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.
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
Power MOSFETs are what you need, and should not be too hard to find.
Grey
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.
john curl said:
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.
I doubt that Erno and I have too many differences of opinion. We have known each other since 1975. We have compared notes.
john curl said:
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."
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
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
Hi,
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,😉
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,😉
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
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
jewilson said:
Yep, I used to use the HA-2539 in radar baseband circuits back in the Reagan era, around '83-'84 🙂 .
Hi,
Grey,
This has been and still is being done for phono stages; using a 2SK170 or similar as the input and half a 6922 on top of it.
You can do this in single ended or differential just as easily.
You can also come up with a number of variations on this theme depending on what it is you're looking for...
In tube parlance one could compare a FET to a triode whereas a cascode of triodes would look like a pentode minus the space charge noise.
I wasn't condemning cascodes in any way, just pointing to the fact that you're looking at a topology with totally different characteristics than a simple FET alone has.
Best, 😉
Grey,
This has been and still is being done for phono stages; using a 2SK170 or similar as the input and half a 6922 on top of it.
You can do this in single ended or differential just as easily.
You can also come up with a number of variations on this theme depending on what it is you're looking for...
In tube parlance one could compare a FET to a triode whereas a cascode of triodes would look like a pentode minus the space charge noise.
I wasn't condemning cascodes in any way, just pointing to the fact that you're looking at a topology with totally different characteristics than a simple FET alone has.
Best, 😉
Yep, that's why I noted the difference in topology. Comparing apples and oranges makes things more difficult.
Grey
Grey
fdegrove said:
maybe I didn't get it.
was the "more distortion" still due to the paralleled jfets, and not to cascoding?
Hi,
First of all noise and distortion are two distinct items....
That said, putting two devices in // should noticeably lower the noise.
Putting those in cascode configuration will give you way more gain, hence much more distortion too.
Note also that putting none //ed devices in cascode will yield roughly the same gain but at the expense of a much higher noise penalty.
There's more to the picture than what I'm saying here but it should give you some idea.
As usual, no such thing as a free lunch.
So, to answer your question, the "more distortion" is coming from the cascode topology, not the //ing.
Cheers,😉
First of all noise and distortion are two distinct items....
That said, putting two devices in // should noticeably lower the noise.
Putting those in cascode configuration will give you way more gain, hence much more distortion too.
Note also that putting none //ed devices in cascode will yield roughly the same gain but at the expense of a much higher noise penalty.
There's more to the picture than what I'm saying here but it should give you some idea.
As usual, no such thing as a free lunch.
So, to answer your question, the "more distortion" is coming from the cascode topology, not the //ing.
Cheers,😉
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