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1. Serial Number s Tested by _ o Z Volts TP1 __ Volts TP2 __ o o Volts TP3 _ _ Z Volts TP4 _ _ _ o oS o Volts TP5 _ Volts TP6 Comp switch out Volts TP1 _____ Comp switch in Volts TP7 __ _ o Gain test Pass Fail Compression test Pass Fail Attenuator test Pass Fail Frequency response test HF 3dB extension Pass Fail Frequency response test LF 3dB extension Pass Fail Phantom 48 supply test Pass Fail N A Listening test Pass Fail Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved SOURCE MUSIC Features Variable Compression Ratio Threshold and Release Time Microphone preamplifier Line amplifier 90 db gain Two inputs Fast attack Smooth recovery 30 db of compression Compact Fully automatic Simple installation A Division of amp BRIDGING ALTEL A LANSING PY Ling Altec Inc 438C Compressor Amplifier INPUT POWER AMPLIFIER AMPLIFIER LOUDSPEAKER S The 438C has been specifically designed for public address use but its outstanding quality also finds many broadcast recording and communi cation applications It is a dual purpose unit providing both high gain microphone preamplification and the complete facility of the Altec auto matic compression line amplifier Its gain and output ore such that it will drive any Altec power amplifier to full output from the
2. be suitable for connection to the appropriate mains vol tage 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 User Access amp Servicing Phcedrus 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 RISK OF ELECTRIC SHOCK Ph drus Audio products should NEVER be connected to the DENAT DEEN external power supply or in any other way energised when the case is opened and or the circuit board is accessible Ground earth issues Ph drus 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 Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved 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
3. event will Phaedrus 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 Phaedrus 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 Ph drus Audio Ltd will incur the shipping cost to the customer and the customer is responsible for the shipping costs to Phaedrus Audio Ltd s designated after sales service of fice For defective products that are out of warranty the customer is responsible for all shipping costs to and from Phaedrus Audio Ltd s designated after sales service office Extended warranty and out of warranty services Various services are available from Phadrus Audio Ltd These include repair services for equipment once the warranty period has expired and the ability to extend the warranty period These are PHAMULUS REP Repair of a defective PHAB preamp charged ata flat rate PHAMULUS MISEAJOUR Mise jour for PHAMULUS PHAE EXTWARR Extends standard warranty see above by a further year A mise 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 Tf extended warranty is ordered after the in
4. units when not in use Connecting the PHAMULUS The PHAMULUS is a line level device designed to be used at standard professional audio levels of OVU 4dBu The input is bridging or high impedance The output is low impedance The PHAMULUS uses a push pull tube amplifier for its output stage just like a little tube power amp so it MUST be operated into the correct load of 600 ohms Because most modern equipment provides for high impedance inputs Phaedrus Audio has chosen to install the 600 ohm load within the PHAMULUS unit In fact it forms part of the output attenuator more about this below If you do NOT require the load to be installed internally speak to Phaedrus Audio or your local representative Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved o meter att n ph drus sm threshold PHAMULUS du tube compressor A PHAMULUS stereo link line output line input power supply Typical connection set up for the PHAMULUS Introduction Compressors Limiters and Limiting Amplifiers Compressors and limiters enjoy a fundamental similarity In a limiter the knee of the compression curve the threshold is set way up the transfer function after which a very high degree of compression is imparted on the input signal curve D in illustration In a compressor a lower degree of compression compression ratio is imparted ata lower threshold curve E Most modern compressors provide
5. 0 Phaedrus Audio Ltd 2011 All rights reserved output level dBu maximum threshold limiter 15 0 me a 10 0 ra minimum threshold compressor E L 0 0 Vs 27 21 13 7 1 5 input level dBu 5 0 10 0 Alternatively because we are here talking of a vari mu variable gain compressor it s instructive to see these characteristics in terms of gain at various input levels measured using static 1kHz tones This is illustrated here O Compressor mt Limiter 21 143 7 1 dBu input level The dynamic response of the PHAMULUS is illustrated in the oscillogram below Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved Meet the PHAMULI Sees Pheedrus PHAMULUS tube co Phaedrus Audio PHAMULI The Phaedrus Audio PHAMULUS has a unique stereo link feature meaning that two PHAMULI yes that s the real plural form of PHAMULUS may be linked together to forma stereo compressor Ideal for mastering duties and adding that final magic When ordering a PHAMULT you will be supplied with a special link cable which should be installed between the two units and connected to the stereo link connector on each unit One unit will operate on the LEFT channel the other on the RIGHT channel Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved stereo link line output line input power supply Stereo Operation When operating the PHAMULI asa stereo pair carefu
6. 1 All rights reserved The much vaulted EMI RS124 at heart a modified Altec EMI s variable release user control has been adopted in the design of the Phaedrus Audio PHAMULUS such that the control allows for adjustable release times between 0 1 sec much faster than the original Altec and 1 3 sec This faster response widens the applications of the Phaedrus Audio PHAMULUS to include compression duties for amongst other things drums PHAMULUS Front panel controls compression meter decay release control indicates the degree of compression minis Altec 436 value giving more flexibilit input attenuator controls the level of drive 5 att n ph drus m threshold PHAMULUS u tube compressor meter zero trim compressor limiter switch output attenuation switches between 10dB pad fixed amp variable threshold The PHAMULUS front panel The front panel controls of the PHAMULUS and their application are illustrated in the diagram above Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved Controls Input Gain Control This control is used to adjust the input level to provide the desired average compression as indicated on the compression meter Release Control The time interval required to restore full gain following a condition of compression is adjustable in the range of 0 1 seconds to 1 3 seconds by means of this control Output attenuator One criticism of ten levelled at the Altec 436 is that i
7. OF THE SETTINGS OF ANY EARTH OR GROUND LIFT SWITCHES General Safety Instructions Do not operate Phaedrus Audio equipment near any source of water or in excessively moist environments Keep your Phaedrus Audio equipment away from babies children and pets Do not let objects do not fall or liquids be spilled into the enclosure Situate the Phaedrus Audio equipment away from heat sources or other equipment that produce heat Ensure Pheedrus Audio equipment has adequate ventilation Improper ventilation will cause overheating and can damage the equipment When cleaning Ph drus 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 Phaedrus Audio equipment Ph drus Audio equipment should be serviced by qualified service personnel or returned to Phaedrus 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 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 12AU7 or ECC82 The 12 AU7 is a popular miniature 9 pin medium
8. User Manual for phaedrus phaedrus audio com N e i e N meter att ye phaedrus m threshold M PHAMULUS u tube compressor PHAMULUS dy tube compressor FAMULUS n pl li the attendant of a sorcerer This manual is also available as a pdf download Go to www phaedrus audio com RISK OF ELECTRIC SHOCK DO NOT OPEN Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved Preface About Phaedrus Audio phaedrus phaedrus audio com Phcedrus Audio was formed to design manufacture and sell high quality products for the professional and semi prof essional audio market Phaedrus 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 Phaedrus Audio s aimis to reproduce the quality and character of classic equipment but ina modern highly reliable and cost effective way Chapter 1 Background The Phaedrus Audio products came about because two recording musicians wanted to own classic audio gear from the nineteen sixties The PHAMULU S is a continuation of that ambition a high quality compressor based on the Altec 436C compressor The Altec 436C Construction or star earths grounds and curly tracks Giv
9. asure of variable gain as illustrated below anode current Suppose that for the moment the radio set is receiving a strong signal In this circumstance the filtered AGC control voltage taken af ter the detector is large and this is so arranged to Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved slide the input signal to the left hand portion of the curve in the figure above If the signal level drops initially the volume of the signal drops too But ina fraction of a second the usual control delay is about 1 4 of a second the AGC control vol tage falls and biases the operating range of the valve to the right of the characteristic thereby restoring the RF level and with it the recovered audio So important was this development in radio sets that radio valves are especially designed to maximise the curvature of the transfer characteristic these types of vales being known as vari mu valves the u mu relating to the coefficient of gain Another term for vari mu is remote cut off which describes the fact that the cut off point for the valve the negative voltage at which the anode current ceases is made as remote as possible from OV thereby prolonging the curvature of the Vg Ia characteristic The alternative to remote cut off valve is a sharp cut off device Audio valves are sharp cut off types which means that the Vg Ia characteristic is made as linear as possible This is one of the reasons why radio
10. controls to adjust both compression threshold and ratio so it is logical to speak of these units as compressor limiters because they can function as either output However in older tube based units such as the equipment the Phadrus Audio PHAMULUS is based upon these functions are combined in a gradual characteristic curve F The user operation of this equipment therefore largely relies on how hard the equipment is driven to alter the dynamic range of the incoming signal By applying low level drive by means of reducing the drive via the input attenuator this equipment will only impart a gentle compression characteristic Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved By driving the equipment with a high level signal by simply advancing the input level control more and more of the dynamic range will be compressed In fact the distinction between the term limiter and compressor was not well defined until the nineteen seventies so you will often find classic compressors were referred to by their manufacturers as limiters limiting amplifiers or compression amplifiers Vari mu compressors the Altec Lansing 436 and the EMI RS124 The most famous classic tube based compressors are of the vari mu family in which advantage is taken of technology developed for automatic gain control AGC for radio equipment In a radio set the radio wave strength may vary due to reflections and obstacles this being especiall
11. d Release Time Output atten meter trim Power Supply 12V AC 4 Watts Tubes 6BC8 12AU7 Phaedrus Audio Ltd reserves the right to alter these specifications without notice Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved Declaration of Conformity The Manufacturer of the Products covered by this Declaration is Pheedrus 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 Phaedrus Audio PHAMULUS dy compressor The basis on which conformity is being declared The manufacturer hereby declares that the products identified above comply with the protection requirements of the EMC directive and with the principal elements of the safety objectives of the Low Voltage Equipment directive and that the following standards have been applied IEC INTERNATIONAL STANDARD 60065 Audio video and similar electronic apparatus Safety requirements The technical documentation required to demonstrate that the products meet the requirements of the Low Voltage Equipment directive has been compiled and is available for inspection by the relevant enforcement authorities The CE mark was first applied in 2011 Richard Brice Technical Director October 2011 Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved Appendix TEST PRO FORMA Product PHAMULUS
12. en their vintage the Altec 436Cs were built on a metal chassis with the components hardwired onto the valve tube bases and tag strips Although Ph drus 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 Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved Every Ph drus Audio product is hand assembled and individually tested A test pro forma is provided with every unit Phaedrus Audio offer comprehensive service for products both inside and outside of their warranty period star earth point natural contour tracking with curved corners and shortest paths Chapter 2 Warranty and Service Please register your purchase with Phaedrus Audio at www phaedrus audio registration htm This will simplify service amp 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 Phaedrus Audio Ltd product contact support phaedrus audio com TELEPHONE 44 207 193 4609 Skype ID phaedrusaudio 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 ha
13. eriod is mandated by applicable local laws If equipment fails due to such defects within this period Phaedrus 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 Phaedrus 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 Phaedrus 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 Phaedrus Audio Ltd There are no user serviceable parts inside This limited warranty isinvalid 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 Phaedrus 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 Phaedrus Audio Ltd Phaedrus Audio Ltd makes no other warranties expressed or implied of merchantability fitness for a particular purpose or otherwise Phaedrus Audio Ltd liability is limited to repair or replacement by Phaedrus Audio Ltd at its sole discretion and inno
14. gain dual triode valve tube and is used in many instrument and hi fi amplifiers Both triodes are used in cascade in the Phaedrus Audio PHI product The 12 AU7 is also known in Europe under its Mullard Philips tube designation 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 Ph drus Audio selects and recommends the long anode plate version long anode of the ECC82 ECC802S or 12AU7 valve for application in the PHI DI Box 6BC8 and 6BZ8 tubes The tube chosen for the vari mu dy element in the Pheedrus Audio PHAMULUS is the 6BC8 6BZ8 tube This was the tube in the original Altec circuit The 6BC8 6BZ8 tubes are meduim mu semi remote cutoff twin triodes originally intended for applications in television receivers Peni Because theses tubes arrived late in the age of thermionics and were Trg employed in the most successful consumer product of all time the ii i i television there exist many many of these tubes as NOS new old stock at reasonable prices This underlies Phaedrus Audio s decision to adopt this tube Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved Valve tube lifetime You should replace the valves in the tubes in the Phaedrus Audio products only when you start to notice changes in the sound quality If the gain of the preamplifier decreases noticeably then
15. input of any quality dynamic microphone Its unique compression assures high intelligibility and freedom from blasting or damage to the power amplifier and speakers due to sudden high level sounds The 438C utilizes variable threshold compression ratio and release time controls permitting maximum flexibility in virtually any type of installation Operationally the 438C will compress signals as much as 30 db This auto matic compression with its rapid attack time of 50 milliseconds can be used to compensate for the differences in loudness between users in paging or recording and to provide over all system protection and smoothness In addition to this microphone input there is a 600 ohm line bridging input for wired music and other program sources The proper balancing of these two inputs results in automatic fading of background music material during paging Use of the bridging input alone provides straight forward compression of line program material Removal of the 6AL5 tube from the circuit permits the 438C to operate as a simple microphone preamplifier and line amplifier without compression The hinged front panel of the 438C contains all controls fuse pilot light and compression meter For remote monitoring of the compressing action the Altec 6049 meter may be ordered When opened the front panel exposes all components and wiring for easy service All connections are made to simple barrier terminals on the rear of the unit A
16. itial warranty has expired then the intervening period is charged without exception Please contact your dealer or Phaedrus 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 Phaedrus Audio Ltd limited warranty please see complete details online at www phaedrus audio com Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved Chapter 3 Safety General Before using any piece of equipment manufactured by Phaedrus 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 Phaedrus Audio Ltd products are available as line items from Phaedrus Audio They are PHAE PSU 1 10 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 Phaedrus Audio PHAB PHAME and PHI units A direct current DC supply is NOT suitable and will damage the unit If a supply is sourced elsewhere than from Ph drus Audio it must have the following specifications 12V AC 12 Watt supply or greater gt or 1000mA supply current Must
17. lly match the input control setting on the two units for a balanced stereo image that isn t pulling to one side The setting of the comp threshold switch must be consistent between both units too and if depressed the threshold pres sets should be set to similar settings The Decay controls interact so that the knob which is set to the LOWER setting fastest response time will predominate Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved Chapter 6 Circuit description OUTPUT V2 12AU7 The Phaedrus Audio PHAMULUS circuit works like this large negative signal peaks on the anodes of the push pull output stage V2 cause double diode V3 to conduct and drag down the bias volts to the input stage V1 This action is damped by the time constant formed by R9 P2 and C4 which allows for some adjustment of the compressor s release time The level threshold at which the compression action is required is adjustable too by means of the simple expedient of biasing the cathodes of the double diode V3 witha constant bias derived from the slider of P3 The compression action is obtained because as signal peaks depress the bias volts on V1 the stage slides further down the input valve s curved Vg k Ia characteristic the required vol tage change for a given change in anode current is reduced and the stage gain is lowered The compression control signal itself remains inaudible because it is applied equally to b
18. oth input valves whereas the music signal is applied differentially in other words the common mode control signal is cancelled out in the differential amplifier Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved Specifications Electrical connections h drus na conventions Inp ut not PN aroun MA re Unbalanced 3 pin XLR Connector Balanced 3 pin XLR Connector oy i Pin 1 Ground Pin 1 Ground 7 ia Pin 2 Signal Pin 2 Signal hot ve left send Pin 3 Connect to Pin 1 Pin 3 Signal cold ve right return Outp ut commoniground screen 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 PHAMULUS Specification Type Compressor Amplifier Gain 30dB reduced to 20dB with output attenuator engaged Frequency Response 1 5 dB 40Hz to 15kHz Max Output Level gt 20 dBu as straight amplifier at 1kHz Harmonic Distortion At 25 db of compression Less than 1 5 Noise Level 74 dB below rated output 111 dBm equivalent input noise Input Impedance 15k bridging transformer earth free Load Impedance Normally bridging 600 ohms if ordered specially Maximum Compression 30 dB Attack Time 50 milliseconds Release Time 0 1 to 1 1 seconds Threshold Adjustable O dBm to 16 dbm output Compression Ratio 2 1 at O dBm threshold 4 1 at 16 dBm threshold Controls Gain Threshol
19. 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 PHILTER and PHI use oxide cathodes which 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 few years service Ph drus Audio Ltd can provide suitable valves tubes as spares which af ter a burn in period are screened for best performance in your Phaedrus 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 PHAE 6BC8 Selected 6BC8 type valve Please contact your dealer or Pheedrus Audio Ltd for current prices Chapter 5 Instructions for use Phcedrus Audio s 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 will operate at optimal specifications Please therefore allow these products to warm up for at least 5 to 10 minutes before using themin your signal chain To prolong the life of your valves tubes it is recommended that you turn off these
20. three conductor power cord is pre wired for ease of installation This outstanding multi function unit occupies only 3 2 of standard rack space or it may be table or wall mounted in the compact Altec 12495 cab inet The high quality fully automatic compressor operation and wide range of function are obtained in this Altec amplifier at an extremely low price which makes it economically sound to use the 438C instead of less effective simple preamplifiers and line amplifiers 1515 S Manchester Ave Anaheim Calif New York Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved 4380
21. ts output level is very hot making it difficult to interface with other equipment This is a valid criticismand the EMI engineers wisely added an output attenuator to the Altec chassis for the RS124 The PHAMULUS also incorporates a controlled impedance attenuator ata fixed 10dB attenuation selectable by a front panel push switch Compression meter The compression meter indicates the average degree of compression being applied to the signal in dB Of course your ears are the judge but this indicator will help you to find the optimum degree of compression to be applied to your trsck or mix Comp threshold switch and threshold preset Note that the Phaedrus Audio PHAMULUS also features a front panel push switch which enables quick selection between a factory set threshold level compressor position and a variable threshold level via the right hand preset labelled limiter position On shipping the threshold preset is setatits maximum which if never adjusted again gives a simple way of switching between two regimes one compression suited for tracking the other switch depressed more suited for mastering or applying over a sub mix Alternatively with the switch depressed the threshold preset control may be adjusted for greater flexibility The compressor characteristics as shipped are illustrated in the figure below Adjusting the threshold preset gives a family of characteristics between these two extremes Ver 1
22. valves make poor devices for audio applications except as the basis for compressors Of course we might justifiably call volume compression by the equally valid term audio automatic gain control or audio AGC so itis not surprising that the earliest compressors adopted the radio technology and applied to audio signal circuits The only difference betweena volume compressor and an AGC circuit is that in the case of the former itis the audio signal itself which is rectified and used as the control signal rather than the modulated carrier Altec Lansing 436C The Altec Lansing 436C compressor amplifier above dates from the nineteen sixties The circuit design of the Phaedrus Audio PHAMULUS is based upon this circuit EMI RS124 A famous but rare EMI developed variant of the Altec Lansing 436 exists which included a modification to the input circuit to include a matched impedance attenuator and added an output attenuator both designed to operate at EMI s quaint 200 ohm interface standard Importantly froman artistic point of view the original Altec circuit only included a variable release time preset RELEASE so this was added as a user control by the EMI engineers This greatly increased the scope of the compressor s duties to which has been attributed some Beatles magic The entire unit was retro fitted with an EMI designed escutcheon which thinly disguised its Altec heritage Ver 1 0 Phaedrus Audio Ltd 201
23. ve an RMA number before you return the equipment to Phaedrus 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 Phaedrus 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 Pheedrus 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 Ver 1 0 Phaedrus Audio Ltd 2011 All rights reserved Phzedrus Audio Ltd s Limited Warranty This limited warranty is valid only if you purchased the product from Phaedrus Audio Ltd of froma Phaedrus Audio authorized dealer in the country of purchase a list of authorized dealers can be found on Pheedrus Audio website www phaedrus audio com or by contacting sales phaedrus audio com Phaedrus 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 p
24. y true for mobile radios in cars and aeroplanes With amplitude modulation AM radio this is problematic because a change in the signal strength causes a change in detected output and thereby reproduced volume and this is very disturbing to the listener This was a major problem with early wireless sets and a solution was eagerly researched The solution to the problem came in the shape of a post detector filter circuit witha time constant long enough that it removed all the audio modulation and left just an overall DC voltage value dependent on the average amplitude of the RF carrier This signal was then used to adjust the gain of the radio frequency or intermediate frequency amplifiers prior to detection By this means the overall value of the radio carrier was held steady and the volume of the detected modulation always remained that of the broadcast signal How was the variable gain accomplished The answer lies in the inherent non linearity of the valve tube which is deliberately exploited in radio valves to offer the AGC function All valves tubes have a power law transfer charac teristic That is to say that a given change in grid voltage does not result in an equal change in anode current over the operating range of the valve By arranging the RF carrier to work only over a proportion of the overall characteristic and sliding the incoming signal over the entire operating characteristic it is possible with any valve to derive a me

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