Non-Phool jellybean audio-frequency JFET

[Ecnerwal]

Trying to pick a JFET that's still being made with reasonable low noise
characteristics for simple audio preamp type duties. No golden-ear BS,
just things that can be measured. Mic preamps, instrument pickup
preamps, a FET to have in the junkbox for unknown things yet to be
cobbled in the small-signal audio range. Cheap is also good. Jeorg cheap
would be even better ;-) Old-fashioned packaging would be nice, but its
unlikely these days, I think.

Mouser has one item that comes up with Audio JFET (that's not a jfet
input something else...) Audio FET gets a few more, but most are class D
power devices.

Toshiba 2SK880 in "irritatingly tiny" package. Par for the course these
days and I have adapted to soldering irritatingly tiny if I have no
other choice in packaging. 43 cents for 1, $29.50 for 100 Looks to be 5
years old judging by the datasheet date.

One that is mentioned in some older web circuits that's still marginally
available (in the 150% larger SOT23 only) is the J201, which seems to
have somewhat worse noise numbers. 23 cents for 1, $21.90 for 100, and
all of its relatives in other packages are already obsolete, so it may
not be long for this world, either?

Digi-key's search is as usual near useless (or it and I search
differently), and Newark comes up with a bunch of class-D power fets
that probably won't like non-switching use (If I have even a vaguely
correct recollection of what "class D audio amp" means. Looks like I do
per 5 seconds of checking my memory)

There are of course lots of RF parts that have no specs below 100Khz, or
1 Mhz, or 1 Ghz, depending on part. Perhaps some of them work fine for
audio. Anyone care to clue me in?

 

[amdx]

BF862s, really. They're the cat's pajamas.

Cheers

Phil Hobbs

Just because I was curious I searched BF862 preamp and found a
high performance phono stage that uses 8 parallel BF862s.

http://www.synaesthesia.ca/LNschematics.html

3/4 downpage

I'm clueless as to whether it is a good circuit.

Mikek

 

[amdx]

BF862s, really. They're the cat's pajamas.

Cheers

Phil Hobbs

Hey Phil,
Do you have a compliment to the BF862?
Mikek

 

[amdx]

Hey Phil,
Do you have a compliment to the BF862?
Mikek

Ok, ok, Hello BF862, you've got some really nice parameters there.
Do you have a complement for the BF862?
Mikek

 

[amdx]

Don't I wish. Try a BF862 in an inverted cascode with some nice quiet
PNP transistor, e.g. a 2N5087 at low frequency or a BFT92 at high
frequency.

Cheers

Phil Hobbs

Ya, most of this is over my head, I was just thinking about
replacements for an AM radio antenna preamp circuit I know of.
Is this a physics problem building a P type to match an N type?
Thanks, Mikek

 

[Ecnerwal]

They fit the mold of what I was thinking might be the case - an "RF"
part with application elsewhere. I had actually found Phil's earlier
postings praising them (and verified that they were still available)
before I posted, but was not at all clear after looking at a datasheet
that cut off frequency specs at 0.1 Mhz on the low end if they'd work
reasonably for audio, given that they were being touted for RF in car
radios and a lot of what Phil does goes up there a ways.

Quite a price variance between digikey/newark/mouser, with mouser
winning strongly (27 cents for 1, 23.3 for 100.)

Given a lack of directly comparable graphs I have a hard time telling if
the slightly more expensive (43/29.9) and smaller (smaller not being an
advantage for me as a one-off tinkerer) 2SK880 has a _slightly_ lower
corner frequency or not really, but I suspect it's not enough of a
difference to actually matter (not going phoolish), given that I'm
mostly looking for a reasonable part to have on hand and play with in
the same way I have a pile of 3904/6, LM833, etc.

Now to see how I do with Larkin's x-acto/copper foil prototyping
technique, since I can't see getting boards made for every iteration of
goofing around I want to do, and these things are seriously small...

 

[Phil Allison]

"Ecnerwal"

Trying to pick a JFET that's still being made with reasonable low noise
characteristics for simple audio preamp type duties.

** JFETs have many desirable properties and one HUGE drawback.

The sample to sample parameter spread is massive - so much so that it is
normal to select devices for a given circuit so that bias / operating point
conditions will be met.

If you need diff pairs with low input offsets - then be prepared to waste
a lot of FETS.

FET input op-amps and matched FETs on a chip are the way to go.



..... Phil