An advice on JFET

wazabee · 25th January 2008, 12:08 PM

Hello,

what do you think about this jfet: Vishay 2n4856a ?
Is it usable in ZEN V4 or jfet BOZ ?

jupiterjune · 26th January 2008, 01:07 AM

Wazabee-
I have used great big switching jfets from Vishay like that in a cascoded BZLS. It sounds amazing! ( I mostly used J105's -- but they are really expensive now).

I particularly like the high Vgs off ratings of such parts -- with a Vgs off of 4.6 or so, you can feed a rull voltage CD standard output signal to the jfets with no attenuation and no feedback.

JJ

jupiterjune · 26th January 2008, 01:15 AM

Another thought--they will probably work better cascoded. I used J176's in my mini-Aleph. It is -1 dB at 20kHZ -- due to gate capacitance I assume.

JJ

GRollins · 26th January 2008, 03:08 AM

The capacitance of that part is quite low...much lower than an IRF9610, for instance. I doubt that your high frequency rolloff is due to the JFET.
However, to get much current through these parts, you'll need to consider cascoding just to keep the heat dissipation down. It may claim 1.8W, but I certainly wouldn't want to try to pull that much heat out of a TO-18 case.

Grey

jupiterjune · 27th January 2008, 12:37 AM

As usual, I can't trust my memory when it comes to numbers. I used J174's in the mini-A, not J176's.

Ignoring good scientific method, I also changed the output devices at the same time that I dropped in the jfets. I changed from IRF634's to IRFP140's (checked that number).

JJ

metalman · 28th January 2008, 05:35 PM

Interesting! Just last week I ordered (in quantity) J174's with the idea of using them as input devices for a modified Aleph 4 I am building, and also for some preamp ideas. The J174 was the only P-channel JFET I could find that was linear, had a high enough transconductance, low enough noise, and most importantly a high enough drain current to drive multiple power MOSFET's.

Good to see that someone else has had the same idea.

Cheers, Terry

jupiterjune · 29th January 2008, 01:09 AM

Metalman-

Let us know how it turns out. I didn't do anything special to make that conversion, I just plopped them in. (actually, now that I think about it, I can't remember if I changed the differential pair current source or not...). At any rate, you might have to use a slightly different value for the resistor (392 ohms) between the differential pair drain and PS rail -- to adjust the DC offset.

JJ

nobody special · 29th January 2008, 03:42 PM

How about the RF JFETs? Is there any reason not to use them? The J309 has good transconductance, and the curves also look pretty good. Of course, it's an N-channel, so no Aleph, unless you want to use P-channel outputs.

jupiterjune · 29th January 2008, 04:21 PM

J308/J309/J310 are the same 'series' (that's the word on the spec. sheet).
There's some preamp named after a large vegetable that uses the J310 -- that's where I first heard of them. I've got a board built up with J310's, but I haven't listened to them yet.

The J309 has lower Vgs off numbers, (and probably lower gate capacitance). You might need to use source degeneration resistors or feedback or input attenuation if you are feeding them a 2.2 V CD output signal.

JJ

nobody special · 29th January 2008, 04:30 PM

Oh... I take it you mean the large round orange variety preamp that seems to be the latest group project? I haven't looked much at it... don't have a lot of time these days.
I'd be interested in hearing your experiences with the J310.

25th January 2008, 12:08 PM	#1
wazabee diyAudio Member Join Date: Mar 2007	An advice on JFET Hello, what do you think about this jfet: Vishay 2n4856a ? Is it usable in ZEN V4 or jfet BOZ ? __________________ l_l_lasabi

29th January 2008, 03:42 PM	#8
nobody special diyAudio Member Join Date: Oct 2002 Location: USA	How about the RF JFETs? Is there any reason not to use them? The J309 has good transconductance, and the curves also look pretty good. Of course, it's an N-channel, so no Aleph, unless you want to use P-channel outputs. __________________ Nobody

29th January 2008, 04:30 PM	#10
nobody special diyAudio Member Join Date: Oct 2002 Location: USA	Oh... I take it you mean the large round orange variety preamp that seems to be the latest group project? I haven't looked much at it... don't have a lot of time these days. I'd be interested in hearing your experiences with the J310. __________________ Nobody

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26th January 2008, 01:07 AM	#2
jupiterjune diyAudio Member Join Date: Nov 2006	Wazabee- I have used great big switching jfets from Vishay like that in a cascoded BZLS. It sounds amazing! ( I mostly used J105's -- but they are really expensive now). I particularly like the high Vgs off ratings of such parts -- with a Vgs off of 4.6 or so, you can feed a rull voltage CD standard output signal to the jfets with no attenuation and no feedback. JJ

26th January 2008, 01:15 AM	#3
jupiterjune diyAudio Member Join Date: Nov 2006	Another thought--they will probably work better cascoded. I used J176's in my mini-Aleph. It is -1 dB at 20kHZ -- due to gate capacitance I assume. JJ

26th January 2008, 03:08 AM	#4
GRollins diyAudio Member Join Date: Feb 2001 Location: Columbia, SC	The capacitance of that part is quite low...much lower than an IRF9610, for instance. I doubt that your high frequency rolloff is due to the JFET. However, to get much current through these parts, you'll need to consider cascoding just to keep the heat dissipation down. It may claim 1.8W, but I certainly wouldn't want to try to pull that much heat out of a TO-18 case. Grey

27th January 2008, 12:37 AM	#5
jupiterjune diyAudio Member Join Date: Nov 2006	As usual, I can't trust my memory when it comes to numbers. I used J174's in the mini-A, not J176's. Ignoring good scientific method, I also changed the output devices at the same time that I dropped in the jfets. I changed from IRF634's to IRFP140's (checked that number). JJ

28th January 2008, 05:35 PM	#6
metalman diyAudio Member Join Date: Jun 2003 Location: Calgary, Alberta	Interesting! Just last week I ordered (in quantity) J174's with the idea of using them as input devices for a modified Aleph 4 I am building, and also for some preamp ideas. The J174 was the only P-channel JFET I could find that was linear, had a high enough transconductance, low enough noise, and most importantly a high enough drain current to drive multiple power MOSFET's. Good to see that someone else has had the same idea. Cheers, Terry

29th January 2008, 01:09 AM	#7
jupiterjune diyAudio Member Join Date: Nov 2006	Metalman- Let us know how it turns out. I didn't do anything special to make that conversion, I just plopped them in. (actually, now that I think about it, I can't remember if I changed the differential pair current source or not...). At any rate, you might have to use a slightly different value for the resistor (392 ohms) between the differential pair drain and PS rail -- to adjust the DC offset. JJ

29th January 2008, 04:21 PM	#9
jupiterjune diyAudio Member Join Date: Nov 2006	J308/J309/J310 are the same 'series' (that's the word on the spec. sheet). There's some preamp named after a large vegetable that uses the J310 -- that's where I first heard of them. I've got a board built up with J310's, but I haven't listened to them yet. The J309 has lower Vgs off numbers, (and probably lower gate capacitance). You might need to use source degeneration resistors or feedback or input attenuation if you are feeding them a 2.2 V CD output signal. JJ

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