Quote:
Originally Posted by
studer58
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I guess I wasn't clear; the biasing of the FET for a 3-wire capsule is done externally by the selection of components by the mic builder.
A 2-wire capsule has the FET source internally connected to ground, so only the drain of the FET and ground are available to the mic builder. This is exactly how the JLI-61A is made.
A 3-wire capsule has both the drain and source connections of the FET available to the mic builder, and the biasing components are external to the capsule and selected by the mic builder using a scope to achieve an unclipped and symmetrical waveform. The capsule maker doesn't do any biasing of the FET. The JLI-61A can be converted to a 3-wire capsule by severing the trace that connects the source pad from the ground/case (also shown in image 3 below). The JLI-61A is essentially the same capsule as the Panasonic capsule that's now discontinued.
https://www.linkwitzlab.com/sys_test.htm
As an example, shown are the Primo EM200 (used in the Line Audio CM4), and it's 2-wire version, the EM204. What's inside the dotted lines are what's in the capsules ("IC" is the FET). The components outside the dotted lines are supplied by the user/mic builder.
For optimum performance the actual supply voltage and resistor values need to be determined by test as individual FET characteristics are quite variable; the values shown are just starting points (though I've gotten very good results from the EM200 using just the values shown, in my particular examples).