ADDAC605 Spectral Tilt (DIY)

ADDAC605 Spectral Tilt

The TILT EQ concept was first introduced in the 1970's by Quad Ltd, featured on the company’s model 34 preamplifier.
The tilt control tilts the frequency spectrum of the audio signal by simultaneously boosting the treble and cutting the bass frequencies, or vice versa, making it a simple to use but powerful EQ.

As stated by Quad Ltd, it was designed to produce a very gradual change in balance across the musical spectrum without changing the overall subjective level.
This gradual fall from Bass to Treble, or vice versa, means there’s no sudden change and hence no ‘coloration’ added to the sound, the sound will remain entirelly natural but with a slight added bottom or high end. In practice, it rotates most of the audio-range frequency response around a 1Khz axis, so as to change the overall balance of the sound from warm to cool without affecting either the apparent volume or the "color" of the sound.

Here we introduce this idea into the modular world in a stereo configuration.
CV input with attenuverter was added for the Tilt control as well as a Spectral Motion switch that allows stereo spectral matching or inverted spectrum use of the Tilt control: Left=Right matches both channels direction, i.e. when left channel is tilted to the low end the right channel will also be tilted to the low end 
Left ≠ Right inverts the Right channel spectrum, i.e. when left channel is tilted to the low end the right channel will be inverselly tilted to the high end, or vice-versa.
This possibility is extremmelly interesting when applied as a stereo effect, in other words a spectral EQ panner.