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Linear Systems Matched LSJ74/LSK170 JFET pairs (Grade B)
Linear Systems Matched LSJ74/LSK170 JFET pairs (Grade B)
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Old 24th April 2018, 09:50 PM   #11
rsheptak is offline rsheptak  United States
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Glad to hear about alternate sources for matched pairs of lsk170BL/lsj74BL since the DIYAudio store seems to be out of them more often than they are in-stock. They are currently out of the matched pairs, and all BL and GR grades of the lsj74.

rus
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Old 28th October 2018, 05:35 PM   #12
Alex Cazanescu is offline Alex Cazanescu
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Good evening!
I need an advise regarding the LINEAR SYSTEMS JFETS LSJ74 and LSK170. Unfortunately I’m a beginner in this field and I think I’ve made a mistake. I intend to build the F6 amp and I have ordered LSJ74 Grade A and LSK170 Grade B.I had the feeling they have to be matched only on each type of JFET ( 2 matched LSJ74 and 2 matched LSK170) , now I realize that all 4 of them have to be matched. Am I right? In this situation do I have any chance to match them somehow? what is the range anyway (10 %...)?

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PS: sorry for my English …it’s another thing that I have to improve :-)
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Old 28th October 2018, 09:35 PM   #13
Nelson Pass is offline Nelson Pass  United States
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Linear Systems Matched LSJ74/LSK170 JFET pairs (Grade B)
For the F6, we will see a small amount of 2nd harmonic creep in with
mismatched P and N, but it isn't very serious, and since we routinely
adjust 2nd harmonic at the output stage, you should be able to null
that if you want.
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Old 29th October 2018, 04:30 PM   #14
Alex Cazanescu is offline Alex Cazanescu
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Dear Sir , thank you very much for your kindness.
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Old 21st February 2019, 07:24 PM   #15
EUVL is offline EUVL  Europe
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Default Idss and Yfs Distributions

For those who no longer have access to NOS Toshiba’s JFETs, the Linear System equivalent (LSK170 and LSJ74) becomes the only alternatives.

However, little data or statistics have been published about these substitutes.
Earlier, we have published some noise data for the LSK170 compared to the original 2SK170.
Recently, we have gained access to 50 pairs of the LS devices (B grade), and have taken some measurements of them.

So here they are.
They would no doubt be of interest to those who intend to use them instead of paying extra for the NOS Toshiba’s.

Firstly the Idss and Yfs distribution of both the N.channel and P-channel JFETs.
As can be seen, there is quite a bit of spread in Yfs for the same Idss.
This is especially so for the LSJ74.

Furthermore, while the LSK170s have very similar Yfs as the Toshiba’s, the LSJ74s have much lower Yfs (~25mS) than the 2SJ74 (~40mS).
This should be taken into account when trying to substitute the 2SJ74s directly with 2SK170s.


Patrick

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Attached Images
File Type: png LS Yfs vs Idss.png (93.5 KB, 295 views)
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Old 21st February 2019, 07:27 PM   #16
EUVL is offline EUVL  Europe
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Default How about the matching yield ?

A matching pool of 50 pairs is a reasonable size for hobbists, especially considering the asking price of the LS JFETs.
Because of the large difference between the N and P channel devices in transconductance, we can only aim at matching Idss of all 4 devices.
And then transconductance between the N devices, and between the P devices respectively.
The so called NN-PP match.

The first thing to notice from the distribution shown above is that there are no N devices below 7mA Idss, and no P devices above 11mA Idss.
Those outliners will not find NN-PP partners, and can therefore serve as as NN and PP pairs only.
For those with Idss between 7mA and 11mA Idss, only 14 quads of NN-PP can be found; i.e a reject rate of 42%.

Those 13 quads can further be classified as :

3x __ Class 0 __ Idss to < 0.05mA; NN-PP Yfs <1.5%
7x __ Class 1 __ Idss to < 0.1mA; NN-PP Yfs < 3%
2x __ Class 2 __ Idss to < 0.2mA; NN-PP Yfs < 3%
2x __ Class 3 __ Idss to < 0.3mA; NN-PP Yfs < 5%

In our experience, the yield will only improve significantly with a much larger pool, e.g. 500 pairs.


Patrick
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Old 21st February 2019, 07:29 PM   #17
EUVL is offline EUVL  Europe
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Default Why is matching of Yfs important ?

Suppose you want to use a pair of matched LSK170Bs in the input differential pair of Borbely EB602 headphone amplifier.
The LSKs have perfectly matched Idss of say 8mA, but different Yfs by say 3%. The diff pair in the EB602 is biased by a CCS of 4mA.
So in case of perfectly matched LSK’s, each device will be biased at 2mA.
With a 3% mismatch in Yfs, they are more like 1.9mA and 2,1mA respectively, at the same Vgs of about -0.6V.
So a perfect match pair is now only 10% matched, when not operating at the matching current.

And yes, you will have a better match when using devices with Idss closer to the operating current.
But you will most likely also find out that trying to get LSK170A is a much more difficult and expensive exercise.

Not all matched quads are born equal……


Patrick
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Old 21st February 2019, 07:31 PM   #18
EUVL is offline EUVL  Europe
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Default Curve tracing

Two LS JFETs with 8mA Idss were selected, curve traced and compared to the same from Toshiba.
As can be seen, while the LSK170 has essentially identical characteristics as the 2SK170 with the same Idss,
the much lower Yfs of the LSJ74 mentioned above can clearly be observed here again.


Patrick

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File Type: png 2SK vs LSK.png (57.9 KB, 290 views)
File Type: png 2SJ vs LSJ.png (57.7 KB, 298 views)
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Old 21st February 2019, 09:24 PM   #19
Russellc is online now Russellc  United States
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Linear Systems Matched LSJ74/LSK170 JFET pairs (Grade B)
Quote:
Originally Posted by Dennis Hui View Post
That's a lot of jfets you need. Should you also be interested in the original Toshibas,
you can try contacting Spencer (FET Audio | Hi-End Audio Projects)
Last time I checked with Spencer, 9.5 idss was as low as he had in matched pairs. Punkydawgs may still have some?

Russellc
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Old 22nd February 2019, 12:32 AM   #20
vdi_nenna is offline vdi_nenna  United States
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Linear Systems Matched LSJ74/LSK170 JFET pairs (Grade B)
Bought another set of quads from punkydawgs today.
Already shipped. Pricey but they've got them.

Toshiba 2SJ74 + 2SK170 LAB MATCHED QUAD to 0.03mA AND 4mV (7ma range) | eBay
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Last edited by vdi_nenna; 22nd February 2019 at 12:35 AM.
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