Impedance is a good point. However, are these interface pres really "obtaining" noise at high gain, or is the inherent noise merely more apparent? I'm actually rather inexperienced, so your learned wisdom would be much appreciated.
The only cases where I noticed a real improvement were:
- Noisy lines between mic and pre
- Microphones with unduly high impedance
I've never experienced it but I could think it would be helpful with a preamp that has not enough gain, or is very noisy.
It is known since ages that in order to provide the best S/N ratio, the first element in the chain must be the one with the lowest intrinsic noise. A typical FetHead, Cloudlifter... has an input noise of about -125dBu, when many preamps have a significantly better figure, particularly at high gain, so in many cases, if the preamp has not enough gain, it is better to put gain after it than before it.
Of course it supposes clean connections between all the elements of the chain.
Indeed, most preamps gain control are not that good in terms of resolution when used close to their maximum gain; that's where operability becomes dominant over pure performance.
Some people believe that turning a knob over the 4 o'clock position is bad practice.
There's a few other threads in this group where I explain why I had to use an in-line booster, basically because I had a case of both noisy lines and wrong impedance mic (2500 ohms instead of advertised 200). Originally I used a FetHead (1st version) and was not very pleased with the result.
I then decided to look at designing one, which had none of the Fethead issues. It does what I need and there are now about a hundred of them in use, apparently with great satisfaction.
Me, I just kept one, that I use only with said wrong Z mic.
The one I designed has an input impedance of 40 kohms, but I haven't noticed any sonic difference with some of the other preamps I have.
I agree that too low an input impedance is detrimental to the sonic quality. In particular, I had a GT "The Brick", which input Z is about 500 ohms and it muffles the sound of dynamic and ribbon mics. I also have a few variable-Z preamps and it is obvious that the low settings never result in good performance.