I only record on one channel - its film sound recorded with one mic.
I use a very low-noise home-made pre-amp for the mic. I take power from the rechargeable battery in this pre-amp to drive the Tascam. Digital noise from the Tascam was coming down the power lead connecting the pre-amp to the Tascam, into the pre-amp where it was being picked up by the pre-amp circuit and sent as audio noise into the Tascam down the audio connector. I fixed this by adding three 3.3 ohm resistors in series with the power line to the Tascam, with three .1uF capacitors from the junctions of the resistors to ground, thus filtering out the digital noise. This has reduced the noise from quite loud down to almost imperceptible, barely noticeable above the mic hiss. Yesterday I thought it was perfect, but re-testing has shown it's not. This is disappointing. Now I'm wondering how to remove the last bit of noise?