Purple MC76 User manual

Purple Audio Inc.
MC76 Peak Limiter
Effective with serial number 001
The MC76 is a Monaural FET Peak Limiting Amplifier

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 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 Regulated 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
*Built 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 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 work, may like to
throw things, don't care.... etc. 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 and shipper 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 costof replacement packing
materials.

The carton we ship out contains:
*One MC76 Limiting Amplifier
*This Manual
*One six foot (two meters) IEC Cable
*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 unit 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 a smallwindow 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 should 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, ø+
PIN 3 = Signal "Cold", Negative, ø-
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 shouldNOT carry
current and are generally connected at only one end of any given wire. For your
convenience and to assure that the chassis can beeasily 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 unit. Since the inputs and outputs are transformer balanced thereis 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 consistent phase from input to output connection of
PIN 3 to audio common is the preferred way to unbalance the output unless youWANT 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 straight up to 12 O’clock. 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 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 referenced 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 release 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-clockwise 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 operation 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 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 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 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, 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 - service techs in the USuse a popular Xcelite screwdriver called a
"greenie".
STEREO INTERCONNECTION OF TWO MC76 UNITS
Two or more MC76 limiters my be connected 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) 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 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 limitersare interconnected, the Attack controls on both units will
interact, as will the Release controls. Since the timing 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 limiter to maximum and to use the
release control on the other unit to control release time.
A given pair of MC76's will normally track properly through at least 10 dB of gain
reduction. In some instances, transconductance of the two FET's (Q1) will differ to the extent
that equal gain reduction will not be obtained as limiting is increased. This conditionis
more apt to be present in MC76's with widely different serial numbers, as the FET's within
the same factory production run are usually well matched. Should this anomaly be
observed, it will be necessary to select FET's for Q1 which match more closely 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 sound 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 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 20µsec to 800µsec. 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 multiple MC76 Limiters. 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
Compressors 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. 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 occurs 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 dynamic
range of an individual track in amulti-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 different 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 2 dB, this would represent a 4:1 compression
ratio.
Amplifiers with compression ratios of up to 8:1are often considered to be Compressors,
while those with ratios higher than 8:1 are called Limiters. The MC76 can functionas a
compressor or a limiter since it has compression rations 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 that 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 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 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 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
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 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 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
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
- there 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 settings
S/N Ratio: >81dB at threshold of limiting 22Hz - 22KHz unweighted
Attack Time: Less than 20µsec for full limiting action
adjustable to 800µsec 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 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

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 MC76 can operate safely over a temperature range of
0°C to +50°C
The MC76 can be stored over a temperature range of
-20°C to +60°C
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 - Legends are silk screened in white
Weight: Unit - 12 pounds Boxed - 15 pounds
Accessories Included: IEC Power Cord and Manual
Optional Accessories: None