jfets for preamp

[kzeprf22]

Hi,
Does anyone knows any good audio Complementary jfets (n-p channel) capable of operating with voltages (VDS >= +- 30 Volt). These will be used in the first stage of a preamp.

Thanks

[brutepuppy]

I believe the PASS "B1" LSK170 ultra low noise is good up to 40 volts.

[lineup]

the best overall pair is Toshiba 2SK170BL / 2SJ74BL
.. they are reasonably in price, can be found and are good for audio = lownoise
... probably the most used .. ever
they are rated SK170 =40 Volt and 2SJ74 =25 Volt
so you will better have to cascode with some higher voltage bipolar transistor
Another pair is:
2SK246 =40V and 2SJ103 =50V

The problem is often to find the P-JFET transistor.

See for example this good supplier:
http://www.ampslab.com/components_fets.htm

[kzeprf22]

they are rated SK170 =40 Volt and 2SJ74 =25 Volt
so you will better have to cascode with some higher voltage bipolar transistor

Hi thanks, for the reply. Can you give an example how to cascode with BJTS

[lineup]

http://diyaudioprojects.com/Solid/Je...ga-Le-Monstre/

In the above, you see 2k resistors divide 12 Volt into two 6 Volt parts.
At the 2SC1775 Emitter is like 5.3 Volt
and so the voltage across the 2SK170 is only like ~5 Volt.

The current from 2SK170 just passes trough the 2SC1775 and gets to the 1k resistor.
In this circuit 2SC1775 and 2SA872 cascodes the complementary JFET pair.

If you use 2 equal resistors, for example 5k, to divide 30 Volt
than each JFET will have like 14-15 Volt drain-source.

Cascoding with bipoar transistors is very often used for to help JFETs.
Not only because they can not take the high voltage
but also because they work best at a certain lower voltage.

In the Hiraga above, the reason is not high voltage (12Volt)
for cascoding .. it is because the JFETs perform better = at 5-6 Volt.


Lineup

[kzeprf22]

Thanks for pointing out this schematic, I have though a question. Why in the schematic we take the output out of the BJT collector and not from the jfet drain as we would do if we didn't cascade them and use the BJT just for consuming power? If we take the utput out of the BJT collector, then we are not take advantage of the quality of the jfets (my opinion).

[lineup]

It is true that some minimal quality will be lost
as the current will have to pass the BJT, as any component will degrade signal a bit.

But in this case, the designer has figured out, that the gain in quality of operating the JFET at the lower voltage chosen
gives a better total result,
than if operating the JFET at higher voltage would have done.
Even counted for the current now has to pass another BJT.

2SK170 works not at best, when the drain-source voltage get above 10 Volt something
It can work, for sure, upto 40 Volt .. but there are some parameters that will be less good.
I do not know exactly WHAT, but some other may tell us the correct reason.

[AndrewT]

The cascode does a double function.
It limits the voltage on the input transistor.
It also operates as a common base amplifier.
We take the output from the collector of the common base amplifier.
You can also use a Jfet as the common gate amplifier with similar results. See Borbely for examples.
There is a third advantage to using the cascode loading of the input transistor; namely, it keeps the input transistor at near constant voltage and this results in almost no change in capacitance of the input device resulting in elimination of much of the non-linearity that device capacitance causes.

[leapcat]

Originally posted by kzeprf22
Thanks for pointing out this schematic, I have though a question. Why in the schematic we take the output out of the BJT collector and not from the jfet drain as we would do if we didn't cascade them and use the BJT just for consuming power? If we take the utput out of the BJT collector, then we are not take advantage of the quality of the jfets (my opinion).

Because the BJT's are just on. The JFETS still control the signal.
(IMHO)

[jacco vermeulen]

Originally posted by lineup
the JFETs perform better = at 5-6 Volt.

http://www.diyaudio.com/forums/showt...72#post1657272