After a bit of tinkering, I've come up with the following mods to the CM60 circuit board.
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It will help to have the CM60 schematic and board overlay from
this thread to hand.
Mod A
Replace R1 and R2 (1G, probably) with 2G resistors - this lowers the low-frequency noise by 2-3dB. I used
these from Mouser - they are
tiny and a real pain to solder.
Mod B
Replace 2SK208Y FET (Q1) with a 2SK209GR. This increases gain, and lowers noise, in conjunction with the bias mods (C and E) below.
Mod C
Correctly bias the FET, for best headroom. The board has space for a trimmer, but as I didn't have one that was small enough, I used trial and error with different resistors. You can just push one through the holes (no need to solder, yet) and measure the voltage across R5 (or equivalent pins on the switch, or C10).
Putting a resistor as shown in the photo puts an additional resistor in parallel with R3, which raises the bias current. Ideally (in conjunction with mods B and E) you need 1-1.3mA bias current, i.e. 2-2.6V across R5. (
This post has more detailed data on JFET biasing for the Schoeps circuit).
For my 2SK209, I ended up with an 82K resistor which gave 1.3mA. It took a
long time for the current to settle on its final value, so patience is required.
Mod D
Raise capsule polarisation voltage, for increased sensitivity and reduced noise. The PCB allows you to fit a trimmer in place of R9, but again I just experimented with a few fixed-value resistors (in parallel with R9). A 2K7 resistor gave about 58V measured on R7, up from 45V or so on the original board.
Mod E
Raise FET operating voltage, for increased headroom/lower distortion. This requires an
increase in the value of R10, as discussed above, so I replaced the original 150R R10 with a 2K7 resistor. Working voltage is just over 12V working voltage, up from 7.2V.
R10 doesn't need to be surface-mount - in the photos it's wired between the +ve end of C12 and one of the R10 pads.
The CM63 already has the equivalent of mods D and E, and mod C is just adjusting the trimmer. The board will need a good cleanup afterwards, especially if your soldering is as neat as mine ;-).