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Headphone Amplifier Measurements

Reviewed on: SoundStage! Solo, November 2019

I measured the RH-5 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. Except as noted, I used the unbalanced output.

Frequency response

This chart shows the frequency response in the left and right channels at 1mW into a 32-ohm load. Channel tracking is good, with the right channel 0.015dB louder at 1kHz, and 0.095dB less roll-off in the right channel at 20kHz. Response measures -1.0dB at 20Hz, -0.53dB at 20kHz, and -5.66dB at 75kHz. This shows the unbalanced output; the balanced output was essentially the same except for -1.91dB additional roll-off at 10Hz. This measurement was taken with gain set to 3. At gain 2, output is reduced by 4.34dB at 1kHz. Gain 1 reduces the output by an additional 3.78dB.

Frequency response

This chart compares the Rogue’s frequency response with 1mW unbalanced output into 32-, 250-, and 600-ohm loads. Into 250 ohms, the numbers are -0.04dB, -0.45dB, and -4.85dB, respectively. Into 600 ohms, the numbers are -0.02dB, -0.45dB, and -4.78dB, respectively. The bass roll-off into 32 ohms, and the treble roll-off into all loads, isn’t impressive -- tubes at work here! -- but in both cases it’s modest enough that you’d be unlikely to notice it.

THD vs. power output

This chart shows the unbalanced output of the Rogue vs. total harmonic distortion (THD) into 32-, 250-, and 600-ohm loads at 1kHz. Rated power is 3.5W into 32 ohms; the distortion level, the frequency, and whether the spec is at the balanced or unbalanced output are all unspecified. Into 32 ohms, output is 1.9W at 0.5% THD and 2.0W at 1%; while this may (or may not) fall short of the spec, it’s certainly plenty enough power to drive any headphones to very high volume. Into 250 ohms, the output is 0.37W at 0.5% THD and 0.39W at 1%. Into 600 ohms, the numbers are 0.16W and 0.17W, respectively. Solid performance here.

Distortion

Here you can see the harmonic distortion spectrum and noise floor of the Rogue, referenced to 5.66Vrms (1W) output at 600Hz into 32 ohms. This shows what I’d expect from a hybrid amp. The sonically benign (because it’s exactly one octave above the fundamental tone) second-order harmonic is stronger than we’d probably see with an all-transistor amp, but it’s not as dominant as we’d probably see in an all-tube amp.

Output impedance is rated at less than 0.1 ohm at 1kHz; I measured 0.07 ohm. This means that the output impedance of the amp will not significantly interact with the reactance of the headphones, so you’ll get consistent response no matter what type of drivers your headphones use.

. . . Brent Butterworth
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Reviewed on: SoundStage! Solo, October 2019

I measured the Lehmannaudio Linear USB II 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 Linear USB II’s frequency response with 1mW output into a 32-ohm load in the left and right channels. Channel matching is excellent, with the right channel measuring just 0.01dB lower in level than the left, and the two having neatly overlapping curves out to 40kHz.

Frequency response

Here you can see how the Linear USB II’s frequency response differs into 32-, 250-, and 600-ohm loads. Into 32 ohms, the response measures -0.01dB at 20Hz, -0.14dB at 20kHz, and -1.18dB at 75kHz. Into 250 ohms, the numbers are -0.02dB, -0.11dB, and -1.15dB, respectively. Into 600 ohms, the numbers are -0.02dB, -0.11dB, and -1.16dB, respectively. From a frequency-response standpoint, the Linear USB II’s response can be described as load-invariant, meaning the amp’s tonal balance won’t change depending on the headphones you use.

THD vs. power output

This chart shows the output of the Linear USB II vs. total harmonic distortion (THD) into 32-, 250-, and 600-ohm loads with a 1kHz signal. Rated power is 400mW into a 60-ohm load and 200mW into 300 ohms, both at unspecified distortion at an unspecified frequency. Into 32 ohms, the power/distortion curve looks peculiar because it rises rather quickly to a plateau of typically 1.6% THD, and doesn’t hit its “clipping knee” until 1.35W at 2.1% THD. It hits 0.5% THD at 127mW, and 1% THD at 176mW. This is the only deviation I found from excellent measured performance. With higher-impedance loads, the Linear USB II performs more as I’d expect. Into 250 ohms, output at 0.5% THD is 327mW, and output at 1% THD is 347mW. Into 600 ohms, output at 0.5% THD is 141mW, and it’s 149mW at 1% THD.

Distortion

Here you can see the harmonic distortion spectrum and noise floor of the Linear USB II, referenced to 2.59Vrms (210mW) output at 600Hz into 32 ohms. This is pretty typical of a conventional solid-state amp, with much stronger odd-order (3rd, 5th, 7th, and so on) harmonics than even-order (2nd, 4th, 6th, etc.) harmonics. Odd-order harmonics are more objectionable because they occur at non-harmonic intervals to the fundamental tone, but this measurement was done at a far higher level than you’d encounter in normal listening.

Output impedance at 1kHz measures 5.6 ohms, close to the rated 5 ohms. This relatively low output impedance ensures that the amp’s output impedance will have very little interaction with the reactance of the headphones or earphones, so the headphones or earphones will deliver the frequency response they were designed for (assuming they were designed using an amp with low output impedance).

. . . Brent Butterworth
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Reviewed on: SoundStage! Solo, September 2019

I measured the Nickel 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.

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This chart shows the Nickel’s frequency response with 1mW output into 32-, 250- and 600-ohm loads. Into 32 ohms, the response measures -0.51dB at 20Hz, -0.26dB at 20kHz, and -2.80dB at 75kHz. Into 250 ohms, the numbers are -0.48dB, -0.22dB, and -2.49dB, respectively. Into 600 ohms, the numbers are -0.47dB, -0.21dB, and -2.50dB, respectively. For a tiny, portable amp like this one, these are respectable numbers.

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This chart shows the matching of the Nickel’s channels at 1mW into a 32-ohm load. The right channel is 0.069dB lower in level at 1kHz than the left channel, a negligible difference, and the shapes of the response curves match precisely within the audioband.

THD vs. power output

This chart shows the output of the Nickel vs. total harmonic distortion (THD) into 32-, 250-, and 600-ohm loads at 1kHz. Rated power is 250mW into 32 ohms, THD and frequency unspecified. Output into 32 ohms is 150mW at 0.5% THD and 156mW at 1% THD; the highest output I was able to measure is 203mW at 10% THD. Output into 250 ohms is 32mW at 0.5% THD and 34mW at 1% THD. Output into 600 ohms is 14mW at 0.5% THD and 15mW at 1% THD.

Distortion

Here you can see the harmonic distortion spectrum and noise floor of the Nickel, referenced to 174mW output at 600Hz into 32 ohms. The distortion is predominantly odd-order (3rd, 5th, 7th harmonics, etc.), although I had to push the amp a little past its limits to even see any even-order (2nd, 4th, 6th, etc.) harmonics.

I measured no-load gain at 5.4dB, a little lower than the rated 6.5dB. Output impedance at 1kHz measures 0.78 ohm. This means the amp’s output impedance will not interact significantly with the reactance of headphones or earphones, and thus won’t alter their frequency response.

. . . Brent Butterworth
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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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Reviewed on: SoundStage! Solo, April 2019

I measured the Liquid Platinum 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. Because I didn’t have the necessary four-pin XLR adapter that would allow me to measure the balanced output, the measurements below are all with the amp in single-ended mode (using the 1/4” TRS headphone output). I have the parts on order to build the adapter and hope to add those results later. Meanwhile, I was able to measure the frequency response of the HiFiMan HE6se headphones from the balanced and unbalanced outputs, and the results were identical.

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This chart shows the Liquid Platinum’s frequency response with 1mW output into 32-, 250-, and 600-ohm loads. Into 32 ohms, the response measures -0.014dB at 20Hz, -0.041dB at 20kHz, and -0.276dB at 75kHz. Into 250 ohms, the numbers are -0.015dB, -0.031dB, and -0.307dB, respectively. Into 600 ohms, the numbers are -0.015dB, -0.027dB, and -0.232dB, respectively. These are very good results.

THD vs. power output

This chart shows the single-ended output of the Liquid Platinum vs. total harmonic distortion (THD) into 32-, 250-, and 600-ohm loads. Note that Monoprice’s power ratings are specified at 33, 56, 150, and 300 ohms, so my measurements are not directly comparable, but Monoprice’s specs seem well in line with my results. Output into 32 ohms is 1.71W at 0.5% THD and 1.85W at 1% THD. (Monoprice’s most comparable rating is 1.78W into 33 ohms, THD unspecified.) Output into 250 ohms is 275mW at 0.5% THD and 289mW at 1% THD. (Monoprice’s most comparable rating is 230mW into 300 ohms, THD unspecified.) Output into 600 ohms is 117mW at 0.5% THD and 122mW at 1% THD.

Distortion

Here you can see the harmonic distortion spectrum and noise floor of the Liquid Platinum, referenced to 1.5Vrms (1W) output at 600Hz into 32 ohms. Distortion is low, and no particular distortion harmonic dominates the spectrum; the first several harmonics are down in the -72dBFS range (plus or minus a couple of dB) relative to the level of the fundamental tone. (For reference, -70dBFS equates to 0.03% harmonic distortion.) We can see some 60-cycle AC hum and its harmonics, typically in the range of -70dBFS. The noise floor of the amp at this level is down around -92dBFS, which is pretty good for a device with tubes in the signal chain.

I measured output impedance of the 1/4” headphone jack at under 0.3 ohm at 1kHz, which is about as low as I can measure with my voltage divider; Monoprice rates it at 0.07 ohm. Either way, it’s easily low enough that the output impedance won’t react significantly with the reactance of the headphones, and thus won’t change their frequency response.

. . . Brent Butterworth
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Reviewed on: SoundStage! Solo, June 2019

I measured the Monoprice 24459 using a Clio 10 FW audio analyzer and a Neutrik NL-1 Minilyzer. I measured only the unbalanced output; for some reason I couldn’t figure out, the amp always went into protection mode when I connected the balanced output into a load resistor. Note that my focus with these tests is on measurements that confirm these devices’ basic functionality.

Frequency response

This chart shows the Monolith 24459’s frequency response with 1mW output into a 32-ohm load using the coaxial digital input. (Measurements with 250- and 600-ohm loads produced effectively identical results.) With the Normal digital-to-analog (DAC) filter, response measured -0.057dB at 20Hz, -0.227dB at 20kHz, and -1.470dB at 40kHz. With the Slow1 filter, the numbers were -0.057dB, -0.307dB, and -6.007dB, respectively. With the Slow2 filter, the numbers were -0.060dB, -0.975dB, and -4.307dB, respectively. These measurements were taken with a 192kHz digital signal, which the coax input accepts, but the digital circuitry is brick-wall filtered at about 40kHz (consistent with Monoprice’s published frequency response), so the effective resolution is actually 96kHz. Note the +1.2dB ringing of the Normal filter at 37kHz. From a technical standpoint, this isn’t impressive, but it won’t be audible. The ringing nearly disappears with the Slow1 and Slow2 filters.

Frequency response

This chart shows the effect of the two different analog-to-digital converter’s filter settings on the frequency response. Both were measured with 1mW output into a 32-ohm load using the unbalanced analog input, with the DAC filter set to Normal. (Measurements with 250- and 600-ohm loads produced effectively identical results.) With the Normal analog-to-digital (ADC) filter, response measured -0.057dB at 20Hz, -0.067dB at 10kHz, and -0.344dB at 30kHz. With the Slow1 filter, the numbers were -0.010dB, -0.139dB, and -0.922dB, respectively. Thus, the difference between the two filters might be just barely audible. (I cite the response here at 10kHz and 30kHz instead of my usual 20kHz and 40kHz because of the slightly non-smooth characteristics of the response curves.)

THD vs. power output

This chart shows the unbalanced output of the Monoprice 24459 vs. total harmonic distortion (THD) into 32-, 250-, and 600-ohm loads. Note that Monoprice’s power ratings are specified at 16, 32, 150, 300, and 600 ohms, so some of my measurements are not directly comparable. Output into 32 ohms was 1420mW at 0.5% THD and 1475mW at 1% THD. Output into 250 ohms was 183mW at 0.5% THD and 190mW at 1% THD. Output into 600 ohms was 77mW at 0.5% THD and 79mW at 1% THD. (Monoprice’s ratings are 1360mW into 32 ohms, 150mW into 300 ohms, and 73mW into 600 ohms, all with THD unspecified). These are very high numbers for a headphone amp, indicating that the Monolith 24459 should have no problem driving any headphones currently available.

Distortion

Here you can see the harmonic distortion spectrum and noise floor of the Monolith 24459, referenced to 6.295Vrms (1.24W) output at 600Hz into 32 ohms. (I used this odd output number as a reference because, as best I can tell, the amplifier stage just barely starts to distort before the unit’s analog-to-digital converter stage clips. Any higher and the distortion becomes very high; any lower and there’s not enough distortion to see the harmonic content.) Harmonic distortion is predominantly odd-order, which is much more audible than even-order distortion, but with the 3rd harmonic at -78.2dBFS and the 5th harmonic at -79.4dBFS (both just slightly over 0.01% distortion), and the distortion occurring only at an extremely high output level, I think the chances of any listener actually hearing this are zero. Note also that the noise floor was generally at about -120dBFS. This is excellent performance.

I measured the output impedance of the unbalanced headphone jack at less than 0.5 ohm, which is as low as I could measure without triggering the amp’s protection circuit. In my opinion, an output impedance of less than 1 ohm is a good standard for headphone amps because it prevents the headphone amp from significantly interacting with the headphones’ impedance in a way that alters the headphones’ frequency response.

. . . Brent Butterworth
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Reviewed on: SoundStage! Solo, April 2019

I measured the Schiit Audio Fulla 2 using a Clio 10 FW audio analyzer and a Neutrik NL-1 Minilyzer. I used the Fulla 2’s analog input for all these measurements, because I haven’t yet found a way to get digital test signals from the Clio 10 FW to USB DACs. Note that my focus with these tests is on measurements that confirm these devices’ basic functionality.

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This chart shows the Fulla 2’s frequency response with 1mW output into 32-ohm and 600-ohm loads. (Frequency response at 250 ohms is not shown because it almost perfectly overlapped with the response at 32 ohms.) Into 32 ohms, the response measures -0.011dB at 20Hz, -0.031dB at 20kHz, and -0.085dB at 75kHz. Into 600 ohms, the numbers are -0.009dB, -0.044dB, and -0.186dB, respectively. These are excellent results, comparable to those of a good high-end analog preamp.

THD vs. power output

This chart shows the output of the Fulla 2 vs. total harmonic distortion (THD) into 32-, 250- and 600-ohm loads. Note that Schiit’s power ratings are specified at 16, 50, 300, and 600 ohms, so some of my measurements are not directly comparable. Output into 32 ohms is 320mW at 0.5% THD and 340mW at 1% THD (Schiit’s rating is 360mW into 32 ohms, THD unspecified). Output into 250 ohms is 50mW at 0.5% THD and 51mW at 1% THD. Output into 600 ohms is 21mW at 0.5% THD and 22mW at 1% THD. These numbers are all very impressive for a $99 DAC-headphone amp.

Distortion

Here you can see the harmonic distortion spectrum and noise floor of the Fulla 2, referenced to 1V RMS output at 600Hz into 32 ohms. Distortion is very low, with the second harmonic slightly higher in level than the third; I’d say this would make the Fulla 2 sound “tubey” if the distortion at this output level and load were high enough for you to hear, but that second harmonic is at -79dB. You can also see that the noise floor of the amp is way down around -110dB.

I measured output impedance of the headphone jack at 3.2 ohms at 1kHz; Schiit rates it at 0.5 ohm. Note that this measurement, made with a potentiometer used as a voltage divider, is not super-accurate, and any output impedance in the low single digits is low enough not to react significantly with the reactance of the headphones, and thus won’t change their frequency response.

. . . Brent Butterworth
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Reviewed on: SoundStage! Solo, January 2019

I measured the iFi Audio xCAN using a Clio 10 FW audio analyzer and a Neutrik NL-1 Minilyzer. Except as noted, I used the xCAN’s unbalanced analog input and unbalanced analog output, because I don’t yet have an adapter for 2.5mm balanced outputs I can use for measurements. Note that my focus with these tests is on measurements that confirm these devices’ basic functionality, and that gauge the efficacy of any special features and functions that might be measurable.

Frequency response modes

This chart shows the xCAN’s frequency response with all processing off, and with XBass II engaged in its three different modes (Bass, Presence, and Bass+Presence), with 1mW output into a 32-ohm load. With processing off, the response measures -0.14dB at 20Hz and -0.19dB at 20kHz. Bass mode boosts response by 9.96dB at 20Hz. Presence mode boosts response in a 4.12dB peak centered at 1288Hz. Frequency response did not change in 3D+ mode, and also did not change with 250-ohm and 600-ohm loads.

THD vs. power output

This chart shows the unbalanced output of the xCAN vs. total harmonic distortion (THD) into 32-, 250- and 600-ohm loads. Note that iFi’s power ratings are specified at 16, 50, 300 and 600 ohms, so some of my measurements are not directly comparable. Output into 32 ohms is 320mW at 0.5% THD and 336mW at 1% THD (iFi’s rating, in S-balanced/unbalanced mode, is 380mW into 32 ohms, THD unspecified). Output into 250 ohms is 46mW at 0.5% THD and 49mW at 1% THD. Output into 600 ohms is 20mW at 0.5% THD and 19mW at 1% THD.

Distortion

Here you can see the harmonic distortion spectrum and noise floor of the xCAN, referenced to 3Vrms output at 600Hz into 32 ohms. The third harmonic at 1.8kHz is slightly more predominant than the second harmonic, which will sound a little more objectionable than an amp (like a typical tube amp) with predominantly second-harmonic distortion, but if you actually dare to listen at 3Vrms (280mW into 32 ohms), the distortion from the headphones will likely be far louder than the distortion from the amp.

I measured the unbalanced output impedance at 1.2 ohms at 1kHz; iFi rates impedance at <2 ohms for balanced and <1 ohm for unbalanced output. Regardless, the output impedance is low enough not to react significantly with the reactance of the headphones, and thus won’t change their frequency response.

. . . Brent Butterworth
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