When I was researching this stuff last year, I read that many of the cheaper interfaces apparently actually
do run the line inputs through the interface's own mic pre's, just using a kinda pad on the inputs.
Supposedly, the gain knob does not actually function as a "trim", but rather as an actual gain control, because even when switched to "line" (vs. mic) the input signal is going through the mic preamp circuit.
From what I read, the thing to look for is an interface where, when you switch the input to "line", the gain control has no effect, and/or where there is a separate "trim" knob.
For example, the M-Audio M Track Plus info says, "The 1/4" input can be set to receive line level signal, or switched to allow an electric guitar, bass or any other instrument level source to be plugged directly into the M-Track Plus, while the gain knob ensures the proper input level."
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My interp of that is that the line-level signal is still going through the mic pre of the interface. At this point, I only use mine instead of my laptops' sound cards playback, so they work for me (paid US$90 or so when they first came out.)
As I recall, the Focusrite 6i6
might have a "straight through" line input because it "boasts two preamp-equipped front panel mic/ line/instrument inputs, and two rear-mounted balanced TRS line level inputs. There are four rear-mounted balanced TRS outputs, and the six in/out spec is completed by rear-mounted stereo S/PDIF in/out."
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