- Written by John E. Johnson, Jr.
- Published on 15 May 2014
The OPPO HA-1 Headphone Amplifier On the Bench
I tested the headphone amplifier using its coaxial digital input, since this has been the standard digital connection in the past. The results should be similar to the other digital inputs, except that the USB DAC input (for music played through your computer) has the ability to decode higher sampling rates.
All distortion measurements are within an 80 kHz bandwidth. I set the volume control to 0 dB. The output was loaded with 47 ohm ceramic resistors. The 4-pin balanced headphone jack was connected to the Audio Precision for the measurements.
At 16/44.1 sampling, and the 1 kHz signal at 0 dB, there was 0.02% THD+N. This is higher than the spec sheet, but that's the way it usually is. The important thing is the array of peaks. The odd-ordered peaks, through the seventh, are higher than the even-ordered through the sixth. However, distortion peaks overall, are small (I didn't hear any harshness or congestion at all, even at the loudest volume I could stand).
The corresponding graph at 24/192 sampling showed a lower measured distortion, and only a second and third harmonic.
The rest of the distortion measurements, shown below, used test signals at – 5 dB.
At 1 kHz, and 16/44.1 sampling, THD+N was 0.017% as it was at 0 dB. This means the noise component is the dominate portion of the number. Notice however, that the number of harmonic peaks has decreased, compared to the test signal at 0 dB.
At 24/192, distortion was 0.003%.
At 16/44.1, using 19 kHz and 20 kHz test tones, there were three peaks on either side of the fundamentals. If there were a B-A peak at 1 kHz, it was buried in some noise.
Correspondingly, at 24/192, there was only one peak on either side of the test tones. This is audiophile grade performance.
IMD with 16/44.1 sampling, was 0.009%. The spectrum is excellent.
The peaks for the 24/192 IMD test were non-existent, as shown below, and in the second graph which has a 0 - 20 kHz scale. These results are more like the output of a player or preamplifier rather than what is actually a speaker power amplifier.
The measured frequency response with 16/44.1 sampling was 10 Hz – 20 kHz, ± 0.1 dB. The left channel's output was 0.2 dB higher than the right channel.
And, for 24/192, the frequency response was 10 Hz – 90 kHz, - 0.9 dB.