Gus said:
The two resistor work with C8 and C9 as a RC filter for the capsules charge voltages.
So IMO you picked the two resistors that have the smallest effect if any at all unless they are broken.
In your microphone what are C1 and C2? If they are Al electrolytics they might have dried out with age if they are tantalums I would leave them alone unless they are bad.
What kind of cap is C3 in your microphone?
Are the gain stage resistors metal film?
NOTE the nice table in the upper right for the resistor values for the JFET used.
Thank you gentleman for lending your knowledge here. That does make sense to me that it is the filter for the polarisation voltage. Hoever, why would we not want to use high quality, low noise, and closely matched parts? If the resistors have higher noise here, would that not still contribute to the overall noise. Also why would they use 22 nF on one line and 10 nF on the other? When I looked at the next generation MV 692 schematic, they used 22 nF on both lines. I was planning on upgrading those old Eastern ceramic caps to new COG 1% or 5% parts.
C1 and C2 are polarised aluminum electrolytics. Could someone please explain their function in this circuit? I'm having a hard time understanding C2 and it's positive polarity facing toward the transformer primary. I was thinking about replacing this with a Nichicon Muse Bipolar of the same value. Similarly, I'm not sure what's going on this C2 either with it's Positive end on the Drain of the Fet. Is this coupling the signal from the drain? I was also thinking about replacing this with a Muse bipolar of the same value.
C4 and C5 are also polarised aluminum electrolytics. They are 1 uf "Tesla" brand. I was planning on replacing them with new 4.7 uF Vishay 2021. I think that the increase in capacitance could be beneficial here in what seems to be a ripple suppression role.
I'm not sure what C3 is exactly but it looks similar to other small polystyrene types I've seen with a clear coating over a silver body. All the pF value caps in my mic are the same type.
I'm not sure if the gain stage resistors are metal film or not as they look quite different from modern western resistors. Based on pictures I could find, it looks like they could be either old soviet metal film or carbon film resistors.
It sounds like the old soviet FET is probably the biggest contributor to noise in this circuit and I would really love to reduce the noise of this mic and improve the clarity of the sound. The noise isn't unbearably loud but it is noticeably louder than modern condensor mic's and with compression, it can certainly be distracting. I could look into finding a replacement FET with Ugs in the range of the table and replace the old biasing resistors for the FET with new low noise metal films.
Let me know what you think!