Purple Audio MC77 1176 Style FET Compressor

The Purple Audio MC77 supercedes the MC76 re-engineered 1176 type FET 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 600Ω “build out” resistors to keep a constant 600Ω load on the source and primary of the input transformer. Clarostat changed carbon manufacturers and the new carbon manufacturer was unwilling or 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; both the originals and replacement parts are drawn in the schematic that you can download below. At the same time, we looked at feedback gathered from the hundreds of satisfied MC76 users. Based on that feedback, we incorporated useful new features into the revised unit, which was designated the MC77.

NEW features in the MC77:

• True Bypass via sealed relay with front panel switch
• Improved stereo linking with front panel switch
• Sidechain insert loop or key input with front panel switch
• Buffered VU meter can monitor input or output level at +4dBu
• LED meter lights that don’t burn out as incandescents do
• Heavy gauge black stainless steel enclosure
• 115v/230v operation switchable from rear panel (serial# 600)

Features inherited from the MC76:

• Discrete Transistor Audio Path Electronics
• Single Element Class A Output Amplifier
• Transformer Balanced XLR Inputs and Outputs
• Zener Shunt Regulated Audio Power Supply
• 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 of 45dB (full gain with no limiting)
• Ruggedized Design – PCB stiffener, chassis mount transformers
• Purple Anodized Aluminum Front Panel
• 3 year warranty