Manley Labs Stereo Variable Mu Limiter Compressor Mastering Version with T-Bar Modification

The Manley Variable Mu Limiter Compressor has become a standard in mastering studios. It works by using the remote cutoff or rebiasing of a vacuum tube to achieve compression. The precious vintage Fairchild 670 also uses this technique and is one of few other all-tube compressors to do so.

Even the side-chain has glowing rectifier bottles. The two 6BA6 dual triode tubes are at the center of the peak-reducing and compression action constantly being rebiased by the vacuum tube rectified side-chain control voltages, which cause this tube to smoothly change its gain. 

You might notice that the Variable Mu Limiter Compressor has a ganged input control, but do not jump to conclusions that it is mono-unfriendly. There are separate threshold and output controls to make compensations with, plus you can always adjust your individual source levels elsewhere. The advantage of the stereo input control becomes dramatically clear when you switch to Link mode, allowing the Variable Mu to shine in its ideal applications: final mix, 2-track, or mastering limiting and compression.

The Mastering Version of the Variable Mu Limiter Compressor uses expensive Greyhill rotary switches with gold contacts where conductive plastic pots were used. The steps are determined with a large number of 1% precision metal-film half-watt resistors. The best conductive plastic pots only have 10 or 20% tolerance.

The ten-fold improvement in precision helps a great deal in left/right matching. There is a subtle audible improvement with stepped switches as well. Audiophile hi-fi often uses that technique to wring out the last drop of performance from a preamp. The Attack Time has been slightly extended in both directions compared to a regular Variable Mu and divided into 11 steps.  

The newer Variable Mu units use 5670 tubes instead of the 6386. By now the availability of the original USA GE 6386 is poor; Manley Labs doesn't have any left at all, and what they do have are not usable due to noise, microphonics, bad side-to-side matching, etc. Up to about 6 dB of limiting, the 5670 tube sounds about the same. After that point, the 5670 version tends to sound more squashed than the original 6386 version.

To solve these problems, the T-Bar Mod uses a pair of 6BA6 pentodes wired as single triodes to replace each dual triode 5670 (or 6386). The 6BA6 T-Bar Mod is the preferred system to use in the Manley Variable Mu to perfectly match each phase half section and each stereo set, so it is possible to select low-noise and low-microphonic sets for a low cost, and because the action of the 6BA6's so closely resemble the smooth 6386 limiting curves.