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Not pertaining to Knif compressors specifically, but what 'is' variable vs pure Mu? Dynamics Processors (HW)
Old 11th September 2011
  #1
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Not pertaining to Knif compressors specifically, but what 'is' variable vs pure Mu?

All I know is that the Fairchild 660 and 670 are in this ballpark, and a lot of things are 'based' on them somehow.

And I know there are sounds associated with each, but I doubt I could pick them out blind.
Old 11th September 2011
  #2
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shortstory's Avatar
Mu refers to the bias voltage of the tubes in the circuit. if it's a variable Mu design, it means that as you change the available settings, the biasing of the tubes in the circuit is being changed too in order to affect the signal.
Old 11th September 2011
  #3
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Quote:
Originally Posted by recorder2 View Post
All I know is that the Fairchild 660 and 670 are in this ballpark, and a lot of things are 'based' on them somehow.

And I know there are sounds associated with each, but I doubt I could pick them out blind.
mu is the gain of a triode
at least when i studied tubes
(read the name!!)

wikipedia says its the gain
use variable mu tubes that vary gain to achieve a soft knee type of compression

VARIABLE-MU TUBES :
have the ability to reduce their mu, or (µ), as the input signal gets larger.
[less gain = compression !]

As the mu (µ) decreases, the likelihood that the tube will be driven into cutoff decreases. (For an amplifier, this may appear to be self- defeating, but it isn't.) The idea is to amplify large input signals as much as possible without causing the tube to cutoff or create distortion
[but note that this does cause some distortion just not as much as saturation or clipping]
Old 11th September 2011
  #4
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HandCrafted Lab's Avatar
 

Quote:
Originally Posted by recorder2 View Post
All I know is that the Fairchild 660 and 670 are in this ballpark, and a lot of things are 'based' on them somehow....
Yes, that is common principle of variable gain compressors, with different side chains though. But main idea is same throughout - biasing of working mode of tube with specific gain curve (so called remote cut-off curve). If signal is growing up, the side chain send "commands" to the compression tubes - hey, please decrease signal", and compression tubes moves working point into lower gain field. To avoidance of distortions in wide dynamical range (can happen in "usual" tubes due to biasing) is need to use these particular tubes, with remote cut-off characteristics. In simple words, usual tubes give like 2-3dB undistorted, remote cut-off tubes - more.
Also, these tubes can be triodes (6386 is most famous), or pentodes, there is many tubes. All tubes have own sounds, of course.
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