The Stereo Variable MU Limiter Compressor Mastering Version from Manley Labs is a dynamic signal processor using a 5670 dual-triode tube for peak reduction and compression. The tube is constantly re-biased by the vacuum tube rectified side-chain control voltages, resulting in a smooth change in gain. This mastering version replaces all conductive plastic pots with Greyhill rotary switches. The 1% metal-film resistors on the Greyhill switches not only provide recallability, but also deliver an improvement in sound quality.
The compressor setting offers a soft-knee 1.5:1 ratio, while the limit mode offers a sharper knee with ratios ranging from 4:1 to 20:1. With higher limiting settings, the knee will start to soften. Distortion effects can be achieved by turning up the input and lowering the output while using little or no compression. The side-chain high-pass filter uses a 1-pole, 6 dB/octave filter that attenuates 1 to 3 dB at 100 Hz and 4 to 6 dB at 50 Hz. Once engaged, the compressor / limiter will not respond as strongly to low frequency content and will prevent audible "pumping" when processing bass-heavy music.
The input of the Variable MU is ganged, but can still process two independent channels with separate threshold and output controls. With the link switch engaged, the unit operates in true stereo mode and excels at processing 2-track, final mix, or mastering compression and limiting. Handmade in the USA, the unit is housed in a 2U steel chassis with a CNC-machined 1/4" faceplate that is also inlaid with laser-engraved inserts. The Variable MU ships with an IEC power cable.
- Stepped Greyhill rotary switches with 1% precision metal-film resistors
- Manley input and output transformers with nickel laminations in mu-metal cases
- Balanced inputs and outputs (600 Ω)
- Independently regulated B+ and heater supplies
- Bypass Switch: Hardwire true bypass
- Input Attenuator: -4 to +4 dB stepped switch
- Stepped Attack: 15 to 90 ms
- Threshold: 24 step range
- 5-Step Recovery Switch: 0.2, 0.4, 0.6, 4, 8 sec
- Compression Ratio: 1.5:1
- Limit Ratio: 4:1 to 20:1
- Large Illuminated Sifam Meters: Displays the amount of dB gain reduction
- Stereo link switch