Fun with FETs

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I remember the first time I heard the sound of a FET (Field Effect Transistor) in comparison to a BJT (Bipolar Junction Transistor). It was quite a revelation. It happened not with individual devices but with an integrated circuit op-amp.

Most op-amps “back in the day” were BJT based and that’s what we were used to listening to. Our first op-amp was the iconic 709C designed by wild man engineer Bob Widlar of the Widlar Salute fame pictured here.

The 709C was a great sounding op-amp if you knew how to manage it (probably as great as Bob Widlar if you knew how to manage him). The next best sounding op-amp didn’t come along until the classic 5532.

Within a few years of the heyday of op-amps a new type appeared. These were touted as steps above traditional BJT based designs because they had a FET at their front end. FETs were better for many applications because they are essentially voltage amplifiers—like vacuum tubes are—and BJTs are current amplifiers. The first one I ever tried in our phono stage was a classic, one we still use today in servo applications. It was the TL082, a design that has outlived most of its competitors.

Fortunately for us, the TL082 is a drop-in part for just about any circuit. If you are using an 8-pin socket you could quickly and easily slip in a TL082, a NE5532, a 301, or a 741 without changing a thing and simply listen to the differences they present (the 709C needed some tender love and care to optimize it. Steps not needed or useful on these others).

Knowing the TL082 had a FET input meant there was a chance it might mirror some of the sonic qualities of a tube. And when we tried it, we were right. Compared to the 709, or 5532, the FET op-amp was smooth as silk. Warm, easy on the top end, not as etched as the others. Wow. At first, we thought it might be a miracle chip, something we could just start using in our designs and raise the level of performance by multiple notches.

But time is cruel to less than great performers. After living with the part for a few days we began to miss the details of the music, the upper harmonics seemed rolled off, the life had been sucked dry and in its place was a warm substitute. But warmth cannot replace details lost.

Which lead us to an entirely new path I’ll tell you about tomorrow.