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Two quotes from the manual of the dbx ProVocal digital modeling microphone preamplifier...
"Drive 0 to 99
This parameter controls the input drive of the signal flowing into the microphone preamp model."
"Gain - 0 to 99
This parameter controls the input gain of the signal flowing into the microphone preamp model."
If I hadn't marked the difference in bold, you might not have noticed. The lack of clarity in the manual would certainly cause confusion for a new user.
All microphone preamplifiers must possess a gain control, whether they are simple analog or complex digital modeling. And if digital, then the gain control must always operate in the analog section of the circuit, before any digital processing is applied.
The reason for the placement of the gain control before the digital processor is that the signal from a microphone is weak and needs special handling. If it were digitized at the input, then the resulting signal would be very noisy.
In the case of the dbx ProVocal, the input signal meets a fixed 23 dB gain stage, then a variable +30 dB to + 60 dB stage, which is where the gain control is found. There is also a -20 dB pad before the first gain stage, in case of very high signal levels from the mic.
But then the confusion sets in, because the dbx ProVocal has another two controls labeled 'gain', and then there is 'drive' too.
The answer to this riddle is in the digital modeling. The real gain control has to be set so that the signal from the mic is boosted up to the optimum level for further processing. After that, this unit mimics, or models, the sounds of various preamp types. Presumably these are modeled after real sought-after units, but they are simply identified here as types 0 to 9.
Any mic preamp will display subtle changes in sound quality at different gain settings. For vintage tube preamps these differences will be clearly audible. So the gain function in the digital modeling section of the ProVocal models these changes.
The manual isn't at all clear on the nature of 'drive'. But tube microphones make further changes to the sound quality when they are operating close to their upper signal level limits. This is dependent to an extent on the gain, but also on the signal level coming from the mic. So in this case an additional drive control is justified.
The dbx ProVocal provides an interesting range of sonic timbres and is probably best viewed in this light, rather than being a modeler for specific preamp types.
All you need do as a user is first make sure you set the analog gain correctly, then apply digital processing so the result sounds good to your ears.