The Mu amp bias or the DMM ? Sounds great, wierd bias readings.

Started by petemoore, August 13, 2008, 01:56:40 AM

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petemoore

  Mu Amp bias reads 2.2v...where the source/drain meet.
  Looks great, went together nicely, but with a 10k/10k divider feeding a 10meg to gate of top Jfet.
  And it sounds great too.
  I theorized...
  The DMM was drawing current or somehow otherwise giving a false reading [which I still sorta think, but not as a consequence of current draw..
  Bing  :icon_idea: I was measuring the source, and it should have enough current available to hold bias on the bottom Jfet Drain or feed a little current to the DMM...
  So...I typed enough stuff, maybe you can help me figure out if the DMM [which otherwise seems to read voltages fine] could be reading incorrectly somehow.
Convention creates following, following creates convention.

Steben

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petemoore

  Nope...I measured the 10meg by having it paralleled with 100k, and that dropped it from 99k to 98k.
  The 10k's were only measured for their ability to make 1/2v in the V divider.
  The 1k...is probably a 1k...according to the color coding.
  The divider reads 4v4 [about 1/2v], but the other end of the 10meg reads 2.2v..as not connected to anything much or according to the schematic connections w/transistors in sockets.
  There seem to be no stray connections anywhere around the board from the gate side of the 10meg.
  So it boosts and sounds great so far, hafta move on and try another one now...I've had schtuff that makes no sense before sound great.
  It seems like it's only a problem for the DMM...I'll test the circuit sound more through more amplifier today.
Convention creates following, following creates convention.

R.G.

R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

petemoore

The DMM has an input resistance too.
  Yupp...didn't make much sense, didn't add up...I decided to just trust the ears and see what you thought..
Thanks RG !

 
Convention creates following, following creates convention.