Audio Research Corporation writes: “Details, details. (part one). Beyond the beauty of the workmanship inside each chassis, there are may details that are not as apparent that contribute to the sound quality.
A previous post covered burn-in, grading and matching for tubes, but we also test and grade every transistor to maintain the best sound quality and consistency, regardless of the circuit. The transistors in every lot we receive meet spec, but they can vary just like tubes.
Audio Research has up to 23 grades of FETs, finding that a specific grade sounds best in certain circuits. Sometimes we can use most of the grades for a particular FET, but with other FETs only one or two grades can be used, turning what would be a one dollar FET into a ten dollar FET. Use the wrong grade and it will degrade sound quality. We color-code the FETs according to the grade, one by one, even in lots of 1,000. In the photo with the tiny black JFETs, there are two color stripes (white and violet). In the photos of the larger MOSFETs with the heat sinks, there are three colored stripes (orange, yellow and green). This also is helpful in service because it is easier for a tech anywhere in the world to know exactly what is needed.
MOSFETs are mounted to heat sinks for cooling, but some of the heat sinks can ring slightly, so we damp them. In one photo the heat sink has a cover, in another the heat sink has a damping band. These are from two different amps, with each one sounding best with its particular damping, and where it is situated. The band sounds best in one spot!
Just like tubes, careful grading and matching provides greater consistency and sound quality, even if you can’t always see it.