MC77 Manual & Schematic
Contents
1. 30 dB and measure THD of the resulting output signal Adjust R86 until the minimum amount of distortion is achieved 10 CLEANING THE LIMITER The front panel of the MC77 may be cleaned with a non abrasive cleanser such as Formula 409 or Fantastic applied with a soft clean cloth Additional protection of the anodized panel can be afforded through a light application of a spray wax preparation such as Pledge NOTE NEVER SPRAY THE PANEL DIRECTLY AS THE CLEANSER OR WAX MAY ADVERSELY AFFECT CONTROLS OR METER AND CAN CONTAMINATE CIRCUIT BOARDS IF IT PENETRATES THE CHASSIS 11 REPAIRS AND WARRANTY We will service and repair the unit free of charge for three years from the date it is shipped We track warranty by serial number therefore make sure to leave the serial number tag on the unit We will not repair damage that results from misuse abuse or neglect We do not guarantee that the purple color of every unit will match the purple of every other unit Should a malfunction occur please email the service department at service purpleaudio com You will receive instructions for return Be sure it is well packed in a sturdy carton with shock absorbing material Tape a note to the top of the unit describing the malfunction and instructions for return We will pay one way return shipping cost on any in warranty repair Because of specially selected components in this product field repairs are not authorized during the warranty period Attempts2. 1 volt as a zero reference and to make things worse people still use the dBm scale to discuss voltage levels across what they are measuring A dBm level is a measurement of power that references to lmW dissipated into a given load and is almost always measured indirectly as a voltage developed across a known load of 600 ohms If someone tells us they are measuring 4dBm at the input terminals of a device with a 10K ohm input impedance what are we to think We prefer the dBu scale because we are measuring voltages Oftentimes we are not measuring across a 600 ohm line AND later when we read these specifications over the phone we don t have to keep saying dB little v Input Constant Impedance Input Attenuator Transformer Balanced and Floating Input Impedance 600 ohmsat all input gain settings Maximum Input Level 30dBu with limiting active and input cranked up full Prolonged exposure to signal levels in excess of 30dBu may burn up the input attenuator Maximum Gain 45dB 1dB with limit action switched off Frequency Response 15Hz to 80KHz 1dB typical Output Floating Transformer Balanced Output Load 150 ohms or higher Load Dependence The Output Level as measured while driving into a bridging load will drop by no more than 1 2 dB when a load of 600 ohms is added there will be a loss in headroom Maximum Output Level 24 dBm into 600 ohm load 12 1 4 volts RMS 30 dBu into bridging load 24 1 2 volts RMS PURPLE AUDI
3. is for monitoring Gain Reduction Out for monitoring I O level Second switch In for Input level Out for Output Level Both referenced at 4dBu Note Input level is monitored across the 600 Ohm input You may notice a level drop depending on source This is normal loading and not a metering error Bypass The third switch is for system bypass this connects the input directly to the output so the user can quick A B limiter in vs limiter out Unit defaults to bypass when the powered off Power Switch The bottom switch is marked ON Gain Reduction 0 Adjust Screwdriver accessible adjustment via small hole in front panel between input and output level controls PURPLE AUDIO MC77 MANUAL 2002 REV 1 EFFECTIVE WITH SERIAL NUMBER 500 PAGE 6 OF 12 6 BACK PANEL CONNECTIONS XLR Pin Assignments PIN 1 Chassis Earth Ground PIN 2 Signal Hot Positive PIN 3 Signal Cold Negative Wiring systems vary from facility to facility PIN 1 may or may not be used to terminate a shield or drain wire In general this pin and the shield or drain wires connected to it are not to be used to Ground or Earth the unit Shields and Drain Wires should NOT carry current and are generally connected at only one end of any given wire For your convenience and to assure that the chassis can be easily grounded earthed there is a Chassis Ground Lug on the back of the unit and the center pin of the IEC power inlet is connected to th
4. to perform repairs may invalidate the warranty Even if your unit is out of warranty we recommend that you return it to the factory for repairs Our experienced personnel supported by special test equipment will be able to find and eliminate any problem in the most efficient way 12 NEWS For current information about Purple Audio visit our website at http www purpleaudio com PURPLE AUDIO MC77 MANUAL 2002 REV 1 EFFECTIVE WITH SERIAL NUMBER 500 PAGE 12 OF 12 PRE AMP 600 Ohm T Attenuator OUTPUT AMP gt meter buffer J2 JUMPER 1 R26 C15 TOP 2 DECK POT IN EARLY UNITS 68K C4 0047 XLR M P2 A H n 3 100uf 25v C10 R2 amp R3 6200 1 4W NC BLUE 1uf 50v L c816 Pin 2 Wine P2 Relay RL1 5 W IN4003 D43 7 R24 Meter buffer 2N3391A 1uf Q6 DS2 AROMAT c600 2M7 UTC 0 12 or Equivalent Q3 100uf 25v 1 NW 2N3391A INLOW 6 e o a 2N3053 BLACK j JUMPER Q4 Q5 15 2N3391A 2N5088 C7 RIA Front deck amp R1B 3 R11 BOTTOM 3 DECK POT IN LATE C819 D C601 1uf 82 1uf 50v R23 c12 100uf 25v 4 ment R 250K 2W 200K 100uf 25v WH EBLACK GREY Bypass relay controlled by y Output F P XLR F p3 bypass on meter switch assembly IN HIGH 5009 2000 R12 Lcs Audio Taper Rid 910 100uf 25v RIA e 4 50009 Lin WHITERED VIOLET Sea Naa IN4148 R15 6K8 R32 22 39 WN s T Aromat TQ2E RL4 R1B 22 R Loop Relay 5K Z Tap Se parE C6 IN LOW Disto
5. O MC77 MANUAL 2002 REV 1 EFFECTIVE WITH SERIAL NUMBER 500 PAGE 3 OF 12 SPECIFICATIONS ELECTRICAL continued Distortion Less than 0 5 THD moise 22Hz 22KHz with limiting activeRelease time set to 1 1sec fast release times will increase distortion at low frequencies NOTE The VU meter is now buffered from the audio and adds no appreciable distortion S N Ratio gt 81dB at threshold of limiting 22Hz 22KHz unweighted Attack Time Less than 20usec for full limiting action adjustable to 800usec with front panel control Release Time Minimum 50msec adjustable to 1 1sec maximum for 63 recovery with front panel control Threshold and Ratio With the input control fully clockwise and the output gain control set 10dB below maximum at about 6 on the dial The overall gain before limiting should be 35dB 1dB The threshold of limiting is defined here as 1dB of limiting at 20 1 the threshold of limiting will occur with an input of 24dBu 2dB and yield an output level of 10dBu at 12 1 the threshold of limiting will occur with an input of 25dBu 2dB and yield an output level of 9dBu at 8 1 the threshold of limiting will occur with an input of 26dBu 2dB and yield an output level of 8dBu at 4 1 the threshold of limiting will occur with an input of 30dBu 2dB and yield an output level of 7dBu Note lower ratios make the limiting action kick in sooner PURPLE AUDIO MC77 MANUAL 2002 REV 1 EFFEC
6. Purple Audio Inc 38 62 Eleventh Street LIC NY 11101 Tel 718 482 8494 Fax 718 482 8159 purple purpleaudio com www purpleaudio com MC77 Limiting Amplifier Effective with Serial Number 450 Operation and Service Manual Table of Contents 1 Description 2 General Definitions 3 Specifications Electrical 4 Specifications General 5 Front Panel Controls 6 Back Panel Connections 7 Unpacking and Installation 8 Operating Instructions 9 Maintenance 10 Cleaning 11 Repairs amp Warranty 12 News 13 Schematics PURPLE AUDIO MC77 MANUAL 2002 REV 1 EFFECTIVE WITH SERIAL NUMBER 500 PAGE 1 OF 12 1 DESCRIPTION The MC77 replaces the MC76 re engineered 1176 type FET Limiter After selling hundreds of the original MC76 units we incorporated new features based on user feedback The MC77 isa gain limiting amplifier Units of this type have been used for many years to provide precise and automatic control of peak signal levels in recording studios disc mastering facilities broadcast stations and sound reinforcement applications Gain reduction in the MC77 is accomplished using a Field Effect Transistor FET as a voltage controlled variable resistor shunt The FET is the first active component in the signal chain Large amounts of limiting can occur without large increases in distortion There are four compression ratios that can be used to best complement the program material and the specific application Atta
7. TIVE WITH SERIAL NUMBER 500 PAGE 4 OF 12 SPECIFICATIONS ELECTRICAL continued Power Requirements 100 125 Volts AC or 200 250 Volts AC selectable At 115 Volts the current draw is 1 2 mA typical and the unit should be fused for 1 8 Amps At 230 Volts the current draw is 1 4 mA typical and the unit should be fused for 1 16 Amps Line Frequency can be 50 60Hz Temperature The MC77 can operate safely over a temperature range of 0 C to 50 C The MC77 can be stored over a temperature range of 20 C to 60 C 4 SPECIFICATIONS GENERAL Dimensions 2U 19 rack mounting enclosure 19 wide 3 1 2 high Depth behind front panel is 8 not including connectors Finish Front Panel is 1 8 thick aluminium purple anodized with some variation in color this can be viewed as a feature and lends a certain individuality to each unit Case finish is black powder coat Legends are silk screened in white Weight 12 pounds loose 15 pounds shipping weight PURPLE AUDIO MC77 MANUAL 2002 REV 1 EFFECTIVE WITH SERIAL NUMBER 500 PAGE 5 OF 12 5 DESCRIPTION OF FRONT PANEL CONTROLS Input Level Control Continuously adjustable rotary attenuator Output Level Control Continuously adjustable rotary potentiometer Attack Time Continuously adjustable rotary potentiometer Ranges from the slowest attack time of 800 microseconds to the fastest attack time of 20microseconds at the full clockwise rotation Limiting In Out The At
8. able Warranty Card PURPLE AUDIO MC77 MANUAL 2002 REV 1 EFFECTIVE WITH SERIAL NUMBER 500 PAGE 8 OF 12 8 OPERATING INSTRUCTIONS Start with all controls at mid rotation Insert Out Link Out Ratio 8 Meter GR with Bypass out Adjust input to show some gain reductions Switch Meter to I O OUT Adjust Output level for levels around 0 VU The Attack Time of the MC77 is the time it takes for the detector circuitry and FET to respond to a signal as it exceeds the threshold This parameter is variable by the user since the adjustment may have a significant effect on the sound of the program material The Release Time may be defined as the time it takes the limiter to return to its normal gain after the signal which has caused the gain reduction has dropped below the threshold The release time is variable for the same reason that Attack time is and can substantially alter the sound of the program material passing through the MC77 Together Attack and Release times will define much of the sound or coloration in any given application For the dirtiest sound push in all four Ratio buttons attack counter clockwise but not off Release clockwise For Bypass Mode Push in Bypass Button Insert places a differentially balanced send and return into the normal connection from the top of the output pot to the side chain detector and gain control circuitry This loop allows EQ filters and other external processors to be used to modi
9. c17 Ww C24 L C23 C805 1uf Attack Time Control F P ECG5202A Q10 2N5088 29 Blue 3300uf 63 GREY f u vt if C27 022 3300uf 35v Q 2N5088 Q7 R42 1uf 50v 2N5088 s E ERS 1N4936 ORANGE i sm c19 R54 470 ceo7 tur BLUE 180K 10uf 35v ij a L cs gt ay ane 9 se AAA Red 19 x R37 Null Test 470K luf K 470uf 25v CR2 R55 mi R43 czs fg C25 _ JERE TNAD36 Ly yd 25K 1 2W Release Control F P Blue 20 T 3300uf 35v ee Pie FDH333 R64 Q13 2N5088 FUSE ie 4 een R56 1K51 4W TAN nee CRTPINADSC 7 Violet 5M 1 2W R57 oV 1001W 100 1W enter tappe Dual Winding 270K 10V Ripple 5mv with load aay De Tsa bc Mea Sage My L Ca i 10puf 25v R59 Black 18 R35 R68 Lf R75 R70 2K 10K gt t c20 FDH333 Mv YNY 2K Null Adj P C B 4K7 Q Bias Adj P C B c20 DE Q j R6O Ouf 35v Bias for 1db attenuation 7 3K9 Copyright 1997 permission given 4 Relays MC77 to copy with proper acknowledgement Vv J 1 6 used to bypass relays 1 2 120V 1 8A SB 240 1 16A SB Purple Audio MC76 CR401 CR402 5 Jumper External WW 3K6 IN5229B Meter Buffer Meter switch PIN 3E INPUT gt j AAN IN PIN 2 INPUT gt R401 oe or 28 ORANGE 22uf 25v OE L 7 er 29 Bi
10. ck Time is continuously adjustable from less than 20microseconds to 800microseconds The fast attack time response is independent of peak frequency content or duration Over the audio band the full limiting action will stabilize in under a half cycle Release time is continuously variable from 50msec to 1 1sec Release time is defined as the time it takes for the gain to recover to within 63 of the normal non limiting gain Provisions have been made for the interconnection of two MC77 limiters for stereo operation 2 GENERAL DEFINITIONS Compressors and Limiters are used in many applications A technical application for these devices is when the dynamic range of program material is too large to be processed by succeeding equipment when the peak to peak amplitude is too large for the headroom of the following equipment Compressors and Limiters are used to reduce the dynamic range of the program material so it fits through the equipment and systems that follow Good examples are broadcast transmitters and public address systems A non technical application is as a production or creative tool Any device that changes dynamic range will cause some subjective change in the sound of the program material Reducing the dynamic range of an individual track in a multi track recording and making it more consistent makes it easier to control in the mixdown process Its applications as a production tool are limitless The two different terms Compressors an
11. d Limiters generally refer to the degree to which the dynamic range is reduced The relationship of input level change to output level change is called the Compression Ratio If for example an increase of 8 dB input signal level should cause the output to increase by 2 dB this would represent a 4 1 compression ratio Amplifiers with compression ratios of up to 8 1 are often considered to be Compressors while those with ratios higher than 8 1 are called Limiters The MC77 can function as a compressor or a limiter since it has compression ratios from 4 1 to 20 1 Signals at levels below the threshold will not be affected by the compression limiting action Any change in input level below threshold will result in a corresponding and equal change in output level while increases in input levels above the threshold will cause a controlled decrease in amplifier gain resulting in a reduced increase in the output level in proportion to the compression ratio The MC77 Input Level control adjusts the amount of signal to be processed above the threshold and hence the degree of compression or limiting PURPLE AUDIO MC77 MANUAL 2002 REV 1 EFFECTIVE WITH SERIAL NUMBER 500 PAGE 2 OF 12 3 SPECIFICATIONS ELECTRICAL Note the dB voltage level measurements listed here are referenced to 0 775 Volts RMS We have seen dB measurements referenced to 0 775 Volts RMS labeled interchangeably as dBu or dBv note the little v while dBV a big V uses
12. e C406 22uf 25v RL2 LEVEL GND C402 RL2 27uf 25v W IN4003 D44 RL 1 BYPASS GND C403 120uf 16v POWER INLET 22uf 25v LOOP RELAY WIPER PCB VOLTAGE SWITC eee Loop Return Amp x 22uf 25v 8 R502 1 22uf 25v es al E310 jl R209 203 2 6 22R 202 lt INSERT RET P2 22uf 25v LINK RELAY BOT 22uf 25v A201a C501 LOOP RELAY BOTTOM INSERT RET P3 22uf 25v 30V O Voltage Divider R316 10K INSERT SEND P3 C304 R317 470uf 16v 10K L All relays close on gnd closure Insert loop driver INSERT SEND P2 OF OUTPUT Pox C303 Copyright 1997 permission given 470uf 16v to copy with proper acknowledgement
13. e chassis as well PINS 2 amp 3 of the input and output XLR connectors must be connected to get signal in and out of the unit Since the inputs and outputs are transformer balanced there is no problem unbalancing either the input the output or both Because the input attenuator ahead of the input transformer takes PIN 3 of the input XLR to signal common is the preferred way to unbalance the input To maintain consistent phase from input to output connection of PIN 3 to audio common is the preferred way to unbalance the output unless you WANT to flip the phase Input amp Output XLR Standard connections Transformer isolating Insert Send amp Return XLR a differentially balanced send and return into the normal connection from the top of the output pot to the side chain detector and gain control circuitry This loop allows EQ filters and other external processors to be used to modify how the limiter responds i e as a vocal stressor or with a low cut filter to limit bass and improve its definition This insert is always sending return is bypassed until activated Stereo Interconnection Two 1 4 Jacks marked Link Input and Output are used to connect two units together for stereo mode utilizing the link control on the front panel Using two 1 4 TS or TRS cables connect the Out jack on each unit to the In Jack on the other unit IEC Power Inlet Accepts standard IEC type power cable and includes Fuse Holder see back panel markings for f
14. fy how the limiter responds i e aS a vocal stressor or with a low cut filter to limit bass and improve its definition This insert is always sending return is bypassed until activated Link connects input of the compression amplifier to the output of the Link summing amplifier Link out is a buffered output that is also run into the summing amplifier with the Link In signal This summed output is then run into the compression amplifier PURPLE AUDIO MC77 MANUAL 2002 REV 1 EFFECTIVE WITH SERIAL NUMBER 500 PAGE 9 OF 12 9 MAINTENANCE METER ZERO ADJ UST Allow 15 minutes warm up Push the GR meter function switch The VU meter should read 0 on the VU scale since the limiting function is disabled the attack control knob is turned to OFF If the 0 indication has drifted beyond or 1 dB with no signal it should be adjusted This may be done through the hole in the front panel located between the Input and Output level controls using a small screwdriver The trim pot is straight back through this access hole and requires a small slotted screwdriver INTERNAL SERVICE ADJUSTMENTS These controls have been set at the factory and should not require adjustments except after service work If re calibration is necessary the test procedure that follows should be performed very carefully and adjustments performed in the exact manner and order specified Before attempting any calibrations the limiter should be on for at least 15 m
15. inutes This avoids subsequent drift WARNING The full AC line voltage is present at several points inside the chassis Be careful to avoid personal shock when you work with the MC77 with the covers removed At worst you can be killed and at the very least it will make you unhappy POWER SUPPLY The positive DC voltage should be 30 volts or 5 the test point to measure the level is top trace end of R51 The negative DC voltage should be 10 volt 5 the test point to measure the level is the right hand end of R90 Q BIAS ADJ USTMENT R59 internal trimpot This is a very important parameter to assure the linear operation of the limiter Therefore the adjustment should be performed very carefully Set the controls as follows Input 24 mid rotation Output 24 mid rotation Attack full CCW switched to off position Release full CW Compression ratio 20 1 Meter mode GR Q bias adjustment full CCW PURPLE AUDIO MC77 MANUAL 2002 REV 1 EFFECTIVE WITH SERIAL NUMBER 500 PAGE 10 OF 12 MAINTENANCE continued Apply a signal 1 KHz O dB to the input and adjust the output to read 11db as read on an external meter Slowly turn the Q bias adjust R59 CW until a drop of 1 dB occurs and the external meter reads 10db This places the gain reduction FET Q1 slightly into conduction GAIN REDUCTION METER TRACKING R75 internal trimpot Due to interaction of the adjustments this procedure may ha
16. rtion Adj 200pf gt OUT HI XLR M P3 FON 3 NWN W IN4003 D41 R28 INSERT RETURN gt oe K k INSERT LOOP DRIVE R1C 27K J3 Jumper Ratio PCB 5000 Lin R22 TO LINK CIRCUIT R21 R19 R78 o lt ____ FROM LINK CIRCUIT 47K 1 4W 56K 1 4W 56K 1 4W 68K 1 4W 56K 1 4W Top In A R1A FRONT R1B MIDDLE R1C REAR 30V 50ma Ripple 25mv with load Link Relay R61 R62 R63 Aromat TQZE RL 3 470 1 4W 560 1 4W 1K5 1 4W W IN4003 D40 Q22 PS3569 f NWY VN Tuf a je a I J R87 CRIS 200R 4 1 8 1 12 1 R45 201 24V INS252B ha 10M 1 4W Controlled by Switch on rear of Attack Switch on attack For use w out relay SIDECHAIN NO 430V ie ar Ree ko Oo ont c814 f C18 Aromat TQ2E RLS F Power Switch gt f lt __ R80 75 ohm 3W 1500 1 f R47 Comp off J6 R38 uf T Alt O9ufZ50 4 T3 IEC NEUTRAL w IN4003 D42 Jumper Lo NO AN 4 een 1uf bd 44K2 CR28 e lt TO METER SWITCH 3 28 Orange CKC3001 P 4 t 4 BLUE WHITE Voltage elector Shunt Zener C81 FL E809 p CR41 N4936 30v 10w tuf T F YELL Q8 TO METER SWITCH
17. tack Time control has a switch position at its full counter clockwise rotation This turns limiting action off Release Time Continuously adjustable rotary potentiometer Ranges from the slowest release time of 1 1sec to the fastest release time of 50msec at the full clockwise rotation Insert Engage for Insert Key input mode Side Chain Insert Loop Switch places a differentially balanced send and return into the normal connection from the top of the output pot to the side chain detector and gain control circuitry This loop allows EQ filters and other external processors to be used to modify how the limiter responds i e as a vocal stressor or with a low cut filter to limit bass and improve its definition This insert is always sending return is bypassed until activated Link Engage for stereo mode New Stereo Link Circuit sums the audio feeding the control circuitry of a pair of MC77 limiters instead of simply joining the FET busses This eliminates the need for alignment and the additive effect of attack and release times Linked Stereo limiting is now plug and play and the limiting action is now just as fast as in an unlinked monaural MC76 or MC77 Compression Ratio Four Position Interlocked Push button Switch allows selection of 4 1 8 1 12 1 and 20 1 compression ratios Some users enjoy pushing more than one of these buttons at once Meter Function The VU meter is switchable between I O level and Gain Reduction Top switch In position
18. use rating Audio amp Chassis Ground Audio ground is connected to the audio circuit Chassis ground is connected directly to the steel case and the IEC ground These are shipped from the factory linked leave these linked unless you have a reason to separated your audio and chassis grounds If you do separate them leave the link on one of the lugs otherwise you will lose it PURPLE AUDIO MC77 MANUAL 2002 REV 1 EFFECTIVE WITH SERIAL NUMBER 500 PAGE 7 OF 12 7 UNPACKING AND INSTALLATION Your MC77 was packed with care at the factory The packaging was designed to protect the unit from rough handling However once the unit leaves us it may pass through the hands of those who have no idea what is in the box may not be happy with their job or may like to throw things Despite the heavy duty carton and foam blocks the unit was packed with we recommend you inspect the box and its contents immediately for any sign of damage that may have occurred in transit If there is damage you should save all the packing materials and contact your vendor immediately We recommend you keep the packing materials Murphy s law states that once you throw away the box the likelihood you will need it back will increase in proportion to the square of the distance from your service center multiplied by the cost of replacement packing materials The carton we ship out contains One MC77 Limiting Amplifier This Manual One six foot two meters IEC C
19. ve to be repeated to achieve satisfactory tracking Setup No signal Disconnect R44 Voltmeter across R74 Calibration 1 Zero MC77 GR meter with R71 Thru front Panel Zero Adjust 2 Adjust pot R75 for 0 0 Volts across R74 3 Repeat 1 amp 2 until both conditions are met 4 Set controls as follows Input 24 mid rotation Output 24 mid rotation Attack full CW Release full CW Compression ratio 20 1 Meter mode GR 5 Apply a signal 1 KHz Odb 6 Set output control for Odb as read on an external meter 7 Set attack full CCW off position Set input control for 10db as read on an external meter 8 Turn the attack control OFF CCW and readjust the output level control for 0 if necessary 9 Repeat 7 amp 8 until the output drops 10dB whenever the attack control is turned ON 10 Reconnect R44 PURPLE AUDIO MC77 MANUAL 2002 REV 1 EFFECTIVE WITH SERIAL NUMBER 500 PAGE 11 OF 12 MAINTENANCE continued SIGNAL PREAMP LINEARITY This control R86 is in the feedback loop of the amplifier and affects the operation of Q1 It will never be necessary to perform any adjustment of R86 unless resistors in this section of the circuit have been replaced If adjustment is required set the controls as follows Input Full CW Output to number 18 on the front panel Attack full CCW switched to off position Release Full CW Compression Ratio 20 1 Meter mode GR Apply an input signal 500 Hz
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