• MANLEY input & output transformers with nickel laminations in mu-metal cases with flat frequency response from 20Hz-25KHz
• BALANCED INPUTS & OUTPUTS (600 Ohms)
• Fully differential ALL-TUBE circuitry
• Variable Gain vacuum tube: 5670 x2 Standard or 6BA6 x4 T-BAR Mod
• Amplifying vacuum tubes: 5751 or 12AX7 x2
• Output vacuum tubes: 12BH7 x2 current production since 4/2016
• Output vacuum tubes: prior to MSLC94006 use 7044 or 5687 x2
• Side Chain Rectifier Diodes: 12AL5 x2
• Independently regulated B+ and Heater supplies
• Hardwire BYPASS switch
• Stepped switch INPUT attenuator as of 10/2011. (older units use Silent dual-ganged potentiometer)
• RECOVERY 5 steps: 0.2s, 0.4s, 0.6s, 4sec., 8sec.
• Variable ATTACK: 25msec-70msec
• Continuously variable THRESHOLD
• LIMIT (4:1 to 20:1) or COMPRESS (1.5 to 1)
• HP SC: High Pass Side Chain Filter -3dB @ 100Hz
• Large ILLUMINATED Sifam METERS (older units before serial number MSLC61642 shipped before 12/2003 use: 26V 1.2W FESTOON LAMPS; Manley's Part Number: VAR016B) Order spare bulbs using our parts order form. (newest units after serial number MSLC61642 shipped after 12/2003 use white LED lighting)
• STEREO LINK SWITCH - Several units can be linked for Surround (custom order )

• Maximum gain: 35dB
• Max. output: +30dBu (26Vrms) 26dB Headroom
• <0.1% THD @ 1KHz Noise floor: -85dB typical
• Power Consumption (120/240VAC): 80 watts
• Unit is factory set for 100V, 120V or 220-240VAC operation for original destination country's mains voltage.
• Operating Mains Voltage changeable with power transformer changeover switch and fuse value change.
• Mains Fuse Value for 100~120VAC operation: replace with 1.25A as of 10/2011
• Mains Fuse Value for 220~240VAC operation: replace with 0.6A (600mA) as of 10/2011
• Mains Fuse Type: MDA or MDL SLO-BLO Time delay 1 1/4" x 1/4"
• Mains Voltage Frequency: 50~60Hz
• Dimensions: 19” x 3 1/2” x 10” (chassis occupies 2u) Power transformer protrudes 3.5" out the back of the chassis.
• Shipping Weight: 23 lbs.

The Manley Variable Mu is unique because its compression is achieved by re-biasing the gain of a vacuum tube, as opposed to using a VCA, optical cell, or FET. This "variable-mu" design is what gives the unit its signature sonic character: an incredibly smooth, musical, and transparent gain reduction curve that gets more gentle as the signal goes deeper into compression. This all-tube, all-transformer design is responsible for its immense headroom, rich harmonic content, and ability to "glue" a mix together in a way that is distinctly musical.

The Variable Mu offers two distinct operating modes. In "Compress" mode, the unit has a gentle, soft-knee ratio that starts at 1.5:1. This mode is ideal for subtle gain reduction, adding warmth and cohesion to a mix or sub-group without affecting dynamics too heavily. In "Limit" mode, the unit employs a sharper knee and a ratio that can increase to 20:1. This more aggressive mode is perfect for catching and controlling fast, transient peaks, making it an excellent tool for mastering and achieving modern loudness.

The High Pass Side Chain (HP SC) filter is a crucial feature that prevents the compressor from "pumping" or over-reacting to bass frequencies. When engaged, the filter tells the compressor's detector circuit to ignore sounds below 100Hz. This means that a kick drum or a heavy bass line will not trigger the gain reduction, allowing you to compress the rest of the mix—like vocals or guitars—without the low end of the track causing unwanted "breathing" or rhythmic side effects.

The Variable Mu utilizes a specific selection of vacuum tubes to achieve its sonic signature. The variable gain stage uses a 5670 dual triode tube. The amplifier stage uses a 5751 or 12AX7, and the output stage uses a 12BH7, 7044, or 5687, depending on the unit's age. This all-tube signal path, combined with Manley's custom-wound transformers, is the heart of its classic analog sound.

Absolutely. While the Variable Mu is most famous for its use on the mix bus and for mastering, its musical and transparent compression makes it a powerful tool for tracking individual instruments. It is an excellent choice for adding a smooth, professional sheen to vocals, giving a bass guitar a solid and consistent foundation, or taming the dynamics of an acoustic guitar, all while adding a touch of classic tube warmth.

The "T-Bar" mod is a factory modification that replaces the 5670 variable gain tube with four 6BA6 tubes. This modification provides an even smoother limiting curve, more gain, and a sound that is even closer to that of a highly-coveted vintage limiting amplifier. This modification is standard on all modern Variable Mu units, providing an enhanced sonic experience.