Vari-Mu Compressors

 In TSM Studio Blog
  • Although less common amongst most compressor builds today, Variable Mu (Tube) compressors have been around for over 60 years and continue to be a staple in top recording studios across the world.
  • Vari-Mu compressors are unique in how the ratio of compression increases with gain-reduction, meaning the louder a transient’s peak is, the more heavily the signal will be compressed. In regard to reaction time among other forms of compression, Vari-Mu compressors are slower than VCA or FET compressors and maintain a soft-knee style of compression that is geared towards “gluing” a mix together.
  • Vari-Mu compressors heavily rely on their tubes to control gain-reduction. Because Vari-Mu compressors’ signal processing relies so heavily on their tubes and often use warmer transformers than OPTO and FET compressors, they’re known to add a signature “color” to a source.
  • Due to their medium reaction time, Vari-Mu compressors are typically not the first choice when seeking to add “punch” to a mix. However, the additional harmonics generated from Vari-Mu compressors are known to add excitement to overheads, room microphones, electric guitars, basses, vocals, and source signals that may sound “thin”.
  • Vari-Mu compressor parameter controls typically include an input knob that allows you to either boost or attenuate the signal feeding into the compressor. The effect of driving the input knob to achieve tube overdrive is a very common and effective use of Vari-Mu compressors.
  • As with most compressors, the Vari-Mu model offers a threshold control that functions similarly to the peak-reduction control on OPTO compressors (refer to our OPTO compressor article here). This threshold parameter will dictate how much gain-reduction is applied. 
  • On certain Vari-Mu compressor models, the attack and release settings are linked together and controlled using the time constants knob (as seen on the Fairchild 670).
  • Examples of Vari-Mu compressors include:
  • Fairchild 670
    • Manley Variable MU
    • Slate VBC-FG-Mu
    • Chandler Limited RS124
    • Altec 436B