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DIY Stompboxes => Building your own stompbox => Topic started by: jellyjams on February 12, 2018, 09:18:05 PM

Title: About those heckin J201s
Post by: jellyjams on February 12, 2018, 09:18:05 PM
So I'm new to the DIY pedal game, and as you know by now it's near impossible to obtain (reasonably priced) J201s without fiddling with SMD and adapter boards, which is unfortunate since I see them in a lot of high-gain FET designs *ehemROGehem*. I've stocked up on more common FETs (5457, 5458, 5486, 5952, MPF102, K30A-GR) I can get from my local electronics stores (I live overseas), but based on the datasheets it seems that nothing comes close. My question is: how do you sub the J201 without compromising stuff such as gain and tone and whatever mojo everyone raves about it?
Title: Re: About those heckin J201s
Post by: Rob Strand on February 12, 2018, 09:51:22 PM
Quote
based on the datasheets it seems that nothing comes close. My question is: how do you sub the J201 without compromising stuff such as gain
They kind of stand alone.

Look at JFETS with low Vgs_off.  You could sort through them using a JFET matcher.
Maybe try these:  2N5484, 2N5485, 2N5457, MPF102.
Title: Re: About those heckin J201s
Post by: jellyjams on February 13, 2018, 04:00:10 PM
I've read up some more and it seems like Idss only really matters to get a tube-like character and the gain merely comes from the source and drain resistors based on that value; is my understanding correct?
Title: Re: About those heckin J201s
Post by: Rob Strand on February 13, 2018, 04:19:19 PM
Quote
source and drain resistors based on that value
The resistors are definitely linked to IDSS but you have some room to play with.
Title: Re: About those heckin J201s
Post by: PRR on February 13, 2018, 05:02:10 PM
> the gain merely comes from the source and drain resistors based on that value; is my understanding correct?

Incomplete. There is resistance "inside the Source". The true gain is lower than the ratio of resistors you see.

Knowing "nothing", you can approximate this by assuming the internal source resistance is "around 1K". However this can be 100 Ohms for a fat FET at high current or 3K for a wee one.
Title: Re: About those heckin J201s
Post by: knutolai on February 13, 2018, 05:09:42 PM
I'd say go for the adapter board solution. I don't really see the problem with it. If you were to buy them through somewhere like SeeedStudio you could get roundabout 100 boards for ~25$. I could draw up a panelized 100x100mm PCB (gerber .zip-file) if the community is interested, though it has probably already been done. Ordering the manufacturing is very easy.

MMBFJ201 is the SOT-23-3 package J201 correct?
Title: Re: About those heckin J201s
Post by: smallbearelec on February 13, 2018, 11:22:17 PM
A JFET that is similar in VGSoff spec to the J201 is this one:

http://www.smallbear-electronics.mybigcommerce.com/fairchild-pf5102/

It is still available in through-hole, and I will do my best to stock up on them before/if they go obsolete.

I have several thousand of the J201 in TO-92, have had to raise the price and not sell bulk. So I will be able to supply small lots for DIY for some years. Going forward, I will put some $ into the SMD version, already have Surfboards for breadboarding:

http://www.smallbear-electronics.mybigcommerce.com/sot-23-to-to-92-surfboards/
Title: Re: About those heckin J201s
Post by: jellyjams on February 14, 2018, 02:33:00 AM
I've just measured all the JFETS I have, and fortunately all my 5457s have |Vp| < 1V. Now what's so special about the tiny tiny Idss of the J201?
Title: Re: About those heckin J201s
Post by: rockola on February 14, 2018, 02:38:02 AM
The SMD J201 is also a bit worrisome, Mouser is showing delivery dates for backorders well into 2019.

Just received a few SMD J201s from Tayda yesterday - they sure are tiny. I may have to give in and get those reading glasses soon.
Title: Re: About those heckin J201s
Post by: alanp on February 14, 2018, 02:45:55 AM
http://www.diyguitarpedals.com.au/shop/index.php?main_page=product_info&cPath=6&products_id=52 (http://www.diyguitarpedals.com.au/shop/index.php?main_page=product_info&cPath=6&products_id=52)

diygp.au sell SMD J201s, plus adaptor boards so you can use them where the PCB expects a through-hole J201 :) (The SMD package version has not been discontinued, IIRC.)
Title: Re: About those heckin J201s
Post by: rankot on February 14, 2018, 06:09:19 AM
I've just measured all the JFETS I have, and fortunately all my 5457s have |Vp| < 1V. Now what's so special about the tiny tiny Idss of the J201?

According to ROG site, Idss it not of big importance for "Fetzer Valve" circuit.
Title: Re: About those heckin J201s
Post by: slashandburn on February 14, 2018, 06:29:21 AM
I had this dilemma a month or so ago, in the end I gambled on some 2n5457 and j201 on ebay from China.  I've yet to use the j201s (I put them in the wrong drawer and only found them again yesterday) but I've been steadily working my way through the bag of 5457's and had no problems. Also all of them made it through the jfet Matcher without any smoke or fire and surpringly a lot of them paired up pretty well. They might well all be fakes but so far they're at least functional fakes.

YMMV but the Chinese sellers on ebay could possibly be an option.
Title: Re: About those heckin J201s
Post by: EBK on February 14, 2018, 08:00:30 AM
Just another friendly reminder that SOT-23 transistors are veroboard compatible.  One example I've found:
(http://i.imgur.com/baVGeoC.jpg)
Title: Re: About those heckin J201s
Post by: jellyjams on February 14, 2018, 08:47:22 AM
According to ROG site, Idss it not of big importance for "Fetzer Valve" circuit.

Oh you're right; I must've gotten lost in the sea of equations ;D So Idss only affects the value of the S/D resistors (while keeping the ratio) and the current draw of the device. Does that mean I have (and only have) to change those resistors to make the 5457 sound like J201, given they have the same Vp?
Title: Re: About those heckin J201s
Post by: reddesert on February 14, 2018, 02:00:21 PM
J201s don't have mojo in particular. They might very well be from a similar silicon process as the 2N5457 and the ones that come out with smaller Vp are labeled J201s. The issue is that with a JFET amplification stage, you have to both bias the drain somewhere near half of Vsupply, and use the ratio of drain/source resistors to get the desired gain and clipping. So you don't have total freedom to select the gain ratio. With a small Vp like the J201, you can get gains of several times, but with the 2N5457, the stage may have a lower gain. There is an example in Section 11 at the bottom of the Fetzer Valve page, http://runoffgroove.com/fetzervalve.html (http://runoffgroove.com/fetzervalve.html).

The problem with finding a sub for the J201 is not that I_dss matters in the Fetzer Valve circuit, but that for a given batch of JFETs, I_dss and V_p are highly correlated. To find a JFET with small values of V_p, it also winds up having a small I_dss. The attached image shows the results of measuring small batches of JFETs with the Runoffgroove FET tester shown on the Fetzer Valve page.

(https://s18.postimg.org/bx0chk9hh/fet_alltypes_props.png) (https://postimg.org/image/bx0chk9hh/)

The J201, 2N5457, and J113 came from Smallbear; the 2N5458 from Tayda; and the "off-J201" were from a US seller on ebay. You can see that the "off-J201" are close, but probably out of spec.
Title: Re: About those heckin J201s
Post by: PRR on February 14, 2018, 04:37:55 PM
> J201, 2N5457, J113 2N5458 "off-J201"

The J113 is a Very Big Chip. That's why it runs big current.

The others could very well be the same size chip, just processed differently (channel thinness). And yes the tail-end of "J201" could be true J201/similar but rejected for high Vp (yet sold anyway).
Title: Re: About those heckin J201s
Post by: Rob Strand on February 14, 2018, 05:23:53 PM
Quote
Oh you're right; I must've gotten lost in the sea of equations ;D So Idss only affects the value of the S/D resistors (while keeping the ratio) and the current draw of the device. Does that mean I have (and only have) to change those resistors to make the 5457 sound like J201, given they have the same Vp?

Assuming all other things are equal, if you double Idss you halve Rd and Rs but it's not really the whole story.

Even J201's vary from unit to unit.  The great majority of DIY circuits using J201's have an adjustable Rd.
The adjustment lets you set the drain voltage, which is one of the more important aspects for the right sound.
The Rd adjustment allows you to tune out most of the variables at once.
[Edit:  However, the result usually has a different gain.]

The point where you get to tweaking Rs is when you realize you can't making everything equal with different JFETs.
If Rs is not bypassed by a cap, and you want the same gain, then you can tweak Rd for the correct gain.
If Rs is bypassed by a cap then very roughly want the DC voltage across Rs as least as big as the circuit you are trying to match.
[Edit: Sometimes you can keep Rd the same and adjust Rs to get the correct the source voltage.  This method can often give a better gain match.]

The easiest way to do it is try a few Rs values,  (accurately) adjust Rd so the DC voltage on the drain is right, then choose the one the sounds the best.

Title: Re: About those heckin J201s
Post by: Rob Strand on February 15, 2018, 01:04:04 AM
Just for fun I put up some results on the effect of variations in J201's.  It shows the effect adjustments in RD and RS has on the preamp behaviour.:
http://www.diystompboxes.com/smfforum/index.php?topic=119707.0
Title: Re: About those heckin J201s
Post by: jellyjams on February 15, 2018, 03:49:21 AM
What exactly does the bypass cap do?
Title: Re: About those heckin J201s
Post by: Rob Strand on February 15, 2018, 04:04:59 AM
Quote
What exactly does the bypass cap do?
If the bypass cap is big then the preamp will have a higher gain than without it.

However, if the cap isn't that big  then the gain at low frequencies will be small low and the gain at high frequencies will be high.  If the frequencies are in the bass-range then it will act kind of like a bass cut.  If the frequencies are in the mid-range it acts like a presence boost, and if the frequencies are high it acts like a treble boost.   The fetzer page has some comments about this.
Title: Re: About those heckin J201s
Post by: thermionix on February 15, 2018, 04:12:58 AM
What exactly does the bypass cap do?

Boosts the gain of frequencies over xxx.

Quote
CE-2 (in progress)

Me too.  Mine's actually done, just waiting on MN3007 and MN3101 to get here, pop 'em in the sockets, set the trimmer, and (hopefully) that's it.  I already have the knobs on, and the battery installed.
Title: Re: About those heckin J201s
Post by: jellyjams on February 15, 2018, 04:18:06 AM
If the bypass cap is big then the preamp will have a higher gain than without it.

However, if the cap isn't that big  then the gain at low frequencies will be small low and the gain at high frequencies will be high.  If the frequencies are in the bass-range then it will act kind of like a bass cut.  If the frequencies are in the mid-range it acts like a presence boost, and if the frequencies are high it acts like a treble boost.   The fetzer page has some comments about this.

I understand that's what it does, but I don't understand how/why it does it.
Title: Re: About those heckin J201s
Post by: reddesert on February 15, 2018, 04:32:41 AM
It's often useful to think about the reactance of a capacitor. This is basically the equivalent resistance to signals of a given frequency: reactance = 1/(2 * pi * f * C), where f is the frequency, C is the capacitance, and the units of reactance are ohms. So the cap behaves like a frequency dependent resistor, low resistance at high frequency.

Putting the bypass cap in parallel with the source resistor gives high frequency signals an easier path to ground. Effectively, it lowers the source resistance at high frequencies. Since the gain is ~ Rdrain/Rsource, that makes the gain higher at higher frequencies.

The bypass cap can often be found in the tube amp gain stages that the JFET amps are modeled after.
Title: Re: About those heckin J201s
Post by: Rob Strand on February 15, 2018, 04:39:39 AM
Quote
I understand that's what it does, but I don't understand how/why it does it.

The simplest explanation for the unbypassed case is the gain is the ratio of the drain resistor to the source resistor,

Av   ~ - Rd   / Rs

However the JFET acts like there is a resistor in series with the source  with value rs  = 1 / gm.  Where gm is called the transconductance.    With this included you get a more accurate expression,

Av  =  - Rd / (Rs + rs)

When you bypass the source resistor this becomes

Av = - Rd / rs

Since the denominator is smaller it means the gain is higher.

When you add the bypass cap is creates a transition between the two gains.   The frequency where the transition occurs depends in the value of the cap and the values of Rs and rs.

It's possible to explain why the gains are what they are a different way.  The input voltage effectively appears across the total source resistance; (rs + Rs) for unbypassed and rs for bypassed.   That means a current  vin / (rs + Rs) flows in the drain resistor.   For a JFET the drain current and the source current are the same value so that means the  source current is also vin/(rs + Rs).   Now observe that the source current flows through the resistor Rd.   The output voltage = Rd * source current.   So that means the output voltage is Vout =  Rd * (Vin / (Rs + rs) ).   The gain is Vout/Vin   = Rd /(Rs + rs).   So that shows where the results above come from.

For more formal reasoning you need to find some PDF's on the gain of common-emitter amplifier with an without bypassed emitter resistors.  Most of this stuff is explained regarding transistors (BJT's)  not JFETs but the ideas are exactly the same.

Most formal stuff will have more maths.   In the above I've started with the least mathematical way to "explain" it.


Title: Re: About those heckin J201s
Post by: Rob Strand on February 15, 2018, 04:42:23 AM
Quote
t's often useful to think about the reactance of a capacitor. This is basically the equivalent resistance to signals of a given frequency: reactance = 1/(2 * pi * f * C), where f is the frequency, C is the capacitance, and the units of reactance are ohms. So the cap behaves like a frequency dependent resistor, low resistance at high frequency.

The point where the gain starts to rise is
    f_low  = 1/(2* pi * Rs * C)
and the point where it starts to level off again is,
     f_hi    = 1/ (2* pi * Rt * C)
where Rt  = Rs in parallel with rs (=1/gm).
Title: Re: About those heckin J201s
Post by: fryingpan on February 15, 2018, 06:50:41 AM
If the bypass cap is big then the preamp will have a higher gain than without it.

However, if the cap isn't that big  then the gain at low frequencies will be small low and the gain at high frequencies will be high.  If the frequencies are in the bass-range then it will act kind of like a bass cut.  If the frequencies are in the mid-range it acts like a presence boost, and if the frequencies are high it acts like a treble boost.   The fetzer page has some comments about this.

I understand that's what it does, but I don't understand how/why it does it.
You know how emitter degeneration stabilises BJTs, but lowers voltage gain? The idea behind the bypass cap is that DC doesn't see it (so it's an emitter degeneration stage for biasing purposes) but AC, according to the size of the cap, might have a direct path to ground (so it's closer to a regular common emitter amp for AC signals, so higher gain). If the cap isn't large enough though, you'll have varying gain within the audio range. My undergrad EE studies basically skip JFETs as they are niche devices (maybe obsolescent?) but I guess the underlying idea is the same.
Title: Re: About those heckin J201s
Post by: thermionix on February 15, 2018, 03:32:56 PM
An unbypassed (cathode/emitter/source) resistor is a form of NFB.