a case against the j201?

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a case against the j201?

Postby RnFR » 04 Jun 2010, 10:03

i recently found this in the book Solid State Guitar Amplifiers by Teemu Kyttala(available for download on the net here- http://fileshare.eshop.bg/downloadsm/55 ... ttala.html - thanks teemuk! great book! :thumbsup ).


Note: One should avoid using JFETs with low IDS – especially in buffering circuits.
For example, J201 is a commonly used FET - probably because it was once used in
the famous “Till” guitar preamplifier and “FET Preamp Cable”, both designed by
Donald Tillman. However, the gate cutoff voltage VGS (OFF) of a J201 is about the
lowest amongst all depletion mode JFETs and with moderate source resistor values
this device can’t even handle input signals that are greater than few hundred
millivolts peak-to-peak. This FET is a horrible choice for buffers and basically for
common source circuits as well: In an equal circuit, a higher current FET, like J309,
can handle input voltages higher than 1 VPP and even offer slightly greater gain.
Although popular, J201 is really not that marvelous device. Note that Donald Tillman
originally substituted a higher current model with a J201 solely because of improved
noise performance. Let this be a lesson to you: Always base your component selection
principles on circuit theory – not to a fact that a particular component was used in
some famous circuit! It might have worked there – likely it will not work as well in
another application.



now, i know that the j201 has been in widespread use in diy stompboxes for quite a few years now(mostly due to ROG and DIYS), but this paragraph does make me wonder if there are better devices out there for our purposes- especially since i recently found a stash of j309s that i picked up in a sample order ages ago. :wink: i've tried using the 309s in buffers and mu-amps and have had great results so far. considering that there are a number of FETs out there, does anyone have any experience with anything besides the usual j201,2n5457, 2n3819(?) fare? and what characteristics do you look for in a JFET?

BTW- i just have to say again that this is a great book. even if you aren't building a solid state amp, the preamp section has lots of good stuff for the intermediate stompbox builder. thanks again, teemu! great job!!! :thumbsup :thumbsup :thumbsup
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Re: a case against the j201?

Postby bajaman » 04 Jun 2010, 10:06

I like the Toshiba 2SK117 :wink:
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Re: a case against the j201?

Postby RnFR » 04 Jun 2010, 10:30

bajaman wrote:I like the Toshiba 2SK117 :wink:
bajaman

:lol: i knew that! is it just because of tone? or is there some specific device characteristic that you find preferable?
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Re: a case against the j201?

Postby lolbou » 04 Jun 2010, 10:57

RnFR wrote::lol: i knew that!


Bought this reference because of bajaman too... :lol: I'll have a look at the datasheet to check the values mentionned in the quoted article above.

OT: any chance for a classic download link for this book? I can't manage the parted archive ok... Thx
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Re: a case against the j201?

Postby SpencerPedals » 04 Jun 2010, 12:29

It does also state the J201 was used because of improved noise performance, which would seem like a very reasonable discriminating factor. Assuming of course that the difference between the two in that category is appreciable.
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Re: a case against the j201?

Postby mictester » 04 Jun 2010, 13:49

lolbou wrote:OT: any chance for a classic download link for this book? I can't manage the parted archive ok... Thx


I think I have the whole thing as a pdf. I'll check later when I'm at home, and if so, I'll let you know, and I'll put it up on Rapidshare. :thumbsup
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Re: a case against the j201?

Postby RnFR » 04 Jun 2010, 14:11

lolbou wrote:
RnFR wrote::lol: i knew that!


Bought this reference because of bajaman too... :lol: I'll have a look at the datasheet to check the values mentionned in the quoted article above.

OT: any chance for a classic download link for this book? I can't manage the parted archive ok... Thx

once you download all 4 parts and unzip, it comes up in one pdf. no worries! :wink:
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Re: a case against the j201?

Postby bajaman » 05 Jun 2010, 00:46

here is the data sheet for the 2SK117 - low noise - very transparent sounding jfet :wink:
cheers
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Re: a case against the j201?

Postby Duckman » 05 Jun 2010, 02:25

Great download, RnFR! Thanks!
I use 2SK246, very similar to 2SK117
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Re: a case against the j201?

Postby cpm » 05 Jun 2010, 08:52

so the J201 is useful for a lower current, lower voltage, higher gain than most of other FET. That's a good reason for the use it has found among us.
That article is making a point, but its talking about buffers, which has another set of requeriments.

I use the BF245 series, which i can buy locally, and for may ears sounds fairly well.
I dont like how J201 sound in some applications, but sometimes you just need that low Vgs
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Re: a case against the j201?

Postby modman » 07 Jul 2010, 08:50

I experimented with 2N5457 and J201 vs SK117 in the Baja Black Toast, and the latter does bias differently:
viewtopic.php?f=13&t=2752&p=35438&hilit=sk117#p35438
Where DIY schematics dictate a J201, I always keep 2N5457 and -8 ready, often using these instead of J201. No distorion in your AMZ Minibooster...
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Re: a case against the j201?

Postby Silent Fly » 07 Jul 2010, 09:04

bajaman wrote:I like the Toshiba 2SK117 :wink:
bajaman


+1
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Re: a case against the j201?

Postby marin » 12 Aug 2010, 17:10

I recall correctly that ROG guys found out while doing the Fetzer thing that J201 had more gain while 2Nxxxx couldn't make unity gain. Neither could SK117. So horses for courses.
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Re: a case against the j201?

Postby swampnoise » 20 Aug 2010, 12:29

Years ago I started using 2SK246 as a replacement for the 2n5457 if I ran out of them. It turns out the 2SK246 is more consistent in Vp & Idss and plainly sounds better to me than the 2n5457.
I use it a lot for active on board electronics & other applications where high headroom are desirable.
Would love to try out the 2SK117 though!
I believe these lovely toshiba fets are no longer made, so we've got to be quick in stocking up on them.

In my batch of 2n5457 I also have some with gold plated legs. They also have a different logo, although I don't know of which manufacturer they are. These also seem to be more consistent & better than the run of the mill 2n5457. Anyone know more about these?
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Re: a case against the j201?

Postby bajaman » 20 Aug 2010, 13:25

Hello swampnoise :)
I have not tried the 2SK246 yet, but what is it about these Toshiba fets that makes them sound so dynamic and transparent :hmmm:
I realise that if you want high gain at low DC voltages , then you need to use J201 fets, but somehow I always felt they were way too dark and muddy sounding - I guess it has lot to do with their low current and high internal impedance. :?:
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Re: a case against the j201?

Postby swampnoise » 20 Aug 2010, 13:58

I am not sure, but think the audio grade 2SK117 is lower noise than the general purpose 2SK246. Never had a noise problem with the latter though.
You are probably right about low current and high internal impedance of the j201 interfering with transparency.

Always thought it was just the low Vp of a typical J201 compressing the initial pick attack a little too much.
I do like using the j201 however if I need to bring really low signals up a bit, like after a lossy filter or with a very weak pickup signal. It's nice to have the choice though!

It seems that linear integrated systems is remaking some nice obsolete toshiba fets. Probably not without reason!
I don't know how to get hold of them, and they probably will be heavily overpriced?
I sent them an email requesting for a supplier of a certain matched pair they make, but never got an answer..

By no means trying to hijack this thread, but anyone know if there is a suitable p channel complement for the j201?
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Re: a case against the j201?

Postby coldcraft » 20 Aug 2010, 17:45

I'm a big fan of the 2N5457, but only for sonic reasons. I don't hear any distortion as a buffer, and it clips quite nicely in a SSRP.
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Re: a case against the j201?

Postby coldcraft » 20 Aug 2010, 17:54

I found these comments in a Bajaman JFET Buffer thread from RG Keen. The seem to suggest that a low VGS is GOOD for buffers biased to 1/2 Vcc. Am I reading it backwards?

R.G. wrote:
bajaman wrote:ps: if your effect does not have a suitable half rail voltage (4.5v with 9v circuits), then just use two 100k resistors in series, strung between the 9v rail and ground. Then connect the 1M input resistor to the junction of these two resistors and voila - all done :wink:

Some high-output guitars will distort on this because the source of the JFET is going to be 0.5 to as much as 3V above the bias voltage. N-channel JFETs bias with their sources higher than the gate.

Do the same biasing procedure, but put a voltmeter across the source resistor. Tweak the bias resistors to get the source close to half of your supply voltage. It doesn't have to be precise, but you do give yourself maximum headroom if it is.

If you have only single coils, you may not need this. Likewise, it depends on your JFET. The J201 has the lowest Vgs I've ever found, 0.3 to 1.0, so it almost doesn't matter on a J201. The 117 can have up to 5V if I remember right, so if you get a worst-case device, you could be bumping up against the power supply all the time.

That ... might... sound good, as this is what the Dallas Rangemaster does with germanium transistor, but it's not a transparent buffer if it does.

Anyway, something to think about.
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Re: a case against the j201?

Postby swampnoise » 20 Aug 2010, 21:58

Hey coldcraft,

I think what RG means here is that there is simply a smaller variation in pinchoff/cutoff voltage in the low Vgs types than in the higher Vgs types. This makes biasing them less critical, but not necesarily better!

For instance pinchoff in a J201 can vary from 0.3 to 1.5V, a difference of a mere 1.2V. While in a higher Vgs type, lets take a 2n5457 here, pinchoff can vary from 0.5 to 6V, a difference of 5.5 V (that's more than half the 9V powersupply!)

This is why it's less important to worry about the correct bias of say a J201 in a buffer. But that doesn't mean it's better than a correctly biased 2n5457 in the same application!

Remember, the max input headroom of a Jfet will never be higher than twice it's cuttoff voltage..
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Re: a case against the j201?

Postby JohnBlakeArnold » 06 Mar 2014, 02:43

Can't download from original link, so here in entrity is: Teemu Kytala Solid State Guitar Ampilifiers

http://www.johnblakearnold.com/solidstate_guitar_amplifiers_teemu_kyttala_v1.0.pdf

Upfront, my ISP sucks balls, so this simple 415 pg. book in pdf format takes about 16 minutes to load into browser window on typical DSL connection.
However, once loaded in browser can be immediately saved through scrolling to lower right hand side and using the PDF save icon which looks like a disk.
Otherwise, once the page finally loads, right click on page and use "save to"

It is a pain, but the document in its great fullness is there, and it is excellent.

Thanks.

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Dirk_Hendrik (06 Mar 2014, 09:41)
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