This is no typical high-gain distortion device, nor a 'decimator' or bit crusher.
I was searching for something that sounded a bit like my old tube bass preamp that I like using for fattening up vocals or drum tracks.
Because good outboard gear is expensive and sonically equivalent software simulators are rare (and expensive as well), I started to lurk behind the curtain, thinking of creating something to fit my personal needs.
I simply could not find suitable simulators out there.

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Well, what is going on inside analog gear, especially when it comes to the fact, that tube equipment always sounds different as solid-state devices?
Many people say 'oh, it's soft saturation that leads to more harmonics'. It took me quite a long time to realize that this is just some part of the story.
Apart from generating the 'right' kind and amount of harmonics, analog circuits are actually doing a lot of dynamic action. Considering a vacuum tube, this behaviour partly depends on the tube itself and also on the electrical parts surrounding it.

Now, THD is an approach to a very few of such aspects. The plugin is intended for a mixer's insert path to act as a preamp stage. It smoothes out the transients, gives more overall gain and warms a bit.

You might also want it to simulate a tape machine driven to the limit to achieve that typical warmth & fatness associated with analog recordings. THD uses similar methods to alter the frequency response of 'hot' tapes as analog recorders do.

As always, trust your ears. It may sound good for some applications, but unsuitable for others.