Phaedrus Audio PHILTER User manual

Ver. 2.0 - ©Phædrus Audio Ltd. 2015 All rights reserved.
User Manual for
PHILTER – Vintage passive EQ
This manual is also available as a pdf download, Go to: www.phaedrus-audio.com

Ver. 2.0 - ©Phædrus Audio Ltd. 2015 All rights reserved.
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 practices 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, 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 way.
Chapter 1 - Background
The Phædrus Audio products came about because two, recording musicians wanted to own 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 well as
cheap! So, we set about 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 which the mixers of this vintage
are organised.
In the famous EMI REDD mixers, these
amplifiers were either the German
manufactured V72(S) amplifiers or the,
very rare, EMI built, REDD.47 amplifiers.

Ver. 2.0 - ©Phædrus Audio Ltd. 2015 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 which the
German amplifiers use are no longer in current production and are becoming 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). This PHILHARMONIC amplifier forms the heart of the Phædrus Audio
LONDON Mark II console. The PHILHARMONIC amplifier circuit is described in detail below in section Circuit
descriptions.
Mullard published many reference designs which were used by EMI in developing the REDD amplifiers. At
Phædrus Audio, we went back to these same references to develop our products.
The prototype London Console (Mark II)

Ver. 2.0 - ©Phædrus Audio Ltd. 2015 All rights reserved.
Construction - or star earths (grounds) and curly tracks
Given their vintage, the V72 amplifiers and the REDD.47 amplifiers were built on a metal chassis, with the
components hardwired onto tag-strips. Although Phædrus Audio equipment uses printed circuit boards to ensure
consistent performance and reliability, the equipment follows "classic" practices such as star earths and "natural
contour" tracking, just like hook-up wire.
Every Phædrus Audio product is hand assembled and individually tested. A test pro-forma is provided with every
unit. Phædrus Audio offer comprehensive service for products both inside and outside of their warranty period.

Ver. 2.0 - ©Phædrus Audio Ltd. 2015 All rights reserved.
Chapter 2 - Warranty and Service
Please register your purchase with Phaedrus Audio at www.phaedrus-audio/registration.htm . This will simplify
service & repair should you need this service. Your name will be placed on our mailing list (unless otherwise
requested) for future updates and new product announcements.
Service
If you experience a problem with a Phædrus Audio Ltd. product, contact:
[e-mail]
We will diagnose the problem remotely and advise you of the warranty status. If a repair or replacement is
required, we will issue a Return Merchandise Authorization (RMA) number and tell you where 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 write 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. will not
accept responsibility for loss or damage in shipping or for equipment returned without valid paperwork and/or a
valid RMA number. Remember, warranty 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 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
[e-mail]. 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 otherwise. 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. will 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.

Ver. 2.0 - ©Phædrus Audio Ltd. 2015 All rights reserved.
Extended warranty and out-of-warranty services
Various services are available from Phædrus Audio Ltd. These include repair services for equipment once the
warranty period has expired, and the ability to extend the warranty period. These are:
PHILTER-REP - Repair of a defective PHILTER equaliser, charged at a flat-rate
PHILTER-MISEAJOUR - Mise à jour for PHILTER*
PHAE-EXTWARR - Extends standard warranty (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 warranty is ordered after the initial warranty has expired then the intervening 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 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.
Follow the warnings indicated on the unit, as well as in the operating instructions.
Selection of PSU
Suitable PSUs for the Phædrus Audio Ltd. products 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 PHILTER. A direct current (DC) supply is NOT suitable and will
damage the unit. If a supply is sourced elsewhere than from Phædrus Audio, it must have the following
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 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. Units supplied to other countries should be verified to
comply with the relevant standards which obtain in those territories.

Ver. 2.0 - ©Phædrus Audio Ltd. 2015 All rights reserved.
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.
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.
Ground (earth) issues
Phædrus Audio Ltd.'s products are all powered by external (12V AC) power supplies which should be double
insulated, class-II types (see above). It should therefore be appreciated that,
A CONNECTION TO EITHER THE PHAB, the PHAME, PHILTER 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 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.

Ver. 2.0 - ©Phædrus Audio Ltd. 2015 All rights reserved.
Chapter 4 - Instructions for use
Typical connection set-up for the PHILTER
PHILTER
Application and connections
The PHILTER equaliser is a self-contained, line-level device. The loss due to the passive equaliser circuitry is made-
up in the included, internal line-amplifier. The figure below is a block-diagram, of the unit. The unit is connected
by balanced, earth-free connections on professional XLR connectors. Operating levels are covered below.
One word of warning: remember the line-amplifier makes up for the loss in the passive Q circuits when the
controls are central (and no boost is applied). In a passive equaliser, boost is actually provided by reversing the
loss of the equaliser over a range of frequencies. In other words, when boost is applied, bass frequencies are “un-
cut”. This means that, when large degrees of boost are applied, it may be necessary to reduce the signal level
sent to the PHILTER to stop it overloading.
Overload indicator
To prevent inadvertent clipping due to high levels of boost, the PHILTER contains a clip detect circuit which turns
the blue power LED red when the audio level is 3dB below peak. This indicator should only be allowed to flash red
occasionally in operation.

Ver. 2.0 - ©Phædrus Audio Ltd. 2015 All rights reserved.
Architecture of the PHILTER equaliser
Controls
Bass, Treble and Frequency-select controls
The equalisation controls on the PHILTER are simple and intuitive to operate. Remember, these circuits were
invented well before the complex, analogue equalisers of the nineteen-seventies. When the middle frequency-
control is turned to the two most anti-clockwise positions, the bass and treble controls (marked with the
appropriate music clefs) act as they do on a hi-fi; as a boost and cut control: clockwise to boost (in 2dB steps);
anti-clockwise to cut (in 2dB steps). The central control may be used to modify the character of the treble-
frequency boost. Depending on the frequency setting: 2.5kHz; 3.5kHz; 5kHz; 8kHz; 10kHz; 12kHz, when the treble
control is rotated clockwise beyond the central position, boost is applied with a broad peak in the response at the
indicated frequency. Note that this control is NOT active when treble-frequency cut is applied; this always
remains as a shelving response. The graph below illustrates the 121 separate response curves which result form
the 121 combinations of treble and bass control when the central switch selects the 5kHz, peaking-response.
Note that the cut curves do not exhibit a complementary notch to the boost peak.

Ver. 2.0 - ©Phædrus Audio Ltd. 2015 All rights reserved.
Operating levels
Because of the vintage of the PHILTER design, and its
origin in German manufactured broadcast television and
radio equipment, the
envisaged operating
level for this
equipment is slightly
lower than modern
recording industry
standards. When
feeding downstream
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, with 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 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.