For A2D inputs and D2A output, there are typically two gain controls, one before the converter and another after the converter. The design of the converters establishes the input/output dynamic range, typically around 114 dB which is very good. However, to get that dynamic range, you need to be pushing the input into the converters as high as you can without clipping. This uses the maximum number of bits in the converter and minimizes the noise floor.
As you push the input into the A2D converter higher, you need to reduce the gain after the converter to compensate, so that the resulting input level into the downstream blocks doesnāt change. The input trim controls on Fractal Audio devices like the FM9 do this automatically. During setup you adjust the input trim as high as you can without clipping on the meters. Changing the input trim doesnāt change the input into the amp blocks, it just optimizes the noise floor.
For Quad Cortex, we donāt know what the Input level controls. Is it the level into the A2D converter? Is it the level out of the A2D converter? Or is it both like the Fractal Audio input trim? Given that the QC Input Level does change the overall input gain, it appears the control is either before or after the A2D converter. We donāt know for sure which.
If Input Level is before the A2D converters (the most likely case), then you might want to push the input level as high as possible without clipping, and then use a gain block early in the signal path to reduce the gain by the same amount to preserve overall unity gain. This would maximize the dynamic range and minimize the noise floor, duplicating the behavior of the FM9 Input Level.
If the Input Level is after the A2D converters, then it would be just a digital clean boost which wouldnāt be very useful.
Iām guessing that the QC guitar input was designed to be optimized for guitar input levels with the Input Level set at 0 dB. But guitar pickup levels can vary quite a bit. Low output single coil pickups might not push the A2D converters high enough to minimize noise and loose some of the available dynamic range. High output humbuckers might push the input into the A2D converters into clipping, something that should be carefully avoided. Digital clipping is not pleasant.
So the Input Level can be used to adjust for situations where the guitar pickup output is significantly different than typical levels. Whether you need to use a gain block to preserve unity gain or not depends on what tone you are looking for. If you want your low output pickup to behave the same in QC as it would into the front of a guitar amp, then youād need to use the gain block to preserve unity gain after bumping up the input level. If on the other hand, you want that low output pickup to behave like other higher output guitars, then leave the QC Input level boost.