Purplemc76 Limiter 电路图.pdf

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1、Purple Audio Inc. MC76 Peak Limiter Effective with serial number 001 The MC76 is a Monaural FET Peak Limiting Amplifier RadioFans.CN 收音机爱 好者资料库 Units of this type have been in wide use for many years to provide precise and automatic control of peak signal levels in recording studios, disc mastering

2、facilities, broadcast stations and sound reinforcement applications. Operation of the MC76 Limiting Amplifier should be very familiar to those who have used similar devices in the past. UNITS FEATURE: *Discrete Transistor Electronics *Transformer Balanced XLR Inputs and Outputs *Zener Shunt Regulate

3、d Power Supply *Single Element Class A Output Amplifier *Compression Ratios 4:1, 8:1, 12:1 and 20:1 *Fast Attack Time 20 microseconds to 800 microseconds *Release Time 50 milliseconds to 1.1 second *Gain 45dB (full gain with no limiting) *VU Meter for output level and gain reduction indication *Buil

4、t In Stereo Interconnect with offset adjustment *Heavy Gauge Stainless Steel Enclosure *Purple Anodized Aluminium Front Panel *120/240 volt operation *3 Year Warranty UNPACKING AND INSTALLATION Your MC76 was packed with care at the factory. The packaging was designed to protect the unit from rough h

5、andling. 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 work, may like to throw things, dont care. etc. Despite the heavy duty carton and foam blocks the unit was packed with, we recommend you inspect the box and it

6、s 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 and shipper immediately. We recommend you keep the packing materials. Murphys law states that once you throw away the box the likelih

7、ood 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. RadioFans.CN 收音机爱 好者资料库 The carton we ship out contains: *One MC76 Limiting Amplifier *This Manual *One six foot (two meters) IEC Cabl

8、e *Warranty Card The unit can operate safely and within specification over a range of ambient temperatures from 0 C (+32 F) to +50 C (+122 F). Humidity is generally not a factor unless there is condensation. One way to avoid condensation is to keep the ambient temperature reasonably constant. The un

9、it can be powered from either 100-125 volt or 200-250 volt AC Mains at a line frequency of 50 or 60 Hz. The operating voltage should be set BEFORE you plug the unit in. The power inlet module integrates the voltage selector into the fuse holder. The selected voltage (115V or 230V) will show through

10、a small window in the flip down “door” that covers the fuse holder. The unit should be supplied with an IEC cable applicable to your locality. The fuses and voltage selection should be set up for you, BUT NEVER ASSUME! Fuses: At 115 Volts the unit should be fused for 1/8 Amps At 230 Volts the unit s

11、hould be fused for 1/16 Amps Signal Connections are made via standard XLR connectors. 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

12、 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 ca

13、n 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 the chassis as well. UNPACKING AND INSTALLATION - continued PINS 2&3 of the input and output XLR connectors must be connected to get signal in and out of the

14、 unit. Since the inputs and outputs are transformer balanced there is no problem unbalancing either the input or output or both. Because the input attenuator is ahead of the input transformer taking PIN 3 of the input XLR to signal common is the preferred way to unbalance the input. To maintain cons

15、istent 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. BRIEF OPERATING INSTRUCTIONS Once the unit is set to the correct line voltage, plugged in and audio connected. 1) Set Input and Output Gain controls s

16、traight up to 12 Oclock. Do the same for Attack and Release controls. 2) Pick a Ratio - a good place to start is 12:1 but any of the four available will do - higher ratios will give a harder limiting effect while lower ratios will provide a softer compression effect. 3) Set the meter select to GR or

17、 Gain Reduction. 4) Run the program material of your choice through the MC76. 5) Observe the amount of Gain Reduction on the meter. Adjust Input Gain and observe how GR indication changes and listen to how the program material sounds. 6) Select Meter to +4 (or +8 if 0VU in your facility is reference

18、d to +8dBu) and look at the level coming out of the MC76. Set the Output Gain to a level that makes sense for you and then return the Meter Select to GR after you set the output level. 7) Experiment Note 1: Attack and Release times are quicker with clockwise rotation of controls. If you set the rele

19、ase too quick the bass frequencies can modulate gain limiting at an audio rate. This may not sound good on intruments like a bass guitar. Fast release does sound good in many applications such as where bass frequencies are not a major component. Note 2: If you turn the Attack control counter-clockwi

20、se until it clicks, this will disable the limiter leaving only the gain which can be substantial. (Be mindful of your speakers!) Note 3: You CAN push more than one Ratio button at a time. Note 4: The XLR connectors are wired Pin 2 hot. Note 5: The VU meter should be left in GR mode during normal ope

21、ration as it is unbuffered and will add distortion to the signal if left hanging across the output. OPERATING INSTRUCTIONS The Attack Time of the MC76 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 si

22、nce 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

23、 the same reason that Attack time is and can substantially alter the sound of the program material passing through the MC76. Together, Attack and Release times will define much of the sound or coloration in any given application. METER ZERO ADJUST (Allow 15 minutes warm-up) Push the GR meter functio

24、n 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, it should be adjusted. This may be done through the hole in the front panel located between the Input and Outpu

25、t level controls using a small screwdriver. The trim pot is straight back through this access hole and requires a small slotted screwdriver - service techs in the US use a popular Xcelite screwdriver called a greenie. STEREO INTERCONNECTION OF TWO MC76 UNITS Two or more MC76 limiters my be connected

26、 for stereo or multi-channel operation as follows. First remove signals from both limiters and disable gain reduction by rotating the Attack controls fully CCW. Set the meter function switches for GR mode. Connect one MC76 to the other by plugging a 1/4” “mono” patch cable (like a short guitar cable

27、) from the offset jack on one unit to the direct jack on another unit. Adjust the stereo offset on the MC76 whose offset jack is used until both meters read zero. If the meters cannot be zeroed, reverse the polarity (or “phase” on early units) and the meters can then be zeroed. The same compression

28、ratio should be selected in both units. Each limiter should be set-up separately for threshold and output level before being linked together for stereo operation. When the two MC76 limiters are interconnected, the Attack controls on both units will interact, as will the Release controls. Since the t

29、iming capacitors are in parallel, the fastest attack time will be double that of a single unit. Attack time on either limiter can be adjusted separately to control both units. Maximum release time is the same as on an individual unit. A good procedure is to set the release control on one MC76 limite

30、r to maximum and to use the release control on the other unit to control release time. A given pair of MC76s will normally track properly through at least 10 dB of gain reduction. In some instances, transconductance of the two FETs (Q1) will differ to the extent that equal gain reduction will not be

31、 obtained as limiting is increased. This condition is more apt to be present in MC76s with widely different serial numbers, as the FETs within the same factory production run are usually well matched. Should this anomaly be observed, it will be necessary to select FETs for Q1 which match more closel

32、y in transconductance. This should be handled at the factory. DESCRIPTION The MC76 is a 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 sou

33、nd reinforcement applications. Gain reduction in the MC76 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

34、 are four compression ratios that can be used to best complement the program material and the specific application. Attack Time is continuously adjustable from less than 20sec to 800sec. The fast attack time response is independent of peak frequency content or duration. Over the audio band the full

35、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 multiple MC76 Li

36、miters. When connecting multiple limiters (usually two for stereo) it is necessary to set the balance between the two connected units. This linking circuit is built in to every MC76 and there is a balance or offset control on the back panel next to the interconnection jacks. GENERAL DEFINITIONS Comp

37、ressors and Limiters are used in many applications. A technical application for these devices occurs when the dynamic range of program material is too large to be processed by succeeding equipment, or when the peak-to-peak amplitude is too large for the headroom of the following equipment. Compresso

38、rs 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 occurs as a production or creative tool. Any device that changes dyn

39、amic range will cause some subjective change in the sound of the program material. Reducing 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. The applications as a production tool are limitless. The two di

40、fferent names (Compressors and 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

41、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 MC76 can function as a compressor or a limiter since it has compression rations from 4:1 to 20:

42、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 that above the threshold will cause a controlled decrease in amp

43、lifier gain resulting in a reduced increase in the output level in proportion to the compression ratio. The MC76 Input Level control adjusts the amount of signal to be processed above the threshold, and hence the degree of compression, or limiting. SPECIFICATIONS - ELECTRICAL Note: the dB voltage le

44、vel 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 1 volt as a zero reference. and to make things worse people still use the dBm scale to discuss

45、 voltage levels across what they are measuring. A dBm level is a measurement of power that references to 1mW 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 termina

46、ls 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 dont have to keep saying dB little v Input: Constant Imp

47、edance Input Attenuator Transformer Balanced and Floating Input Impedance: 600 ohms at 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

48、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 - all other things being equal - ther

49、e will be a loss in headroom Maximum Output Level: +24 dBm into 600 ohm load (12-1/4volts) +30 dBu into bridging load (24-1/2 volts) SPECIFICATIONS - ELECTRICAL - continued Distortion: Less than 0.5% THD+noise 22Hz-22KHz with limiting activeRelease time set to 1.1sec (fast release times will increase distortion at low frequencies) - NOTE: The VU meter should be switched to GR mode in normal operation as it is unbuffered and will add distortion to the signal if left hanging across the output in the +4 or +8 se