The PPA headphone amplifier was designed by Team PPA in late 2002 as a community service project. Over the past few years, PPL created a series of pocket headphone amplifiers using opamps and open loop buffers which were the inspiration for Apheared’s #42 and the META42.
Standard headphones have 3 wires: left, right, and ground, which is tied to the negative side of both drivers. Standard headphone amplifiers have 2 channels: left and right, with signal ground or power supply ground used for the return path from the headphone ground wire. Fully balanced headphone amplifiers have 4 channels, require custom 4 wire headphones, and are typically operated in bridged mode, which doubles the output voltage and quadruples the power by using inverting and noninverting pairs, but does not offer symmetrical transfer characteristics due to the difference between inverting and noninverting modes.
?Jung multiloop reduces distortion and increases stability with a high gain inner or local loop and sets the overall amplifier gain with an outer or global loop. There is no significant load on the opamps so they operate with low distortion and confer their linearity to the unity gain open loop output buffers which drive loads with greater stability and lower distortion than closed loop buffers or power opamps.
FET Isolated Power Rails
?The opamps are powered by their own FET isolated rails so audio frequency modulation on the output buffer power rails does not get into the signal. A TLE2426 splits the opamp rails and references signal ground but not output ground. FET Cascode Current Source The opamps are biased into Class A operation using a FET cascode current source which improves sound quality by reducing transient intermodulation distortion and operating further into the linear range than Class AB or B modes. The source resistor RS (R9) trimpot allows for more precise current control, compensation for wide FET IDSS range, and the use of higher IDSS FETs to source lower currents.
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