Single or dual differential?

[thanh]

I want to build a JFET amp but i can't buy any small power JFET in my country.I'm using a BJT amp.

[GRollins]

You're going to have trouble finding power JFETs anywhere in the world.
Power MOSFETs are what you need, and should not be too hard to find.

Grey

[john curl]

Many advanced audio designers use techniques similar to or better than the concepts described in IC designs in previous threads. Often, we invented them first, such as the complementary differential input stage.

[Bricolo]

Originally posted by john curl
Many advanced audio designers use techniques similar to or better than the concepts described in IC designs in previous threads. Often, we invented them first, such as the complementary differential input stage.

Mr Curl,

Since we're talking about input stages here. I don't remember on which thread, you mentionned that paralleled jfets was a bad idea (something related to (non linear) input capacitance)
Can you tell me more about this? I re-read Borbely's 2 articles about jfets yesterday, and it seems that he doesn't agree with you on this point.

[john curl]

I doubt that Erno and I have too many differences of opinion. We have known each other since 1975. We have compared notes.

[Bricolo]

Originally posted by john curl
I doubt that Erno and I have too many differences of opinion. We have known each other since 1975. We have compared notes.

Maybe I misunderstood what he wrote about that.


Here's an extract from the 1st article, on page 4 (right column)
http://www.borbelyaudio.com/ae599bor.pdf


"However, even without RS, the
noise of a single K170 is not low enough
for MC pickups. To achieve lower noise,
you can parallel several of these devices."

[GRollins]

Aye, lad, but note that Borbely is talking noise, whereas John is referring to distortion. Two different things. Just one of those annoying tradeoffs that nature is fond of. Yes, THD and noise are often lumped together in measurements, but that's not to say that they are the same thing. In other words, both John and Erno are right...from different points of view.
Nature cannot be forced, but she can sometimes be seduced. You could cascode the paralleled JFETs to make the Gate capacitance less of a problem, but then you're getting into another topology.

Grey

[fdegrove]

Hi,

You could cascode the paralleled JFETs to make the Gate capacitance less of a problem, but then you're getting into another topology.

Yep, lower noise...far higher gain and a ton more distortion....

MC carts aren't too sensitive about the input C so paralleling selected 2SK170s makes alot of sense.

Cheers,

[lumanauw]

Mr. Curl

I really want to read your writings about power amp. Where can I read them?

[GRollins]

Frank,
I approach cascodes the same way I approach current sources. First I ask myself if there's any other reasonable way to get the job done. If not, I'll toss in a cascode. Trouble crops up when people assume too quickly that the answer is that, yes, the cascode (current source/current mirror/etc.) is indeed the one and only way to get the job done. After a few times of "thinking it through" in this manner, the answer becomes "well, of course I'll use a cascode." It's the 'of course' that I'm always crusading against.
That said, why don't we cascode a couple of 2SK170s with a 6922 or a 6SN7?
Might be fun.

Grey