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[Donn]

A fellow studio owner in Albuquerque recently lent me his U87ai for comparison to mine. Inside I found a Vishay T63YB 10k trim pot. What might be the purpose of this component?

[klaus]

That's one way to arrive at the optimal bias point for the FET. The trimpot takes the place of the fixed source resistor, and is adjusted for the lowest distortion/lowest noise when you feed in a 1K signal into the gate of the FET.
The other way: use a trimpot to find the quiescent point of the FET, and then install that value as a fixed resistor. Advantage: it won't drift like the trim pot tends to.

[Kai]

...and does not introduce distortions like some trimpots do.