Purple Audio MC77 Discrete Class A Limiting Amplifier E 1176 Type Compressor Purple Audio

The MC77 is a standard 19″ two space rackmountable limiter. The MC77 recreates the audio circuitry of the revision E 1176, using modern components matched to the original. One significant component is the input attenuator. The input attenuator that was custom made for the original revisions A-F 1176 and for the Purple MC76 by Clarostat was discontinued in 2002. It was a J series T-pad attenuator with two 600 OHM “build out” resistors to keep a constant 600 OHM load on the source and primary of the input transformer. Clarostat changed carbon manufacturers and the new carbon manufacturer was unable to make the part. Purple temporarily stopped making the MC76 to find a solution. We tried several three deck Clarostat 70 series pots in different configurations to achieve the same loading and to match the taper. We found a solution that matched the original. Based on that feedback, from hundreds of satisfied MC76 users we incorporated useful new features into the new MC77.

Features:

• True Bypass via sealed relay w/front panel switch
• Improved stereo linking w/front panel switch
• Sidechain insert loop or key input w/front panel switch
• Buffered VU meter can monitor input or output level at +4dBu
• LED meter lights that don’t burn out
• Heavy gauge black cold rolled steel enclosure
• Discrete Transistor Audio Path Electronics
• Single Element Class A Output Amplifier
• Transformer Balanced XLR Inputs and Outputs
• Zener Shunt Regulated Audio Power Supply
• Ruggedized Design – PCB stiffener, chassis mounts
• Purple Anodized Aluminum Front Panel
• 3 Year Warranty

The MC77 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 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.

Attack 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.