Ray Samuels Audio Emmeline SR-71 portable headphone amplifier Measurements
The measurements were performed using a fresh set of alkaline batteries. I measured the Emmeline SR-71's maximum gain as 15.54dB, significantly higher than the specified 6dB. Its input impedance was a usefully high 23k ohms from 20Hz to 20kHz. I also found its output impedance to be less than 1 ohm, which is well below the specified 16 ohms, but perhaps contributes to the amplifier's excellent bass quality. DC offset on the amplifier's output was negligible.
The SR-71's frequency response was flat from 10Hz to 100kHz, and only just starts to roll off at 200kHz (fig.1). This response was taken with the volume control set to its 12:00 position; note the 0.25dB channel imbalance. With the volume control set to its maximum setting, the channels were perfectly matched. The channel-separation graph (fig.2) reveals crosstalk increasing at high frequencies with the 6dB/octave slope typical of capacitive coupling between the channels. However, the separation is better than 80dB below 1kHz, which is fine.
Fig.1 Ray Samuels Emmeline SR-71, frequency response at 600mV into 150 ohms (0.5dB/vertical div.).
Fig.2 Ray Samuels Emmeline SR-71, channel separation (10dB/vertical div.).
Noise levels were very low, the SR-71's unweighted, wideband signal/noise ratio ref. 1V output measuring 86dB. Switching in an A-weighting filter increased this ratio to a superb 108.3dB. The decreasing slope of the traces below 1V in fig.3, which shows how the THD+noise percentage present in the Emmeline's output varies with increasing output voltage, shows that even with the low noise level, the actual distortion is even lower. The exception is into 100 ohms, where the distortion starts to rise above 200mV before hard-clipping at 4.5V (1% THD), this higher than required to drive typical headphones to deafening levels.
Fig.3 Ray Samuels Emmeline SR-71, distortion (%)vs 1kHz output voltage into (from bottom to top at 1V): 100k, 10k, 1k, and 100 ohms.
Fig.4 plots the THD+N percentage against frequency into 100k ohms and 150 ohms, taken at 2V, a level where fig.3 indicates that the distortion is starting to rise above the noise floor. The distortion is very low below 1kHz, but starts to rise above that frequency due to the circuit's decreasing open-loop bandwidth. (There is less gain margin available as the frequency rises to allow negative feedback to operate.) Even so, the distortion remains below 0.02% even into 150 ohms.
Fig.4 Ray Samuels Emmeline SR-71, THD+N (%)vs frequency at 2V into (from bottom to top): 100k and 150 ohms.
Fig.5 shows the spectrum of the SR-71's output driving 1kHz at 1V into 8k ohms. The measured THD (actual sum of the harmonics) is just 0.0013%, with the third harmonic the highest in level at –100dB (0.001%). But note that even though they are at very low levels, the fifth, seventh, and ninth harmonics can be seen (circled). Intermodulation distortion was also very low (fig.6).
Fig.5 Ray Samuels Emmeline SR-71, spectrum of 1kHz sinewave, DC–10kHz, at 1V into 8k ohms (linear frequency scale).
Fig.6 Ray Samuels Emmeline SR-71, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 1V peak into 8k ohms (linear frequency scale).
Overall, the Emmeline SR-71 offers excellent measured performance, and my own positive experience of using the amplifier to drive Ultimate Ears UE-5C in-ear headphones echoes WP's enthusiastic comments.—John Atkinson