I'm building a booster with a J201.
Usual thing - Rd of 10k ohms, Rs 10k trimpot (to get Vd of Vsupply/2). Gate grounded by 1 M ohm resistor.
It biased nicely, with the source resistance of just 1k ohms (plenty of gain). That's the FABULOUS FRIEND part.
But the voltage at the source is just 0.25V - which leads me to believe it will clip with just -0.25 V p-p of input. That's the SNEAKY FOE part (at least for me, looking for some clean solid state boost going into the valve preamp.
Maybe this is an old topic? Maybe it's too obvious to be worthy of a comment? Maybe I haven't plugged it in yet and realised that it's barely noticeable? (yes to that one).
Cheers and have a good day.
Any comments welcome.
ps I note that JFETs that "turn on" with the source at 1 to 2 volts (2N5854) only have half the transconductance of the J201. I assume that means the gain is less in a similar configuration (at least until the source resistor is AC bypassed with a capacitor).
If you are using it as booster and not buffer then you will take output from Drain, not Source, right? What is the voltage at D? That is your bias point. I think...
Vsupply =6 V and Vdrain =3.4 V
About 0.25 mA of current.
The biasing resistance on the source is about 1k ohms.
(Vsupply is low because it's the 6 V AC supply for the valve heaters, rectified to DC. However, the low Vgs-on isn't affected much by Vsupply ).
So looks good. Should sound good. Your signal output is floating close to 1/2 supply. Problem?
Quote from: brett on March 06, 2023, 07:45:13 PM
But the voltage at the source is just 0.25V - which leads me to believe it will clip with just -0.25 V p-p of input. That's the SNEAKY FOE part (at least for me, looking for some clean solid state boost going into the valve preamp.
Can I deduce that the Vp of your fet is .5 V?
In that case, any inputsignal higher than .5V wil get clipped. If your guitar has high output pickups, chances are you will get some inputdistortion because of it.
A fet with a higher Vp can solve this. For this, I would go with a 2n5854 and a Vp of say 1V (input voltage swing just got doubled to .5 V p - p)
But you will have to lose on the voltagegain part of your booster - as you mentioned already.
QuoteCan I deduce that the Vp of your fet is .5 V?
I'm getting Idss=0.6mA, VP=0.72V ; virtually typ. values
ID = (6 - 3.4) / 10k = 260uA
IS = 0.25 / 1k = 250uA ; cross-checks ID
So ID = IS ~ 255uA
Calcs with above mentioned params:
ID = 0.6mA (1 - 0.25/0.72)^2 = 256uA ; ignoring channel modulation
The Pros and Cons of the J201 JFET:
The J201 is actually far from an ideal JFET for a lot of designs but it does have the benefit of having, by comparison with many other JFETs, an exceptionally low pinch off voltage, Vgs(off), which makes it ideal for use in circuits running from low power supply voltages such as the 9v supply used in guitar pedals. But it does come at a cost. By having such a low pinch off voltage, it means that it can only handle low input signal levels before it starts to distort. Of course this no good for linear amplifier circuits but can be exploited by those guitar pedal builders who deliberately want to distort the signal. If it is used in buffers such as source followers then the low pinch off voltage means that the input requires biasing correctly to provide sufficient dynamic range. The low pinch off voltage and its high on resistance or low saturation current also means that it is not really suitable for applications where it is used as a voltage controlled resistance such as a phaser or it is used as an analogue switch in the signal path. Often the J201 is cited as a high gain device but in reality there are many other JFETs with superior performance.
In Teemu Kyttälä 's book, "Solid State Guitar Amplifiers", he discusses the issues with use of the J201 in many types of guitar pedal circuits and amplifiers.
The following is an extract from Teemu Kyttälä 's book "Solid State Guitar Amplifiers" that discusses the issues with use of the J201 in many types of guitar pedal circuits.
Solid-State Guitar Amplifiers
Teemu Kyttälä
1st Edition 2008
Link to PDF of book:
https://www.thatraymond.com/downloads/solidstate_guitar_amplifiers_teemu_kyttala_v1.0.pdf
The problems with using devices like J201s:
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 the J201 is really not that marvellous a device. Note that Donald Tillman originally substituted a higher current model with a J201 solely because of improved noise performance. 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.
The following is an extract from Donald Tillman's article that popularised the J201 amongst guitar pedal builders.
A Discrete FET Guitar Preamp
J. Donald Tillman
6 May 2001, updated June 2005
Link to article:
http://www.till.com/articles/GuitarPreamp/
Link to additional 'FET Preamp Cable article:
http://www.till.com/articles/PreampCable/
Technical details:
It's a simple unassuming common source FET stage that looks and performs somewhat like the first 12AX7 stage in a Fender preamp. Q1 is a Siliconix J201 N-channel JFET. You can get a copy of the data sheet from the Siliconix web site. My original design used a Motorola 2N5457 N-channel JFET, which also works well, but the J201 is a lower noise device.
A note about FETs:
I should point out that FETs in general suffer from a serious lack of manufacturing consistancy. The FET VGS and IDSS, the parameters that determine the bias point, can be anywhere over a 5-to-1 range and still be within spec. That's pretty awful, but such is life. It's an engineering accomplishment to design a circuit that can function exactly the same over a wide variation of component parameters. But I can't guarantee that in this situation; there's not enough supply voltage headroom.
The upshot is that while this preamp circuit is designed to work with typical J201 FETs, it will not work with all of them and it would be a good idea to try a handful of FETs and throw out any that don't bias correctly. An easy test is to measure the voltage at the drain of Q1 and if it's between 5.0 and 7.0 volts things are fine.
(https://i.postimg.cc/mcpC5tvr/Tillman-Preamp.png) (https://postimg.cc/mcpC5tvr)
Quote from: Rob Strand on March 07, 2023, 03:01:29 AM
I'm getting Idss=0.6mA, VP=0.72V ; virtually typ. values
With those values of Idss and Vgs(off), then it looks like a drain resistor of 15k and a source resistor of about 680 ohms would bias it to a sensible point.
(https://i.postimg.cc/gwj2Y6HF/J201-Bias.png) (https://postimg.cc/gwj2Y6HF)
Is that a screenshot from an online calculator or an installed program FSFX? I'm not familiar with that interface or any jfet calculator that outputs that much data.
Quote from: cspar on March 07, 2023, 07:56:36 AM
Is that a screenshot from an online calculator or an installed program FSFX? I'm not familiar with that interface or any jfet calculator that outputs that much data.
It is a brilliant JFET bias calculator spreadsheet by Frank_NH, one of the members of this forum.
I did a few minor changes to it and loaded it up on my website for people to use.
You can download it here: www.fleetingspider.com/files/JFET_Calc.xlsx
You can use voltage divider bias to get a higher gate and source voltage, to solve the problem of clipping when the input signal goes too high.
6 volts supply really gives very little headroom for a clean booster. Maybe you can use a voltage doubler or tripler with your 6 volt AC supply?
Thanks everyone.
I note that some people thought the Gate was at -5 V or -0.6 V.
That would be good, but it was at 0.25 V. I "squeezed" it a little further and got 0.27 V without throwing Vd too far away from the often-recommended Vs/2. A possible advantage of this is the associated "cold/cutoff" bias, which to my ears gives more pleasant distortion than "hot/saturation" bias. (I may change my mind about that in the future :icon_wink:).
I got good gain (unmeasured as yet, but approx 20x) with a mild source resistor bypass (680 ohm resistor and 22uF cap). Output goes through a 50kA 'gain' pot and into a 12AX7 triode.
There's mild distortion with the guitar volume cranked. Quite pleasant, to my surprise. And close to clean with the guitar volume at 8 to 9. With the output maxxed, it's capable of driving the 12AX7 triode quite hard, with somewhere between the tasteful Fender blackface breakup and a raucous Marshall 18 watter. The 12AX7 is set up very much like a Fender blackface but lower supply voltage (300V supply, 100k plate resistor, 1.5 k cathode bias resistor).
Overall, I've quite happy.
QuoteThat would be good, but it was at 0.25 V. I "squeezed" it a little further and got 0.27 V without throwing Vd too far away from the often-recommended Vs/2.
The 0.6V is the JFET parameter VP and in-circuit the JFET operates at a gate-source voltage less than that, like 0.25V.
If you measure across the gate and source the meter loading can drop the measurement by a factor of 1/2 for a 1M ohm input DMM. However if you measure the source voltage and the meter loading has (virtually) no effect.
+------------------------+--- +9v
| |
< |
< 6.8k |
< |
| 5uF |
+----||----+--- out |
| | |
|-+ | |
in ---+----->| | |
| |-+ | |
| | | |
< 10M < 2.2k < 51k ---
< < < --- 10uF
< < < |
| | | |
+--------+----------+-------------+--- gnd
The FET is a 2N5457. The voltage gain is subtle, 3dB or so.
The original Till boost.
regards, Jack
Jack, with respect, I had already posted details of the Till in my comment above.
For those interested in the original Till boost, here are the links.
A Discrete FET Guitar Preamp by J. Donald Tillman, 6 May 2001, updated June 2005
http://www.till.com/articles/GuitarPreamp/
Link to additional 'FET Preamp Cable article:
http://www.till.com/articles/PreampCable/
Quote from: FSFX on March 07, 2023, 03:39:56 AM
The following is an extract from Donald Tillman's article that popularised the J201 amongst guitar pedal builders.
A Discrete FET Guitar Preamp
J. Donald Tillman
6 May 2001, updated June 2005
Link to article:
http://www.till.com/articles/GuitarPreamp/
Link to additional 'FET Preamp Cable article:
http://www.till.com/articles/PreampCable/
Technical details:
It's a simple unassuming common source FET stage that looks and performs somewhat like the first 12AX7 stage in a Fender preamp. Q1 is a Siliconix J201 N-channel JFET. You can get a copy of the data sheet from the Siliconix web site. My original design used a Motorola 2N5457 N-channel JFET, which also works well, but the J201 is a lower noise device.
My NatSemi Fet databook says that the 2N5457 was manufactured through a process very similar to that of the J201, just with a larger wafer geometry. For both devices a typical noise voltage of 10nV/Hz is given. These devices were marketed as audio devices, but they are not particularly low noise. Interestingly, the J113, sold as switch, is 6nV/Hz, and the BF245, RF device, is 8nV/hz.
So there isn't much difference in noise performance between these Jfets. I suspect, but that's a bit cynical maybe, Jfets were marketed as 'general pupose or audio' if they were not good for anything else..
Quote from: Clint Eastwood on March 12, 2023, 05:35:33 AM
My NatSemi Fet databook says that the 2N5457 was manufactured through a process very similar to that of the J201 . . .
The differences in JFET geometry, construction and application are fully described in the Fairchild Application Note. AN-6609.
https://www.onsemi.com/pub/collateral/an-6609.pdf
According to ON Semi, previously Fairchild, here are the different internal geometries of the J201, 2N5457 and J111 to J113.
The larger area (and more 'fingers') obviously give higher Idss and lower Ron for the different JFETs.
(https://i.postimg.cc/xJn4p6Ch/JFET-comparison.png) (https://postimg.cc/xJn4p6Ch)
Quote from: Clint Eastwood on March 12, 2023, 05:35:33 AM
Quote from: FSFX on March 07, 2023, 03:39:56 AM
The following is an extract from Donald Tillman's article that popularised the J201 amongst guitar pedal builders.
A Discrete FET Guitar Preamp
J. Donald Tillman
6 May 2001, updated June 2005
Link to article:
http://www.till.com/articles/GuitarPreamp/
Link to additional 'FET Preamp Cable article:
http://www.till.com/articles/PreampCable/
Technical details:
It's a simple unassuming common source FET stage that looks and performs somewhat like the first 12AX7 stage in a Fender preamp. Q1 is a Siliconix J201 N-channel JFET. You can get a copy of the data sheet from the Siliconix web site. My original design used a Motorola 2N5457 N-channel JFET, which also works well, but the J201 is a lower noise device.
My NatSemi Fet databook says that the 2N5457 was manufactured through a process very similar to that of the J201, just with a larger wafer geometry. For both devices a typical noise voltage of 10nV/Hz is given. These devices were marketed as audio devices, but they are not particularly low noise. Interestingly, the J113, sold as switch, is 6nV/Hz, and the BF245, RF device, is 8nV/hz.
So there isn't much difference in noise performance between these Jfets. I suspect, but that's a bit cynical maybe, Jfets were marketed as 'general pupose or audio' if they were not good for anything else..
If tubes would not have the high Usupply/filament issues, jFETs would be called utter crap.
Or am I very obnoxious now? :icon_neutral:
Still, it is not hard to have a low to medium gain BJT stage with a high input impedance and comparable harmonic spectrum. You just need some extra components.
Quote from: Steben on March 12, 2023, 06:58:22 AM
If tubes would not have the high Usupply/filament issues, jFETs would be called utter crap.
I don't think it is just a matter of supply voltage as many JFETs work so much better at 24 to 30 volts anyway.
The only reason that the J201 is popular is that it runs OK on 9v supply.
Tubes will usually need output transformers.
The J201 has a greatly reduced output swing on the Tillman preamp because the drain voltage biases high.
(No problem with the 2N5457, the original part used, despite what the website now shows.)
Here's a J201 version which performs very much like the 2N5457 version.
(https://i.postimg.cc/9rvydJ9y/J201-Tillman-Preamp-sch.png) (https://postimg.cc/9rvydJ9y)
(https://i.postimg.cc/GBjY1R5j/J201-Tillman-Preamp-waveform-1-V-in.png) (https://postimg.cc/GBjY1R5j)
(https://i.postimg.cc/0rjShLdp/J201-Tillman-Preamp-waveform-2-V-in.png) (https://postimg.cc/0rjShLdp)
If I scaled the output 51k load the match is even closer.
Quote from: FSFX on March 07, 2023, 03:39:56 AM
The following is an extract from Donald Tillman's article that popularised the J201 amongst guitar pedal builders.
Do a search in the "First Stompbox Forum" archive and it will show that the J201 was in widespread use on this forum (at its previous site) several years before Don decided to change out the 2N5457 for the J201. I would contend that the J201 popularity came from here instead of his revised article that came along a few years later. :)
Quote from: Rob Strand on March 12, 2023, 07:09:58 AM
The J201 has a greatly reduced output swing on the Tillman preamp because the drain voltage biases high.
(No problem with the 2N5457, the original part used, despite what the website now shows.)
As I recall, at the time the 2N5457 was quite popular but the available devices began to vary all over their spec range and made it difficult for beginning builders to get the best performance from their circuits. The J201s were more consistent, had low noise and gave a bit more boost for what was being built here at the time.
There are still some TO92 jfets available on Mouser at fair prices and anyone trying this circuit could test it with the J113 or the J112 and see what kind of results those fets give.
Best regards, Jack
Quote from: amz-fx on March 12, 2023, 10:19:49 AM
Quote from: FSFX on March 07, 2023, 03:39:56 AM
The following is an extract from Donald Tillman's article that popularised the J201 amongst guitar pedal builders.
Do a search in the "First Stompbox Forum" archive and it will show that the J201 was in widespread use on this forum (at its previous site) several years before Don decided to change out the 2N5457 for the J201. I would contend that the J201 popularity came from here instead of his revised article that came along a few years later. :)
I was really just rephrasing what Teemu Kyttälä said in his book
"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."
Quote from: amz-fx on March 12, 2023, 10:19:49 AM
The J201s were more consistent, had low noise and gave a bit more boost for what was being built here at the time.
There are still some TO92 jfets available on Mouser at fair prices and anyone trying this circuit could test it with the J113 or the J112 and see what kind of results those fets give.
Best regards, Jack
I can confirm that the J113 works very well in this and many other guitar related circuits. They are pretty consistent, have lower noise than J201 (not that I noticed the difference) and a relatively high amplification factor. I have absolutely no beef with the J201, but
do wonder where the myth comes from that it is low noise ???
https://www.interfet.com/jfet-datasheets/jfet-j201-j202-interfet.pdf
https://pdf1.alldatasheet.com/datasheet-pdf/view/121328/VISHAY/J201.html
Quote from: Clint Eastwood on March 12, 2023, 11:28:23 AM
I have absolutely no beef with the J201, but
do wonder where the myth comes from that it is low noise ???
2 possible sources linked.
Quote from: Clint Eastwood on March 12, 2023, 11:28:23 AM...no beef with the J201, but
do wonder where the myth comes from that it is low noise ???
If your source impedance is over about 3kOhms, source self-hiss will exceed J201 hiss. By 5k or 10k, any clean FET you can find (say post-1970) will be "low-noise" relative to the source.
This means E-guitar, MM phono, most tape heads, any rational step-up input-transformer mike input, many-many line inputs.... it's not about the JFET. You can have biasing and overload troubles, but if you get it happy, the FET noise is not a limit.
MC phono and 1:1/1:2 mike transformers do want a fat (high Gm) FET.
Quote from: duck_arse on March 12, 2023, 11:54:20 AM
https://www.interfet.com/jfet-datasheets/jfet-j201-j202-interfet.pdf
https://pdf1.alldatasheet.com/datasheet-pdf/view/121328/VISHAY/J201.html
Quote from: Clint Eastwood on March 12, 2023, 11:28:23 AM
I have absolutely no beef with the J201, but
do wonder where the myth comes from that it is low noise ???
2 possible sources linked.
Well, that's interesting. Interfet says 5nV/Hz, Vishay says 6nV/Hz, and Fairchild (application note 6609) states 10nv/Hz. Looking closer, the latter is measured under different conditions, so that probably explains the difference.
So let's say it is 5nV/hz. This is 'very low noise' according to Interfet and Vishay. But compared to what?
To make a fair comparison, let's look at data for different jfets from the same manufacturer, Interfet.
2n5457 - 7nV/Hz, described as 'Low noise'. This is noisier than the J201, but not a huge difference
2n5484 - 4nV/Hz, 'Low noise'. This device is suitable for VHF applications, equivalent to MPF102, BF245. It has a lower noise figure than the 'very low noise' j201 ???
J113 - 1,2nV/Hz, 'Low noise'. Marketed as mainly for switching and chopper duties.
Clearly, if you call a J201 very low noise, it would be logical to say a J113 is like 'ultra low noise'.
Conclusion: it seems that the confusion about jfet noise performance is caused by inconsistent descriptions in datasheets.
By the way, I looked at the data for the LSK170, made by Linear systems and a very highly regarded (and pretty expensive) audio device. It is has a noise figure of 0.9 nV/Hz, not that much better than the ordinary 50 times cheaper J113 ;)
Quote from: Clint Eastwood on March 12, 2023, 02:24:46 PM.... confusion about jfet noise performance is caused by inconsistent descriptions in datasheets.....
JFET hiss in the audio band at sane impedances is almost all 1/Gm resistance thermal noise. So makes sense that fat switches rate well.
You can't test noise for-sure in production. It takes a closed chamber and time (if you are going to test, some markets expect 1/f zone guarantees). Unless you pay a fortune for slow tested parts, you get lot-tests and a huge margin to cover bad-day variation. And there is considerable more variation in JFETs than in BJTs. So JFETs find few niches. So get sloppy specsheets and pass/fail testing.
Quote from: amz-fx on March 12, 2023, 10:19:49 AM
Do a search in the "First Stompbox Forum" archive and it will show that the J201 was in widespread use on this forum (at its previous site) several years before Don decided to change out the 2N5457 for the J201. I would contend that the J201 popularity came from here instead of his revised article that came along a few years later. :)
Jack, that is absolutely the case!
I put it out there, if a newbie presented a design, which was the same as the Don Tillman Preamp with a J201, and put up the tests voltages there would be quite a few people flagging the bias point is way off. The design with the 2N5457's is textbook.
Quote
As I recall, at the time the 2N5457 was quite popular but the available devices began to vary all over their spec range and made it difficult for beginning builders to get the best performance from their circuits. The J201s were more consistent, had low noise and gave a bit more boost for what was being built here at the time.
And now a lot of J201's are dodgy. As the fake (and out of spec) devices come in we see more and more problems.
QuoteWell, that's interesting. Interfet says 5nV/Hz, Vishay says 6nV/Hz, and Fairchild (application note 6609) states 10nv/Hz. Looking closer, the latter is measured under different conditions, so that probably explains the difference.
To first order the Jfet noise depends only on the drain current bias point, and not on the type, that sets the transconductance (yfs, gm). The high Yfs0 JFETs (ie. low resistance) generally produce less noise because they *can* be operated at a higher transconductance. Low Yfs JFETs are limited by their maximum Yfs. The point is, the operating point can have an effect and datasheet don't compare apples to apples.
On top of that is a whole lot of secondary effects. A major one is 1/f noise. If you have a higher 1/f noise and the JFET noise spec is a 1kHz then it's quite possible that this alone can determine the published noise. That requires the operating transconductance is high enough.
BJTs have similar issues. Some BJTs are quoted as low noise. Some are low noise *for high impedance circuits* because the secondary effect which cause noise in high impedances are reduced. Similarly there's BJTs which are low noise *for low impedance circuits*. These tend to have low base resistance (rbb') which appears in series with the base and adds noise. This is on top of the fact that to first order all BJTs produce the same noise when the operating current is the same.
The source impedance used in spec can have an effect on the published noise as well.
This document gives a reasonable overview,
https://www.vishay.com/docs/70599/70599.pdf
I pulled up some quite old J201 (/J202/J203) datasheets from Motorola. They are quite sparse. The datasheet states: see 2N4220 for graphs. This was common in the Motorola data, it was good because it leaked info about commonality between parts.
When we look at the noise plots for 2N4220 and 2N5457 (Motorola), it's the same plots!
One caveat is the J201 specifically has parameters outside of the 2N4220 range. The J201 have a high channel resistance, in fact they have a higher channel resistance than most JFETs.
Quote from: Rob Strand on March 11, 2023, 07:56:07 PM
QuoteThat would be good, but it was at 0.25 V. I "squeezed" it a little further and got 0.27 V without throwing Vd too far away from the often-recommended Vs/2.
The 0.6V is the JFET parameter VP and in-circuit the JFET operates at a gate-source voltage less than that, like 0.25V.
If you measure across the gate and source the meter loading can drop the measurement by a factor of 1/2 for a 1M ohm input DMM. However, if you measure the source voltage and the meter loading has (virtually) no effect.
Good point. My DMM is 10M input, but thinking back, the difference was there.
Thanks.
The results of others might vary, but I live in a quiet apartment and was recently disappointed to find that this J201 project resulted in a really noisy booster (ie I could hear the amp when I turned it on. I always aim for on-off to sound the same). This is despite the gain of the booster and the preamp+amp being quite low.
What?
As it turns out, these circuits pour ripple through. With a basic plugpack supply, unfiltered, I was getting 0.03mV of output ripple, and with gain of 5 x, then 12AX7 triode boost of 60x, and 6J1 boost of 5x that was about 0.05V output (and 0.05W) into a speaker with 95 dB/W@1m. Around 80dB ! I needed an RC filter of 1k ohm and 220uF to calm it down. It's still noisier than I'd like it to be (maybe 40dB).
I haven't calculated anything concerning JFET noise at the speaker, but I'd be surprised if many people go as far with power supply filtering as my RC of 1kohm and 200uF. Even with this, there's still no chance of me hearing the JFET noise. Yes, my plugback is probably poor (transformer, not switching, Moen brand IIRC).
Here's a similarly filtered booster/buffer.
cheers
(https://i.postimg.cc/F1dMD72j/Booster-Buffer-JFET-general-purpose-schematic.jpg) (https://postimg.cc/F1dMD72j)
> these circuits pour ripple through
Well, yeah. It's got about no PSRR. (The FET 'plate resistance' is over 100k, so ripple is hardly attenuated.)
You can't power simple preamps with crap power.
Quote from: Rob Strand on March 12, 2023, 08:06:25 PM
I pulled up some quite old J201 (/J202/J203) datasheets from Motorola. They are quite sparse. The datasheet states: see 2N4220 for graphs. This was common in the Motorola data, it was good because it leaked info about commonality between parts.
When we look at the noise plots for 2N4220 and 2N5457 (Motorola), it's the same plots!
One caveat is the J201 specifically has parameters outside of the 2N4220 range. The J201 have a high channel resistance, in fact they have a higher channel resistance than most JFETs.
It seems that Fairchild never actually published true graphs of J201 characteristics. The J201 graphs on the datasheet bear no relationship to the numerical date. In fact they are the same as the J211 graphs.
(https://i.postimg.cc/mPrLnqR9/J201-Data.jpg) (https://postimg.cc/mPrLnqR9)
(https://i.postimg.cc/YvyrB3qs/J201-J211-Datasheet.jpg) (https://postimg.cc/YvyrB3qs)
(https://i.postimg.cc/wty6qm1T/J201-Query.jpg) (https://postimg.cc/wty6qm1T)
QuoteAs it turns out, these circuits pour ripple through. With a basic plugpack supply, unfiltered, I was getting 0.03mV of output ripple, and with gain of 5 x, then 12AX7 triode boost of 60x, and 6J1 boost of 5x that was about 0.05V output (and 0.05W) into a speaker with 95 dB/W@1m. Around 80dB ! I needed an RC filter of 1k ohm and 220uF to calm it down. It's still noisier than I'd like it to be (maybe 40dB).
Without using regulators, it's not uncommon to use second order filters to get more attenuation. They are very easy to whip-up when you are testing.
Here's a few common circuits you come across to clean up power rails.
(https://i.postimg.cc/F1L3nJXy/JFET-Preamp-PSU-Filter.png) (https://postimg.cc/F1L3nJXy)
QuoteIt seems that Fairchild never actually published true graphs of J201 characteristics. The J201 graphs on the datasheet bear no relationship to the numerical date. In fact they are the same as the J211 graphs.
The J201 data was never good but that one sure looks all over the place. Do you know what year that data is from?
There are few eras of National Semiconductor and Fairchild, as they went through different owners.
I thought the National Semiconductor 1997 FET databook might be the go. That has graphs but they cover the whole process and the low Vgs(off) case for the J201 isn't represented well.
Quote from: Rob Strand on March 13, 2023, 04:16:10 AM
The J201 data was never good ....
It is not just the J201 as can be seen here too.
(https://i.postimg.cc/WtNSKLtS/PF5102-vs-J111.png) (https://postimg.cc/WtNSKLtS)
Quote from: Rob Strand on March 13, 2023, 04:16:10 AM
Do you know what year that data is from?
I am not sure of the dates as the same data has been re-published a few times and even persisted when ON took over. The National FET databook that I have (1977) seems to only have numeric data.
The graphs for Process 52 in the ON Semi application notes are probably a good representation of the J201 characteristics.
A few years back, I emailed ON Semi to point out the datasheet errors.
QuoteIt is not just the J201 as can be seen here too.
I can see how the link to the J111 etc. graphs works. There's no intent to match the tabulated data. Think of the PF5102 as *another entry* in the J111/J112/J113 tabulated data. The PF5102 covers Vgs(off) from -0.7 to -1.6V, the J113 covers Vgs(off) from -0.5V to -3.0V, etc. The PF5102 has tighter tolerances than the J113 and sits somewhere the near middle of the J113, you could even think of it as a tight tolerance J113. So the graphs for the J111 etc. family still apply. The tabulated data for the PF5102 is fully specified with min and max (no typ.). The tabulated data for the J111 etc are very loose in that there are no maximums on IDSS and no minimum on Rds_on.
QuoteI am not sure of the dates as the same data has been re-published a few times and even persisted when ON took over. The National FET databook that I have (1977) seems to only have numeric data.
The graphs for Process 52 in the ON Semi application notes are probably a good representation of the J201 characteristics.
A few years back, I emailed ON Semi to point out the datasheet errors.
The NS FET Databook (1977) I've seen has all the process info. It's actually *very* similar, if not identical, to the Fairchild/ON info. In the front of the book were tabulated data, not quite complete datasheets.
In the past I've spent a lot of time matching the info on the processes with the JFETs data they correspond too. It's a time consuming and frustrating task. Things don't match up. Datasheets like the J201, J111 which have incomplete tables which don't help. I've also tried to use datasheets from multiple manufacturers for cross-checking and filling in some of the blanks. There's simply not enough info to fill in the blanks, and there's so many things that don't match up between the graphs and the tabulated data. Every time I've done this type of thing I end up tell myself not to do it again now I just chant "Insanity is doing the same thing over and over again and expecting different results."
FWIW, back in reply #19 I posted a J201 version of the Don Tillman preamp which behaves like the original 2N5457 version.
Here's a lower parts count version of the circuit.
(https://i.postimg.cc/8sz7gVST/J201-Tillman-Preamp-sch-V12.png) (https://postimg.cc/8sz7gVST)
The gate bias shift trick can be use to help any low VP JFET (which are now common on ebay) behave like a higher VP JFET. However, if the JFET also has a low IDSS you may have to increase the resistor values to allow the JFET
to clip correctly on the -ve output swing, as I have done in this circuit.
Quote from: Rob Strand on March 12, 2023, 04:54:06 PM
Quote from: amz-fx on March 12, 2023, 10:19:49 AM
Do a search in the "First Stompbox Forum" archive and it will show that the J201 was in widespread use on this forum (at its previous site) several years before Don decided to change out the 2N5457 for the J201. I would contend that the J201 popularity came from here instead of his revised article that came along a few years later. :)
Jack, that is absolutely the case!
I remember being over at Ampage forum when Aron said he was going to start his own forum. I guess that was around 2001-2002? Anyway, yeah I built quite a few of those J201 based circuits that people were presenting here. They were coming out of the woodwork back then. I still use some of those circuits today.
Quote from: Paul Marossy on March 16, 2023, 02:40:54 PMI remember being over at Ampage forum when Aron said he was going to start his own forum. I guess that was around 2001-2002?...
Aron had a "page" before a forum. Back in the 20th century!
https://archive.ampage.org/threads/1/open/000934/Re_Arons_Stompbox_Page_has_moved-1.html
Quote from: Paul Marossy on March 16, 2023, 02:40:54 PM
Quote from: Rob Strand on March 12, 2023, 04:54:06 PM
Quote from: amz-fx on March 12, 2023, 10:19:49 AM
Do a search in the "First Stompbox Forum" archive and it will show that the J201 was in widespread use on this forum (at its previous site) several years before Don decided to change out the 2N5457 for the J201. I would contend that the J201 popularity came from here instead of his revised article that came along a few years later. :)
Jack, that is absolutely the case!
I remember being over at Ampage forum when Aron said he was going to start his own forum. I guess that was around 2001-2002? Anyway, yeah I built quite a few of those J201 based circuits that people were presenting here. They were coming out of the woodwork back then. I still use some of those circuits today.
Jack was the one who introduced the mu amp to guitar pedals. He may have also been the one that introduced the J201 to guitar pedals at the same time. I would have to trawl through the archives (Aron's and/or mine) to confirm it.
QuoteAron had a "page" before a forum. Back in the 20th century!
https://archive.ampage.org/threads/1/open/000934/Re_Arons_Stompbox_Page_has_moved-1.html
And yep, Aron (and Jack, RG) have all had their stuff up on the web from the very early days of the web.
https://www.muzique.com/amz/mini.htm
"In 1995, my site went live on the Internet and became the second guitar effects and schematics site to be available! The first site was called the Leper's Abode and it has since disappeared, leaving my web site as the longest continuously available DIY guitar effects site on the Net. "
Quote from: PRR on March 16, 2023, 05:47:21 PM
Quote from: Paul Marossy on March 16, 2023, 02:40:54 PMI remember being over at Ampage forum when Aron said he was going to start his own forum. I guess that was around 2001-2002?...
Aron had a "page" before a forum. Back in the 20th century!
https://archive.ampage.org/threads/1/open/000934/Re_Arons_Stompbox_Page_has_moved-1.html
Hmm... I guess I actually got involved here after that happened. I think he still hung out sometimes at Ampage too? I'm not sure... was a long time ago now! I just know where Aron started from. I remember the forum moved once or twice too.
I chuckle when I think back to the Ampage days. Someone would make a post there about having figured out the Fuzz Factory, provide a schematic and Z Vex himself would reply with things like "another mangled schematic". lol
I only remember the exact day I made my first working guitar FX because it was the same day my firstborn entered the world (Oct 20, 2001). It was from info I got at this forum and I chased my tail for several years after that building all kinds of stuff, none of which made me a better guitarist... but my sound is hella better now. :icon_mrgreen:
J201, 1998/1999
I didn't dig too hard but there was an old thread from Oct 1998 (on Jack's message board?) discussing Jack's FETMuff.
- The original version of the pedal from 1998 only specified 2N5457's.
- Somewhere along the way, perhaps in the text on Jack's site he may have specified an MPF102
as an alternative. Jack regretted it because people had trouble with the device to device
variations - which is the reason for the thread.
- In the same thread it looks like Jack suggests using a J201 to someone with limited JFET sources.
Here's a 1999 version of the circuit which has the J201 as an option - it seems to be an update which occurred between 1998 and 1999.
https://www.tremolo.pl/Efekty/FET/Fetmuff.gif
In 1999 there's quite a few J201's in use.
In 2000 there's plenty - "everybody's doing it"
I don't have enough info on the Booster at my fingertips (only go back to Feb 2000 version with J201)
Hi all.
I remember my first pedals, which I started building 7 years ago (maybe even earlier). The first was Big Muff, the second was Fuzz Face, and then I got to know the world of J201.
Yes, I remember those thoughts that the pedal sounds like a real amplifier (haha no), and how cool that it is enough to take the original circuit and replace the tube with j201 and adjust it correctly (yes, yes, again not true). Then search for different circuit, J201 as amplifier, Mu Amp, SRPP, combinations of these solutions. I compared it to Zvex Bor/distortron/JHS Charlie Brown and found j201 to be better.
But the moment came and I realized that I want to make circuits meaningful and understand how they work. Then I realized how many shortcomings of J201 and other Jfets.
1) Limited stock. Jfets very quickly begin to distort the signal, give stronger compression, work worse with dynamics (attack with the right hand and volume knob). BJT, Msofet, Op Amp are better, as they are limited only at the extreme points of the supplied power, these devices are closer to lamps in this parameter.
2) Biasing, we know that lamps can be biased cold (cold clipping) or hot (standard). BJT, Mosfet, Op Amp, can also be biased differently, Jfet are much less flexible and have a smaller tuning range, I would say that they cannot or is pointless to use in Cold clipping mode.
3) Constancy/repeatability/stability. Tubes, BJT, Mosfet, Op Amp are stable, we just install them in the finished circuit and everything works, Jfets do not. yes we can use Mu Amp and SRPP but their sound is very different from single cascades and IMHO limited in flexibility.
4) Clear sound Tubes, BJT, Mosfet, Op Amp have a large headroom, Jfet can have this headroom 10 times less (and many times the difference is even greater). we live in the 21st century and many people use active pickups and preamps built into the guitar.
5) BJT, Mosfet, Op Amp are readily available in dip packages, Jfets are mostly in smd. Another significant drawback, during the design.
6) The gain of Jfets is also lower than that of Op Amp, BJT, Mosfets.
Let's forget that Jfets is the holy grail (no) and think coldly, why are we still using it years later and with so many disadvantages and alternatives that do its job better? Amplifiers on Jfet outdated? I think yes. Phasers on Jfet deprecated? definitely Do you need Jfet? I doubt it very much. perhaps there is a need to use it as a switch. hardly more.
Interestingly, in many decades working in electronics, the only place that I have ever seen J201s used extensively is in DIY guitar pedals. The manufacturers probably still continue to manufacture them (well the SMD version) just to satisfy the hobby market and DIY pedal builders who seem to consider the J201 is the holy grail of JFETs and the only JFET to use for everything whilst excluding or ignoring all the other fine JFETs still being manufactured.
From this thread it seems we can gather a bit of history as to how this runt of a JFET managed gain favour with pedal builders and be considered the only device to be used, even mistakenly in such circuits as phasers, analogue switches, tremolos and other applications where its high on-resistance, low Idss and low Vp make it quite unsuitable and a very poor choice of device.
There are places where it is a good choice. The J201 is great for what it was originally intended which is 'low level audio', however, I would not call guitar level signals 'low level' like microphone signals. But then that does mean it is pretty much guaranteed to distort a guitar level signal which is probably what overdrive and fuzz pedals are meant to do anyway.
Probably the only real plus point about it compared to many other JFETs is that its Idss and Vp is so low that it runs quite happily from a 9 volt battery taking very little power supply current whereas a lot of other JFETs require to run at higher currents and voltages for optimum performance.
Is there something akin to J201 hidden in the back of electret mic capsules?
Quote from: anotherjim on March 21, 2023, 10:41:28 AM
Is there something akin to J201 hidden in the back of electret mic capsules?
Most definitely. In fact the TF202 is a device made specifically for electret mics and it has very similar characteristics to a J201 at the bottom of its characteristic range with very low Vgs(off) and Idss.
http://www.unisonic.com.tw/datasheet/TF202.pdf
Quote from: anotherjim on March 21, 2023, 10:41:28 AM
Is there something akin to J201 hidden in the back of electret mic capsules?
There's a whole heap of them. You will find the JFETs for electret microphones have very low IDSS (and low VP). The IDSS is less than half of a J201 and are generally sorted into two or three groups.
(some examples: TF202,TF218,TF222,TF246,TF252, TS788, 2SK3373, 2SK3536)
It's exactly why electret microphones have *<* 500uA specs (not *=* 500uA like most people assume).
^Thanks, I see from the datasheet the mic JFETs include a gate bias network attached to the source. I've wondered how they did the bias.