Leaving Class A — Nelson Pass / 2008

The meters on our amplifi ers are different. They refl ect the current consumption of the amplifi er, and when the amplifi er is operating, they don’t go down to zero like the meters on other amplifi ers. This is because the electrical current consumption of our circuits has a fairly high value at all times, a property called the bias. The bias current runs through the amplifi ers at a minimum value, determining the class of operation – Class B, Class AB, or Class A. Class B has no bias current, Class AB has a moderate bias current, and… More...

Zen I/V Converter — Nelson Pass / 2010

Thirteen years ago Pass Labs launched the D1, a dedicated Digital to Analog converter for high end audio. It was based on balanced PCM63 DAC chips with a current source output, which requires a separate current-to-voltage converter (known as an I/V) to turn its output into the voltage to drive other line level audio equipment. We didn't care much for the op-amp based solutions seen in the manufacturer's application notes and decided to approach this part of the circuit much differently. The PCM63, like other current output DACs, favors driving a very low impedance, preferably ground. You can easily create… More...