2SK170-BL - Low Noise N-Ch JFET TO-92 | Toshiba
MPN: 2SK170-BL β End of Life| Qty | Unit Price | Extended |
|---|---|---|
| 1 | $3.5 | $3.50 |
| 10 | $3.15 | $31.50 |
| 100 | $2.8 | $280.00 |
| 500 | $2.45 | $1,225.00 |
| 1,000 | $2.1 | $2,100.00 |
2SK170-BL Overview
A JFET (junction field-effect transistor) is a voltage-controlled semiconductor device that conducts current through a channel whose width is modulated by the gate-source voltage. In the broader hierarchy, the 2SK170-BL sits within small-signal transistors -> discrete semiconductors -> analog front-end devices. Unlike MOSFETs, JFETs have a Schottky-free p-n gate with very low input bias current and inherently low 1/f noise, which is why they dominate the input stages of premium audio preamplifiers.
Key features of the 2SK170-BL include its low noise figure tailored for audio-band operation, high input impedance typical of junction-gate devices, a three-grade Idss selection system (GR, Y, BL) that lets designers bin devices for consistent bias points, and the ubiquitous TO-92 package for easy hand assembly and prototyping. The BL grade, with the highest Idss range of the family, delivers the strongest transconductance, improving gain-bandwidth and lowering equivalent input noise in moving-coil phono stages and microphone preamps.
Technically, the device achieves its performance through a carefully optimized N-channel silicon junction structure with minimized gate leakage and geometry tuned for audio-frequency transconductance rather than switching speed. Designers routinely parallel devices or cascode them to further reduce noise and stabilize drain voltage, since absolute Vds ratings must be respected.
Typical applications include phono preamplifier input stages, dynamic and condenser microphone preamps, moving-coil step-up head amplifier circuits, and high-impedance instrument buffers. Its low noise and high input impedance make it the classic choice wherever a signal source is weak and impedance is high.
A key design consideration: the 2SK170-BL is discontinued by Toshiba and original-stock parts command premium pricing and encounter counterfeits, so verified sourcing and a pin-compatible substitute such as the Linear Systems LSK170 series should be qualified early in new designs.
This page synthesizes datasheet data, distributor listings, verified drop-in alternatives, and practical design notes not found in the manufacturer datasheet.
Drop-in alternatives for 2SK170-BL β same package, pin-to-pin compatible. Different-package parts requiring PCB rework are excluded.
Variants in this series
Same-series models that are drop-in compatible with 2SK170-BL (same form factor and footprint) β differing in Mounting Type, Package, Configuration, Device Type, Transistor Type.
Quick Comparison Tool β Select alternative parts for side-by-side comparison:
2SK117-BL
β Drop-In β οΈ εζ°εΎ ιͺθ―π Reference alternative (not in catalog)
2SK170-GR
β Drop-In β οΈ εζ°εΎ ιͺθ―β In Stock
$0.5 / Unit
View Datasheet βLSK170C
β Drop-Inπ Reference alternative (not in catalog)
LSK170B
β Drop-Inπ Reference alternative (not in catalog)
LSK170A
β Drop-Inβ In Stock
$0.94 / Unit
View Datasheet β2SK170-BL Maximum Ratings & Electrical Characteristics
| Transistor Type | N-Channel JFET (Junction FET) |
| Technology | Silicon N-Channel Junction Type |
| Primary Application | Low Noise Audio Amplifier |
| Drain Current Rating | 12 mA |
| Drain-Source Voltage | 40 V |
| Grade / Idss Rank | BL (highest Idss grade of 2SK170 family) |
| Package | TO-92, 3-pin |
| Mounting Type | Through-Hole |
| Polarity | N-Channel |
| Configuration | Single JFET |
2SK170-BL Pin Configuration
| Pin 1 | S β Source |
| Pin 2 | G β Gate |
| Pin 3 | D β Drain |
Typical Applications
2SK170-BL is suitable for 6 applications: Phono Preamplifier Input Stage, Microphone Preamplifier Front End, Moving-Coil Step-Up Head Amplifier, High-Impedance Instrumentation Buffer, Guitar Effects Pedal and Instrument Input, Discrete Analog Audio Signal Switching and Volume Control.
Phono Preamplifier Input Stage
The 2SK170-BL is the archetypal input device for moving-magnet and moving-coil phono stages. Cartridge sources present sub-millivolt signals from impedances that can reach kilo-ohms, so input voltage noise and gate leakage directly determine the noise floor. The BL grade's high Idss gives maximum transconductance, minimizing equivalent input noise, and the near-zero gate current avoids DC offset across cartridge coils. Circuits commonly parallel two devices to halve voltage noise, or place the JFET into a cascode with a higher-voltage transistor to keep drain voltage stable while preserving the JFET's low-noise advantage. Substitutes must match the Idss bin to keep bias currents in the datasheet low-noise region.
Recommended
Microphone Preamplifier Front End
Studio condenser and ribbon microphone preamps demand sub-nanovolt-class input noise and very high input impedance. The 2SK170-BL serves as the first amplifying device in discrete front ends, where its junction gate contributes negligible bias current and the BL-grade transconductance suppresses flicker noise in the audio band. Typical topologies pair the JFET with a bipolar second stage or an operational amplifier to add open-loop gain. Because phantom-power rails reach 48 V, designers often cascode the JFET so its 40 V-class drain-source rating is never exceeded. Replacement devices should be Idss-matched to preserve the designed operating point and noise performance of the original circuit.
Recommended
Moving-Coil Step-Up Head Amplifier
Moving-coil cartridges output only tens of microvolts, requiring an active step-up head amplifier with the lowest achievable input noise. The 2SK170-BL's low noise figure and high transconductance make it a preferred discrete solution; multiple JFETs are frequently paralleled, which divides input voltage noise by the square root of device count while multiplying input capacitance - acceptable at MC source impedances. The BL grade's higher Idss supports the milliamper-level bias currents these head amps run for optimal noise. Original-stock devices are prized here for authenticity in restoration builds, while new constructions use LSK170 series parts with equivalent Idss grading.
Recommended
High-Impedance Instrumentation Buffer
Measurement instruments probing piezo sensors, pH probes, and high-impedance photodiodes need buffers with femtoampere-class input current. The junction gate of the 2SK170-BL provides extremely low gate leakage and high gate-source impedance, allowing it to sense megohm-to-gigohm sources without loading. Configured as a source follower, the device delivers near-unity gain with the high linearity needed for precision front ends. The TO-92 package simplifies guarding and cleanliness layout on through-hole prototypes. Designers must keep drain-source voltage within the 40 V class rating and manage leakage paths on the PCB, since board contamination can dominate the JFET's own gate current.
Recommended
Guitar Effects Pedal and Instrument Input
Discrete guitar stompbox designers favor the 2SK170-BL for input buffers, fuzz front ends, and JFET-based amplitude-control stages because it emulates the soft-clipping character and high input impedance that vintage tube circuits present to pickups. The DIYstompboxes audio community extensively documents bias networks around its GR/BL Idss grades; the GR grade suits lower-current roles while BL supports higher-headroom stages. Since the original is discontinued, pedal builders qualify the LSK170 series or 2SK117-BL equivalents, noting that Idss binning differences shift gate bias points and clipping thresholds slightly. Matched-pair selection from the diyAudio Store simplifies consistent batch production.
Recommended
Discrete Analog Audio Signal Switching and Volume Control
JFETs act as voltage-controlled resistors in analog audio paths, and the 2SK170-BL is a classic choice for clickless signal muting and stepped attenuator implementations. With the gate driven well beyond pinch-off the device turns off; near pinch-off its channel resistance varies linearly with gate voltage, enabling low-distortion gain control when a fraction of the drain-source signal is maintained small. Its low gate leakage prevents clicks, and the BL grade's high transconductance keeps on-resistance low for minimal insertion loss. Pairing with the complementary 2SJ74-BL enables symmetric switching circuits with cancellation of control-signal feedthrough in high-end preamplifiers.
Recommended
Recommended Products Summary
Engineering reference data for 2SK170-BL β comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | 2SK117-BL | 2SK170-GR | LSK170C | LSK170B | LSK170A |
|---|---|---|---|---|---|---|
| Package | TO-92 (3-pin) | TO-92 - same | TO-92 - same | TO-92 - same | TO-92 - same | TO-92 - same |
| Brand | Toshiba | Toshiba | Toshiba | Linear Systems | Linear Systems | Linear Systems |
| Lifecycle Status | Discontinued (EOL) | Discontinued (EOL) | Discontinued (EOL) | Active / in production | Active / in production | Active / in production |
| Idss Grade | BL (highest grade) | BL grade equivalent | GR (lowest grade, ~2.6-6.5 mA class) | Grade matched to BL range | One grade below C | Lowest LSK170 grade |
| Primary Application | Low noise audio amplifier | Low noise audio amplifier | Low noise audio amplifier | Ultra low noise audio/acoustic | Ultra low noise audio/acoustic | Ultra low noise audio/acoustic |
| Substitute Documentation | Original device | Community-verified same-family substitute | Same datasheet family | Manufacturer-documented pin-for-pin replacement | Manufacturer-documented pin-for-pin replacement | Manufacturer-documented pin-for-pin replacement |
Key Differentiators
- Highest Idss grade (BL) for lowest noise input stages (vs 2SK170-GR)
- In-production availability of the documented replacement (vs LSK170C)
- Same-family BL-grade substitution path (vs 2SK117-BL)
Design Notes
The 2SK170-BL is discontinued and the secondary market is saturated with counterfeits. Purchase only from sellers offering traceable original stock (Xecor, Lisleapex, diyAudio Store matched lots) and verify Idss on receipt with a simple gate-tied-to-source drain-current test. Measure and record Idss for every device before assembly - genuine BL devices cluster in the highest Idss bin, and fakes typically fall outside the grade range, revealing themselves immediately.
Keep drain-source voltage within the 40 V class rating of the device. In phono and microphone preamps fed from 48 V phantom or higher supply rails, use a cascode transistor above the JFET so the 2SK170-BL never sees the full rail voltage. Bias current should be set from the measured Idss of the specific device - BL-grade parts need individual bias networks or matching to hold the operating point in the datasheet's low-noise region.
Paralleling two or more 2SK170-BL devices divides equivalent input voltage noise by the square root of the device count, at the cost of multiplied input capacitance. This is beneficial for low-impedance sources such as moving-coil cartridges but harmful for high-impedance sources where capacitive loading rolls off treble. Match paralleled devices for Idss within 10% to prevent current hogging, and add individual gate-stopper resistors if used at RF-exposed inputs.
Gate leakage of a junction FET is extremely low, but PCB surface contamination can create leakage paths that dwarf it. Place a guard ring driven by the source potential around the gate net, keep the gate trace short and away from supply rails, and clean flux thoroughly after soldering. On through-hole TO-92 layouts, position the device with adequate clearance so heat from nearby parts does not raise its junction temperature and drift the bias point.
Compliance Information
Original Toshiba 2SK170-BL predates modern compliance reporting and the device is discontinued; the Linear Systems LSK170 replacement series is offered in RoHS-compliant packages per Linear Systems literature.
