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Old 28th July 2013, 05:52 PM   #491
OllBoll is offline OllBoll  Sweden
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Nice, I think I'll do just that =)
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Old 11th January 2014, 03:59 PM   #492
OllBoll is offline OllBoll  Sweden
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I just hooked up an F2J with a Hammond 193V as per post #482. The plan is to compare the sound of the F2J to my L'amps, they are both similar but at the same time different so will be interesting.

The 193V gives a mad efficiency boost to the F2J, my last highest power on the F2J was with 48V rails and then I got ~ 6.5W into 8 ohm. Here I use 24V rails and I get 10W clean with no higher order distortion than 2nd + 3nd into 9.4 ohm, so probably around 12W into 8 ohm. Considering that the vanilla F2J only achieves 5W into 8 ohm I guess big inductors as current sources are pretty nice =)

The current is currently 2.18A and ~ 20.5V drain voltage so about 45W dissipation on the R100 which is ok.
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Old 11th January 2014, 05:58 PM   #493
OllBoll is offline OllBoll  Sweden
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I just did some distortion comparisons on some speakers with the F2J ( 200 ohm in parallel instead of 15.66 so ~ 150 ohm output impedance ) vs L'amp for those who are interested in the L'amp thread.
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Old 13th January 2014, 07:49 PM   #494
OllBoll is offline OllBoll  Sweden
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Hmm... currently the R100 is running at 20V @2.25A => 45W... sounds a bit high or what do you others think? Heat isn't a problem, I can hold my hand on the sink indefinitely but I don't want the jfets to break in the long run =)

I have two matched pairs so I'm considering upping the source resistors to 1 ohm and then do ~ 1.2 - 1.5A on each for ~ 25-30W dissipation on each jfet. My 193V inductor is capped at 3A so I can't go higher than that.

I do have a lot of heatsinking though so I could also double the voltage to 40V at jfet drain and run each device at 35W instead. The question is if I would get power into the right impedance, the speaker is 10-20 ohm with an absolute minimum of 9.5 ohm.

Last edited by OllBoll; 13th January 2014 at 07:53 PM.
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Old 13th January 2014, 07:57 PM   #495
Zen Mod is offline Zen Mod  Serbia
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if it's cooled properly , there is no fuss with 45W
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Old 13th January 2014, 07:58 PM   #496
OllBoll is offline OllBoll  Sweden
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Quote:
Originally Posted by Zen Mod View Post
if it's cooled properly , there is no fuss with 45W
That's good to hear, then I'll probably leave it as it is since it works so well at the moment.
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Old 19th April 2014, 07:33 PM   #497
OllBoll is offline OllBoll  Sweden
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When I re-built two inducted F2Js with the mod for more current I think I had the pot turned in the wrong direction and killed them both on turn-on.

So now I thought I might confirm how enchantment mode should be used to conduct 0 current. Depletion JFETs conduct close to nothing when gate is very negative compared to source, and conducts more the higher V at gate. And if I understand correctly now enchantment mode JFETs conduct nothing if source = gate, and more current the closer gate voltage is to drain voltage?
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Old 20th April 2014, 07:22 PM   #498
OllBoll is offline OllBoll  Sweden
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I think I've found the culprit!

Though the main dumbness was from me =) The biasing of the F2J is of course tailored for ~ 12V drain voltage and I slammed it with 24V so even at lowest setting of pot the gate voltage got too high and the R100 tried to conduct too much current. I copied the arch nemesis bias supply instead which also has that handy zener to bring down the voltage and now it works =)

I did some tinkering first though, didn't want to blow another SS JFET so I chucked in an old IRF240 to ensure that the rest of the amp worked as intended first =)
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