Quality Headphone Amplifier

Posted on May 12, 2012

It is a single stage class A MOSFET design with the right gain and a low output impedance. Here we don't have the limitations of the Zen amps (at least in the single-stage implementations) regarding speaker compatibility. A single stage topology with correct interfacing values misses very few things in the original music message. The gain device in the original Zen amplifier is biased by fixed current source. For this amp, I employed an active current source described in Pass' patent no. 5,710,522 (see Zen Variations Part 2). The benefits of an active source include higher output current, lower distortion and 50% theoretical operating efficiency (compared to the 25% efficiency from a fixed source). This type of current source is featured in the Aleph power amplifiers from Pass Labs.

Quality Headphone Amplifier
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Quality Headphone Amplifier - image 1
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I designed this circuit to obtain the best sound from my Grado SR-325 headphones. Portability here was not imperative, so the high power consumption of a real class A amp was not a big drawback. I developed this project at the beginning of 2002 and it was first presented at DIYaudio.com in February 2002. Since the intellectual property belongs to Pass Labs, this circuit is intended only for notforprofit use, unless the current source topology is altered. I wish to thank Nelson Pass for his open-mindedness and involvement in the DIY audio world; the DIYer community will be always grateful to him! To obtain the wide compatibility with low-output sources, such as tape decks and tuners, I chose at least 10dB gain for the amplifier. This ensures adequate headroom with almost every CD and CD player I've tried. The amplifier's output current depends on the the output stage bias current, which can be set between a few mA and about 280mA. If you have an high-impedance headphone, you don’t need to charge the higher current setup, even if at least 100mA bias current is advisable for the best sound quality. This circuit makes use of feedback, but mainly to estabilish the working point, since the open-loop performance is quite good. R12 forms a little local feedback. It slightly enhances the circuit linearity, without further dropping the input impedance of the stage. The main feedback network (R7, R9, R14) takes care of the...

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