a special quality version for military use. The EF86 tube is
still being produced in Russia and in the Slovak Republic under JJ Electronics brand name
(formerly Tesla). Chinese close equivalents are also still being manufactured.
ECC88
The ECC88 is a frame-grid, sharp cut-off, t in (dual) triode ith an internal,
separating screen. The valve as originally designed for service in the front ends of
VHF radio receivers. The ECC88 (E88CC) tube type is equivalent to the North
American 6DJ8 tubes and the (rare) military branded CV2492. The transposition of
the figures from ECC88 to E88CC as Mullard's ay of denoting a special quality
build and is equivalent to the US 6922. Russian 6N1P tubes and the Chinese 6N1
tube are NOT equivalent. NEVER i stall a 6N1(P) i the Phædrus Audio PHAB or
PHAME product. The ECC88 valve is sill being manufactured in China and in the

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
Slovak Republic under JJ Electronics brand name (formerly Tesla).
12AU7 or ECC82
The 12AU7 is a popular, miniature 9-pin medium-gain dual triode valve
(tube) and is used in many instrument and hi-fi amplifiers. Both
triodes are used, in cascade, in the Phædrus Audio PHI product. The
12AU7 is also kno n in Europe under its Mullard-Philips tube
designation ECC82. This tube is idely available both ne and NOS
and a number of special quality equivalents are available. Current
production of 12AU7 takes place in Russia, Slovakia, and China.
Phædrus Audio selects and recommends the long anode (plate) version
of the ECC82 (ECC802S) or 12AU7 valve for application in the PHI
DI-Box.
Valve (tube) lifetime
You should replace the valves in the tubes in the Phædrus Audio products only hen you start to
notice changes in the sound quality. If the gain of the preamplifier decreases noticeably, then this
is certainly evidence of the onset of valve (vacuum tube) failure. Before this, the tone may become
"dull" and transients may be become "blunted".
That said, the lifetime of a valve (tube) is largely determined by the lifetime of its cathode
emission and the small-signal valves (tubes) used in the PHAB, PHAME and PHI use oxide cathodes,
hich can provide adequate cathode emission for 100,000 hours or more. That's over eleven year’s
continuous use. So do not replace valves (tubes) just because they have seen a fe years service.
Phædrus Audio Ltd. can provide suitable valves (tubes) as spares hich, after a burn-in period, are
screened for best performance in your Phædrus Audio product. These are available as line items:
PHAE-12AU7 – Selected 12AU7/ECC82 type valve
PHAE-ECC88 - Selected ECC88 type valve
PHAE-EF86 - Selected EF86 type valve
Please contact your dealer or Phædrus Audio Ltd. for current prices.
Chapter 5 - Instructions for use
Phædrus Audio’s PHAB, PHAME and PHI products employ valves (vacuum tubes) as the ONLY
ACTIVE DEVICES in the audio path. Valves (vacuum tubes) and their associated circuitry need
time to reach an electronic equilibrium before they ill operate at optimal specifications. Please
therefore allo these products to arm up for, at least, 5 to 10 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 hen not in use.

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
Typical connection set-up for the PHAB
PHAB
Application and connections
Mixer and computer interface manufacturers operate in a very competitive environment. Even a
small mixer must (by definition) contain 8, 16 or perhaps 32 microphone amplifiers. Similarly, a
computer interface designer has to "shoe-horn" a microphone amplifier into a box ith a host of
other features and a noisy, semi-digital environment. Unsurprisingly, these amplifiers are designed
on a careful budget and contain some compromises. Hence, the use of a fe dedicated, stand-alone
microphone preamplifiers, solely designed ith sound-quality in mind, can transform the quality of
your recorded signals. That is the aim of the Phædrus Audio PHAB tube microphone preamplifier:
to offer just such a no compromise microphone amplifier in a product hich incorporates the
standards and qualities adopted in classic recording gear. A block diagram of the PHAB is given
belo . The unit is connected as sho n in the accompanying diagrams.
Architecture of the PHAB preamp'
Controls
Input rotary attenuator, Pad and Gain switch
The PHAB microphone preamplifier operates ith a constant-gain amplifier and attenuates the
signal reaching the amplifier for level control. This is NOT common ith modern equipment; in
hich the gain is adjusted by modifications to the feedback net ork. Yet this approach as
completely standard in the 1950s and 1960s.

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
The Phædrus Audio PHAB tube preamplifier is built around a modular amplifier ith a fixed gain of
+40dB ( ith a s itch available to raise this to approximately +46dB). This amplifier is preceded by
a rotary, s itched attenuator control, offering attenuation of 0 to -30dB in steps of 6dB. A
further pad (operated by a front panel s itch) 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 s itch) are used to set
the overall preamplification level of the unit and feed a suitable electrical level to your
do nstream equipment. (see Operating level section for more information).
Normal operation should be to operate WITHOUT the -10dB pad engaged and ith the
preamplifier gain set to 40dB. Only operate the pad s itch hen the signal from the microphone is
too great for attenuation via the rotary attenuator control: and only operate the +46dB gain
s itch hen the signal from the microphone is too lo , even hen the rotary attenuator is set to
0dB.
HPF (rumble filter)
By virtue of its very high quality input and output transformers and its minimalist, ideband, 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, un anted, very lo 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 ith the
anted signals ith in the microphone preamp' itself, or, being fed to do nstream equipment. This
filter section is engaged by depressing the HPF s itch.
+48V "Phantom" supply
An internal phantom supply is provided to po er the microphone connected to the input. This
supply is engaged by depressing the "+48V" s itch on the front panel of the unit. Although this
supply is designed to "ramp-up" slo ly, it is NEVER a good idea to s itch phantom po er onto a
microphone on a channel ith an open fader because it can create "pops and bangs" hich can
damage electronic equipment, speakers and fray nerves! Al ays mute any follo ing circuit before
s itching on the phantom (+48V) supply. To avoid loud transients, al ays make sure phantom po er
is off hen connecting or disconnecting microphones.This supply is designed to support the
hungriest of microphones.
NOTE: Some microphones may be damaged by having phantom-po er applied to them: ribbon microphones are especially
delicate in this regard. ALWAYS check hat type of microphone is connected, and that it is suitable for phantom-po ering
BEFORE operating the front-panel s itch.

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
PHAME
Application and connections
Architecture of the PHAME preamp'
The unit is connected as sho n in the accompanying diagrams.
Direct injection (DI) provides a clean ay of applying instruments to your mixer or computer
interface. This is especially interesting today, no that there are so many plug-in effects and
amplifier emulations hich run as soft are on the computer host. All that is required, hen
orking this ay, is to get a natural signal into the computer's analogue to digital conversion
circuits. The soft are ill then "take it from there".
How to connect the PHAME preamp'
But if only things ere that simple! The problem is that a normal passive DI-box, or a solid-state
input stage can rarely offer the benefits of a valve preamplifier. (See Voicing and the valve (tube)
amplifier
box.)

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
Voicing and the valve (tube) amplifier
Guitar pickups are made by inding about 8000 turns of ire on a former or bobbin hich surrounds several permanent
magnets. The resistance of the ire is relatively lo , but the inductance is high and the device therefore has a rising
output impedance. The guitar connects to the amplifier by means of a coaxial cable ith the inner, signal ire surrounded
by the earthy "screen". The construction of this type of cable is not a million miles a ay from the construction of a
capacitor and, unsurprisingly, the coaxial cable displays a significant capacitance at audio frequencies. When the inductive
pickup is connected via a capacitive cable, a resonant circuit is formed and e can model this and confirm that this creates
a musically-significant resonant-circuit ell ithin the audio band. (Left trace belo .)
This resonant mechanism is one of the principal voicings for the electric guitarist. Three factors affect the resonant
circuit: the inductance of the pickup; the capacitance of the cable; and the input impedance of the amplifier: essentially
the greater the capacitance and the greater the inductance, the lo er the formant frequency. This is hy expensive
"Custom Wound" pickups and Humbuckers ( hich have more turns and therefore greater inductance) sound considerably
more "chunky" or "fatter" than standard pick-ups. It's the major reason hy cables really do sound different; fatter
cables (because of their lo er capacitance) "singing" better than cheap, thin cables.
The effect of the amplifier input resistance is to damp the resonant circuit. Valve preamplifiers do not do this because of
the common value of 1MΩ for the bias resistor of the first valve. Solid state circuits on the other hand – due to their
higher bias currents and lo er circuit impedances – can have a major damping effect on the input signal, destroying so much
of the character imparted by the resonant circuit (see right-hand trace above). In short, the formant frequency is
absolutely crushed and the character of the voice destroyed. Amongst many other reasons, this, so many solid-state guitar
preamplifier designs fail to have the "spring" and "life" of a good valve amplifier: the precious signal is choked before it
reaches (literally and metaphorically) first base!
Controls
Input attenuator
The input attenuator is a good quality, high-value potentiometer; just like the volume control on an
amp’. Set this control so as to drive the appropriate level to the do nstream equipment; see
Operating Levels section.
Gain select (low/ high)
The gain select s itch should normally be set to the lo position. In the high position it is possible
to overdrive the tube preamplifier circuitry from a guitar. This setting is for lo output
instruments and/ or creative effects only.

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
Ground (earth) lift
Hums and buzzes are the bane of an audio engineer’s life. Unfortunately, it is the nature of a high-
impedance, unbalanced instruments like an electric guitar to be highly susceptible to the AC
electromagnetic fields thro n off by all mains-po ered equipment. Often all that is required to
solve the problem and reduce the hum (and/or buzz) to a manageable level is to orient the
instrument differently so that the pickups no longer intercept so much changing flux. But
sometimes hums and buzzes prove to be particularly stubborn and intractable, regardless of the
guitarist’s pirouetting! When this happens, it is usually because of a ground-loop (or earth-loop).
A ground-loop forms hen an unbalanced, audio signal flo s in a circuit in hich the signal-return
(usually the cable braid), provides a current path hich is in parallel ith the installation earth.
This is a problem because "all earths are not equal". That's to say, the installation earth ill be
carrying certain leakage currents. These currents set up a potential across the installation iring
hich drives a circulating current in the braid of the audio cable. The result is a “hum” voltage
superimposed upon the audio signal.
The adoption of balanced signals, and especially audio transformers should ensure that ground-
loops (or earth-loops) should NEVER occur. Certainly the Phædrus Audio PHAME, by being both
double-insulated and incorporating an earth-free, transformer-balanced output stage can never
create a ground-loop in a correctly ired studio. Ho ever, connection to an earthed (or grounded)
guitar amplifier can cause a circulating current should pin 1 of the output XLR be earthed
(grounded) else here in the installation.
This is the role of the GROUND-LIFT s itch. It interrupts the connection of pin 1 of the output
XLR to the chassis of the PHAME and to the sleeve and screen of the guitar iring. Adjust the
position of this s itch to eliminate any ground-loop “hums”.
Please note, as explained in the Chapter 4, 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 GROUND LIFT" SWITCHES.
Buzz?
The PHAME preamplifier, like a lot of modern audio gear, is designed to operate from a double-insulated po er supply. If
the rest of the equipment in your recording set-up is double-insulated too (for instance, you might be using the PHAME
ith a laptop computer), it is possible for your entire audio installation to operate ithout a mains earth. This represents no
problems from a safety point of vie . Ho ever it can cause the unbalanced, high-impedance input of the PHAME to be very
susceptible to electrostatically induced mains-interference. The tell-tale signs here are a high level audio “buzz” rather
than a “hum” on the output signal, and the phenomenon that the instrument (particularly electric- and bass-guitar) is very
“live” so that simply touching the strings may orsen or improve the buzz.
In this situation, it is recommended that pin 1 of the output connector of the PHAME be connected to mains-earth (ground)
or a technical-earth, and the “LIFT” s itch be left in the out position.

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
Operating levels
Because of the vintage of the PHAB and PHAME
preamplifier designs and their origin in German
manufactured
broadcast
television and
radio equipment,
the envisaged
operating level for
this equipment is
slightly lo er than
modern recording
industry
standards. When
feeding do nstream equipment, the output level should be set to a nominal 0dBu (0.775V RMS).
This means that if monitoring the output on a standard VU instrument, the input attenuator should
be set so that the output level reads around -4VU, ith occasional peaks to 0VU on programme. On
a digital meter, this is equivalent to signal peaks reading +6dB above recording industry standard
+4dBu alignment level. This is equivalent to +10dBu (or -8dBFS on equipment aligned to EBU EBU
R64-1992 standard and -14dBFS on equipment aligned to SMPTE RP155 as illustrated). On a
broadcast standard PPM, peaks should be set to read 6 on the BBC scale, or +8 on the European
EBU scale. Output level is sufficient easily to modulate semi-pro' equipment (mixers, DAT, solid-
state recorders and many sound cards) to 0dBFS.
That said, the headroom on the
PHAB and PHAME is
considerable, and no noticeable
distortion is evident hen
operating the equipment at
standard recording industry
levels of +4dBu=0VU, or even
higher. There is thus considerable scope for
experimentation ith over driving this vintage circuitry to
explore its character in gradual overload. For example,
peaks may be allo ed to reach +18dB above alignment level
ithout clipping. The Phædrus Audio PHAB and the PHAME
preamplifiers may thus be used directly to feed modern A
to D conversion equipment aligned to EBU or SMPTE
standards follo ing the recommendations illustrated
(right).

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PHI
Application and connections
Simple DI units utilise a transformer to balance the unbalanced signal and
feed it to the microphone input of your mixer or microphone preamplifier.
This technique
provides a
reasonably high-
impedance as
presented to the instrument; typically a
fe tens of thousands of ohms. Adequate
for a keyboard, for example: at a push,
sufficient for a bass guitar. But the tone
of an electric guitar is completely destroyed, unless it drives an impedance hundreds of times
greater than can ever be provided by a transformer. The only ans er is to present the instrument
ith the very high input impedance, such as it "sees" hen plugged into a valve amp' (see Voicing
and the valve amplifier
section above). This is the role of the Phædrus Audio PHI DI-Box. The dual
valve (tube) stage acts as an impedance-converter, presenting the instrument input ith an
impedance of approximately 1MΩ. The output of the unit then feeds the microphone input of your
mixer, computer interface, or your PHAB preamplifier.
The Phædrus Audio PHI DI-Box is connected as sho n in the diagrams above. Ensure that the
"LOUDSPEAKER/INSTRUMENT" s itch is in the "INSTRUMENT" position. Ho ever, note that
feeding the output of the PHI into the studio console ill negate some of the benefits of the
PHI’s unique character. Use a premium-quality microphone preamplifier (such as Phædrus Audio’s
PHAB) for the very best results as sho n belo .

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Controls
Instrument/ Loudspeaker switch – LOUDSPEAKER MODE
As a general rule, guitar and bass guitar and certain electric pianos and organs rely on the
distortions in their amplifiers and loudspeaker-cabinets for the character of their sound. Ideally
of course, these instruments need to be captured ith microphones, rather than applying a line-
level signal to the mixer. Ho ever, this is not al ays possible for a host of reasons including:
unacceptable "spill" from other sources; feedback problems; room resonance issues; cabinet
"buzz", inadequate microphone response and so on.
In this situation, it is useful to be able to DI, not the output from the instrument, but the signal
hich drives the loudspeaker. This signal is not modified by the cabinet, but it does have - at least
- the character of the amplifier, its tonal modifications and distortion characteristics, imprinted
upon it. The Phædrus Audio PHI is able to provide this functionality. Ho ever, this loudspeaker
signal is of a very much larger amplitude
than this signal hich leaves the instrument
- it has been amplified after all! It is
therefore necessary to attenuate this
signal before presenting to the DI circuitry
and this is exactly hat is done hen the
"LOUDSPEAKER/INSTRUMENT" s itch is
moved to the "LOUDSPEAKER" position.
The attenuation is -32dB.
YOU MUST SELECT THE “LOUDSPEAKER” POSITION OF THE LOUDSPEAKER/INSTRUMENT
SWITCH WHEN CONNECTING THE Phædrus Audio PHI TO THE LOUDSPEAKER CIRCUIT OF

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
AN AMPLIFIER. NOT SO DOING WILL DAMAGE YOUR Phædrus Audio PHI. FURTHERMORE
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 ith the Ground
lift s itch in the “LIFT” position hen operating the PHI in this ay.
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 po er amplifier. Valve amplifiers especially may be damaged ithout the correct
speaker-load connected.
Flat/ Cabinet switch
The problem ith the signal from the amplifier is that it often contains many high frequencies
hich are not reproduced by instrumental loudspeaker cabinet. The result 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 filter; 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 hich reproduces the electro acoustic band-pass of the loudspeaker cabinet. To
this end, the Phædrus Audio PHI - DI unit, is enhanced by the inclusion ithin the unit of a
loudspeaker-emulation filter hich imprints the response of a typical, closed-back, guitar cab'
onto the signal. The general response of this filter is illustrated above. Essentially the response
falls a ay gently in the bass, peaks in the mid-treble and falls a ay rapidly above about 4kHz as
sho n in the figure.
Ground (earth) lift
A ground-loop (earth-loop) forms hen an unbalanced, audio signal flo s in a circuit in hich the
signal-return (usually the cable braid), provides a current path in parallel ith the installation
earth (ground). Small potentials in the installation earth (ground) iring, drive a circulating current
in the braid of the audio cable. The result is a “hum” voltage superimposed upon the audio signal.

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 ired studio. Ho ever, connection to an earthed (or grounded)
guitar amplifier can cause a circulating current should pin 1 of the output XLR be earthed
(grounded) else here in the installation.
This is the role of the GROUND-LIFT s itch. 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 iring. Adjust the
position of this s itch 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 GROUND LIFT" SWITCHES.
Buzz?
The PHI DI-Box, like a lot of modern audio gear, is designed to operate from a double-insulated po er supply. If the rest
of the equipment in your recording set-up is double-insulated too (for instance, you might be using the PHI ith a laptop
computer and USB interface), it is possible for your entire audio installation to operate ithout a mains earth. This
represents no problems from a safety point of vie . Ho ever it can cause the unbalanced, high-impedance input of the PHI
to be very susceptible to electrostatically induced mains-interference. The tell-tale signs here are a high level audio “buzz”
rather than a “hum” on the output signal, and the phenomenon that the instrument (particularly electric- and bass-guitar) is
very “live” so that simply touching the strings may orsen or improve the buzz.
In this situation, it is recommended that pin 1 of the output connector of the PHI be connected to mains-earth (ground) or
a technical-earth, and the “LIFT” s itch be left in the do n position.
Chapter 6 - Circuit descriptions
PHAB & PHAME
The Phædrus Audio PHAB and the PHAME preamplifiers share roughly the same preamplifier
circuit hich is illustrated in general form belo . The differences mainly lie in the input circuitry
and attenuator controls. Both devices operate ith 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 ( ith a s itch available to raise this to approximately +46dB). The amplifier
is preceded by a s itched attenuator control offering attenuation of 0 to -30dB in steps of 6dB. A
further pad (operated by a front panel s itch) 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 ith the high-gain s itch depressed).

Ver. 1.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
The amplifier block is a t o-stage design incorporating an EF86 lo -noise pentode in the first
stage and a paralleled ECC88 high mutual-conductance double-triode as the output stage. All the
landmark amplifier designs from Mullard's application laboratories specified the EF86. In fact,
Mullard described the EF86 as a lo noise pentode intended for use as a RC coupled AF voltage
amplifiers; particularly in the early stages of high gain audio amplifiers, microphone preamplifiers
and magnetic tape recorders
.
The output stage comprises a dual triode valve in hich the t o devices are ired in parallel. This
paralleling of the t o triodes has the beneficial effects of doubling the mutual conductance and
halving the anode resistance. The latter characteristic is particularly valuable in ensuring a lo
impedance drive to the output transformer.
The output stage directly feeds the output step-do n transformer. The output is balanced and
earth free. A modest degree of negative feedback is taken from the anode of the output valve(s)
to the cathode circuit of the input stage to stabilise gain, extend frequency-response and lo er
distortion. A change to this feedback net ork is employed to effect the gain s itching (from
40dB to 46dB in the PHAB and from 24dB to >30dB in the PHAME), less feedback being applied to
achieve higher gain.
In the case of the Phædrus Audio PHAB microphone preamplifier, the grid of the input pentode is
driven from the input transformer. In the case of the PHAME, this transformer is not fitted.
Performance
At recording industry line-up (0VV=+4dBu) the magnitude of all distortion components on a 1kHz
sine ave input are less then -55dB belo fundamental. This, and the A- eighted noise response
relative to maximum output, is illustrated belo .