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Reviewed on: SoundStage! Solo, June 2019

I measured the Euterpe using a Clio 10 FW audio analyzer and a Neutrik NL-1 Minilyzer. Note that my focus with these tests is on measurements that confirm these devices’ basic functionality. I used the analog inputs; unfortunately, I’m currently unable to interface Clio’s coax digital output to USB-only DACs.

Frequency response

This chart shows the Euterpe’s frequency response with 1mW output into 32-, 250- and 600-ohm loads. The impedance switch on the amp was set to L for the 32-ohm load, and H for the 250- and 600-ohm loads. Into 32 ohms, response measures -8.24dB at 20Hz, -1.9dB at 20kHz, and -20.76dB at 75kHz. Into 250 ohms, the numbers are -7.74dB, -0.67dB, and -14.25dB, respectively. Into 600 ohms, the numbers are -9.91dB, -0.027dB, and -11.01dB, respectively. As you can see, the response curve basically shifts higher in frequency into higher-impedance loads, but in any case, this is an extreme amount of bass roll-off, and a substantial amount of treble roll-off.

Frequency response left and right channels

I don’t normally include this chart because in most headphone amps, the channels are so closely matched that the difference isn’t worth noting. This difference here is, though. The right channel (measured into 32 ohms) is 0.36dB higher in level at 1kHz than the left channel is. Although it’s hard to see without normalizing the two curves at a certain frequency, you can see that the right channel’s frequency response is basically shifted to higher frequencies.

Frequency response with headphones

This is another chart I don’t usually show, but I thought it important to show the effects that the Euterpe’s high output impedance will have on the sounds of a couple of different headphones, so I compared the frequency response of two headphones driven by the Euterpe and by the Musical Fidelity V-CAN (output impedance 5 ohms). The lower traces show the response with the Audeze LCD-Xes, a planar-magnetic headphone that has a largely resistive impedance that makes it relatively insensitive to headphone amp output impedance. Still, the Euterpe’s output impedance (with the impedance switch set to L) is enough to reduce the LCD-Xes’ bass by 2.35dB at 50Hz. With the Beyerdynamic Amiron Homes, a dynamic-driver design, the effect is more pronounced -- the Euterpe reduces the Amiron Homes’ bass by 2.73dB at 50Hz, and also tilts the treble up by about 0.84dB. Bottom line: This amp is not neutral, and it will change the sound of your headphones relative to what you’d hear with most other headphone amps, especially ones with a low output impedance.

THD vs. power output

This chart shows the output of the Euterpe vs. total harmonic distortion (THD) into 32-, 250- and 600-ohm loads. Rated power is 0.9W, into an unspecified load at unspecified distortion at an unspecified frequency. Into 32 ohms, the lowest distortion I measured, at 0.01W, is 0.5%; the amp breaks my 1% THD max at 0.038W, and at the rated 0.9W max output, THD is 5.49%. Into 250 ohms, THD at 0.01W was 0.52%; output at 1% THD is 0.035W, and THD at the rated 0.9W is 5.89%. Surprisingly, the performance at 600 ohms easily bests the performance into lower-impedance loads -- output at 0.5% THD is 0.042W, and it’s 0.165W at 1% THD. At the rated 0.9W, THD measures 2.43%.

Distortion

Here you can see the harmonic distortion spectrum and noise floor of the Euterpe, referenced to 3.1Vrms (0.3W) output at 600Hz into 32 ohms. This is a classic profile of the distortion of a single-ended tube amp, with the second-order distortion predominant. Because second-order harmonic distortion adds a harmonic precisely one octave above the fundamental, it’s less sonically offensive than third- or fifth-order harmonic distortion.

Output impedance at 1kHz measures 51 ohms with the impedance switch set to L, and 350 ohms with the switch set to H. This is extremely high output impedance relative to what I’m used to measuring; with any headphones that exhibit a significant impedance swing (such as earphones with balanced-armature drivers, and large over-ear headphones with dynamic drivers), the amp’s output impedance will interact with the reactance of the headphones or earphones to change the frequency response.

This is an amp with audible frequency response errors and high distortion. There are some audio writers who consider these idiosyncrasies a badge of honor, but I’m not one of them.

. . . Brent Butterworth
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