At the heart of most amplifiers are transistors, and two of the most common types are field-effect transistors (FETs) and bipolar junction transistors (BJTs). On a schematic they may look like building blocks, but in practice they behave differently, and those differences impact how an amplifier sounds.
BJTs are current-controlled devices. They can deliver large amounts of current and are often associated with strong drive capability. FETs, by contrast, are voltage-controlled and behave more like vacuum tubes in their transfer characteristics. Many designers, including me, find FETs to be smoother and warmer in the way they handle signals, while BJTs can offer speed and punch.
Often, we combine the properties of both to make the synergy we're looking for.
Neither device is inherently superior. Much depends on circuit topology, biasing, and how we integrate them into the system. Some amplifiers even combine the two, using FETs in the input stage for their tube-like linearity and BJTs in the output stage for their robust current drive.
For the listener, the difference may show up as a subtle preference—one amp may sound more muscular, another more graceful.
What matters is not the device alone but how it’s used. Understanding the distinction helps explain why amplifiers with similar specs can sound remarkably different.
