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Old 26th October 2010, 07:09 PM   #6381
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Optimist, I presume. The 2SK170 data sheet shows for 5ma and 14.5V, an input current of 1 nano amp, at 25 degrees C Look it up. That would give you 5V across a 5G resistor. So, 5V/20mV means that your jfets have input current 250 times lower than the data sheet, OR you are using lower voltage, perhaps 9V or less. Kind of restricting, isn't it? ;-)
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Old 26th October 2010, 08:49 PM   #6382
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Optimist, I presume. The 2SK170 data sheet shows for 5ma and 14.5V, an input current of 1 nano amp, at 25 degrees C Look it up. That would give you 5V across a 5G resistor. So, 5V/20mV means that your jfets have input current 250 times lower than the data sheet, OR you are using lower voltage, perhaps 9V or less. Kind of restricting, isn't it? ;-)
I would never operate these FET's in the impact ionization region for this application. No point in running it at 5mA either.
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Old 27th October 2010, 12:51 AM   #6383
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John these low and high impedance applications are two different worlds It's best you concentrate on your Blowtorch and avoid these distractions.
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Old 27th October 2010, 12:53 AM   #6384
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Of course, these are two different worlds, but you have not shown concern for specific design situations where a quality resistor could be a project saver. I believe that I have.
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Old 27th October 2010, 09:18 AM   #6385
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Yes, PMA, we have stuff written by marketing people. However, the power amps are based on a transformerless hybrid vacuum tube power amp design first shown to me 40 years ago by Bascom King, the designer of the Hercules power amp, now using mos fets on the driver and output stage. I am pretty sure the input stage uses 2SK170 jfets.
Bascom King has always been a believer in same 'sex' output devices. He was using quasi-complementary output transistor stages, back in 1971 or so, in a Marantz design that he did while there, when complementary output transistors were available and used in other Marantz amps.
For the record PMA, I use 1G ohm resistors or more with jfets. BUT with a SERVO, it might be possible to raise the input resistor on a vacuum tube based condenser microphone input, in order to lower the self noise, BUT there might be a real DC voltage drop, maybe 10V or so. THEN excess noise would be VERY important. Therefore, it is a matter of design application whether excess noise is important.
Even in 1970 or so, B&K used metal film resistors for their hi value input resistors (200 Meg ohms, initially, for the 2619 microphone preamp.
Thanks John... any comments on the BF862?
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Old 27th October 2010, 06:09 PM   #6386
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I believe that Scott Wurcer introduced those jfets to us, and they have been shown by him, and Kirkwood Rough, (who will be at Burning Amp this week in a big way) by measurement with a Quan-Tech noise analyzer, to be very good for microphone input. However, as I recall, they have very low breakdown voltage, so low voltage operation is mandatory.
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Old 27th October 2010, 07:22 PM   #6387
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It is enlightening to directly compare, as much as possible, the data sheets of the BF862 and the 2SK170. Then let us also presume that we are going to make a simple source follower with one or the other of these parts. Let us presume that we are making a 'quality' calibration level microphone, so that we can test loudspeaker distortion, for example. Fair enough?
Now, what voltage would we use, based on the data sheets of these two devices would we use across the jfet we chose for optimum dynamic range and low distortion?
First, it might be noted that the 2SK170 has a voltage rating of 40V. The BF862 of 20V. This should be a clue, but that is not all. Secondly, in general, the nonlinear input capacitance of each jfet should drop with INCREASED voltage as shown on the Ciss-Vds and Crss-Vgd graphs on the 2SK170 data sheet. It fact, Ciss drops from 50pf to 30 pf, IF you put 10V across it. Dare you use any less? Perhaps, but there will be a penalty of 'excessive' nonlinear capacitance, part of which will be minimized by the 'bootstrapping' of the source follower and its relatively high Gm. However this is but a 'band-aid' and not a cure for the non-linear input capacitance, and lower would be better, implying that higher voltage across the jfet would result in lower distortion.
However, what about leakage? Now look at the Igsx-Vgs graph for the 2SK170. It says that you can expect about 10pa at 10V, which is pretty good, and only 50 mV across a 5G resistor. Now, what about a BF862 in the SAME circuit, instead of the 2SK170. The BF862 would have about 10nA or 1000 times more leakage from the data sheet. Therefore, it would have 50 V across the 5G resistor. Perhaps I made a mistake, but this would not work.
It would seem that I have to reduce the supply to less than 7.5V just to make the BF862 work at all and that 6V or less would be even better, yet this is a 20V rated jfet!
Well, what does that do to the circuit if I reduced the voltage to 5V instead?
Then the maximum output of the circuit would be reduced by at least 6dB and the residual distortion might increase somewhat, due to increased input capacitance. Still, the BF862 might work out to be pretty good, overall, perhaps better than the 2SK170, so long as dynamic range is not of great concern.
I built just such a preamp about 38 years ago, and I still have it today. It will measure better than a standard Neumann KM84 mike, that was popular at the time.
I did not know everything then, that I have shown here, but I sort of 'lucked out'. I hope that others don't fall into the gate leakage 'trap' that many interesting jfets have.
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Old 28th October 2010, 07:47 AM   #6388
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Good illustration John, once again thanks.

I was thinking of replacing the sk170 with the BF862 in the first gain stage from a phono amp - MM. So do I see it correct that it could work better here using the BF with lower voltage supply?

What let me linger is the dynamic range you mention. Music is very dynamic, I mean the micro and the macro dynamics, and maybe a change to a BF could be a step back in view of sonic reason.
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Old 28th October 2010, 04:48 PM   #6389
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Sonic reasons are not allowed to be discussed here. I will say that the BF would probably be OK, and operating voltage with only 50K max on the gate is certainly a different condition from 5G, or about 100,000 times different. ;-) Therefore, some care is still prudent, but not so much as in the microphone case.
Still, I would like to thank Ed Simon for the tip on the Dale resistors. My present employer found a cost effective Dale resistor that is completely non-magnetic, that works just great. In our reality, these questions are as serious as what topology to use.
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Old 28th October 2010, 05:01 PM   #6390
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Thumbs down What?

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Sonic reasons are not allowed to be discussed here. .......
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