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Old 9th December 2003, 05:05 PM   #1
gustav is offline gustav  Sweden
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Default mosfet oscillation

Hi there,
hoping for some advice.
Have a big amp with +-100V rails and 8 pairs of Hitachi 2SK176/J56 powerfets. None of the usual cures against HF oscillation are totally effective.
With a 4 ohm load it works ok with up to +-70V supply.

Has anyone used these devices with success?
( have used the 135/50 types earlier without problem but with +-60V supply only)
Regards Jonas
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Old 9th December 2003, 05:12 PM   #2
Bricolo is offline Bricolo  France
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can you post the schematic?
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Old 9th December 2003, 05:49 PM   #3
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What type of oscillations do you have, above over a certian current, only positive half period?
/Per-Anders (my first name) or P-A as my friends call me
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Old 9th December 2003, 06:22 PM   #4
gustav is offline gustav  Sweden
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Typical 50MHz oscillation and yes it happens at some current level, maybee 10A or so. The amp is not up and running just now but I think only one half is effected, could well be the positive side.

I have no "digital" schematic but I am quite shure that only the output stage is involved .
There are gateresistors and there is also some RC-load to ground/heatsink (50 ohms ac-coupled)

Thanks for your interest !
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Old 9th December 2003, 08:46 PM   #5
PMA is offline PMA  Europe
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Did you use the resistors in series with gate of FETs? - sorry you have just replied. How is the feedback loop (poles etc.)?

Do you use output coil and Zobel circuitry?
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