A functional replacement for Toshiba's discontinued 2SK389 low-noise dual JFET is available from Linear Integrated Systems, which devotes itself to specialty semiconductors. In fact, the LSK389 series of 1-nV n-channel monolithic dual JFETs improves upon the original.
When it was available, Toshiba's 2SK389 was used in audio amplifiers and preamps, discrete low-noise operational amplifiers, and battery-operated audio preamps. It also went into audio mixer consoles, acoustic sensors, sonic imaging, instrumentation amplifiers, microphones, sonobuoys, hydrophones, and chemical and radiation detectors.
Linear's devices are monolithic dual versions of the company's LSK170 single-n-channel JFET. Interleaving a pair of JFETs on the same piece of silicon eliminates matching and improves thermal tracking, compared to using separate discretes.
Three grades of IDSS ratings are available: 2.6 to 6.5 mA, 6.0 to12 mA, and 10 to 20 mA. On-chip, IDSS is matched within 10%. Gate threshold offset is 20 mV. Typical voltage noise (en) is 1 nV/Hz at 1 kHz and 2 nV/Hz at 10 Hz. Typical forward transfer admittance (Yfs) is 20 mS. Common-source input capacitance is 25 pF. And, the low input noise-to-capacitance product results in nearly zero popcorn noise.
The LSK389 series includes a thru-hole TO-71 six-lead package as well as a surface mount SOIC-8 for new designs. The thru-hole version isn't a drop-in replacement, however. Its pinout is different from the Toshiba part. Thousand-piece unit pricing is $3.
Linear Integrated Systems