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[Joe Davisson]

The excess noise is caused by low-frequency voltage oscillations produced by interaction of the Si/SiO2 gate oxide. It cannot be filtered out. It's generally only bad in N-channel parts, which is why I switched to P-channel for the Obsidian:

http://www.analogalchemy.com/pedals/obsidian.html
The bummer is the trannies are 5x the cost, but the noise is at least 5x lower. (Funny how you get what you pay for...)

CMOS basically always includes N-channel transistors, so the CMOS circuits are always noisy to some degree. That doesn't make them unusable, but I've been much happier going with the discrete P-channels.

A good example is comparing the CA3140 opamp with the CA3260. The CA3140 has P-channel inputs, and BJT outputs. The noise is very low compared to the CA3260 dual-op, which has CMOS outputs.

-Joe