2SK170-BL

The 2SK170-BL is a high-performance N-channel junction field-effect transistor (JFET) from Toshiba, specifically engineered for ultra-low-noise audio amplifier applications. This device has achieved legendary status among audiophiles and professional audio designers for its exceptional noise characteristics, making it the gold standard for sensitive preamplifier input stages. The ‘BL’ suffix denotes the IDSS grade, with drain current ranging from 6.0mA to 12mA at zero gate-source bias. With a forward transfer admittance (transconductance) of 22mS typical at 3mA drain current, the 2SK170-BL provides substantial voltage gain in single-stage configurations. Its ultra-low noise performance, with an input-referred voltage noise density of just 0.95 nV/√Hz at 1kHz and a noise figure of 0.5dB typical, ensures that even the weakest input signals are amplified with minimal added noise. This makes it particularly well-suited for moving-coil (MC) phono cartridges and high-quality microphone preamps where signal levels can be extremely low. The device’s high input impedance, with gate leakage current of only 1nA maximum, prevents loading of high-impedance signal sources. The 40V gate-drain breakdown voltage provides adequate headroom for most preamplifier designs. The 2SK170 is available in three IDSS grades: GR (2.6-6.5mA), BL (6.0-12mA), and V (10-20mA), allowing designers to select the appropriate operating current for their specific circuit requirements. Packaged in the widely-used TO-92 format, this transistor is easy to work with in both prototype and production environments. The 2SK170-BL is recommended for the first stages of phono equalizers, MC head amplifiers, and any application where preserving signal purity at low signal levels is critical to audio performance.

The 2SK170-BL is an N-channel junction field-effect transistor that operates as a voltage-controlled current device. Unlike bipolar transistors that are current-controlled, JFETs use an electric field to modulate the conductivity of a semiconductor channel. The N-type channel between the drain and source terminals has its cross-sectional area controlled by the electric field from the reverse-biased P-type gate region. When the gate-to-source voltage (VGS) is zero, the channel is fully open and maximum drain current (IDSS) flows. As VGS becomes more negative, the depletion region expands, narrowing the channel and reducing drain current. At the pinch-off voltage (VGS(off) of -0.2 to -1.5V), the channel is closed and drain current falls to nearly zero. The forward transfer admittance (transconductance) Yfs of 22mS represents the change in drain current per unit change in gate voltage, determining the amplification capability. The low-noise characteristics of the 2SK170 stem from its optimized channel geometry and doping profile, which minimize both thermal noise and flicker (1/f) noise. The junction gate structure provides inherently lower noise than MOSFETs because there is no oxide-silicon interface to generate additional noise sources. In audio preamplifier applications, the 2SK170 is typically used in common-source configuration as the first gain stage, where its high input impedance and low noise figure are critical for preserving signal integrity from high-impedance or low-level signal sources such as phono cartridges and microphones. The high transconductance enables high voltage gain in a single stage, reducing the total number of gain stages needed and minimizing accumulated noise.

• Phono preamplifiers for turntables (both MM and MC cartridges)
• Professional microphone preamplifiers and studio recording equipment
• High-end audio preamplifier input and gain stages
• Differential amplifier input pairs for instrumentation and audio
• Buffer stages for impedance matching between high-impedance sources and low-impedance loads
• Guitar and bass preamplifiers for musical instruments
• High-impedance sensor signal conditioning (piezoelectric, photodiode)
• Radio frequency and RF low-noise amplifier front-ends