Schwerkraftmaschine Diode Pressor

  • 01/08/2017
  • Michael Krusch
  • Story
  • Schwerkraftmaschine

Part 2 of our review of the different compression modes in the Schwerkraftmaschine

As the name suggestions, this mode involves diode compression.
Diode compression isn't as well-known as optical compression, but the concept has been around for a while.

The diode compression method

The technology takes advantage of the fact that diodes can not only block or pass a signal, but also feature a small range in between that behaves logarithmically.

When a direct current is applied, the interior resistance of the diode can be modified within this range.
Arrange 4 diodes into a bridge switch, and it suddenly becomes possible to harness that variable resistance in a voltage divider for inverse current signals as well.
Feed the diodes from the side-chain's control signal, and you've got a diode compressor on your hands.

The same basic concept is used by Moog® in their famous cascade filter.

Compared with silicon diodes, those made of germanium have a softer and broader threshold between the block and pass range.
In a rectifier, this would be considered a flaw; for diode compressors it's considered a plus.

The disadvantage is that the characteristic diode curve only covers very small voltage levels.
Otherwise you can end up with highly unattractive distortions.
The audio signal must thus be attenuated prior to the diode bridge and then amplified strongly again afterward. In the process, the strong amplification naturally reinforces the noise inherent to the diodes as well.

Beyond this, the 4 diodes must be selected for optimal characteristic curves. Any deviation within the diodes causes distortions and allows the control signal to interfere with the audio signal path.

Diode Pressor

One famous example of the diode compressor is the Neve® 33609.
Its compression behavior served as the inspiration for the Diode Pressor mode on the Schwerkraftmaschine.

The Diode Pressor has a compressor and a limiter section.
The compressor section works at a ratio of 5:1. This can be reduced using the mix knob.
The limiter works at a ratio of 10:1.
Because the Schwerkraftmaschine is configured using one single threshold knob, studio experience has shown that a gap of 3dB represents a practical gap between the compressor and limiter threshold.
The audio signal is first compressed up to 5:1, and then once an additional 3dB are gained it is limited at 10:1.

Sound

The first thing that should come to mind: drums.
The Diode Presser is outstanding at compressing a drum subgroup. The threshold can sit in the left control range and meter can go to all the way to the left (careful, don't break the needle!).

But taking a broader view, almost anything that compresses well is a candidate. Rock mixes, in-your-face vocals, slapping bass, punchy electric guitars... I think you get the idea.
And best of all, the Schwerkraftmaschine's base tube sound gives its own special feel.

Try it out for yourself.


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