The LSK3S9 by Linear Systems got some attention in the audio diy community, roughly 2004-2010.
It's supposed claim to fame was low noise.
It's still avail today, and bit a pricey.
Not sure why the hype, other than than the manuf advertising this device in certain mags.
Why not simply use two matched JFET transistors?

Here is phono preamp design from 2007. AX magazine:

https://gammaelectronics.xyz/images/audioX_907_797_f2.jpg


https://gammaelectronics.xyz/audiox_09-2007-LP797_.html

 

The LSK3S9 by Linear Systems got some attention in the audio diy community, roughly 2004-2010.
It's supposed claim to fame was low noise.
It's still avail today, and bit a pricey.
Not sure why the hype, other than than the manuf advertising this device in certain mags.
Why not simply use two matched JFET transistors?

Here is phono preamp design from 2007. AX magazine:

https://gammaelectronics.xyz/images/audioX_907_797_f2.jpg


https://gammaelectronics.xyz/audiox_09-2007-LP797_.html

Because you'd have to spend time matching them.
Just look at the datasheet for an average JFET.
https://www.onsemi.com/download/data-sheet/pdf/2sk3557-d.pdf
The Idss (drain current at Vgs=0) can vary from 10mA to 32mA and the cutoff voltage can vary from -0.3V to -1.5V.
Even if you bought a dozen, you might not find a matched pair.

(Though I do think it's a bit daft buying a matched pair and wiring them in parallel - if you were using it as a long-tailed pair, it would make more sense)
The 47k load resistance will be the dominant source of noise above the LR time constant of the cartridge (usually about 150Hz), so the factor of √2 reduction in noise by paralleling them might be a waste of time.

 

Because you'd have to spend time matching them.

Note that even the LSK359 had to be selected/matched for in the batch the author noted:

Q1
The LSK3S9 is advertised in audioXpress as a “mV (per _/Hz) Low Noise Dual JFET,” by Linear Integrated Systems, (510) 490-9160 or (800) 359-4023. The datasheet (linearsystems.com) specifies 0.9nV _/Hz typical, 1.9 maximum at 1kHz, 2mA, 10V.
I measured six samples; four had noise less than 1nV/ _/Hz, but the other two were 2.18 and 2.83 nV _/Hz. All had an IDSS within the 6-12mA spec for the “B” category (LSK389B TO-71 used here). However, considering the advertised 1nV/ noise, you should request units meeting this number. The price as of last fall was $7.29 (1-99) and $5.87 (100-999).

 

47k resistor has a voltage noise of 27nV/√Hz at 25°C, who cares about the odd 1 or 2nV in the transistor?

 

Not sure why the hype, other than than the manuf advertising this device in certain mags.
Why not simply use two matched JFET transistors?

It's mentioned in the datasheet. In addition to IDSS and threshold voltage matching, they track better thermally.

 

Better thermal tracking is good reason to choose the monolithic device. Damn, these phono stages can get noisy and hummy quick. I might DIY this unit after all.

 

Better thermal tracking is good reason to choose the monolithic device. Damn, these phono stages can get noisy and hummy quick. I might DIY this unit after all.

Yes, but you can get a 2SK3557 much cheaper!