The Stereo Variable MU Limiter Compressor with M/S and T-Bar Mod from Manley Labs is a dynamic signal processor using two single-pentode tubes wired in triode for peak reduction and compression. The tube circuit is constantly re-biased by the vacuum tube rectified side-chain control voltages, resulting in a smooth change in gain.
The Mid/Side modification (also known as the Vertical/Lateral or Sum/Difference) allows for stereo encoding and decoding, as well as image enhancement processing capabilities. Instead of an active circuit, the left switch changes the wiring of the input transformer, while the right switch changes the output wiring. When both M/S switches are activated, the stereo signal is encoded and decoded providing the ability to compress the mid, sum or mono part of the sound on channel one, while channel two's compressor controls the side or difference. The Mid/Side switches replace the high-pass side-chain position on the front panel. The HP/SC are ganged and moved to the power switch position, while the power switch has been moved to the rear of the chassis.
The T-Bar modification uses a converter board with two 6BA6 single pentodes wired in triode mode to take over the duties of the single-envelope dual-triode 6386 or 5670 tubes. Each of the single-pentode tubes are matched for low noise and low microphonics before being installed onto the adapter board. Direct A/B comparisons and testing have found the sound and performance of the dual 6BA6 to be nearly identical to the original 6386 tube. Comparing the dual 6BA6 to the 5670 sounds less "squashed" with fewer artifacts as the amount of limiting is increased.
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.