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Chip Amps Amplifiers based on integrated circuits

49830 tweaking
49830 tweaking
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Old 3rd June 2009, 03:35 AM   #1
traw is offline traw  United States
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Join Date: Sep 2005
Default 49830 tweaking

Hopefully not redundant of another thread, but intent is to explore 49830/49810 (or their cousins) tweaking...

Been playing with 49830 using irfp240/9240 combo or the fairchild equivalent another time - 4 total.

How touchy is the gate resistance (aware of the 1645 doc)? Have sent 2 channels up in smoke. About 2-3 minutes running on the bench, gate resistors went up in flames. Also noticed one of the pair biasing little harder than the other, like 90mv versus 70mv. Had Vgs matched them albeit at low current. Seems my N side was prone... Looking at the board design, notice one gate resistor fed off trace from chip little quicker than other but never thought trace would add much resistance - say maybe 3/4 inch longer path. Both times burned, first was with 75ohm and again with 100ohm. Had built another with 221 ohm and no problems... of course this leads me to believe need higher gate resistance. Question is are the symptoms reflective of that being the culprit? (before send another up in smoke).

Anyway... on the tweaking. Was trying lowest reasonable gate resistance as was trying to get more "air" out of the sonics. Likely trying too many variables at once - bypass caps, comp cap size, irfp240 versus fairchild 19n20c etc... I notice the 1850 ref design uses 22ohm with the toshiba's. Pass F5 uses 47ohm, though different animal.

Can be having one cap onboard for local power supply 'buffering' rather one per fet be a culprit? Board manufacture problem? (maybe jumper with wire the input of the gate resistors to lower the trace resistance that may be there).

Any suggestions on impact of various tweaks?

Lastly... what is the purpose of the resistance between the gates, shown as 12k on the typical application of the lme49830 datasheet then as 2.2k in the an1850 doc.

Pic attached of disconnected smoked channel where can see the destruction from the smoked gate resistors.
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