JFET preamp, battling 1/f noise

I should have mentioned, my circuit looks a lot like the one referenced here:

** Beware - Google Groper on the loose

** Yeah.

Use a low noise fet instead of a switching type.

2N5457, 2N5459, 2SK30A etc .....

....... Phil

Thanks for the tips. Would you mind explaining "Google Groper?"

** No problem.

** Been waiting for someone to ask me that.

It's no spelling mistake.

Think of someone " groping about in the dark " and then the other usual meaning of "groping".

Seems a very apt title to give the hordes of moronic, sociopathic, anonymous, trolling Googlegroups posters that have been recently inflicted on usenet and are now *hell bent* on looting and destroying what little of any value was left after the Hotmail onslaught.

BTW

If you desire even the tiniest bit of respect - learn to post correctly for usenet.

Click on "options" and then "reply, quote the previous text and poster's name.

........ Phil

What he _really_ means is that he doesn't want anyone new here, so that USENET will die a slow death from starvation, and moderated, for profit 'forums' will be the only way to find information. He's just being cranky with Google Groups because it's bringing in a lot of newbies and because the broken Google interface gets in the way of making an intelligent looking reply.

But do learn what a real USENET reply should look like and how to coerce Google into doing it for you -- read the link in my tagline for more information.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

I think what he really means is that he hates everyone, and actually welcomes an enlarged population that he can hate.

John

"Tim Wescott"

** Where did you get THAT asinine idea from - Tim ?

Same place the sun permanently don't shine ?

BTW

I do not contribute to any on line "forums" - they are all crap AFAIK.

** It does a LOT worse than that:

It brings in .. " hordes of moronic, sociopathic, anonymous, trolling Googlegroups posters that have been recently inflicted on usenet and are now *hell bent* on looting and destroying what little of any value was left after the Hotmail onslaught. "

** You missed the main, BAD issue with GG posts - there is often no clue as to whom or which post they address.

When a GG post comes along late in a thread, the post appears in a list ( created by OE ) that depends on its time and date of arrival - not its relevance to the post immediately above it.

This is why it is ESSENTIAL for ANYONE quote the prior post - so readers know WHO is being addressed.

It is pig arrogant and damn lazy not to do so.

....... Phil

Thanks for that useful link about posting from Google. I had no idea, as I assumed that all newsreaders organized threads in pretty much the same way as Google Groups.

On an amusing note, fear of newbies ruining the Internet is as old as the Internet itself, if not older. I remember a time when many Internet users seriously believed that any commercially motivated use of the 'net was unacceptable. There were people who would flame anybody with "aol.com" in their e-mail address.

** Get a *real* news reader and news account and join usenet.

See just how wrong you are.

** This is NOT the internet - pal.

This is usenet, a non-commercial, open public forum paid for by its users.

** While some commercial operators are innocuous enough on usenet - most soon become a real MENACE to the openness and original purpose of most news-groups. Particularly in the world of audio. ** Same as you would *expect* folk to heckle someone using a public park or other public speaking arena for commercial gain. It plain stinks.

........ Phil

The BF862 is also a good choice...

--
Uwe Bonnes                bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------

"Uwe Bonnes"

** I looked it up:

It's an RF type in a tiny SMD pack.

Likely give the OP a coronary.

Are you sure it has a low NF at audio frequencies?

........ Phil

I'll wager is does quite well. The combination of 0.8nV with only 10pF is rather nice, much better than a 2N5457 or 2sk30.

--
 Thanks,
    - Win

"Winfield Hill"

** Hypothetical *wagers* made by *wankers* = voodoo science.

The OP simply needs a low noise, audio J-fet he can install in lieu of a TO92 switching type in a musical instrument pre-amp.

The commonly available, TO92 J-fets I listed provide a 1 dB, or better, noise figure in this kind of app.

...... Phil

Summarizing what I think that I have learned so far: The JFET itself is likely to be the only palpable source of 1/f noise, assuming a reasonably designed circuit. For newbies like me (mere physics PhD),

1/f noise is something that's just there empirically, for which a deeper understanding than a simple parametric model is of little practical use.

I found one device, LSK170, with an impressive 1/f corner around 10 Hz. Notably, it is an IC. It appears that JFET op amp chips also have lower

1/f corners than mainstream discrete JFETs. Any ideas as to the reason?

** Notably, it is *not* an IC.

Beware of press releases with purple prose.

** Their noise level at high frequencies is higher.

BTW

Why are you so concerned with this nonsense in an audio band pre-amp ?

Since 95 % of the bandwidth is *above 1kHz* , that is where the audible noise is.

........ Phil

That's a good point. If the 1/f corner is where the 1/f contribution equals the white noise contribution, then the corner could be lowered by raising the white noise, rather than by lowering the 1/f. And as you say below, it's the broadband stuff that contributes to the audible noise.

That's certainly a fair criticism. I am willing to consider the droll little audio problem to be solved, but I would not mind understanding this stuff in a bit more generality, just for the sake of my own education. I do some things with electronics that are not audio related, but which involve signals in the audio band.

** Do a Google on 1/f noise.

Plenty there to be read.

Seems to be some tie in with Chaos theory and fractals .....

BTW

Nice to see you using GG correctly now.

....... Phil

You're correct the JFET is the primary source of 1/f noise, but you're off base in worrying about it as a significant spectral- noise-density issue. Consider analyzing the integrated noise in third-octave bands, and factor in the Fletcher-Munson curves.

The LSK170 is a substitute for Toshiba's 2sk170. Toshiba provides lots of information about the 2sk170, but Linear Integrated Systems provides little detail about their LSK170 replacement. For example, look at Toshiba's plot of voltage-noise vs drain current: the curve goes through 0.9nV at 2mA, which is the only point provided by Linear Systems. After several glances over the years, I haven't seen any advantages for the LSK170 part. BTW, in my opinion, the 2sk170 is an excellent JFET for applications having 20 to 40pF of sensor-node capacitance (it's the old en-Cin-w noise issue). If your capacitance is significantly lower than 15pF another JFET may be a better choice.

--
 Thanks,
    - Win

And a device with 1nV / sqrt Hz is essentially noiseless compared to a guitar pickup !

Graham