Dual transistors and why they suck.
- Admin
- 7 days ago
- 3 min read
If you spend any time restoring vintage amplifiers, you’ve probably encountered the 2SA798. This small, unassuming part shows up in classic gear from Sansui, Pioneer, Luxman, Kenwood, and others—and it’s responsible for more noise issues than almost any other semiconductor of its era.
In this post, we’ll take a closer look at what the 2SA798 is, why it fails, and how to replace it using modern, reliable parts.
What Exactly Is the 2SA798?
The 2SA798 is a dual PNP low-noise transistor packaged inside a single epoxy case. Inside the housing are two matched transistors sharing a common collector—designed for use as a differential input pair, typically at the very front end of an amplifier.
This location is critical. The differential pair sets the noise floor, DC stability, and linearity of the entire amplifier. Any instability or mismatch here carries through the rest of the circuit.
Common applications include:
Power amplifier input stages
Phono preamps
Tone control front-ends
Low-level amplifier circuits where matching is essential
Why the 2SA798 Goes Bad
Like many vintage semiconductors, the 2SA798 tends to age poorly. Over time:
Noise rises significantly (hissing, rushing, crackling)
Internal leakage increases
The two internal transistors drift out of match
DC offset becomes unstable, sometimes wandering by tens of millivolts
These failures often present as subtle background hiss or as dramatic popping and crackling that seems to come from nowhere. Some units will work fine for years—then develop offset drift or noise overnight.
Pinout Reference
If you’re replacing one, this orientation matters:
Front view (flat side facing you)
1 2 3 4 5
B1 C1 E C2 B2
The center pin is the shared emitter, and the outer pins are the base. BCECB.
Modern Replacements for the 2SA798
Because the original part is long discontinued—and genuine NOS pieces are increasingly rare or counterfeit—the best solution today is to build a modern equivalent using two discrete transistors.
Here are the top choices:
1. Matched KSA992 Pair (Recommended)
The KSA992F is a modern, low-noise PNP transistor with characteristics well suited for differential amplifier stages.
To use them as a replacement:
Select two transistors with closely matched hFE values (ideally within 1–2%).
Tie their collectors together.
Thermally bond them using heat-shrink or thermal epoxy.
This provides far better long-term stability than a 40-year-old epoxy dual transistor.
2. Matched 2SA970 or 2SA1015 Pair
These alternatives also work well, especially in phono or tone-stage circuits. They have slightly different electrical characteristics but are quiet, stable, and easy to match.
3. NOS 2SA798 (Use with Caution)
Original parts are:
Expensive
Frequently counterfeited
Subject to the same aging issues as the ones you’re replacing
Unless sourced from a trusted dealer and tested, NOS is generally not recommended.
4. General-Purpose PNPs (Last Resort)
Parts like the SS9015 can technically substitute in less demanding circuits, but noise performance and stability won’t match audio-grade devices. Use only if nothing else is available.
Replacing the 2SA798: Step-by-Step
1. Remove the Original Carefully
Vintage PCBs can be fragile—work slowly and use temperature-controlled tools.
2. Prepare the Matched Pair
Tie the collectors together
Arrange the leads to match the 2SA798 footprint
Bond the two transistors so they track temperature equally
3. Install the Replacement
Insert the five leads into the original holes, checking orientation twice.
4. Power Up Safely
A dim bulb tester or current-limited bench supply is recommended for amplifier work.
5. Verify DC Offset
A properly matched pair often reduces offset dramatically and keeps it stable with temperature changes.
What to Expect After Replacement
Replacing a noisy or drifting 2SA798 usually yields immediate benefits:
Lower noise floor
More stable DC offset
Improved channel balance
No more wandering offset as the unit warms up
This small upgrade can make a shockingly large improvement in overall amplifier performance and reliability.
Final Thoughts
The 2SA798 is a perfect example of a vintage part that simply hasn’t aged well. Fortunately, modern audio-grade transistors like the KSA992 provide a reliable, inexpensive, and superior replacement—once matched and properly installed.
If you restore classic amps or receivers, replacing these dual transistors is one of the best preventive upgrades you can make.
