mosfet oscillation

gustav · 9th December 2003, 05:05 PM

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

Bricolo · 9th December 2003, 05:12 PM

can you post the schematic?

peranders · 9th December 2003, 05:49 PM

What type of oscillations do you have, above over a certian current, only positive half period?

gustav · 9th December 2003, 06:22 PM

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 !

PMA · 9th December 2003, 08:46 PM

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?

9th December 2003, 05:05 PM	#1
gustav diyAudio Member Join Date: Sep 2003 Location: stockholm	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

9th December 2003, 05:49 PM	#3
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	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 Tube Buffered Gainclone in work \|Thread

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9th December 2003, 05:12 PM	#2
Bricolo diyAudio Member Join Date: Nov 2002 Location: Grenoble, FR	can you post the schematic?

9th December 2003, 06:22 PM	#4
gustav diyAudio Member Join Date: Sep 2003 Location: stockholm	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 !

9th December 2003, 08:46 PM	#5
PMA diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	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?