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Old 18th March 2011, 01:04 PM   #11481
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Quote:
Originally Posted by SY View Post
Really? I don't remember the spec, but if memory serves, it wasn't anything outstanding (maybe 10-15 nV/rtHz?). Were they actually better in practice?
Hi SY,

The NPD5564 was a big-die monolithic JFET pair with lots of interdigitaled gates. Looks a lot like the LSK389. If memory serves me, the NPD5564 was equal or better than the LSK389/2sK389. National made really good discrete stuff back then. Beware, I think there are "5564" devices available that are not monolithic pairs; not sure about them.

I used the NPD5564 in my MOSFET power amplifier with error correction in 1983.

Cheers,
Bob
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Old 18th March 2011, 01:09 PM   #11482
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Originally Posted by Bob Cordell View Post
Actually the first mention of it that I recall was from Marshall Leach in the late 1970's when he published his article on a moving coil preamp. He specifically mentioned the unusually low Rbb of the 2N4401/4403 pair.

Cheers,
Bob

Those are about 40 Ohm rbb' @ 1mA the PNP does a bit better. Very good for that, considering they are mainstream cheap and available. Not high FOM but still the little swing won't be bothered in such a stage by Early non linearities, Ib will have to be blocked in SE or trimmed out in symmetric also with any better hfe BJT anyway. 4 are still needed to be shunted for low MC IMHO. Higher capacitance in such very low swings brings down speed but I don't think strongly impacts THD too?
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Old 18th March 2011, 02:48 PM   #11483
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Quote:
Originally Posted by Bob Cordell View Post
Hi SY,

The NPD5564 was a big-die monolithic JFET pair with lots of interdigitaled gates. Looks a lot like the LSK389. If memory serves me, the NPD5564 was equal or better than the LSK389/2sK389. National made really good discrete stuff back then. Beware, I think there are "5564" devices available that are not monolithic pairs; not sure about them.

I used the NPD5564 in my MOSFET power amplifier with error correction in 1983.

Cheers,
Bob
They were sold as low Ron switches, a search now can turn up others but the low freq noise must be checked. The J305 family can be selected for very low noise and costs pennies.
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Old 18th March 2011, 03:01 PM   #11484
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This was also true with tubes and jfets, BUT jfets were very noisy then, and tubes took a lot of real estate and current. Often, a transformer was a better solution, but not always.
In today's case we are paralleling low noise jfets. This is by far the easiest way to lower noise without a transformer. In this example, 4 jfets in parallel would be a 6 dB lowering of noise, not bad for perhaps 1 sq inch of extra space, and some added current. 8 jfets would give us 9dB, and 16 jfets would give us 12dB or the equivalent of an ideal transformer with a gain of 4.
Now, what about input capacitance times Rin? Well, it would go up 16 times, and that is a lot. However, what if we used a X4 transformer? It would also go up 16 times. Therefore, it is a draw, with regards to nonlinear capacitance.
.
Let me help you out John, you are more right than you think and don't even realize it. The distortion is a wash only at the second order for higher order the paralleled devices win. To make an extreme example, say you had a perfect input stage that clipped at 20mV. Your 100uV Ortophon would probably never clip it, but add a 30:1 step up transformer now you need 0.6V of input range for the same headroom. That's why valves and transformers were made for each other. If you want to stick with linear open-loop stages your's is the way to go (in this case). Now you can buy me a coffee.

BTW paralleling two resistors to get 1/2 R was known a lot longer. At the first mention of rbb being a limiting noise source I would consider the rest obvious.
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Old 18th March 2011, 03:04 PM   #11485
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Well, you are in my playground now. First, let me apologize in advance if I 'correct' a few comments here a little too sternly, it is my 'habit', not my intent.
I discovered the low noise potential of the 2N4401, 2N4403, and the 2N4405 (even better) when I was at Ampex in 1968, with a Quan-Tech noise analyzer. Except for Ampex, it was a well kept secret until 1971, when I sent an LTE to 'WW'. They thought I was some sort of 'nut' but they passed my letter to H.P. Walker, and he added a footnote in his second paper on noise, pointing out my recommendation of the 2N4403. This was the first public mention of a medium to low rbb' device used for low noise input stages.
In 1973, the Levinson JC-1 was released with 4 x 2N4401 and 4 x 2N4403 in noise parallel, giving about 0.4nV/rt Hz input noise, at the NY AES convention. Nobody thought it possible without a transformer.
Later, in 1979, I found that the Fairchild 2A, output transistor pair, especially made for transistor radios, were made on the same assembly line as other smaller devices, rather than with the larger power devices. They were the equivalent of about 4 X 2N4403 or 4 X 2N4401 for each part, and the SOTA head amp was born with just one PE8550 and one PE8050 per channel, again giving about 0.4nV/rt Hz input noise.
As far as jfets are concerned, the 2N3564, and its cousins, is an old military part, just look at the part number. It originally was a dual matched NC geometry by Siliconix, or Process 51 by National. Later, National made a monolithic dual called Process 96. I have at least 1000 of them, here in my lab.
However, these are compromised devices, noise wise, compared to the Toshiba 2SK170 or the 2SK389 which are somewhat quieter, have more Gm, but at the disadvantage of somewhat higher non-linear input capacitance. I used my first NPD5566, (Process 96) commercially, in the Vendetta Research Electronic Crossover, released about 1983 or so. Before this, I used even quieter parts in the Levinson JC-2 input stage such as the J110, in 1973. Unfortunately a process change destroyed the usefulness of these parts for low noise, by 1975.
Toshiba has led the way, up until now, with the best low noise jfets ever actually available for sale in quantity. Unfortunately, they are pulling out of the business.
National is gone, and so is Siliconix.
Linear Systems is trying, and mostly successful in filling the gap.
Other companies are competing to fill the gap left by Toshiba. Let us hope, soon.

Last edited by john curl; 18th March 2011 at 03:28 PM.
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Old 18th March 2011, 03:08 PM   #11486
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Quote:
Originally Posted by SY View Post
Were they ?

At least preferable to 2N5564/5/6, U440/1, and even the monolithic clone LS844 (so they say )

Anything with an S sounds even prettier (drift numbers vary).
Attached Files
File Type: pdf NPDS5565-NPDS5566.pdf (148.4 KB, 80 views)
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Old 18th March 2011, 03:14 PM   #11487
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Higher noise than I remembered. 50nV/rt Hz.

I had pretty good luck with some old Siliconix switching FETs with low Rds(on). I think they were 2N5114 or 5116. As a demonstration of their quality control, the last batch I bought all had the wrong pinout. Quiet, though, worked great as an input buffer for a phono stage.

The problem with high gm devices like the Toshibas or LS is the high input capacitance. Some MM cartridges don't care but some do (mine unfortunately being one of the latter).
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Old 18th March 2011, 03:14 PM   #11488
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Originally Posted by scott wurcer View Post
Your 100uV Ortophon would probably never clip it, but add a 30:1 step up transformer...
That should be...

Oh nevermind.

se
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Old 18th March 2011, 03:21 PM   #11489
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One might surmise that a 'power' mosfet
would be very quiet due to the statistical
noise cancellation of the thousands of
cells paralleled.
IIRC that was the method used to make this device:
LM394 - SuperMatch Pair [Obsolete]
(only hundreds of bipolar 'cells' in parallel)
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Old 18th March 2011, 03:37 PM   #11490
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Originally Posted by john curl View Post
Joshua, you are right, but we do NOT use paralleled jfets with MM, without active buffering.
What do you mean by active buffering?
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