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Old 4th May 2003, 09:36 AM   #1
mothman is offline mothman  United States
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Default LM3886TF Grounding Question

Hello,
First off I'm a humble WebTv user or I'd provide a schematic to clarify my question.
Anyway the isolated LM3886tf I'm using,and I do have the NSC data sheets,has a separate ground pin 7 that apparantly the LM3875 that most posted Gainclone schematics does not have.
My Clone is a one box unit.Dual secondary trans -25-0-+25 with separate Schottky bridge for each rail and 2x2200uf per side located close to the chip.
My grounding scheme has two stars per channel one power between the supply caps and one signal at the feedback cap.The PSU has a separate wire from each bridge that are connected to form a central star.
Bear with me I'm gonna get to the point real quick like.
My signal star has the RCA and pot(one per channel) connected at the feedback cap in turn connected to the power star via separate wire.I have the chip pin 7 connected to the power ground.
Finally the power ground from each channel is connected to the central star along with the speaker grounds.This center star is also connected directly to the chassis with the earth/safety ground at the exact same point with no decoupling measures.Amp runs very quiet by the way.
OK aside from any comments on my grounding in general my main question is would you consider the chip pin 7 a signal or power ground best connected at which point?

Thanks in advance for any help,
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Old 4th May 2003, 11:42 AM   #2
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Hi Mothman,

after a quick skim of the datasheet - hmm, there's not a lot of info. However, looking a the Equivalent Schematic you can see that the ground appears to be used in three places: mute function, input/VAS biasing and feedback network. This is thus a signal ground and should be connected to your signal star.

James
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