I bought Marantz PMD 661 recorder last year and find it a great piece of equipment. There is one thing that I would love to know and that is why does the noise floor never go below -79db even when recording in 48Khz 24Bit.
I made a test recording in line level with the gain turned down to Zero to see what the true noise floor is, but it only goes to minus 79db. I measured this by opening it in a stereo editor and having a look at how high the level goes to. Although I am unlikely ever to hear this noise floor, I feel it should be lower than this, even the 16 bit noise floor is lower. Does anyone know why this is?
I'm not sure, but something does come to mind; dithering.
Dithering is the deliberate addition of a very tightly defined (and very low, basically 1 bit p-p) amount of a specific type of noise to the signal. That greatly improves quantisation errors at the expense of raising the noise floor, which has the subjective effect of reducing what you hear as noise (which sounds counter intuitive, but it's a well established technique and I've heard some tests that have been quite impressive).
I couldn't be sure, but that's my guess. TBH -79dB is pretty good and it's much more likely that if you have noise problems they'll be from background sound, mismatched equipment or electronic interference than the recording.