Wednesday, September 20, 2017

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I measured the Blue Ellas using a G.R.A.S. Model 43AG ear/cheek simulator, a Clio 10 FW audio analyzer, a laptop computer running TrueRTA software with an M-Audio MobilePre USB audio interface, a Musical Fidelity V-Can headphone amplifier, and an Audio-gd NFB-1AMP amplifier for the distortion measurements. This is a “flat” measurement; no diffuse-field or free-field compensation curve was employed.

Frequency response

The Ellas’ frequency response is unusual in two ways. Judging from the right-channel response, it’s flatter than usual, with a much narrower peak in the 3kHz area than I’m used to seeing. The other unusual thing is that the response of the left channel doesn’t match that of the right. I tried reseating the left earcup many times, and even repeated my measurements a week later, but couldn’t get a closer match than is seen here. It could be because the left channel’s acoustics are different from the right’s. I’ve seen this before in active headphones, and assume that it’s because of the space consumed by the battery in one earpiece. It’s worth noting that in Blue’s first headphones, the Mo-Fis, which share the Ellas’ basic design and are also internally powered, the channels were much better matched.

Three modes

This chart shows the Ellas’ frequency response in passive, active, and + (bass boost) modes. The response in active mode is the same as in passive, but the level in passive mode is 8.1dB lower. The response in + mode shows a bass boost about two octaves wide, centered at 65Hz.

Frequency response

This chart shows the results of adding 70 ohms of output impedance to the V-Can’s 5 ohms, to simulate the effects of using a typical low-quality headphone amp. Only the results in passive mode are shown, because in active mode the impedance of the source component doesn’t affect the response. Using a higher-impedance source has only a barely measurable, probably inaudible effect on the Ellas’ response.

Frequency response

This chart shows the Ellas’ measured right-channel frequency response compared with that of Oppo Digital’s PM-3s -- like the Ellas, one of only a few closed-back planar-magnetic headphones available -- and NAD’s Viso HP50s, my comparison standard for midpriced closed-back headphones. Out of curiosity, I also included Blue’s original headphones, the Mo-Fis, which have dynamic drivers instead of the Ellas’ planar magnetics. The Ellas have more or less the flattest measured response of the bunch, though that doesn’t necessarily mean they’re the flattest-sounding.

Waterfall

The spectral-decay (waterfall) chart shows a resonance only at around 500Hz, though overall there’s less bass resonance than I usually see in these charts.

THD

The total harmonic distortion (THD) of the Ellas was higher than I’m used to measuring. At 90dBA, the THD is low above 50Hz, and of course most music has little content below 50Hz. The THD gets very high below 60Hz at 100dBA, but this is an extremely loud level that few people would want -- or be advised -- to listen to for more than a few seconds. I repeated this measurement a week later with a fresh calibration and got essentially the same result. The measurements were roughly the same in passive and active modes, so this distortion is apparently coming from the driver.

Isolation

In this chart, the external noise level is 75dB SPL; the numbers below that indicate the degree of attenuation of outside sounds. For comparison, I’ve included the isolation plots of the Oppo Digital PM-3s (a closed-back planar magnetic design), the NAD Viso HP50s (a closed-back dynamic design), and the noise-canceling Bose QC25s. The Ellas’ isolation is below average for a closed-back design, though they do attenuate the upper mids and treble by 14-19dB.

Impedance

The Ellas’ impedance in active mode, while specified as 10 ohms, is actually beyond my Clio analyzer’s limit of 1500 ohms. (The Clio is designed to measure the impedance of speakers, not electronics.) In passive mode, it’s flat at 54 ohms, with almost perfectly flat phase response. Electrical phase varies more in active mode, but this shouldn’t affect the Ellas’ sound.

The sensitivity of the Ellas, measured between 300Hz and 3kHz with a 1mW signal, was 95.8dB in passive mode (calculated for the specified 50 ohms passive impedance), 95.3dB in active mode (calculated for the specced 10 ohms active impedance). Note that because these sensitivity measurements are calculated for different headphone impedances, they use different signal voltages -- 0.22V for 50 ohms, 0.1V for 10 ohms -- and thus are not comparable. In my frequency-response measurements, cited above, I found that switching to active mode boosts the Ellas’ output by 8.1dB with the same test-signal voltage. In either mode, though, the Blues’ sensitivity was a little lower than average; the Ellas should play loud enough in active mode, but passive mode might be a little too quiet to reach satisfying levels with some music in some environments.

. . . Brent Butterworth
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