• Gianluca Sperti

A jFET is for ever

JFET's are widely used in our BeCubes with different purposes as they are flexible and exhibit high performance, superb linearity and low noise. Despite the acclaimed success in the audio Industry due to the sound quality and clarity of jFET based electronics, the production of 2SK170 and 2SJ74, the most famous and audio oriented FET's, was discontinued by Toshiba a decade ago. They can still be found on the web even thou care must be taken as fake transistors are now populating eBay and certain online shops; existing stocks are exhausting and fewer appears available day after day: they are becoming like some rare vacuum tubes and getting really expensive. So, if you have some, look after them as the most precious piece of jewellery.

Fortunately Linear Systems from Fremont California has been producing transistors for 30 and more years and is offering classical single and dual monolithic FETs as well as new products. John Hall (1932-2014) was a pioneer of the technologies that allowed the birth of many devices we use daily and after 45 years of work in the semiconductor industry and over 60 patents, he founded Liner Systems.

We at AudioDinamica prefer differential topologies and the LSK389, a very low noise dual monolithic jFET, comes handy to boost the performance of our phono preamplifiers as it contains in the same package two tightly matched devices that share the same substrate: this allows a sensible reduction of distortion, thermal drift and DC offset while assuring exceptionally low noise and high gain in a simple effective and accurate sounding design.

We also use the LSK489, a dual monolithic jFET entirely developed at Linear System: when used as input differential stage, its very low capacitance facilitates the interfacing with other devices and eliminates intermodulation distortion due to non linearities of the input impedance. The effect on the sound can suddenly be recognised as better focus and accuracy and a superior and natural dynamic.

Linear Systems also offers the p-Channel LSJ74, complimentary to LSK170; its production rate is much lower then the n-Channel due to the complexity of the manufacturing and the very high quality standards Linear System is imposing. Used as complimentary output buffer, LSJ74 and LSK170 can offer very low output impedance and a vanishing distortion. Needless to say that this buffer sound so neutral that we can hardly find acceptable replacements. Clearly n-Channel and p-Channel jFET's need to be matched for a perfect buffer, we do that meticulously selecting from large batches of jFETs the best pairs.

There are multiple way to skin a cat and even more to build a preamplifier or an amplifier. BeCubes are entirely based on jFET's with no compromise on quality and regardless of the associated costs. If you want to experience the sound quality and accuracy of a fully differential jFET device give us a call, we'll be happy to help you.

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