Recommend JFETS for audio

If I were to do a project from scratch anybody have any recommendations for a specific JFET to use?

Please state the specific application in which you desire to use them. (input stage, voltage amp stage, output, etc.)

Common picks by professionals:
2SK170, N-CH
2SJ74, P-CH
2SK389, Dual N-CH
2SJ109, Dual P-CH
2SK246, N-CH
2SK30, N-CH
2N5564, Dual N-CH
2N5457, N-CH

My picks, FWIW:
Low noise JFETs for input stage
PN4392, N-CH
PN4393, N-CH
Medium gm JFETs for voltage amplification
J270, P-CH
J271, P-CH
Small signal MOSFETs for output stages
ZVN3310A, N-CH
ZVP3310A, P-CH

Cheers,
Tamas

Thanks that's exactly what I meant :green:. Recommended JFETs for audio and their application Thanks.

And J112 for switching ?

As used a lot and for instance also like in the electronic switching pt-II article in EW-feb2004 of Douglas Self. If it's OK to provide scans of recent magazine articles I can provide.

Bye,

Peter

Thats great stuff Tamas - cheers! :thumb:

Just a little bump up, with a question.

I have been reading the Borberly JFET articles and am planning on experimenting with a few discrete JFET opamp designs.....he mentions paralleling the differentional input pair to reduce noise.

After reading the Hardy 990 document, it became clear that there would be a substantial improvement in noise performance by using a device like the monolithic LM394 bipolar transistor chip.

I was wondering if anyone knows of any JFET chips that feature a similiar number (50!) of transistors paralled on a chip? Or would a couple of 2SK389 still offer comparable noise performance compared to the LM394?

I'm going to compare specs from the datasheets...........but thought I'd ask.

Cheers Tom

[quote author="Viitalahde"]This is GREAT stuff! :thumb:

Just what I need to get myself started.. I wanna play with discrete op-amps once I have some time.

Thanks![/quote]

Yea me too. I had to build an input and output stage for a discrete opamp as an electronics projects as an undergraduate. We had to use BJTs. It worked (passed signal), but I'm sure it would've sounded like crap. Everyone else in the class thought it was the worst project, but I loved it. I'd like to give an honest attempt to try and build a nice discrete opamp. Maybe one day !?!

[quote author="TomWaterman"]I was wondering if anyone knows of any JFET chips that feature a similiar number (50!) of transistors paralled on a chip? Or would a couple of 2SK389 still offer comparable noise performance compared to the LM394?
[/quote]

Tom,

The advantage of JFETs is their low, for practical purposes non-existent, current noise, and medium to high voltage noise. This makes them excellent input devices for high source impedances.
The BJT based, LM394 has just the opposite characteristics. It has high current noise, but very low voltage noise. This makes them suitable for coupling to low input impedances.
There are no known dual JFETs, to me, with dozens of devices paralleled inside. Traditionally when you need really low voltage noise engineers reach for BJTs.
You usually do not need both current and voltage noise to be ultra-low in order to optimize noise performance. First, you need to make a decision as to what sort of impedance you will be interfacing with. Then you need to make sure that the total noise that includes both current and voltage noise does not exceed your goals. You can find examples for the math on the web.

As you notice John Hardy uses the JH990 with a low impedance input transformer because the opamp will give you the least noise if the source impedance (Rs) is around 1k Ohm.
If you to try it paralleling two 2SK389 FETs should give you both excellent current and voltage noise figures (paralleling two devices gives you only 30% of voltage noise reduction). Depending on the source impedance it may not be necessary at all.

That FET is really unique (and now obsoleted by Toshiba) in that it has unusually low input voltage noise and has high gain. There are extremely low noise JFETs like the 2SK30, but their gain is miserably low to make decent first differential amp stages out of them. The closest contender FETs with decent gain are the 2N5566 and 2N4393 with two to three times of the 2SK389 noise. You would have to hook eight of them in parallel to get the voltage noise figure of the 2SK389.

Did I answer your question? It is eleven at night and I am just rambling on.

Cheers,
Tamas

Hi Tamas!
Thats excellent - just what I was wondering - thank you muchly!

I remember thinking something similar when reading the 990 PDF. I was thinking the 990 would be better suited to coupling to a low impedance mic in a preamplifier design....whereas JFETs would be more suitable to high impedance line inputs? Not that the 990 makes a poor line level device.....or JFETs an unsuitable choice for a pre but .....oh I don't know what I'm saying :razz: rambling...

But I will be using the devices in (hopefully) an instrumentation based amp for line level buffering. I'm choosing JFETs for their low noise and good RF rejection....there will probably digital control in the same box and I don't want to contaminate my audio path with RF. The main amplifer in the buffer will be a Forssell992. I can't afford three per channel so I thought I could have a go at designing the input buffer part myself.

You can find examples for the math on the web.

Click to expand...

I'll look around.....just getting into this, the Borberly and Pass documents were a good intro.

If you to try it paralleling two 2SK389 FETs should give you both excellent current and voltage noise figures (paralleling two devices gives you only 30% of voltage noise reduction). Depending on the source impedance it may not be necessary at all.

Click to expand...

Cheers! Thats something I will try for sure then.....is the 2SK389 obselete? Bloody hell that doesn't bode well - what about a bunch of matched 2SK170s? I noticed that they are/were used in Freds 992.

Did I answer your question? It is eleven at night and I am just rambling on.

Click to expand...

You most certainly did - once again thanks! I love this place... :guinness:

Cheers
Tom

is the 2SK389 obselete?

Click to expand...

Yes, I know it totally sucks. There are still stocks available, but the price is going up every few months.

what about a bunch of matched 2SK170s? I noticed that they are/were used in Freds 992.

Click to expand...

Yes, if you match Idss within a few percent it should work fine. I think Fred buys a thousand 2SK170s at a time and sorts them.

Tamas

Hey Tamas - that sure sucks.....

I will try to get a load of 389s before there all gone......maybe more people would be interested so we could get a load at once????

I have drawn a very very rough schematic with the help of the borberly papers and planned on the 389s in many configurations - I like the idea of the matching already done for me!! Should be able to get really low noise and offset with these.

Any one got a good source for 2SK389s (or the JFets in general) - I can't find them (389s) at Farnell?

Cheers Tom

Try...

http://www.bdent.com/

They usually have the duals and singles in stock. I've never purchased from them, so I can't vouch for them as a company.

I use a pair of the 2SK170 single in place of the K389 all the time, and yes, I do buy them in 10k quanities. However, I still use the K389 in critical locations. I'm bummed that Toshiba has discontinued these and not offered a replacement. They better not to that to the K170/J74 parts!

Good luck.

Thanks a lot Fred!

It seems to me that all the good audio parts are being discontinued - don't you find it worrying that you may not be able to build your 992 in the near distant future?

10k - thats a nice large amount, how long does that last?? :grin:

Fred if I may be so bold - I have been looking at your 991 schematic and was wondering how different the 992s are that I have? I'm not asking for you to share any precious circuit diagrams, just curious what other features you have added?

Cascodes, current mirrors etc

Cheers Tom

[quote author="TomWaterman"]It seems to me that all the good audio parts are being discontinued [/quote]
Types as SK170, and the others from Toshiba were produced
for low-noise inputs in casette recorders.
Now, casette players are obsolette, low noise low
impedance devices in mass production are obsolette.

And studio usage? It is too small amount to produce
it by normal mass producer.

xvlk

Hi!

So what else can be used when they are all gone??

Tom

[quote author="TomWaterman"]Hi!

So what else can be used when they are all gone??

Tom[/quote]

The 2SK170 is still listed as production device on the Toshiba web site. I am planning to get 1k of them before the end of the year.

I think there are still a few good n-channel JFETs out there. For a dual device the 2N5566 works well, but you have to account for the lower gain. Also this is a pricey part at $6 a pop. For single devices the 2N4393/PN4393 or 2N4392/PN4392 work and these are made by several vendors. Cheap but need a lot of sorting.
I am sure Fred knows more about possible substitutes than I do.

Hi Tamas

Sorry I missed your reply there for a while.....

Thanks for the suggestions - when are you planning to buy some 2SK170s? I may be up for a few to make your order up.....what kind of price are they each? I guess I might be able to swing a hundred...maybe.

Cheers Tom

Tom,

I am looking at the November time frame to get one thousand. At that quantity they cost US$0.59 each. Currently I am concentrating stashing away a couple of hundred of 2SK389s.

Tamas

I was looking at the PRR's thread suggesting using a 48V supply and I can't find data sheets for the 2SK170's so can any of the suggested fets run this high?

[quote author="ChumpitO"]I can't find data sheets for the 2SK170[/quote]

Look Here

Also are you guys looking at 2SK170GR? There are 4 different Idss groups for that one. 2SK30GR is the same Idss btw, but "GR" is the highest for K30 while K170 goes to "V" (10 to 20 mA) Typical noise figure is 0.5dB for either part but max is 2dB for K170, 5dB max for K30. We have used Toshiba 2SK30GR for several years in audio products at my daytime job, with very good results. I used this part in a CountryHick DI and it has good gain and is quiet!

Peace!
Charlie

Thanks for the tip Charlie!

I'll check them out..

Tamas, let me know if you want a hand adding to your total buy of the 2SK170s - I may be in for 1-200 at that price if its not too much trouble.

As a general rule how many JFets would one go through to get, say 20 pairs well matched? I'm guessing its a lot..

Cheers Tom