when/where would you use a 2n5457 instead of a J201 ?

[Derringer]

I have both, some mpf102's as well

I've used J201's all over the place and 2N5457's when a schematic calls for one.
I've played with 2n5457's in some of my own designs, but they've always sounded weak to me ... and that's probably because I'm just used to what J201's do.
But, I don't know why a design would call for one over another.
Educate me please.


Thanks,

Bill

[CynicalMan]

2N5457s have a lower Vp and a higher Idss than J201s. MPF102s have lower Vp and higher Idss still. I'm not going to go into the whole electronics lecture about what each of these do, but for us that means that they require a lower Vgs to turn off for switching, in a common-source setup they have lower gains than J201s, and they make slightly better buffers. I'm sure one of the EEs here could tell you volumes about the stuff I've glossed over, but that's the difference between them AFAIK.

This is good reading:
runoffgroove.com/fetzervalve.html

[edvard]

Have you tried swapping in J201's in the 2N5457 circuits?
Not everybody is looking for more gain in FET circuits.
Could be the lower gain figure makes for a different tone, or could even be the circuit is an older one.
Back in circa '98 to 2000 when all I could find was Jaime Heilman's archive, 2N5457's were gold.
I've only noticed J201's being called for fairly recently.

[R.G.]

J201s are a very special purpose JFET. Their very low Vgsoff and high transconductance make them high gain devices. They have a quite low Idss, so they're of little use for switching any significant amount of current. They are, in a word, high gain, low current amplifiers.

If you need a wider Vgsoff, like in many phasers, or need them to be a linear voltage controlled resistor for somewhat larger analog signals, they're going to have problems. They work best with small input signals.

And they can't switch all that well or all that much current. But sometimes it's enough for guitar signals, which are in general small compared to line level and up.

For good linear voltage controlled resistors, you want a large Vgsoff. We're limited in the effects biz by needing to use (generally) 9V or less for a power supply. Contracting a whole welter of considerations into one statement, JFETs with Vgsoff of about 3-6V max work best in 9V pedals as variable resistors and switches.  But for amplifying small signals, the J201 works well.

This is why the 2SK30, 2N5292, 2N5485, 2SK117, and a few others keep showing up in pedals as signal switches and variable resistors - their Vgsoff is about 3-6V max, and their Idss is big enough.

Unfortunately, the number of considerations to be made for coming to these broad generalities is large.

[Derringer]

Unfortunately, the number of considerations to be made for coming to these broad generalities is large.

understood

I'm making an analogy in my head right now that J201:12AX7 as 2N5457:12AU7
the J201 gives more voltage gain which makes the fet better suited for amplification and overdriving ... similar to a 12AX7
whereas something like the 2N5457 will have relatively higher current output, less of a voltage swing, which makes it better suited to being a buffer ... ala a 12AU7

ok, I need to look up and study exactly what Idss, Vgsoff, and Vp stand for and implicate though
I think once I read through the Fetzer article a few more times I'll be a bit more enlightened.

as always, thanks folks

[JDoyle]

The difference between the two is in the sorting - they both come from the same die/pattern and are then sorted. Those that fit the specs of a J201, become J201s, those that fit the specs of 2N5457, become 2N5457s. Those that don't fit either specs will become 2N5458s, 2N5459s, 2N4338s, J202s, J204s, etc., depending on which specs they DO fit.

[R.G.]

The difference between the two is in the sorting - they both come from the same die/pattern and are then sorted. Those that fit the specs of a J201, become J201s, those that fit the specs of 2N5457, become 2N5457s. Those that don't fit either specs will become 2N5458s, 2N5459s, 2N4338s, J202s, J204s, etc., depending on which specs they DO fit.

That's pretty close. They can get closer than that. They make a wafer and dice it up into several hundred transistors.These they sort into J201s, J202s, J203s and probably junk anything else.There are differences in the length, width, and doping profiles of the channel and gate regions that make a long-channel device different from a short channel device, and a heavily-doped, high conductance channel different from a lightly doped, high resistance device.

But they do sort heavily within wafers and get different proportions of family member A, B, C, D, ...