Schwerkraftmaschine Classical VariMu

  • 02/02/2017
  • Michael Krusch
  • Story
  • Schwerkraftmaschine

Part 9 of our review of the different compression modes in the Schwerkraftmaschine

Vari-Mu is probably the oldest form of compression. And it's the method of control at the heart of the Schwerkraftmaschine.

How does Vari-mu control work

The name "Vari-Mu" comes from the tube parameter µ (pronounced 'mu'), which in simple terms describes the amplification of the tubes.

A Vari-Mu (or Vari-µ) tube offers different levels of amplification for different cathode biases.
From a general electrical engineering standpoint, it's not actually considered a positive attribute in a tube.
But it does have practical applications when it comes to controlling amplification.

The system works by adding the negative control voltage to the audio signal. Depending on the control voltage, the tubes will then exhibit higher or lower amplification. But that control voltage must ultimately be released again afterwards. To allow for this, two different control levels are established. One level is fed the out-of-phase audio signal, with the difference between the two levels then amplified.
If both tubes have the same characteristic curve, then the control voltages cancel each other out, leaving the audio signal untouched.

Because the working point can be repositioned very quickly, Vari-Mu compressors can achieve very quick reaction times.

It sounds simple, but in practice the details can make a big difference.

It starts with the tubes. It's no easy task to find tubes where both triode systems have the same characteristic curves.
Buying a couple of sweet NOS tubes off eBay and sticking them into a Vari-Mu compressor usually ends in disaster. The tubes must meticulously preselected and calibrated.


The benefits of Vari-Mu compression are its quick response and soft knee. Compression doesn't ramp up immediately at full force when it hits the threshold, but rather starts with a lower ratio which then rises as the compression increases.

The side-chain control signal is also produced using rectifier tubes, which have a softer characteristic curve than their germanium or silicon brethren.

Taken as a whole, this lets Vari-Mu compressors achieve highly organic compression. The harmonics added in through a symmetrical control level differ from the asymmetrical levels, creating a pleasant spectrum of overtones that imbues the music with greater liveliness and definition.

There's a good reason why devices from this period remain in use even today. And in some cases are sold for the kind of sums that would buy a well-equipped compact car.

Schwerkraftmaschine as the most modern form of the Vari-Mu compressors

The Classical Vari-Mu mode on the Schwerkraftmaschine brings together all of these properties.
As a descendant of the Vari-Mu compressor, the Schwerkraftmaschine inherently brings that nice tube sound, supplemented by the control character of the classic device itself.

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