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Old 19th March 2011, 04:02 AM   #11521
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> This is because it is a P channel

In the case of jfets you may want
to rethink that statement a little bit .
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Old 19th March 2011, 04:20 AM   #11522
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Quote:
Originally Posted by john curl View Post
Good parts usually cost some amount of money, if you want performance. Just like auto tires. By the way, SY what was the pin out that looked wrong?
No it's the esotericness of the application, in other words volume of sales. The BF862 was designed for AM car radios, it beats the SK170 on every spec and costs 5 cents in quantity.

If a car maker designs a rim that takes a totally unique tire size you can bet they cost a premium.
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Old 19th March 2011, 05:10 AM   #11523
jlsem is offline jlsem  United States
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Quote:
esotericness
esotericity.
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Old 19th March 2011, 07:41 AM   #11524
SY is offline SY  United States
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@John: Yes, it's a p-channel, which is what's needed in that design. The ones I used, as I mentioned, seemed pretty quiet for MM use, quieter than the J271 they replaced. I don't claim that it's universally true for all 2N5114/16. I'd love to find a reliably quiet p-JFET for a publishable design- do you have a recommendation?
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Old 19th March 2011, 08:10 AM   #11525
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Originally Posted by hitsware View Post
Well then their ya go ! .......
A vacuum is a quieter
space than a solid ! ........
You won't get below the background noise level of the big bang. Liquid nitrogen isn't cold enough. Try liquid helium.

How fortunate microphone preamps don't have these problems. Do you think their designers know something phono cartridge preamp designers don't?
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Old 19th March 2011, 12:30 PM   #11526
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They don't have these problems? Are you sure?
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Old 19th March 2011, 01:29 PM   #11527
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Of course, microphone preamps have similar problems. That is why Wilson Audio and Crystal Clear Records hired me to design their microphone preamps, many decades ago.
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Old 19th March 2011, 01:35 PM   #11528
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You won't get below the background noise level of the big bang. Liquid nitrogen isn't cold enough. Try liquid helium.

How fortunate microphone preamps don't have these problems. Do you think their designers know something phono cartridge preamp designers don't?
Pure physics, the brownian noise of the air on a stretched membrane is easily computed. It is rarely possible to make a useful microphone that reaches the limit of audibility, but the electronics piece of the problem is relatively easy. A picture is valuable to make this point, these are 1" B&K curves actally presented in nV for comparison.
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Old 19th March 2011, 01:38 PM   #11529
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esotericity.
Too much Facebook.
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Old 19th March 2011, 02:10 PM   #11530
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Well SY, I think that any P channel input is a poor choice. I said the same thing to Scott Wurcer, a few years ago, because he made the same 'mistake'.
Now what is the problem? In this case, ALL ELSE BEING EQUAL, P channel devices have LESS Gm and higher 1/F noise.

A perfect example is the PS geometry that SY used and its 'complement' NC geometry.
In National Semi terms, it is process 88 vs process 51.
Typically, the N channel is 3 times quieter at 100Hz than its P equivalent.
Does that mean that P channels should not be used EVER? Of course not!
As I said before, I have been using the same basic P channel part since 1973. I used them in EVERY Levinson JC-2, microphone preamp, master recorder, Vendetta Research power supply buffer, etc, etc. I have more than 1000 of them sorted to 1 ma Idss in plastic trays. Yet, I don't normally use them below line level for input stages, because they are more noisy.
Now, could you, SY, have lucked out? Yes. 1/F noise is somewhat mysterious, and is dependent on processing. WHY do you think that Linear Systems has had such a hard time getting the P channel J74 out to the public? Lack of interest? Oh no! They just have a hard time figuring how Toshiba did it as well as they did.

I realize that some scoff at innovation here, as they usually come by it, once it is established, and then it becomes OBVIOUS.
BUT, back in the old days, jfets were virtually written off as very noisy, due to what seemed to be, inherent 1/F noise, and many textbooks, even later ones, would warn against using them for ANYTHING below 100K ohm or so source impedance.
However, like the paralleling of devices was a revelation to me in 1966-7, the same journal talked about jfet noise, especially in the 2N3819, a common N channel part that can be fairly noisy. That was all that was available then. Look at the part number, they go up with time, so newer parts have higher numbers.

Guess what? Some professor, somewhere, measured 100's to 1000's of these devices and found 1 device with virtually no 1/F noise in the audio band! This was a REVELATION, because it showed that it was POSSIBLE to make a low noise 2N3819, or a similar device. You just had to get the yield up. Look what followed? NC and PS geometries, and the NI geometry, the very heart of the Levinson JC-2.
Later, Toshiba made even better parts, with higher Gm, better match, and LOWER 1/F noise.

Now, batch to batch, SOME batches of P channel devices will look similar to N channel devices, but I would not recommend it to the general public. All else being equal, if you get a design that can be 'REVERSED' in power supply voltage, then do it. Put an N channel jfet on the input and the P channel jfet on the second stage. It is just good engineering practice. It is the reverse with bipolar transistors. There, you want to use a pNp transistor at the input and an nPn transistor for the second stage. This is because the capitalized letters are the BASE MATERIAL of the bipolar transistor. Again, N is better for noise.
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