hf noise

[rgrayton]

What have you found to be the most likely cause of HF (I would speculate >12KHz.) noise? I have not performed any tests or investigation, yet.
Thanks,
-BG

[Nordic]

LM3875 datasheet

Inadequate power supply bypassing will manifest
itself by a low frequency oscillation known as “motorboating”
or by high frequency instabilities.

These instabilities can be eliminated through multiple bypassing utilizing a large tantalum or electrolytic capacitor (10 ìF or larger) which is used to absorb low frequency variations and a small ceramic capacitor(0.1 ìF) to prevent any high frequency feedback through the power supply lines.

If adequate bypassing is not provided the current in the
supply leads which is a rectified component of the load
current may be fed back into internal circuitry. This signal
causes low distortion at high frequencies requiring that the
supplies be bypassed at the package terminals with an
electrolytic capacitor of 470 ìF or more.