You haven't said how the 'oscillation' manifests itself but ...Have you got 2 oscillators in the same mike body?This is a really bad idea. Use one oscillator to power both capsules. 2 oscillators will beat with each other and generate stuff in the audible band.
Any chance you could re-wire the rectification / filtering / pattern-switching to something like this?http://audioimprov.com/AudioImprov/Mics/Entries/2015/12/20_Modding_a_BM-800_Mic_files/Mic%208%20TLC.jpg
Khron's suggestion is from Henry Spragens Schoeps like schematic, published some time ago on his audioimprov.com website. I can confirm it works, as I have built several mics using this design with no problems at all.So, I'm with Khron's suggestion... give it a try!EDIT: On your schematic, I see that the font side of capsule is connected direct to ground. How is it going to be polarized that way?One common approach for multi-pattern designs, is to have the front side of the capsule always connected to the polarization voltage (+50 to +60V) output from the DC-DC converter, the center plate is always to the input FET gate thru a small poly cap (1nf), and finally, a switch controlling how the rear side gets the polarization voltage: +50 to +60V from the DC-DC converter for Omni pattern, unconnected for cardioid pattern, and -50 to -60V from inverse DC-DC converter output for 8 figure. This is clearly seen on Herny's schematic. Take a look...Regards.HL
That sort of resistive divider being used to derive the capsule bias works only in tube mics, where you have a +100-120v with a good few mA available, so putting that through those 51meg resistors is no issue (an insignificant additional load for the "main" power-supply).Unfortunately, this sort of oscillator used in phantom-powered solid-state circuits is far less potent, current-wise, so that's most likely where your problem lies.
If anything, you'd want LARGER value resistors (which would pass / draw much less current than the 51M ones).
with only one oscillator, I get good signal from both capsules.
The bad news is that my S/N ratio is now terrible. There's pretty significant background noise, far too much to be usable in a studio environment.Is that a consequence of using a single oscillator to power both sides of the microphone?