What's wrong with those MOSFET's

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Bensen

Bensen

Member
Joined 2003
Hi all,

I,ve just build two Crescendo amps using the 2sk1530 and 2sj201 Toshiba FET's as output devices. Now I was planning to parallel the output FET's.
So, I started to measure the VGS on the normal way to match output devices. I used a resistor of 37ohm and a supply voltage of 15V.
Normally the VGS should be around 4V if I'm not mistaking. But I measured on both the 2sj201 devices, approx 1.85Vdc?? BTW, after one hour it still was 1.85V (0.35A).

Those FET's just came out off perfectly working Crescendo amps.

So, wat the hell is wrong with the FET's?
Do I have to buy new one's?

Cheers

Ben
 

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jan.didden

jan.didden

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linearaudio.net
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Bensen said:
[snip]
So, I started to measure the VGS on the normal way to match output devices. I used a resistor of 37ohm and a supply voltage of 15V.
Normally the VGS should be around 4V if I'm not mistaking. [snip]Ben
Click to expand...

Maybe you are mistaken? Maybe it is normal that they carry a few 100mA with a Vgs of less than 2V? Did you check?

Jan Didden
 
Bensen

Bensen

Member
Joined 2003
Thanks guys,

I think I've found it. When I take a look at the graphs in the datasheet off the 2sj201 I found the attached graph. But I'm not shure if this graph can be used in this case.
But, anyway...

I'm glad the FET's are OK.

One more question. Am I wright when I say that it's best to match the FET's at a current the same as the quiescent current used in the amp?

Ben
 

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UrSv

UrSv

Member
Joined 2002
janneman, it does take a little bit of experience with components to know that "all FETs are not created equal" as the Bible says. Until some familiarity is ganined then some help is not bad. I just happened to know them fairly well since I have a box full and Bensen already contacted me about buying some. Secondly I think that on a forum where the Pass products come in huge numbers and all of them use the same type FETs as well as people listing their Vgs values almost every day it may very well be an easy misconception to expect 4 V everywhere...
 
jan.didden

jan.didden

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Joined 2002
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Ben,

OK, so maybe I overreacted. But "I dit take a look at the datasheets. But I never printed out more than the first page off it. All those graphs are a little strange for me. I'm just out off shool, and I studied Electromechanics and not Electronics.", and you're going to build a couple of Crescendo's? I admire your courage. Good luck (I mean that).

Jan Didden
 
jan.didden

jan.didden

AX tech editor
Joined 2002
linearaudio.net
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