Where do i put FET biasing pot?

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kingkorg

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Apr 15, 2017
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Hi guys,

Where do I put biasing trimmer in this configuration? Should I replace just r3, or r4, or both with middle pin connected to r1? Someone suggested bypassing r4, and tweaking just r3. What are advantages/disadvantages?
 

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R3 sets the bias. You could replace it with a trimmer plus a series resistor. The series resistor is needed to set a limit the adjustment to a sensible range.

Cheers

Ian
 
Thanks, will do. I just got confused by this thread and Dave's (aamicrophones) post where he suggests r6 (r4 in this schematic) should be bypassed.

https://messageboard.tapeop.com/viewtopic.php?t=81385 
 
kingkorg said:
Thanks, will do. I just got confused by this thread and Dave's (aamicrophones) post where he suggests r6 (r4 in this schematic) should be bypassed.

https://messageboard.tapeop.com/viewtopic.php?t=81385

R4, in conjunction with the 47K connected to the f]drain of the FET, sets the gain of the stage. Unbypassed there is some local negative feedback that will reduce distortion in the FET stage. Bypassing R4 will increase the gain and the distortion.

Cheers

Ian
 
With the jumper instead R4, deemphasis network works better, more close to the real U87. That's why most chinese mikes with deemphasis network is much brighter. 
 

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