Recommendation report : apparatus to amplify headphones and test sound quality Ellis, George
The purpose of this project was to prototype a hi-quality headphone amplifier, and to measure the amp’s performance using a dummy (pure resistive) or real headphone load. The idea was to either demonstrate how a headphone amp could quantifiably improve sound quality played out of a source, or to at least investigate typical usage scenarios and metrics for measuring sound quality. Key metrics identified for characterising the amp’s performance were: input and output impedance, signal-to-noise + distortion (SINAD), and the gain response over audio frequencies. The original discrete amplifier design chosen for the project was changed a more forgiving LM1875 op amp design during the prototyping stage due to deficiencies in parts ordered. The second amp was measured to have 21 Ω input and < 1 Ω output impedances, and a noise floor at -85 dB(VRMS) relative to the audio signal. Qualitatively, the amplifier was able to drive audio transducers from computer Line Out with impedances from 8 Ω to 100 Ω without noticeable noise or degrading audio quality. It was concluded that a medium-quality audio amplifier is not necessarily suitable for all applications. Recommendations include further testing and continued prototyping of the more ambitious (but trickier) hi-fi design.
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