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2SA970 and 2SC2240 Are Obsolete: How to Choose Safe Modern Replacements
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posted by Vincent Zhang
KSA992 and KSC1845 are practical starting points for many low-noise audio stages, but current rating, gain, pinout, bias, and circuit function must all be checked before fitting them.
Toshiba's 2SA970 (PNP) and 2SC2240 (NPN) became familiar choices in phono stages, tone amplifiers, preamplifiers, and power-amplifier input circuits. The original Toshiba devices are obsolete: Toshiba's discontinuation notice lists multiple 2SA970 and 2SC2240 variants, and Mouser separately lists the Toshiba 2SC2240 as obsolete with no manufacturer replacement.[11][3] Parts carrying the old number may still appear from other manufacturers or as old stock, but the printed number alone does not establish the same noise performance, gain grade, or traceable origin.
Why Replacement Needs Care
A transistor substitute is acceptable only when it fits the circuit, not merely because the polarity and package look similar. A technician must compare collector-emitter voltage, collector current, power dissipation, gain at the actual operating current, frequency response, capacitance, noise behavior, and lead arrangement. The circuit must then be checked for correct bias and stability after installation.
The key limitation: 2SA970 and 2SC2240 were rated for 100 mA collector current, while KSA992 and KSC1845 are rated for 50 mA.[1][2][4][5] This usually causes no problem in low-current input and voltage-amplifier stages, but it rules out an automatic substitution wherever fault or operating current can approach 50 mA.
What the Original Parts Were Designed to Do
The Toshiba datasheets describe both devices as low-noise audio transistors with 120 V collector-emitter ratings, 100 MHz typical transition frequency, and hFE grading of GR (200–400) or BL (350–700). The 2SC2240 datasheet specifically recommends it for the first stages of equalizer amplifiers and specifies low 1/f and pulse noise.[2] The 2SA970 provides the corresponding PNP characteristics.[1]
These details explain why a general-purpose transistor chosen only by voltage and current may produce more hiss, change the DC operating point, or alter high-frequency stability. Restoration work should preserve the electrical role of the original part.
The Practical Replacement Pair
For through-hole, low-current audio stages, the most useful starting pair is KSA992FTA for 2SA970 and KSC1845FTA for 2SC2240. onsemi describes both as audio-frequency low-noise amplifiers and lists them as complementary devices.[4][5] Their 120 V VCEO, 100 MHz typical fT, 500 mW dissipation, and 300–600 hFE range align well with many original low-signal applications.
Use the full suffix when ordering. As checked on the publication date, authorized-distributor pages list KSA992FTA and KSC1845FTA as active parts.[8][9] The January 2026 onsemi datasheet lists KSA992FTA and KSA992FATA while marking KSA992FBU and KSA992FBTA discontinued.[4] DigiKey also marks KSA992FBU obsolete.[10] A purchase specification that says only “KSA992” is therefore incomplete.
Figure 1: The common replacement route for low-current audio stages. Suitability still depends on the circuit.
Datasheet Comparison
| Parameter | 2SA970 | KSA992FTA | 2SC2240 | KSC1845FTA |
|---|---|---|---|---|
| Polarity | PNP | PNP | NPN | NPN |
| VCEO | −120 V | −120 V | 120 V | 120 V |
| IC maximum | −100 mA | −50 mA | 100 mA | 50 mA |
| Power dissipation at 25°C | 300 mW | 500 mW* | 300 mW | 500 mW* |
| DC gain | 200–700 at 2 mA | 300–600 at 1 mA (F) | 200–700 at 2 mA | 300–600 at 1 mA (F) |
| Typical fT | 100 MHz | 100 MHz | 100 MHz | 100 MHz |
| Typical Cob | 4.0 pF | 2.0 pF | 3.0 pF | 1.6 pF |
| Lead numbering | 1 E, 2 C, 3 B | 1 E, 2 C, 3 B | 1 E, 2 C, 3 B | 1 E, 2 C, 3 B |
*The onsemi 500 mW figure is tied to its stated PCB and ambient test conditions. Apply temperature derating and the actual board layout; never treat package dissipation as a guaranteed operating target.[4][5]
Pinout and Package Checks
The four datasheets number the leads 1–2–3 as emitter–collector–base. That makes the suggested through-hole pair mechanically convenient in many boards. Still, confirm the drawing for the exact manufacturer and suffix in hand. Flat-face orientation, formed leads, third-party versions, and PCB silkscreens can create mistakes even when the family name is familiar.
Figure 2: Datasheet lead numbering for the cited Toshiba and onsemi through-hole parts. Confirm the viewing direction and exact device suffix.
Gain Matching and Differential Pairs
Original GR and BL grades overlap the KSA992/KSC1845 F range, but gain limits are specified at different test currents. Do not assume that two random devices will match at the current used by the amplifier. For a differential input pair, select two transistors of the same part and production type, then compare hFE and VBE at or near the circuit's collector current. Closely matched VBE generally matters more to DC offset than a headline gain number.
Replace both members of a differential pair together. If the original pair was thermally coupled, restore that thermal contact with electrically insulating material as required by the design. After warm-up, measure DC offset and confirm that it remains stable.
A 2026 Semiconductor BOM for New Designs
A restoration and a new PCB have different priorities. A restoration should disturb as little of a proven compensation network as possible. A new design can specify currently orderable parts throughout the signal path, provide proper heatsinking and footprints, and validate stability around the selected devices. The following BOM is a practical starting architecture for a conventional discrete class-AB audio power amplifier; it is not a complete schematic or a substitute for load-line, safe-operating-area, thermal, and loop-stability analysis.
| Circuit position | Earlier part | 2026 recommendation | Package | Design note |
|---|---|---|---|---|
| NPN input differential pair | 2SC2240 | KSC1845FTA | TO-92 | Good fit around 0.5–2 mA; match the pair at operating current. |
| PNP input differential pair | 2SA970 | KSA992FTA or KSA992FATA | TO-92 | FTA is F grade, 300–600; FATA is FA grade, 300–470, both specified at 1 mA.[4] |
| NPN tail or small-signal current source | 2SC2240 class | KSC1845FTA | TO-92 | Only when calculated current, voltage, and dissipation fit the 50 mA device. |
| PNP current source or mirror | 2SA970 class | KSA992FTA/FATA | TO-92 | Choose the gain grade deliberately; thermally couple mirror devices where required. |
| VAS NPN | 2N5551 | 2N5551TA (ammo) or 2N5551TFR / 2N5551TF (tape-and-reel) | TO-92 | Retain the installed device on a proven PCB. For a new board, specify one complete orderable code; the Y-suffix NPN versions are discontinued.[14] |
| VAS PNP | 2N5401 | 2N5401YTA (ammo) or 2N5401YBU (bulk) | TO-92 | Retain the installed device on a proven PCB. Recalculate dissipation and compensation before selecting a different VAS device.[15] |
| Driver NPN | — | MJE15032G | TO-220 | Provide a suitable heatsink and check SOA at the actual load. |
| Driver PNP | — | MJE15033G | TO-220 | Complementary partner to MJE15032G. |
| Output NPN | MJL3281A | MJL3281AG | TO-264 | Retain; the G suffix denotes the Pb-free orderable package. |
| Output PNP | MJL1302A | MJL1302AG | TO-264 | Complementary partner to MJL3281AG. |
| Output emitter-sharing resistor | 0.1 Ω | 0.1 Ω / 5 W, if calculations confirm it | Low-inductance power resistor | One per output transistor; specify tolerance, pulse rating, temperature coefficient, spacing, and flameproof construction. |
Figure 3: A device-level architecture for a new design. It does not prescribe the circuit topology or compensation values.
Why the VAS Should Not Be Changed Casually
In the earlier example, the voltage-amplifier stage used 2N5551 and 2N5401 at roughly 5–20 mA. Current onsemi ordering tables list 2N5551TA, 2N5551TFR, and 2N5551TF, together with 2N5401YTA and 2N5401YBU. The January 2026 2N5551 revision marks 2N5551YTA and 2N5551YBU discontinued, so those Y-suffix NPN versions should not be specified for a new design.[14][15] The listed onsemi families are rated at 160 V/600 mA for 2N5551 and 150 V/600 mA for 2N5401, with 625 mW dissipation at the stated 25°C board condition. Those maximum numbers do not prove that a VAS operating point is thermally safe. Calculate PD = VCE × IC at idle and under signal, apply ambient-temperature derating, and verify junction temperature.
Keeping 2N5551/2N5401 on an existing mature PCB avoids changing several variables at once. Cob, fT, current gain, VAS current, the Miller capacitor, open-loop gain, device placement, and trace parasitics all affect the loop. A faster replacement can turn a clean square wave into overshoot or ringing and, in the worst case, ultrasonic oscillation that overheats the drivers or output stage. A new VAS device can be worthwhile on a new PCB, but it requires a fresh compensation design followed by simulation and bench verification into resistive and reactive loads.
Pinout Summary for the 2026 BOM
The E-C-B drawing above applies only to the cited 2SA970, 2SC2240, KSA992, and KSC1845 devices. It must not be copied across the complete amplifier BOM. Use the exact manufacturer's package drawing when assigning every PCB footprint.
| Device family | Pin 1 | Pin 2 | Pin 3 | Package note |
|---|---|---|---|---|
| 2SA970 / 2SC2240 | Emitter | Collector | Base | Archived Toshiba TO-92 drawings.[1][2] |
| KSA992FTA/FATA / KSC1845FTA | Emitter | Collector | Base | onsemi TO-92 leadformed packages.[4][5] |
| 2N5551TA/TFR/TF / 2N5401YTA/YBU | Emitter | Base | Collector | Confirm the selected onsemi case and shipping suffix.[14][15] |
| MJE15032G / MJE15033G | Base | Collector | Emitter | The TO-220 metal tab is also collector.[12] |
| MJL3281AG / MJL1302AG | Base | Collector | Emitter | The TO-264 mounting surface is collector.[13] |
Drivers, Output Devices, and Emitter Resistors
For a new board, MJE15032G and MJE15033G form a credible driver pair. onsemi explicitly describes them as high-frequency audio-amplifier drivers and specifies 250 V VCEO, 8 A continuous collector current, 50 W dissipation at a 25°C case, and 30 MHz minimum fT.[12] The current and power ratings cannot be used simultaneously without checking the safe-operating-area curves, heatsink, case temperature, drive duty cycle, and second-breakdown limits.
There is no compelling device-level reason to remove MJL3281A/MJL1302A from this architecture. The current onsemi datasheet rates the complementary pair at 260 V, 15 A continuous collector current, and 200 W at a 25°C case. It also specifies NPN/PNP gain matching within 10% from 50 mA to 5 A and identifies high-end consumer and professional audio amplifiers as intended applications.[13] Specify the complete Pb-free orderable codes MJL3281AG and MJL1302AG.
A 0.1 Ω / 5 W emitter resistor on every output transistor is a reasonable starting value for current sharing in some parallel class-AB output stages. It is still a circuit value, not a universal component substitution. Calculate resistor heating with P = I2R for continuous and program peaks, then derate for enclosure temperature and restricted airflow. The resistor must also satisfy tolerance, pulse-energy, flameproof, and low-inductance requirements. Physical placement should keep heat away from small-signal devices and electrolytic capacitors.
A Safe Replacement Procedure
- Identify the circuit role. Determine whether the part is an input transistor, current source, voltage-amplifier device, muting switch, or driver. The recommendation here targets low-current linear audio stages.
- Measure before removal. Record supply rails, transistor terminal voltages, idle current, and output DC offset when the unit can be powered safely.
- Check worst-case stress. Confirm VCE, normal and fault collector current, device dissipation, ambient temperature, and available derating. Keep useful margin below every absolute maximum.
- Confirm all three leads. Use the actual datasheets and continuity from the schematic or PCB traces. Do not rely only on the flat face or silkscreen outline.
- Select and match parts. For paired input devices, measure at the intended operating current. Buy traceable parts from an authorized distributor.
- Power up with current limiting. Use a current-limited bench supply or the service method appropriate to the equipment. Stop if rail current, bias, or DC offset is abnormal.
- Verify DC and AC behavior. Set bias as the service manual requires, allow thermal stabilization, then check noise, gain, distortion, and unwanted high-frequency oscillation.
When KSA992/KSC1845 Are Not Suitable
Do not use this pair when the circuit can exceed 50 mA, when the transistor must dissipate more heat than the real layout permits, or when a particular low-noise condition is essential but has not been verified. A driver or voltage-amplifier stage may need a higher-current device such as a KSA1013/KSC2383 class part, but those devices have different gain, bandwidth, capacitance, noise intent, and often a different pin arrangement. They require a fresh circuit-level comparison rather than a second generic substitution.
For new surface-mount designs, onsemi's FJV992 and FJV1845 retain the 120 V, 50 mA class in SOT-23 packages.[6][7] They may be useful on an adapter board, but an adapter changes lead mapping, parasitic capacitance, creepage, and mechanical reliability. Treat it as a small redesign.
Practical conclusion: KSA992FTA and KSC1845FTA are well-aligned modern choices for many 2SA970/2SC2240 low-current audio positions. Approve the substitution only after confirming that the 50 mA current ceiling, gain behavior, pinout, thermal conditions, and measured bias all fit the specific amplifier.
Frequently Asked Questions
Can KSA992 directly replace every 2SA970?
No. It is a strong candidate in low-current audio stages, but its 50 mA maximum collector current is half the original rating. Check current, voltage, dissipation, gain, pinout, and bias first.
Can KSC1845 directly replace every 2SC2240?
No. The electrical profile is close for many small-signal stages, but the same 50 mA limitation applies. Verify the circuit and test the finished repair.
Should GR or BL originals be replaced with a specific gain grade?
Compare gain at the circuit's operating current. The onsemi F grade is specified at 300–600 at 1 mA, which overlaps much of the original GR and BL ranges, but grade names and test conditions are not interchangeable.
Must both transistors in a differential pair be replaced?
Yes, that is the preferred repair practice. Use the same part type, match VBE and gain near the operating current, restore thermal coupling, and verify warm DC offset.
Why not buy old-stock Toshiba parts?
Traceable genuine stock can work, but age, storage history, relabeling, and counterfeit risk may be hard to establish. Current authorized-distribution parts provide clearer traceability and repeatability.
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References
- Toshiba, 2SA970 Silicon PNP Epitaxial Transistor (archived datasheet). https://www.mantech.co.za/datasheets/products/2sa970.pdf
- Toshiba, 2SC2240 Silicon NPN Epitaxial Transistor (archived datasheet, November 1, 2007). https://www.mantech.co.za/datasheets/products/2sc2240_toshiba.pdf
- Mouser Electronics, Toshiba 2SC2240-GR(F,T) lifecycle listing. https://au.mouser.com/en/ProductDetail/Toshiba/2SC2240-GRFT
- onsemi, KSA992 PNP Epitaxial Silicon Transistor, Rev. 5, January 2026. https://www.onsemi.com/pdf/datasheet/ksa992-d.pdf
- onsemi, KSC1845 NPN Epitaxial Silicon Transistor, Rev. 3, August 2023. https://www.onsemi.com/download/data-sheet/pdf/ksc1845-d.pdf
- onsemi, FJV992 PNP Epitaxial Silicon Transistor. https://www.onsemi.com/download/data-sheet/pdf/fjv992-d.pdf
- onsemi, FJV1845 NPN Epitaxial Silicon Transistor, Rev. 2, May 2024. https://www.onsemi.com/download/data-sheet/pdf/fjv1845-d.pdf
- DigiKey, onsemi KSA992FTA product and lifecycle listing (accessed September 4, 2026). https://www.digikey.com/en/products/detail/onsemi/KSA992FTA/1048232
- DigiKey, onsemi KSC1845FTA product and lifecycle listing (accessed September 4, 2026). https://www.digikey.com/en/products/detail/onsemi/KSC1845FTA/1047631
- DigiKey, onsemi KSA992FBU obsolete-part listing and KSA992FTA substitute. https://www.digikey.com/en/products/detail/onsemi/KSA992FBU/1048216
- Toshiba, Discontinuation Note DCN 1107, including multiple 2SA970 and 2SC2240 variants. https://www.anglia.com/registration/pcn_ptn/docs/ptn/DCN%201107.pdf
- onsemi, MJE15032/MJE15033 Complementary Silicon Plastic Power Transistors, Rev. 7, December 2024. https://www.onsemi.com/pdf/datasheet/mje15032-d.pdf
- onsemi, MJL3281A/MJL1302A Complementary Bipolar Power Transistors, Rev. 12, October 2024. https://www.onsemi.com/pdf/datasheet/mjl3281a-d.pdf
- onsemi, 2N5551 NPN General-Purpose Amplifier, Rev. 7, January 2026. https://www.onsemi.com/download/data-sheet/pdf/2n5551t-d.pdf
- onsemi, 2N5401 Amplifier Transistor, Rev. 2.1, May 2016. https://www.onsemi.com/pdf/datasheet/2n5401-d.pdf
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