fladd wrote:If I understand you correctly, then there is no such thing as absolute gain.
Yes, there's absolute gain -- how much bigger or smaller the signal is made, overall -- but that is the sum of all gain stages, including that of the digital conversion process. In the case of your two interfaces, I suspect the 2i2 effectively has less analogue gain but more digital gain than the 18i8.
Let me try and explain it with some numerical examples ...
Let's say the output voltage from your microphone is 3mV (0.003V).
If the first preamp boosts that by 46dB the signal level will be raised to 0.6V (in round numbers).
If the second preamp boosts it by 50dB it would produce an analogue output signal level of 0.95V.
...and in the analogue world, the latter is obviously (4dB) louder than the former, as you would expect.
However, let's say that these analogue signals are fed into different A-D converters, and the first converter is set up so that it reaches 0dBFS (clipping level) with an input signal level of 1V. In that case, the input of 0.6V produces a digital output level of -4.4dBFS.
But let's say the second interface is set up with 6dB more headroom so that it delivers 0dBFS only when the input voltage reaches 2.0V. In that case the input signal of 0.95V produces a digital output of -6.5dBFS.
...and consequently the output from the second interface is actually 2dB lower in the digital domain
than the that from the first preamp, despite that first model having 4dB less analogue
And the reason is that although it has 4dB less analogue
gain, it effectively has 6dB more digital
gain (from the conversion alignment) and thus 2dB more gain overall from analogue input to digital output.
This seems to have several very important and rather surprising implications:
(a) For a manufacturer to provide, and for a customer to read the maximum gain value (in isolation) is completely uninformative.
Well, not completely
uninformative. It is still useful to know the available analogue gain range. 40dB is about the minimum you can get away with with close-miked sources. 60dB is typically required for quieter sources or less sensitive mics, and 75dB or more for very insensitive mics with distant placements... for example.
(b) Statements such as "an SM7b needs at least 60dB of gain" are entirely meaningless.
Without a context, arguably, yes. Although in the context of a typical analogue preamp/mixer, it's a handy rule of thumb to estimate the amount of gain required in a typical application.
(c) There is no a priori way of knowing if an interface/preamp will provide enough amplification for a given microphone.
I refer you to my earlier answer re typical 40/60/75dB gain range applications.
(d) Comparing the amount of amplification between interfaces/preamps is not possible without actually testing them against each other.
This one is definitely true because, as I said in my previous response, few manufacturers specify the the analogue-digital conversion alignment, and thus the gain through to the digital domain.
Out of interest, what sort of digital level difference are you getting from your two Scarlett interfaces?