An advice on JFET

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
 
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
 
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
 
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
 
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
 
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
 
sorry for interrupting civil and nice conversation here ( I don't wanna Off Topic Mob pollute any other thread than "mine" ) ......... :clown:


regarding vegetable thingie use of J310 ........ I have impression that J310 is sorta 2SK170V , maybe even on steroids .

'till now - Lucky Luke ( AKA steen) give most proper explanation :

http://www.diyaudio.com/forums/showthread.php?postid=1413932#post1413932
 
jupiterjune said:
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


I have 2sj74v (10-20MA) ( which are not as robust as the J174 ) . Still, I think they would be a choice in the MINI-A . If I calculated correctly there should be 15-18MA through Q1.
 
By reputation, the 2SJ74's are a really great jfet. (I don't own any). If you look at the noise voltage graphs on the spec. sheets of the two jfets, you will see the 2SJ74's have much lower (by at least a factor of 10) noise figures.

Even the better Siliconix jfets available from mouser don't come close (if the spec. sheets are to be believed). An Idss of 10 to 20 mA should be fine for a mini-A, and they should give much, much, better performance than the J174's.

JJ
 
sandstorm33 said:



I have 2sj74v (10-20MA) ( which are not as robust as the J174 ) . Still, I think they would be a choice in the MINI-A . If I calculated correctly there should be 15-18MA through Q1.


jupiterjune said:
By reputation, the 2SJ74's are a really great jfet. (I don't own any). If you look at the noise voltage graphs on the spec. sheets of the two jfets, you will see the 2SJ74's have much lower (by at least a factor of 10) noise figures.

Even the better Siliconix jfets available from mouser don't come close (if the spec. sheets are to be believed). An Idss of 10 to 20 mA should be fine for a mini-A, and they should give much, much, better performance than the J174's.

JJ

even if seems that even any sparrow hangin' in Pass forum knows that - I'll repeat , just in case :

Mini A is scaled down dreky Aleph 3 , or 30
Jfet Mini A can be scaled down dreky Aleph J

Aleph J use 2SJ109BL in input LTP

2SJ74BL is exactly half of 2SJ109BL
 
Zen Mod said:





even if seems that even any sparrow hangin' in Pass forum knows that - I'll repeat , just in case :

Mini A is scaled down dreky Aleph 3 , or 30
Jfet Mini A can be scaled down dreky Aleph J

Aleph J use 2SJ109BL in input LTP

2SJ74BL is exactly half of 2SJ109BL


Only have eight of the devices ..seems they should be matched as they would be in the A-J . Doubtful I could get a match from only eight pieces needing two sets. Noone seemed to be interested in matched devices for the MINI-A. Why??
 
That depends on how you define 'operate differently.' They don't really operate differently, but they do bias differently. JFETs are depletion mode devices. MOSFETs are (generally) enhancement mode devices. Once you make allowances for that, there's not really a whole lot more to worry about. A differential is a differential is a differential. Either the current flows through one side or it flows through the other. This doesn't change depending on the device, be it bipolar, JFET, MOSFET, or tube.
Idss means that you hook the Gate and Source together at ground and hook the Drain to the appropriate polarity voltage (+ for N-ch, - for P-ch). Measure the current that flows through the device. This is pretty much full-on for a JFET unless you intend to forward bias the device (I'd rather not, but if you choose that route watch your Gate current and don't push it very far or your distortion will go way up).
Vgs off is at the other end of the spectrum. You're trying to measure when the device more-or-less quits conducting. While it's a useful number to know, most people grade devices by Idss, not pinchoff. Now, if you happen to know both of them, you'll be in a position to better match devices.

Grey