SS 120R085 Depletion Mode Jfet - Page 25 - diyAudio
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Old 14th July 2012, 04:15 AM   #241
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Well, the relavince to Micheals 1st article and SITs in general is very much the same. If you do the load line thing thru the R085 graph buzzforb made, you see a nice low distortion loadline! I extrapolated a little bit and set an operating point at 2.5A and 16V. I'm not sure 8 ohms dosent look the best. Just playin with graphs though
Hmmm
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Old 14th July 2012, 12:31 PM   #242
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FLG,
Here is a link to a negative voltage generator Nelson suggested.
RO85 Zen
I may try to test the fets lie MR suggested in the article to see if we can get the voltages up a little.
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Old 16th July 2012, 12:00 PM   #243
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Got mine up and running yesterday. Sounded good enough that i want to cean everything up and get test rig worked out. Iam finishing up the GB, so it is goingt o haev to go on the back burner for a couple of weeks while i get everything sorted.
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Old 16th July 2012, 01:27 PM   #244
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Well, I played with your graph quite a bit and settled on an operating point about 15V and 3A That's 45W dissapation!
Maybe go for a 5-6 ohm loadline for starters? Very similar to what MR did with the SITs
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Old 16th July 2012, 01:54 PM   #245
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what's wrong with 45W diss ?


Babelfish J is burnin' 50W in each IRFP150N with ease and only thing sweating is my forehead

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Old 16th July 2012, 02:45 PM   #246
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The IRFP150N is rated at 160W max dissapation. The R085 114W max dissapation. You are running about 31% of max dissapation. My intial q point would be almost 40% of max dissapation. The max Thetaj for the 150 is .95C/W the R085 1.1C/W
Your sweating
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Old 16th July 2012, 03:49 PM   #247
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Based on Nelson's articles, I think 45W is ok. Mine seemed Ok, even if on oversized heatsink. Nelson often speaks of the tendency of Toshiba parts to be able to go beyond specs. I assuming the same here. PLus, I have a nice and quiet desktop fan that can do amazing things to hot amps. What is your rail voltage? I plan on presenting both SE and SRPP version with measurents to boot, as I tihnk this is going to be worthwhile. THe SE version sounded nice to my ears.
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Old 16th July 2012, 05:27 PM   #248
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Of coarse, the V+ will depend on the speaker and Rl (the effective load) of the gain device. With a 8 ohm speaker and a 14 ohm load R, I would see about a 5 ohm LL with 57V V+. deffinatly something that needs to be experimented and played around with a little...
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Old 17th July 2012, 01:56 AM   #249
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Hey Buzz. Why not do some destructive testing. It would be very informative. Try running ond at 100W continuos and see how long it lasts for. Make sure you also record the temperature
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Old 17th July 2012, 02:13 AM   #250
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30$ per fet gets expensive, although i do Sh^t that much out on occasions
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