Phaedrus Audio PHAB, PHAME, PHI User Manual

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User Manual for

PHAB - tube microphone preamp'

PHAME - tube instrument preamp'

PHI - tube DI box

This manual is also available as a pdf download, Go to: www.phaedrus-audio.com

Preface – About Phædrus Audio

Phædrus Audio was 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 prac tices of the fifties and sixties, and are motivated to re-establish the values of the great audio-technology legends of the past with their ideals of transparency, handbuilt quality, and serviceability. Using modern manufacturing techniques and the benefi ts of modern component technology, Phædrus Audio's aim is to reproduce the quality and character of classic equipment bu t in a modern, highly reliable, and cost-effec tive way.

Chapter 1 - Background

The Phædrus Audio PH AB, PHAME and PHI products came about becau se two, recording musicians wanted to own a "classic" console from the nineteen-six ties. Ideally a famous EMI, REDD, all valve (vacuum tube) mixer u sed to record T he Beatles.

But those desks make a collection of hen's teeth look positively prosaic as well as cheap! So, we set abou t researching with the idea of producing a replica console.

In order to do this, first it was necessary to have a design for the amplifier modules around w hich the mixers of this vintage are organised.

In the famous EMI REDD mix ers, these amplifiers were either the German manufactured V72(S) amplifiers or the, very rare, EMI built, REDD.47 a mplifiers.

V72 and REDD.47 modular amplifiers

Ironically, despite the fact that many more V72 amplifiers exist than do REDD.47s, the EF804 valves which the German a mplifiers use are no longer in current production and are beco ming very expensive: whereas the EF86 and ECC88 (E88CC) valves employed in the REDD amplifiers remain in production and are widely available. So, a new amplifier, designed from the common source of Mullard's reference audio designs, and thereby sharing a common heritage with the REDD amplifiers, was developed. We called this amplifier the Phaedrus "PHILHARMONIC" (literally

music loving

Mark II console. The PHILHARMONIC a mplifier circui t is described in detail below in section

Circuit descriptions

). This PHILHARMONIC amplifier forms the heart of the Phædrus Audio LONDON

Mullard published many reference designs which were used by EMI in developing the REDD amplifiers. Phædrus Audio w ent back to these same references to develop the PHAB, PHAME and PHI products.

The prototype London Console (Mark II)

PHAB

The Phædrus Audio PH AB microphone prea mplifier is essentially the front-end of a single-channel of the LONDON console, in which PHILHARMONIC modular-amplifier is combined with a ro tary attenua tor, switchable pad, high-pass filter and gain-swi tching circuitry similar to that employed in the famous six ties consoles.

PHAB microphone preamp’ and PHAME in strument preamp’

PHAME

The Phædrus Audio PH AME is a development of the PH AB in which the very high input impedance of the inpu t pentode valve is exploited to provide a very high quality instrumental preamplifier.

PHI

The Phædrus Audio PHI DI-Box is a produc t designed for a wide range of applications; from live use to the recording studio. Partnered with the PHAB it provides a solu tion for almost any instrumental or vocal preamp' application.

Construction - or star earths (grounds) and

PHI DI-Box

curly tracks

Given their vintage, the V72 a mplifiers and the REDD.47 amplifiers were buil t on a metal chassis, with the components hardwired onto tag-strips. Although Phædrus Audio equip ment uses printed circuit boards to ensure consi stent performance and reliability, the equipment follows "classic" practices such as star earths and "na tural contour" tracking, just like hook-up wire.

Every Phæd rus Audio PHAB, PHAME and PHI product is hand assembled and individually tested. A test pro-forma is provided w ith every uni t. Phædrus Audio offer comprehensive service for products both inside and ou tside of their warranty period.

Chapter 2 - Warranty and Service

Please register your purchase wi th Phaedrus Audio at www.phaedrus-audio/registration.htm . This will simplify service & repair should you need this service. Your na me will be placed on our mailing list (unless otherwise requested) for fu ture upda tes and new product announcements.

Service

If you experience a problem with a Phædrus Audio Ltd. product, contact:

support@phaedrus-audio.co m TELEPHONE: +44 207 193 4609 Skype ID: phaedrusaudio

We will diagnose the problem remotely and advise you of the warranty sta tus. If a repair or replacement is required, we will issue a Return Merchandise Authoriza tion (RMA) number and tell you w here to send the uni t to be repaired. You MUST have an RMA number before you return the equipmen t to Phædrus Audio Ltd.'s support service. Be sure to write RMA number on outside of shipping box and to include your name, address, phone nu mber, a copy of original sales invoice and a detailed description of the problem. Phædrus Audio Ltd. will not accept responsibility for loss or damage in shipping or fo r equipment returned w ithout valid paperwork and/or a valid RMA number. Remember, warranty is void if product serial numbers have been removed or al tered, or if the product has been da maged 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 YOU R PROOF OF PURCHASE COVERING YOUR LIMITED WARRANTY. LIMITED WARRANTY IS VOID WITHOUT SUCH PROOF OF PURCHASE.

Phædrus Audio Ltd.'s Limited Warranty

This limited warranty 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 website www.phaedrus-audio.com, or by contacting sales@phaedrus-audio.com. Phædrus Audio Ltd. warrants that the equipment it manufactures shall be free from defects in material and workmanship for a period of one (1) year from the original date of purchase; unless a longer minimum warranty period is mandated by applicable local laws. If equipment fails due to such defects within this period, Phædrus Audio will, at its option, repair or provide a replacement for the defective part or product. Valves (vacuum tubes) are excluded from the one-year warranty, but are warranted for 90 days from day of purchase. This warranty 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 warranty is invalid if the factory-applied serial number has been altered or removed from the product. This limited warranty is extended exclusively to the original buyer (customer of Phædrus Audio Ltd., or authorized retail dealer) and is not transferable to anyone who may subsequently purchase the product. No other person (retail dealer, etc.) shall be entitled to give any warranty promise on behalf of Phædrus Audio Ltd. Phædrus Audio Ltd. makes no other warranties, expressed or implied, of merchantability, fitness for a particular purpose or otherwi se. Phædrus Audio Ltd. liability is limited to repair or replacement by Phædrus Audio Ltd., at its sole discretion and, in no event, will 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 law, damage for personal injury, even if Phædrus Audio Ltd. has been advised of the possibilities of such damages.

Shipping Charges

For any hardware defects experienced by the customer while the product is under warranty, Phædrus Audio Ltd. wi ll 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 warranty 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 equipmen t once the warranty period has expired, and the ability to extend the warran ty period. These are:

PHAB-REP - Repair of a defective PHAB preamp', charged at a flat-rate PHAME-REP - Repair of a defec tive PHAME preamp', charged a t a flat-rate PHI-REP - Repair of a defec tive PHI DI box, charged at a flat-rate

PHAB-MISEAJOUR - Mise à jour for PHAB preamp'* PHAME-MISEAJOUR - Mise à jour for PH AME preamp'* PHI-MISEAJOUR - Mise à jour for PHI DI box* PHAE-EXTWARR - Extends standard w 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 well as a re-test to performance specification.

If extended warran ty is ordered after the initial w arranty has expired then the in tervening

period is charged without exception .

Please contact your dealer or Phædrus Audio Ltd. for current prices.

Warranty service conditions are subject to change without notice. For the latest warranty terms and conditions and additional information regarding Phædrus Audio Ltd. limited warranty, please see complete details online at

www.phaedrus-audio.com .

Chapter 3 - Safety

General

Before using any piece of equip ment 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. Follow the warnings indicated on the unit, as well 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

A 12V AC supply is required for the Phædrus Audio PHAB, PHAME and PHI units. A direct current (DC) supply is NOT suitable and will damage the unit. If a supply is sourced el sewhere than from Phædrus Audio, it must have the following specifications:

• 12V AC, 12 Watt supply or greater (> or = 1000mA supply current)

• Must be sui table for connec tion to the appropria te mains voltage

• Must be Class-II, double-insulated

• Must have 30% regulation or better

• Must contain one-shot thermal fuses, resettable fuses, or Polyswitches for over-current

protection.

UK and Euro units should comply with EN 60950-1 and CE requirements and only PSUs with UL and CSA approvals should be provided on North American units. Uni ts supplied to other countries should be verified to comply with the relevan t standards which obtain in those terri tories.

User Access & Servicing

Phædrus Audio equipment employs thermionic valve (vacuu mtube) technology and employs hazardous voltages for the hightension supplies. THE USER SHOULD NOT ATTEMPT TO SERVICE THE UNIT. ALL SERVICING SHOULD BE REFERRED TO QUALIFIED SERVICE PERSONNEL OR F ACTORY ONLY. The PHAB and PHAME prea mplifiers and the PHI DI-Box should NEVER be connected to the external power supply or in any

other way energised when the case is opened and/or the circui t 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) were smashed, the exposed vol tages ensure touch-current is limited to a safe value. Nevertheless keep this product away from babies, children and pets. DISCONNECT T HE 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 pow ered by external (12V AC) power supplies which should be double insulated, class-II types (see above). It should therefore be apprecia ted 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 water or in excessively moist environments.

• Keep your P hædru s Audio equipment away from babies, children and pets.

• Do not let objec ts do no t fall, or liquids be spilled, into the enclosure.

• Situate the Phædrus Audio equip ment 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 equip ment.

• When cleaning Phædrus Audio equip ment, remove all connections to the unit; including power and gently wipe with a clean lint-free cloth; if necessary, gently moistened w ith lukewarm or distilled water. Use a dry lint-free cloth to remove any remaining moisture. NEVER use aerosol sprays, solven ts, 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 ob jects) have fallen into the enclosure; or liquid has fallen into, or been spilled into the unit; or the unit has been expo sed to rain or high humidity; or the unit does no t operate normally or exhibi ts 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 PH AB 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 selec ted to give the best possible performance. We offer versions of all products with NOS (rather than current production) valves. But these must be c hosen carefully, and be pre-screened. Replacing the supplied valves (tubes) with different, collectible, or “high-end” valves (tubes) types may not affec t any improvement and might cause deterioration of performance.

EF86

The EF86 is a low 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 designa tion 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 na me (formerly Tesla). Chinese close equi valents are also still being manufactured.

ECC88

The ECC88 is a frame-grid, sharp cu t-off, twin (dual) triode w ith an in ternal, separating screen. T he valve was 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 was Mullard's way of denoting a special quality build and is equivalent to the US 6922. Ru ssian 6N1P tubes and the Chinese 6N1 tube a re NOT equivalent. NEVER install a 6N1(P) in the Phædrus Audio PHAB or PHAME product. The ECC88 valve i s sill being manufactured in China and in the

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 a mplifiers. Bo th triodes are used, in cascade, in the Phædrus Audio PHI produc t. The 12AU7 is also know n in Europe under i ts Mullard-Philips tube designa tion ECC82. This tube is widely available both new 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 when you start to notice changes in the sound quality. If the gain of the preamplifier decreases no ticeably, then this is certainly evidence of the onset of valve (vacuum tube) failure. Before this, the tone may beco me "dull" and transients may be become "blunted".

That said, the lifetime of a valve (tube) is largely determined by the lifetime of its ca thode emission and the small-signal valves ( tubes) used in the PHAB, PHAME and PHI use oxide cathodes, which can provide adequa te cathode emission for 100,000 hours or more. That's over eleven year’s continuous u se. So do not replace valves (tubes) just because they have seen a few years service.

Phædrus Audio Ltd. can provide suitable valves (tubes) as spares which, af ter a burn-in period, are screened for best performance in your P hædru s Audio product. These a re 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 produc ts employ valves (vacuum tubes) as the ONLY ACTIVE DEVICES in the audio path. Valves (vacuu m tubes) and their associated circuitry need time to reach an electronic equilibrium before they will operate at optimal specification s. Please therefore allow these produc ts to warm up for, a t least, 5 to 10 minutes before using them in your signal chain. To prolong the life of your valves (tubes), it is reco mmended that you turn off these units when not in use.

Typical connection set-up for the PHAB

PHAB

Application and connections

Mixer and computer in terface manufacturers operate in a very competi tive environ ment. Even a small mixer must (by definition) contain 8, 16 or perhaps 32 microphone a mplifiers. Similarly, a computer in terface designer has to "shoe-horn" a microphone a mplifier into a box w ith a host of other fea tures and a noisy, semi-digital environment. Unsurpri singly, these amplifiers are designed on a careful budget and contain some compromises. Hence, the use of a few dedicated, stand-alone microphone preamplifiers, solely designed with sound-quality in mind, can transform the quality of your recorded signals. That is the aim of the Phædru s Audio PHAB tube microphone preamplifier: to offer just suc h a no compromise microphone a mplifier in a product which incorpora tes the standards and qualities adopted in classic recording gear. A block diagram of the PHAB is given below . The unit is connected as shown in the accompanying diagrams.

Architecture of the PHAB preamp'

Controls

Input rotary attenuator, Pad and Gain switch

The PHAB microphone prea mplifier operates wi th a constant-gain amplifier and attenua tes the signal reaching the amplifier for level control. This is NOT co mmon with modern equipment; in which the gain is adjusted by modifications to the feedback network. Yet this app roach was completely standard in the 1950s and 1960s.

The Phædrus Audio PH AB tube prea mplifier is built around a modular a mplifier with a fixed gain of +40dB (with a switch available to raise this to approximately +46dB). T his a mplifier is preceded by a rotary, switched a ttenuator con trol, offering attenua tion of 0 to -30dB in steps of 6dB. A further pad (operated by a fron t panel switch) is available, to increase the attenuation to -40dB; thereby matching the gain of the modular a mplifier and reducing the overall unit gain of the equipmen t to uni ty. These con trols (the input attenua tor, pad and the gain switch) are used to set the overall preamplification level of the unit and feed a suitable elec trical level to your downstream equipment. (see

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 mic rophone is too great for attenua tion via the rotary attenua tor control: and only operate the +46dB gain switch when the signal from the microphone is too low, even when the rotary attenua tor is set to 0dB.

Operating level

sec tion for more information).

HPF (rumble filter)

By virtue of its very high quali ty input and outpu t transformers and its mini malist, wideband, valve circuitry, the pass-band of the Phædrus Audio PHAB preamplifier is extended in both the bass and extra-high f requency ranges. Due to this, unwanted, very low frequencies, due to traffic or airconditioning "rumble" may be picked up by the microp hones and a mplified. The high-pass filter (HPF) filters ou t these frequencies and prevents them from either, intermodulating with the wanted signals with in the microphone preamp' itself, or, being fed to downstream equip men t. 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. Although this supply is designed to "ra mp-up" slow ly, 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 phan tom (+48V) supply. To avoid loud transien ts, alw ays 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.

PHAME

Application and connections

Architecture of the PHAME p reamp'

The unit is connec ted as shown in the accompanying diagrams.

Direct injection (DI) provides a clean way of applying instruments to your mixer or co mputer interface. This is especially interesting today, now that there are so many plug-in effects and amplifier emulations which run as software on the computer host. All that is required, w hen working this way, is to get a natural signal into the computer's analogue to digi tal conversion circuits. The sof tware will then "take it from there".

How to connect the PHAME preamp'

But if only things were 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 prea mplifier. (See

amplifier

box.)

Voicing and the valve (tube)

Voicing and the valve (tube) amplifier

Guitar pickups are made by winding about 8000 turns of wire on a former or bobbin which surrounds several permanent magnets. The resistance of the wire is relatively low, 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 with the inner, signal wire surrounded by the earthy "screen". The construction of this type of cable is not a million miles away 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 we can model this and confirm that this creates a musically-significant resonant-circuit well within the audio band. (Left trace below.)

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 lower the formant frequency. This is why expensive "Custom Wound" pickups and Humbuckers (which have more turns and therefore greater inductance) sound considerably more "chunky" or "fatter" than standard pick-ups. It's the major reason why cables really do sound different; fatter cables (because of their lower 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 lower 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 soli d-state gui tar preamplifier designs fail to have the "spring" and "life" of a good valve ampli fier: the preci ous signal i s choked before it reaches (literally and metaphorically) fi rst base!

Controls

Input attenuator

The input attenuator is a good quality, high-value potentiometer; just like the volu me control on an amp’. Set this con trol so as to drive the appropriate level to the dow nstream equipment; see

Operating Levels

sec tion.

Gain select (low/ high)

The gain selec t switch should normally be set to the low posi tion. In the high position it is possible to overdrive the tube preamplifier circuitry from a guitar. This setting is for low output instruments and/ or crea tive effec ts only.

Ground (earth) lift

Hums and buzzes are the ban e of an audio engineer’s life. Unfortunately, it is the nature of a highimpedance, unbalanced instruments like an elec tric gui tar to be highly susceptible to the AC electromagnetic fields thrown off by all mains-powered 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 pa rticularly stubborn and intrac table, regardless of the guitarist’s pirouetting! When this happens, it is usually because of a ground-loop (or earth-loop).

A ground-loop forms when an unbalanced, audio signal flows in a circuit in which the signal-return (usually the cable braid), provides a current path which is in parallel with the installation earth. This is a problem because "all earths are no t equal". That's to say, the installation earth will be carrying certain leakage currents. These currents set up a po tential across the installation wiring which drives a circula ting current in the braid of the audio cable. The result is a “hum” vol tage superimposed upon the audio signal.

The adoption of balanced signals, and especially audio transformers should ensure tha t groundloops (or earth-loops) should NEVER occur. Certainly the Phædrus Audio PHAME, by being both double-insulated and incorpora ting an ea rth-free, transformer-balanced output stage can never create a ground-loop in a correctly wired studio. How ever, 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 PHAME and to the sleeve and screen of the guitar wiring. Adjust the position of this switch to elimina te 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 UP ON PROVIDE A PROTECTIVE OR SAFETY EARTH (GROUND). T his advice ob tains IRRESPECT IVE OF THE SETTINGS OF ANY "EARTH OR GROUND LIFT" S WITCHES.

Buzz?

The PHAME preamplifier, li ke 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 PHAME with a laptop computer), it is possible for your entire audio installation to operate wi thout a mains earth. This represents no problems from a safety point of view. However i t can cause the unbalanced, high-impedance input of the PHAME to be very susceptible to electrosta tically i nduced mains-i nterference. The tell-tale signs here are a hi gh level audio “buzz” rather than a “hum” on the output signal, and the phenomenon that the instrument (parti cularly electric- and bass-gui tar) is very “live” so that simply touching the strings may worsen or improve the buzz.

In this situati on, i t 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” switch be left in the out position.

Operating levels

Because of the vintage of the PHAB and PHAME preamplifier designs and their origin in German

manufac tured broadcast television and radio equipment, the envisaged operating level for this equipment is slightly lower than modern recording industry

standards. When feeding downstrea m equipment, the ou tput level should be set to a nominal 0dBu (0.775V RMS). This means that if monitoring the outpu t on a standard VU instrument, the input attenuator should be set so that the output level reads around -4VU, w ith occasional peaks to 0VU on programme. On a digital meter, thi s is equivalent to signal peaks reading +6dB above recording industry standard +4dBu alignmen t level. This is equivalent to +10dBu (o r -8d BFS on equipment aligned to EBU EBU R64-1992 standard and -14dBFS on equip men t aligned to SMPTE RP155 as illustrated). On a broadcast standard PP M, 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, solidstate recorders and many sound cards) to 0dBFS.

That said, the headroom on the PHAB and PHAME is considerable, and no noticeable distortion is evident when operating the equipment at standard recording industry

levels of +4dBu=0VU, or even higher. There is thus considerable scope for experimentation with over driving this vin tage circuitry to explore its character in gradual overload. For exa mple, peaks may be allowed to reach +18dB above alignment level without 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 following the reco mmendations illustrated (right).

PHI

Application and connections

Simple DI units utilise a transformer to balance the unbalanced signal and feed it to the microphone inpu t of your mixer or microphone preamplifier. This technique provides a reasonably high-

impedance as presented to the instrument; typically a few 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 gui tar is completely destroyed, unless it drives an impedance hundreds of times greater than can ever be provided by a transformer. The only answer is to present the instrumen t with the very high input i mpedance, suc h as it "sees" when plugged into a valve amp' (see

and the valve a mplifier

valve (tube) stage ac ts as an impedance-con verter, presenting the instrument input with an impedance of approximately 1MΩ. The output of the unit then feeds the microphone input of your mixer, computer in terface, or your PHAB preamplifier.

section above). This is the role of the Phædrus Audio PHI DI-Box. The dual

Voicing

The Phædrus Audio PHI DI-Box is connected as shown in the diagrams above. Ensure that the "LOUDSPEAKER/INSTRUMENT" swi tch is in the "INSTRUMENT" po si tion. However, no te that feeding the output of the PHI into the studio console w ill negate some of the benefits of the PHI’s unique character. Use a premium-quali ty microphone preamplifier (suc h as Phædrus Audio’s PHAB) for the very best results as shown below.

Controls

Instrument/ Loudspeaker switch – LOUDSPEAKER MODE

As a general rule, gui tar and bass guitar and certain elec tric 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 cap tured with microphones, ra ther than applying a linelevel signal to the mixer. How ever, this is not always 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 si tuation, it is useful to be able to DI, not the output from the instrument, but the signal which 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. How ever, this loudspeaker signal is of a very much larger amplitude than this signal which leaves the instrument

- it has been therefore necessary to attenuate this signal before presenting to the DI circuitry and this is exactly what is done when the "LOUDSPEAKER/INSTRUMENT" swi tch is moved to the "LOUDSPEAKER" position. The attenuation is -32dB.

YOU MUST SELECT THE “LOUDSPEAKER” POSITION OF THE LOUDSPEAKER/INSTRU MENT SWITCH WHEN CONNECTING THE Phædrus Audio PHI T O THE LOUDSPEAKER CIRCUIT OF

amplified

after all! It is

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, i t is important to observe the correct polarity of the signal fed from the amplifi er 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 addi tion, as a precaution, ALWAYS operate with the Ground lift switch in the “LIFT” position when operating the PHI in this way.

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 power amplifier. Valve amplifiers especially may be damaged without the correct speaker-load connected.

Flat/ Cabinet switch

The problem with the signal from the a mplifier is that it often contains many high frequencies which are not reproduced by instrumental loudspeaker cabinet. The resul t 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 fil ter; steeply attenua ting the treble above abou t 4kHz, and gently rolling off the bass.

The signal from the a mplifier may therefore be very greatly improved by the incorporation of an electrical filter which reproduces the electro acoustic band-pass of the loudspeaker cabinet. To this end, the Phædrus Audio PHI - DI unit, is enhanced by the inclusion within the unit of a loudspeaker-emulation filter which 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 away gently in the bass, peaks in the mid-treble and falls away rapidly above about 4kHz as shown in the figure.

Ground (earth) lift

A ground-loop (earth-loop) forms when an unbalanced, audio signal flows in a circuit in which the signal-return (usually the cable braid), provides a current path in parallel w ith the installation earth (ground). Small potentials in the installation earth (ground) wiring, drive a circulating current in the braid of the audio cable. The resul t is a “hum” vol tage superimposed upon the audio signal.

The adoption of balanced signals, and especially audio transformers should ensure tha t groundloops (or earth-loops) should NEVER occur. Certainly the Phædrus Audio PHI, by being both double-insulated and incorpora ting an ea rth-free, transformer-balanced output stage can never create a ground-loop in a correctly wired studio. How ever, 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 elimina te 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 UP ON PROVIDE A PROTECTIVE OR SAFETY EARTH (GROUND). T his advice ob tains IRRESPECT IVE OF THE SETTINGS OF ANY "EARTH OR GROUND LIFT" S WITCHES.

Buzz?

The PHI DI-Box, li ke a lot of modern audio gear, is desi gned 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), i t is possible for your enti re 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-i mpedance 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 touchi ng the strings may worsen or improve the buzz.

In this situati on, i t 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” switch be left in the down posi tion.

Chapter 6 - Circuit descriptions

PHAB & PHAME

The Phædrus Audio PH AB and the PHAME preamplifiers share roughly the same preamplifier circuit which is illustrated in general form below. The differences mainly lie in the inpu t circuitry and attenua tor con trols. Both devices opera te wi th a constant-gain a mplifier and a ttenuate 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 approximately +46dB). T he amplifier is preceded by a switched attenua tor control offering a ttenua tion of 0 to -30dB in steps of 6dB. A further pad (operated by a fron t 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 a ttenuator (pot') to ensure a very high inpu t impedance. This is vital to ensure the best tone from all electric instruments, and especially elec tric and bass guitars. The fixed gain of the amplifier is +24dB (>30d B with the high-gain switch depressed).

The amplifier block is a two-stage design incorporating an EF86 low-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 applica tion laboratories specified the EF86. In fact, Mullard described the EF86 as

a low noise pentode intended for u se as a RC coupled AF voltage amplifiers; particularly in the early stages of high gain audio amplifiers, microphone preamplifiers and magnetic tape reco rders

The output stage comprises a dual triode valve in which the two devices are wired in parallel. This paralleling of the two triodes has the beneficial effects of doubling the mutual conductance and halving the anode resistance. The latter charac teristic is pa rticularly valuable in ensuring a low impedance drive to the output transformer.

The output stage directly feeds the output step-down 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, ex tend frequ ency-respon se and lower distortion. A change to this feedback network is employed to effect the gain switching (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 inpu t pentode is driven from the inpu t transformer. In the case of the PHAME, this transformer is no t fi tted .

Performance

At recording industry line-up (0VV=+4dBu) the magnitude of all distortion components on a 1kHz sine wave input are less then -55dB below fundamental. This, and the A-weighted noise response relative to maximum output, is illustrated below.

High Pass or Rumble filter (HPF)

The input stage in the PHAB is fed by a 1:7 transformer. When a capaci tor is included in series with the transformer primary (it is usually shortcircuited out of circuit), the capacitor forms a half-section fil ter with the primary inductance of the transformer primary; resulting in a sharp cu toff, high-pass filter. The response of the PH AB 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 effec t, before the signal from the instrumen t is applied to the transformer, i t is buffered by a dual cathode-follower circuit. The first stage runs a t 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 overco me the excita tion 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 a ttenuation 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 acco mplished with a single-pole, RC respon se. The upper skirt of the passband is i mplemented by means of a single-terminated LC filter.

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 – Connect to Pin 1 Pin 3 – (-) Signal

PHAB Specification

Circuit configuration: 3 valve (vacuum tube) desi gn Output: Balanced, transformer coupled Maximum output level: +18dBu @ 1kHz Input: Balanced, transformer coupled Maximum input level: +8dBu (without PAD), +18dBu (with PAD) at 1kHz Gain: +40dB (± 1dB) - normal setting, approximately +46dB - high setting Attenuation: -30dB to 0dB in steps of 6dB; minus a further 10dB, by engaging PAD Frequency response: 40Hz to 15kHz, ±1dB: 15Hz to 30kHz ±3dB Distortion: <0.2%, 1kHz @ -4VU (0dBu), gain 40dB : <1%, 40Hz @ -4VU (0dBu), gai n 40dB Noise: <-120dB referred to input Power supply: 12V AC Power consumpti on: 8 Watts Phantom Supply: +48V DC, up to 5mA

PHAME Specification

Circuit configuration: 3 valve (vacuum tube) desi gn Output: Balanced, transformer coupled Maximum output level: +18dBu @ 1kHz Input: Unbalanced, high-impedance Maximum input level: Effecti vely infi nite. Signals >2V RMS not recommended. Gain: 24dB – low-gain setting, >30dB – high-gain setting Attenuation: Continuous. Max > -60dB Frequency response: 40Hz to 15kHz, ±1dB: 15Hz to 30kHz ±3dB Distortion: <0.2%, 1kHz @ -4VU (0dBu), gain 40dB: <1%, 40Hz @ -4VU (0dBu), gai n 40dB Noise: <-120dB referred to input Power supply: 12V AC Power consumpti on: 8 Watts

PHI Specification

Circuit configuration: 2 valve (vacuum tube) desi gn Output: Balanced, transformer coupled Maximum output level: XX, just prior to waveform clippi ng @ 1kHz Input: Unbalanced, high-impedance Maximum input level (INSTRUMENT): 2V RMS Maximum input level (LOUDSPEAKER): 100V RMS Gain: -18dB Power supply: 12V AC Power consumpti on: 2 Watts

Phaedrus Audio Ltd. reserves the right to alter these specifications without notice.

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 – microphone preamplifier; Phædrus Audio PHAME – instrument preamplifier; Phædrus Audio PHI – DI-Box.

The basis on which conformity is being declared:

The manufacturer hereby declares that the produc ts 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: Date:

Richard Brice, Technical Director

January 2011

Appendix - TEST PRO-FORMA

Product: PH AB / PHAME Serial Number: ________________

Tested by: ________________

V1 volts: ___________

Volts TP1: ___________ Volts TP2: ___________

Gain set: ______dB (40dB or LOW GAIN position)

High ain test Pass / Fail

Distortion test: Pass / Fail

PAD test: Pass / Fail

Frequency respon se test: HF (-3dB) extension: Pass / Fail

Frequency respon se test: LF (-3dB) ex tension: Pass / Fail

Phantom (+48) supply test: Pass / Fail / N/A

Listening test: Pass / Fail

Product: PH I Serial Number: ________________

Tested by: ________________

Gain test Pass / Fail

Distortion test: Pass / Fail

Frequency respon se test: HF (-3dB) extension: Pass / Fail

Frequency respon se test: LF (-3dB) ex tension: Pass / Fail

Listening test: Pass / Fail

Phaedrus Audio PHAB, PHAME, PHI User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual