JFET shootout

[Gus]

Something I want to test is one reason a tube microphone can sound a little different than a simple one gain stage JFET and this does have something to do with this thread
If you minimize the drain to gate cap you reduce(remove) the semiconductor capacitance from drain to gate that make every simple JFET amp like a 84 87 have a charge amp cap inside the JFET so this is why a RF type JFET works well there.

If you bootstrap or cascode or SF (as others posted) or use a front end of the FET47 the drain to gate capacitance is minimized however you need to think about the Vgs at the ID you are working the device at.

A tube has vacuum capacitance between the elements. Some well respected tube microphones have caps in the feedback networks.

 

[Екатерина]

Oh, this is a good direction for discussion. I have used 2SK373 2SK246 before, but unfortunately they have stopped production. 2SK246 gives me a sense of granularity, which is impressive

 

[Ricardus]

I have what might be fake 2N3819’s. One channel in my lathe electronics was noisier than the other. It would also intermittently cause a low frequency noise. After chasing connections and checking for dirty pots I zeroed in on the Feedback Amplifier. It is a high gain circuit like a microphone preamp. I figured the 2N3819 at the front end might be the culprit.

I looked in my stash and I saw some that looked like the had an A suffix. That usually means low noise so I figured I’d give it a shot. No bueno. Didn’t pass signal. Fake? I put in a regular 2N3819 and it worked and was quieter than the previous 2N3819. I changed all the bjt’s also and that made it even quieter. Now the two channels match well noise wise.

Fake 3819s are everywhere. Like every one I ever bought on FleaBay was fake. I mean there might be some legit dealers that I am not aware of, but I made ebay give me refunds on every fake order I did.

I have found some legit NOS ones at some vintage electronics places over the last few years.

 

[Mr. Moscode]

Plate to grid capacitance provides a negative feedback loop which reduces gain as the frequency goes up. It's called the Miller effect. Same thing in a Jfet, drain to gate capacitance. It's what you're talking about but I haven't seen it mentioned by name.

Has anyone tried a J310 jfet? Very high transconductance. Use two in cascode to get past Mr Miller. I used it for high gain mm pickup input in a phono preamp. Op-amps use cascode input stage for that reason, I presume.

Extra, extra, read all about it!: Miller effect - Wikipedia

 

[kingkorg]

Behringer B1, TLM103

 

[Mr. Moscode]

Yup. Exactly. With a bootstrap. I hadn't seen Miller mentioned earlier, but I may have missed it.

I've used a cascode follower to drive the output transformer in a Neuman U47. Piano chords went from being heard as chunks to the individual notes. Walt Sear and I did an A-B test at Sear Sound in NYC sometime back in the 80s. His beautiful studio was loaded with 47s. Walt preferred the stock version, but my customer was happy.

 

[abbey road d enfer]

Yup. Exactly. With a bootstrap. I hadn't seen Miller mentioned earlier, but I may have missed it.

Check the first line of post #11. Minimizing Cgd means implicitly minimizing Miller effect.
I believe most of the members who post in this thread know about Miller effect, even if they don't mention it by name.
Miller is the Nr1 reason for cascoding or bootstrapping.

 

[Mr. Moscode]

Yes indeed. Come to think of it, that's why pentodes, wired as triodes, are used in tube mics. Grid1 to grid 2 has has less capacitance than G to P in a triode, I would think.

I married big mosfets to a tube driver. To get extended bass I bootstrapped the output stage to itself.

 

[Gold]

Fake 3819s are everywhere. L

I have a few different vintages in my 2N3819 drawer. I know I got some from an ancient parts bin which I know are good. I only had a few of the ones that turned out to be bad. I threw the ones that looked like the bad one away.

 

[Gus]

I did not use the term miller for a reason
Often people think of it as a high frequency roll off
I wanted people to think about what I was posting
In a charge amp like setup like some microphones it sets part of the gain along with the capsule

Also note I posted about vacuum capacitance vs the JFET capacitance

 

[thor.zmt]

It depends..
One example is Primo EM23 capsule witch was measured having an SNR of 84 dB(A). Sensivity is roughly -40 dBV @ 94 dB SPL. Self noise is 10 dB(A). The capsule related noise floor is therefore -124 dBV(A). If we use a preamp with -122 dBV(A) we sacrifice the good performance, the overall self noise would be 14 dB(A). A preamp with -130 dBV(A) would nearly preserve the excellent capsule SNR and would lead to 11 dB(A) overall self noise.
So what is wrong with getting the best out of a system? Or the other way round, what amount of self noise is acceptable?

For practical use I go by the 3dB rule. A 3dB improvement that is essentially cost neutral and causes no extra complications (like unobtanium parts) is worth having. Less than 3dB only if free.

And a 3dB increase in cost (and of circuit complexity/assembly etc. these all can be costed) should produce at least a 6dB improvement.

Of course, this is for consumer/prosumer applications. For the "money is no object" case, sure why not. Even better if the Fet is in a metal can case with gold plated leads, costs at least 15 USD each and is near impossible to get.

Thor

 

[ricardo]

@ricardo: Sorry, but EIN is THE one and only criterium for noise spec. because it includes the gain of the circuit automatically. No question, the gain itself is influenced by the JFET Cin, but also the output noise. But this may be better visible without the noisy 1G resistors.
....

You are of course right Guru MicUlli. Mea maxima culpa.

But I'm not sure your circuit measures this. There's a couple of points I would contest but I'm too lazy so will confine myself to what might teach me something new.

Therefore devices with high forward transconductance gm AND low excess noise capacitance Cen are preferred. Unfortunately Cen has to be checked by measurement. Cen is correlated with the input capacitance Cin of the JFET but cannot be expressed analytically.

I've never seen this 'excess noise capacitance Cen' before. Both Guru Scott Wurcer and Great Guru Baxandall, in what I remember of their articles, and also some private correspondence, stop short at sorta your Unch in their analysis of FET noise.

Anyone know if Guru Wurcer brings this up in his Linear Audio articles.? Beach bums don't buy books.

If you've got a reference to a web source explaining this (or even a dead tree reference), I would be very grateful.

Your measurements support this mechanism. I can't for the life of me remember if I found the same thing when I last did similar measurements in da previous Millenium

 

[McIrish]

Sorry, I didn't read this entire thread, but be aware that the currently available 2N3819 FETS from Mouser (Central Semi) have absolutely nothing to do with the original 2N3819. They will not be a drop in replacement for the old ones. They bias completely different. I ran into that with the bansai KM84 kit. Those FETs are trash. Don't use them. I used LS846 instead. Worked great and I would recommend it. It biases close to the original 2N3819.

 

[MicUlli]

You are of course right Guru MicUlli. Mea maxima culpa.

But I'm not sure your circuit measures this. There's a couple of points I would contest but I'm too lazy so will confine myself to what might teach me something new.

I've never seen this 'excess noise capacitance Cen' before. Both Guru Scott Wurcer and Great Guru Baxandall, in what I remember of their articles, and also some private correspondence, stop short at sorta your Unch in their analysis of FET noise.

Anyone know if Guru Wurcer brings this up in his Linear Audio articles.? Beach bums don't buy books.

If you've got a reference to a web source explaining this (or even a dead tree reference), I would be very grateful.

Your measurements support this mechanism. I can't for the life of me remember if I found the same thing when I last did similar measurements in da previous Millenium

No problem at all
Most of the low noise circuits with JFETs operate with low input impedances. I havent found literature regarding noise estimations when using pF source impedances so i did my own research..

 

[ricardo]

"I've never seen this 'excess noise capacitance Cen' before. Both Guru Scott Wurcer and Great Guru Baxandall, in what I remember of their articles, and also some private correspondence, stop short at sorta your Unch in their analysis of FET noise."

I havent found literature regarding noise estimations when using pF source impedances so i did my own research..

Is 'excess noise capacitance Cen' your own invention?

The only stuff on pF LN stuff I know of is GG Baxandall's 1967 Wireless World article (in MicBuilders) and some posts of Guru Wurcer in MicBuilders circa 2006. He might have reprinted some of it in his Linear Audio articles.
I have some private correspondence on the subject with both Gurus.

Guru MicUlli, if Cen is your own invention, it deserves a groupDIY White Paper on the subject so we can quote & reference it and give you due credit.

 

[MicUlli]

"I've never seen this 'excess noise capacitance Cen' before. Both Guru Scott Wurcer and Great Guru Baxandall, in what I remember of their articles, and also some private correspondence, stop short at sorta your Unch in their analysis of FET noise."

Is 'excess noise capacitance Cen' your own invention?

The only stuff on pF LN stuff I know of is GG Baxandall's 1967 Wireless World article (in MicBuilders) and some posts of Guru Wurcer in MicBuilders circa 2006. He might have reprinted some of it in his Linear Audio articles.
I have some private correspondence on the subject with both Gurus.

Guru MicUlli, if Cen is your own invention, it deserves a groupDIY White Paper on the subject so we can quote & reference it and give you due credit.

Good heavens, no!
Cen is NOT my own invention, channel induced current noise in JFETs is mentioned throughout electronics standard books. But my thought was to figure out how large it can be

 

[Mr. Moscode]

So Cen should be Cicn? If not, what's Cen?

 

[MicUlli]

Sorry,
not visited this thread a long time. After examining several JFET datasheets in detail and comparing it to my findings it turns out that
Cen is approximately 2x Cin (Cin = Cgs+Cgd) at the JFET operating point.
BR MicUlli

 

[Mr. Moscode]

For those of you not familiar with the book "The art of electronics (by P. Horowitz and Winfred Hill) I would warmheartedly recommend reading Chapter 8 (Low Noise Techniques). There is also a chapter (8.6.1, ED3) about Low Noise Design with JFETS and a quite comprehensive list of the lowest noise J-FETs available (page 516 in ED3). Generally thorough and well-written text and much about JFET noise optimization. Highly recommended read!

I think I have an earlier version. What's the publication date of yours? Is it the most recent? It is a great resource.

I'm wondering why the J310 Siliconix is never mentioned around here except by me,

 

[AUDIO FREQ]

Is the LSK389 and LSK489 worth a mention?