The Manley Labs Variable MU is also available in a MASTERING version. Featuring fully detented logable steps for accurate recall of settings and 1% metal film resistors sealed gold contact Grayhillswitches. 

It is not possible to upgrade a 'normal' VARMU to a Mastering VARMU. 

Other options available;

HIGH PASS FILTER FUNCTION
SURROUND SOUND LINKING
MS ENCODING/DECODING
T-BAR MOD (PERMITTING USE OF 6386 TUBES)

Specifications

• 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 using one each 
  5670, 5751, 7044, & 12AL5 per channel 
• Independently regulated B+ and Heater supplies 
• Hard-wire BYPASS switch
• Silent conductive plastic dual INPUT attenuator 
• 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) 
• Large ILLUMINATED Sifam METERS
• 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 
• UK supplied units factory set for 220-240VAC operation 
• Mains Voltage Frequency: 50~ 60Hz 
• Dimensions: 19” x 3 1/2” x 10” (occupies 2u) 
• Shipping Weight: 10.5kgs (23lb)

PDF Files
{Download Instruction Manual}