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Old 14th March 2012, 04:36 PM   #3421
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Dejan,

Quote:
Originally Posted by dvv View Post
The limited choice of locally available FETs is dictated by the requirements of Japanese TV sets and some audio, I think.
As said, 2SK246 (BL preferred) and matching 2SJ103 can be used. It is a tradeoff. Transconductance is very low (next to 2SK170/2SJ74), but so are parasitic capacitances and pinchoff voltage are high, so they are easier to bias, more resistant to RF, plus they have their zero tempco at around 1mA and at higher currents can be used offset Bipolar transistors drift diamond transistor style, so there is some mileage there.

Quote:
Originally Posted by dvv View Post
They are mostly in the trip of "if it ain't a FET of some kind, it can't play music well". Well, maybe a tube or two, but that's it.
Tubes, Fets and BJT's are all devices that have uses. What they precisely do best varies. I like my tubes and like J-Fets for small signal, but I never really got on well with Mosfet output Amp's.

For solid state I feel compound structures that mix low capacitance lateral or J-Fets as "drivers" to BJT's allow a more design freedom and remove some of the probelms in designing SS Amp's rather neatly.

For a real extreme VAS, how about 2SK246 & PNP Rush cascode (and same on the other rail)? Now that should be sufficiently freaky to make everyone scratch their heads... You could do the same Fet Follower drives common base BJT trick in the input stage too... Many interesting options if you combine Bipolar and Fet...

Ciao T
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Old 14th March 2012, 04:50 PM   #3422
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T is correct DVV. The 2SK246-j103 combination should be adequate in this case, just buy a number of them, maybe 50 or more each for easy matching. They are CHEAP!
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Old 14th March 2012, 05:08 PM   #3423
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DVV please send me your email address and I will send you what I am about to use from Parasound, which is the A21. The JC-1 is bigger and better, but it is TOO powerful for my needs. You can learn and compare from what I send you. Mostly you will find more fets where you put bipolar transistors.
This design, derived from the Ampzilla, is pretty good, but it is about 40 years behind the times. Yes we were making as good, for all practical purposes, 40 years ago, as prototypes, just like you are doing, in fact, I had been using the comp-diff bipolar input stage since 1968, when I worked at Ampex. BUT about 1972, I shifted to the complementary differential jfet input stage that I developed independently as well, and since then, I have never gone back to bipolar input stages, for all the reasons that T describes so well.
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Old 14th March 2012, 05:11 PM   #3424
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Quote:
Originally Posted by bcarso View Post
That seems eminently plausible --- somehow the footnote got lost. As far as which is superior, it depends on whether you want a negative tempco of collector current or not. I recall an opamp using standard two-diode bias
which did so partly to make the transconductance of the nondegenerated input pair more constant with temperature.

To clarify: the LED bias will correspond to a low-tempco, the two diodes to a negative tempco, the latter being appropriate to some extent for stabilizing a diff pair gm.

Brad
Actuallay, back-to-back, with some thermal compound, two transistors make a great current source - extremely thermally stable.

The problem with LEDs is that they are rather different between them, depenind which you happen to have. Much like ground - theoretically, ground is ground is ground, but we all know what happens if you carry that view into a design - ground loops, osciallations, niose, distortion.

I just went through a cycle of very poor quality LEDs. I use them as visual indicators of the filter status. In their day, in the 70ies, LEDs were sad to have a life expectancy of 80 years, and I lived to see that shrunk down to 14 days in some cases. Chinese products of godawful quality, don't last longer than about a month. Back to Germany for my material sourcing.

Classic diodes, like 1N4148, are FAR more reliable and last forever. As do the ancient ZPD (0.6W) and ZY (1.3 W) series of zener diodes.
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Old 14th March 2012, 05:20 PM   #3425
dvv is offline dvv  Serbia
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Quote:
Originally Posted by john curl View Post
T is correct DVV. The 2SK246-j103 combination should be adequate in this case, just buy a number of them, maybe 50 or more each for easy matching. They are CHEAP!
Muchas gracias for the confirmation, John. As you may remember, I mentioned the fact that Thorsten and I have "known" (via the Internet) each other for some 11 or 12 years now, from other forums. This is why I tend to heed his advice - experience has taught me that I'll be the better for it.

And if bought Thorsten just one beer for every piece of advice he gave either me or someone else over time, Thorsten would be in a contual stupor. But, I am forever the optimist, I still believe that some day, somewhere, somehow, I will get to buy him beer in person. Even if sometimes I don't quite sound the optimist.

Just one note - 50 is NOT a safe number. I could get lucky, true, but I hate haphazard attempts, so it's more like 100. Usually, out of a population of 100, one can count on getting 4, perhaps 5, really well matched pairs. To me, "really well matched" means +/- 2% or better.

I still grieve the day Analog Devices discontinued their MAT series - those were down to 1% easy. Costly, but well worth the money if you wanted outstanding.
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Old 14th March 2012, 05:27 PM   #3426
dvv is offline dvv  Serbia
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Quote:
Originally Posted by john curl View Post
DVV please send me your email address and I will send you what I am about to use from Parasound, which is the A21. The JC-1 is bigger and better, but it is TOO powerful for my needs. You can learn and compare from what I send you. Mostly you will find more fets where you put bipolar transistors.
This design, derived from the Ampzilla, is pretty good, but it is about 40 years behind the times. Yes we were making as good, for all practical purposes, 40 years ago, as prototypes, just like you are doing, in fact, I had been using the comp-diff bipolar input stage since 1968, when I worked at Ampex. BUT about 1972, I shifted to the complementary differential jfet input stage that I developed independently as well, and since then, I have never gone back to bipolar input stages, for all the reasons that T describes so well.
I'm at dvv@beograd.com , with an (almost) unlimited capacity to send or receive.

Ampex, huh? I loved those 70ies machines, they actually left tanks envious of their build quality. But over here, Studer/reVox was King, Ampex machines were rare, and it was local nuke explosion when Radio Belgrade's most prestigious Studio 5 purchased an MCI machine in the late 70ies. They must have had a thousand Studers and reVoxes, with some Nagra thrown in for good measure, and then, shock! horror! not a Studer, but an MCI. Wow!

I do miss open reel machines. They were such FUN!
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Old 14th March 2012, 05:40 PM   #3427
dvv is offline dvv  Serbia
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Quote:
Originally Posted by ThorstenL View Post
Dejan,



As said, 2SK246 (BL preferred) and matching 2SJ103 can be used. It is a tradeoff. Transconductance is very low (next to 2SK170/2SJ74), but so are parasitic capacitances and pinchoff voltage are high, so they are easier to bias, more resistant to RF, plus they have their zero tempco at around 1mA and at higher currents can be used offset Bipolar transistors drift diamond transistor style, so there is some mileage there.



Tubes, Fets and BJT's are all devices that have uses. What they precisely do best varies. I like my tubes and like J-Fets for small signal, but I never really got on well with Mosfet output Amp's.

For solid state I feel compound structures that mix low capacitance lateral or J-Fets as "drivers" to BJT's allow a more design freedom and remove some of the probelms in designing SS Amp's rather neatly.

For a real extreme VAS, how about 2SK246 & PNP Rush cascode (and same on the other rail)? Now that should be sufficiently freaky to make everyone scratch their heads... You could do the same Fet Follower drives common base BJT trick in the input stage too... Many interesting options if you combine Bipolar and Fet...

Ciao T
Now you REALLY have me shifting gears into 6th.

Some time ago, I did a constant current source by cascoding two bipolars, with LEDs on their bases, and, as you quite rightly pointed out, it had people scratiching their heads. Now, combining FET and bipolar, that will produce some headaches. Good idea, though. That I have to try.

Completely agreed on MOSFET output stages. They (almost) always leave me with a feeling of unfinished business, somehow I always feel something has been left out, though I can't put my finger on it.

Only two amps with MOSFET outputs I remember I really loved. One is an old Perraux model from 1980 (I think?), rated at 100 wpc, using the old Hitachi devices; somehow, that did it for me, it sounded memorable through all these years. The other is a copy of Goldmund's Mimesis 3 (I think?), a nominally 50 wpc model - knowing the guy who did it, I believe it is a very faithful copy, illegal as it may be. That one also sounded memorable. But in general, I am not amused.

In your last paragraph above, you mention a "Rush cascode"; I am not sure I know what that is, perhaps I do but not under that name, could you elaborate a bit, perhaps with a diagram, if you have the time?
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Old 14th March 2012, 06:04 PM   #3428
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Google "RUSH CASCODE"

It is all there!
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Old 14th March 2012, 06:23 PM   #3429
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Stick with Studer, I switched to Studer analog tape recorders in 1974. In 1979, I 'improved' Studer by throwing away the electronics and making my own for Mobile Fidelity, and later for Wilson Audio in 1983. Studer made great transports, but relatively lousy electronics, getting worst with each succeeding generation. Amazing, but true.
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Old 14th March 2012, 06:35 PM   #3430
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Quote:
Originally Posted by dvv View Post
I still grieve the day Analog Devices discontinued their MAT series - those were down to 1% easy. Costly, but well worth the money if you wanted outstanding.
Numbers and packages have changed but they are still there.

IMHO matching of JFET's is overrated, certainly at the buy 20X what you need level.
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Last edited by scott wurcer; 14th March 2012 at 06:48 PM.
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