Excellent sound quality, simplicity, linearity and no multi-stage feedback. It is a single stage class A MOSFET design with the right gain and a low output impedance.
The schematic in figure 1 shows one channel of the headphone amp. The HexFets employed are the usual International Rectifier IRF610, both for the active side and for the current source. The circuit is quite simple, only one gain stage built around an IRF610 (Q2 at the bottom of the schematic), biased by another IRF610 (Q1). The two diodes (D1 and D2) are for over-voltage protection purposes: any type, between few volts and about 15-16V will work fine.
The main idea behind Pass’ variable current source is that the bias current is controlled by the output signal. The “sensing amplifier” is built around Q3. An output resistor, R11, senses the current through the load. Q3 drives the gate of Q1 and modulates the bias current in Q2 according to the “sensed” load current.
The Grados are low impedance headphones, about 30 ohms, and are very efficient too. To get the maximum output swing required by a headphone amplifier, I measured, by a storage scope, the maximum output level at which the headphones began obviously to distort. At 2.7Vpeak (1.9Vrms), there was enormous sound pressure from the headphones. This was definitely plentiful!
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