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Parent Category: Products

Category: Headphone Amplifier Measurements

Created: 20 September 2019

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.

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.

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.

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.

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
brentb@soundstagenetwork.com

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Parent Category: Products

Category: Headphone Amplifier Measurements

Created: 01 August 2019

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.

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.

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.

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.

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%.

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
brentb@soundstagenetwork.com

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Parent Category: Products

Category: Headphone Amplifier Measurements

Created: 20 April 2019

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.

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.

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.

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
brentb@soundstagenetwork.com

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Parent Category: Products

Category: Headphone Amplifier Measurements

Created: 20 April 2019

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.

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.

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.)

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.

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
brentb@soundstagenetwork.com

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Parent Category: Products

Category: Headphone Amplifier Measurements

Created: 10 April 2019

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.

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.

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.

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
brentb@soundstagenetwork.com

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Parent Category: Products

Category: Headphone Amplifier Measurements

Created: 10 January 2019

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.

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.

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.

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
brentb@soundstagenetwork.com

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Parent Category: Products

Category: Headphone Amplifier Measurements

Created: 01 October 2018

Reviewed on: SoundStage! Solo, October 2018

I measured the iFi Audio xDSD using a Clio 10 FW audio analyzer and a Neutrik NL-1 Minilyzer. For all of these tests, I used the xDSD’s coaxial digital input. Note that this is the first DAC-headphone amp I’ve measured for SoundStage! Solo; I’ve decided to focus my efforts on tests that confirm such devices’ basic functionality, and that gauge the efficacy of any special features and functions that might be measurable.

This chart shows the xDSD’s frequency response in its Listen and Measure modes, and with XBass+ engaged, with a 24-bit/192kHz S/PDIF signal and the xDSD set for 1mW output into a 32-ohm load. The response in both modes measured -0.16dB at 20Hz and -0.26dB at 20kHz. Listen mode actually measured slightly better here, with less rolloff above 65kHz; apparently, the switch is mislabeled. The bass boost in XBass+ mode was 6.48dB at 20Hz.

This chart shows the xDSD’s frequency response in Listen and Measure modes, and with XBass+ engaged, with a 16/48 S/PDIF signal and the xDSD set for 1mW output into a 32-ohm load. The treble response at 20kHz in Measure mode is -1.91dB, and in Listen mode -0.32dB. Definitely, the switch is mislabeled. According to the xDSD manual, the Listen filter is “transient-optimized minimum phase” and the Measure filter is “frequency response optimized,” but a filter with -1.91dB rolloff at 20kHz is certainly not “frequency response optimized.”

This chart shows the output of the xDSD vs. its total harmonic distortion (THD) into loads of 32, 250, and 600 ohms. Although iFi specifies the xDSD’s power output into 16, 50, 300, and 600 ohms, which renders most of my measurements not directly comparable, those measurements do suggest that iFi’s specs are on the mark. The xDSD’s output into 32 ohms is 291mW at 0.5% THD and 304mW at 1% THD; into 250 ohms, the output is 53mW at 0.5% THD and 54mW at 1% THD; and into 600 ohms, the xDSD puts out 22mW at 0.5% THD and 23mW at 1% THD.

Here you can see the xDSD’s spectrum of harmonic distortion and noise floor when driven by a 24/192 S/PDIF signal and referenced to 1.5V RMS output at 600Hz. Note that the distortion profile of the Measure and Listen modes is effectively the same.

I measured the xDSD’s output impedance as 0.8 ohm at 1kHz, which confirms iFi’s rating of <1 ohm. I prefer a headphone amp’s output impedance to be 1 ohm or less; the output impedance will then not react significantly with the reactance of the headphones, and thus won’t affect the ’phones’ frequency response.

. . . Brent Butterworth
brentb@soundstagenetwork.com

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Parent Category: Products

Category: Amplifier Measurements

Created: 01 July 2025

Link: reviewed by Dennis Burger on SoundStage! Access on July 1, 2025

General Information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The NAD C 700 V2 was conditioned for 1 hour at 1/8th full rated power (~10W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The C 700 V2 offers two single-ended RCA analog inputs (one phono MM, one line-level), two digital S/PDIF inputs (coaxial and optical), an HDMI input, an ethernet connection for streaming, line-level subwoofer and pre-outs (RCA), and a pair of speaker-level outputs. For the purposes of these measurements, the following inputs were evaluated: coaxial digital (RCA), and the analog line-level and phono MM unbalanced (RCA).

Most measurements were made with a 1Vrms line-level analog input or 0dBFS digital input depending on the input. The volume control is variable from 0 to 100. The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 80W. For comparison, on the analog input, an SNR measurement was also made with the volume at maximum.

Based on the accuracy and repeatability at various volume levels of the left/right channel matching (see table below), the C 700 V2 volume control is likely operating in the digital domain. Consequently, all analog signals are digitized at the C 700 V2’s inputs so the unit may apply volume, bass management, and tone controls. The volume control offers a total range from -46dB to +32.6dB (speaker-level outputs). Volume increments are in 1dB steps.

Because the C 700 V2 is a digital amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz-90kHz was necessarily changed to 10Hz-22.4kHz for all measurements, except for frequency response and for FFTs. In addition, THD versus frequency sweeps were limited to 6kHz to adequately capture the second and third signal harmonics with the restricted bandwidth setting.

Volume-control accuracy (measured at speaker outputs): left-right channel tracking

Published specifications vs. our primary measurements

The table below summarizes the measurements published by NAD for the C 700 V2 compared directly against our own. The published specifications are sourced from NAD’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was set at its maximum (DC to 1MHz), assume, unless otherwise stated, a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the analog/digital input (unless specified, assume a 1kHz sinewave at 1Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the C 700 V2 was able to sustain 107W (2% THD) into 4 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (10.7W) for 5 seconds for 5 continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the C 700 V2 was only slightly warm to the touch.

Our primary measurements revealed the following using the analog phono MM input (unless specified, assume a 1kHz sinewave at 5mVrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth): 

Frequency response (8-ohm loading, line-level input)

In our measured frequency response (relative to 1kHz) chart above, the C 700 V2 is nearly flat within the audioband (20Hz to 20kHz). At the extremes the C 700 V2 is -0.04dB at 20Hz, and -0.2dB at 20kHz. The C 700 V2 cannot be considered a high-bandwidth audio device as the -3dB point is just past 20kHz. That’s because incoming analog signals are digitally sampled at 44.1kHz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Frequency response (8-ohm loading, line-level input, bass and treble controls)

Above are two frequency-response plots for the analog input, measured at 10W (8 ohms) at the speaker outputs, with the treble/balance controls set at both minimum and maximum. They show that the C 700 V2 will provide a maximum gain/cut of approximately 6dB at 20Hz, and a maximum gain/cut of approximately 6dB at 10kHz.

Frequency response (subwoofer output engaged, 80Hz crossover)

Above are two frequency-response plots for the analog input, measured at 10W (8 ohms) at the speaker outputs, and at the line-level subwoofer output, with the crossover set at 80Hz. The C 700 V2 DSP crossover uses a slope of 18dB/octave, and the subwoofer output is flat down to 10Hz.

Phase response (8-ohm loading, line-level input)

Above is the phase-response plot from 20Hz to 20kHz for the analog input. The C 700 V2 appears to invert polarity (not evident in graph due to scale) and due to the sampling by the ADC and the inherent delays associated with this process, the overall phase shift is significant at 1.5 million degrees at 20kHz.

Frequency response (MM input)

The chart above shows the frequency response (relative to 1kHz) for the MM phono input. What is shown is the deviation from the RIAA curve, where the input signal sweep is EQ’d with an inverted RIAA curve supplied by Audio Precision (i.e., zero deviation would yield a flat line at 0dB). We see a very flat response from 50Hz to 1kHz. There is a 0.5dB rise at 20Hz, then a steep dip below 20Hz (roughly -3dB at 10Hz). There is a slow 0.5dB (at 20kHz) high frequency rise starting at 2kHz, then brickwall filtering just past 20kHz.

Frequency response vs. input type (8-ohm loading, left channel only)

The chart above shows the C 700 V2’s frequency response as a function of input type. The green traces are the same analog input data from the analog line-level previous graph. The red/blue traces are for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple/green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink/orange is 24/192 from 5Hz to 96kHz. The analog-in frequency response is identical to the 16/44.1 digital input (but for slightly more low frequency roll-off), with brickwall filtering just above 20kHz. The behavior at low frequencies is the same across all digital input types: -0.15dB at 10Hz. The behavior at high frequencies for all digital input types is typical. The 24/96 data shows brickwall-type filtering right around 48kHz, while the 24/192 data shows a gentler slope with a -3dB point at 77kHz.

Digital linearity (16/44.1 and 24/96 data)

The chart above shows the results of a linearity test for the coaxial digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the line-level pre-outs of the C 700 V2. For this test, the digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555. The ideal response would be a straight flat line at 0dB. Both digital input types performed similarly, approaching the ideal 0dB relative level at -100dBFS, then yielding perfect results to 0dBFS. At -120dBFS, the 16/44.1 input data overshot the ideal output signal amplitude by 2-3dB, while the 24/96 data overshot by only 0.5dB. The above results were good, so we extended . . .

. . . the test to -140dB. In this instance both the 16/44.1 and 24/96 deviated considerably from the ideal of a flat line at 0dB.

Impulse response (16/44.1 and 24/96 data)

The graph above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the line-level pre-outs of the C 700 V2. We can see that the C 700 V2 utilizes a typical sinc function reconstruction filter, though this test again shows that it appears to invert polarity.

J-Test (coaxial input)

The plot above shows the results of the J-Test test for the coaxial digital input measured at the line-level pre-outs of the C 700 V2. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically,  a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave. In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least-significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc). Note that the alternating peaks are in the test file itself, but at levels of -144dBFS and below. The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

We see very small peaks in the audioband at -140dBFS around 10kHz and 14kHz. This is a very good J-Test result, and an indication that the C 700 V2 DAC has good jitter immunity.

J-Test (optical input)

The plot above shows the results of the J-Test test for the optical digital input measured at the line-level pre-outs of the C 700 V2. We see essentially the same result as with the coaxial input above.

J-Test (coaxial input, jitter 10ns)

The plot above shows the results of the J-Test test for the coaxial digital input, measured at the line-level pre-outs of the C 700 V2 with sinewave jitter injected at 2kHz at the 10ns level. We see the same result as without the extra injected jitter. The optical input yielded a similar result.

J-Test (coaxial input, jitter 100ns)

The plot above shows the results of the J-Test test for the coaxial digital input measured at the line-level pre-outs of the C 700 V2 with sinewave jitter injected at 2kHz at the 100ns level. We see the tell-tale peaks at 10kHz and 14kHz peaking just above the very low -140dBFS level. More evidence of how strong the jitter immunity is for the C 700 V2 DAC. The optical input yielded a similar result.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input)

The plot above shows a fast Fourier transform (FFT) of the C 700 V2’s line-level pre-outs with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of a brickwall-type reconstruction filter. There are effectively no aliased image peaks in the audioband above the -130dBrA noise floor. The main 25kHz alias peak is near -75dBrA. The second, third, and fourth distortion harmonics (i.e., 38.2, 57.3, 76.4kHz) of the 19.1kHz tone are between -75dBrA and -95dBrA.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the analog input swept from 10Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Zoomed in, we can see a maximum deviation within the audio band of less than 0.2dB from 4 ohms to no load, which is an indication of a relatively high damping factor, or low output impedance. The maximum variation in RMS level when a real speaker was used is less, deviating by about 0.1dB within the flat portion of the curve (100Hz to 5kHz), with the lowest RMS level, which would correspond to the lowest impedance point for the load, exhibited around 200Hz, and the highest RMS level, which would correspond to the highest impedance point for the load, at around 2kHz.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency (20Hz to 6kHz) for a sinewave stimulus at the analog input. The blue and red plots are for the left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 71W. The power was varied using the volume control. All three THD plots are relatively flat. The 1W data exhibited the lowest THD values, with values varying around 0.002-0.003%. The 10W data shows THD values around 0.004-0.005%. At 71W, THD values were around 0.05%.

THD ratio (unweighted) vs. frequency at 10W (phono input, MM)

The chart above shows THD ratios as a function of frequency plots for the MM phono input measured across an 8-ohm load at 10W. For this test, the input sweep is EQ’d with an inverted RIAA curve. THD ratios ranged from roughly 0.02% at 20Hz, down to as low as 0.01% at 300-400Hz, then up to 0.02% from 1kHz to 6kHz.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD ratios measured at the output of the C 700 V2 as a function of output power for the analog line-level input for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). Although there are fluctuations before the “knee,” both the 4-ohm and 8-ohm data are close to the same, ranging from 0.002% to 0.05%. The “knee” in the 8-ohm data occurs around 80W, hitting the 1% THD mark at 83W. For the 4-ohm data, the “knee” occurs around 100W, hitting the 1% THD mark at 108W.

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD+N ratios measured at the output of the C 700 V2 as a function of output power for the analog line-level-input for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). There’s a distinct 5dB jump in THD+N (also visible in the THD plot above, but to a lesser degree) when the output voltage is around 1Vrms (i.e., 0.15W into 8 ohms, 0.3W into 4 ohms). This behavior was repeatable over multiple measurement trials. Overall, THD+N values before the “knee” ranged from around 0.01% (3 to 20W into 8 ohms and 10 to 30W into 4 ohms) to 0.05/0.03% (8/4 ohms at the knee).

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the C 700 V2 as a function of load (8/4/2 ohms) for a constant input voltage that yielded 10W at the output into 8 ohms (and roughly 20W into 4 ohms, and 40W into 2 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find increasing levels of THD from 8 to 4 to 2 ohms, with about a 5dB increase from 8 to 4 ohms, and a 2-3dB rise from 4 to 2 ohms. Overall, even with a 2-ohm load at roughly 40W, THD values were fairly flat within the audioband at between 0.01 and 0.02%.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the C 700 V2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Generally, THD ratios into the real speakers were higher at frequencies below 400Hz than those measured across the resistive dummy load. The differences ranged from 0.1/0.006% at 20/150Hz for the two-way speaker versus a constant 0.003% for the resistive load, and 0.01/0.006% at 25/150Hz into the three-way speaker. The three-way speaker did dip as low as 0.001% at 250Hz.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the C 700 V2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Generally, IMD ratios were similar for all three loads, from 0.001 to 0.003%.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the C 700 V2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). We find very similar IMD ratios into all three loads, a constant 0.01% from 40Hz to 500Hz, then down to 0.006% to 1kHz.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input. We see that the signal’s second (2kHz) and third (3kHz) harmonics dominate at -95dBrA and -85dBrA, or 0.002% and 0.006%. The remaining signal harmonics are below -110dBrA, or 0.0003%. Below 1kHz, we do not see traditional peaks from linear power supplies (60/120Hz) because of the switching power supply. All other noise-related peaks are near or below -120dBrA, or 0.0001% (an improvement from the original C700). It appears that the analog signal is digitized with a 44.1kHz sample rate, as peaks can be seen at 44.1kHz, as well as the IMD products with the main signal at 43.1 and 45.1kHz.

FFT spectrum – 1kHz (phono MM input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the analog phono MM input. Signal harmonics are difficult to distinguish from the numerous noise peaks that range in level from -60dBrA, or 0.1%, down to -110dBrA, or 0.0003%, at high frequencies. The signal’s second (2kHz) and third (3kHz) harmonics can be seen, however, at just below and above -80dBrA, or 0.01%.

FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 16/44.1. We see close to the same signal harmonics at 2kHz and 3kHz as with the analog input, as well as the same lower-level signal harmonics at and below -110dBrA, or 0.0003%. The noise floor is low at -130dBrA, and spurious noise peaks are below -120dBrA, or 0.0001%.

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 24/96. We see essentially the same signal harmonic profile as with the 16/44.1 FFT above, but with a slightly lower noise floor due to the increased bit depth.

FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sinewave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and the second (2kHz) and third signal harmonics (3kHz) dominating at around -120dBrA, or 0.0001%. The noise floor on the left channel is roughly 10dB lower than the right channel at -150dBrA.

FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude. The rest of the FFT is very similar to the 16/44.1 FFT above, including the different noise floors between channels.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the analog input. The X axis is zoomed in from 40 Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant (non-signal) peaks are the signal second (100Hz) and third (150Hz) harmonics at around -100dBrA and -85dBrA, or 0.001% and 0.006%, with other signal harmonics seen below the -110dBrA level. Spurious noise peaks are generally below the -120dBrA, or 0.0001%, level.

FFT spectrum – 50Hz (phono MM input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the phono MM input. The X axis is zoomed in from 40 Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. We see a multitude of noise peaks, signal harmonics, and IMD products between the two, at the -60dBrA, or 0.1%, and below level.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the analog input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) are at -95/-110dBrA (left/right), or 0.002/0.0003%, while the third-order modulation products, at 17kHz and 20kHz, are just under -90dBrA, or 0.003%. We also see the main aliased peaks at 25.1kHz and 26.1kHz around -65dBrA, indicating that the C 700 V2 ADC is digitizing the incoming analog signal at 44.1kHz (i.e., 44.1kHz-19kHz = 25.1kHz).

Intermodulation distortion FFT (line-level input, APx 32 tone)

Shown above is the FFT of the speaker-level output of the C 700 V2 with the APx 32-tone signal applied to the analog input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and are at and below the low -120dBrA, or 0.0001%, level.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, phono MM input)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the phono MM input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation products (i.e., the difference signal of 1kHz) are at -90/-100dBrA (left/right), or 0.003/0.001%, while the third-order modulation products, at 17kHz and 20kHz, are around -80dBrA, or 0.01%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, digital 16/44.1)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 16/44.1. We find that the second-order modulation products (i.e., the difference signal of 1kHz) are at -90/-100dBrA (left/right), or 0.003/0.001%, while the third-order modulation products, at 17kHz and 20kHz, are just under -90dBrA, or 0.003%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, digital 24/96)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 24/96. We find that the second-order modulation products (i.e., the difference signal of 1kHz) are at -90/-100dBrA (left/right), or 0.003/0.001%, while the third-order modulation products, at 17kHz and 20kHz, are just under -90dBrA, or 0.003%.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the analog input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the C 700 V2’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very limited bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. Due to C 700 V2’s very limited bandwidth, only the square wave’s fundamental (10kHz) sinewave is reproduced here. In addition, we can see the 400kHz switching oscillator frequency used in the digital amplifier section visibly modulating the waveform.

Square-wave response (1kHz) — 250kHz bandwidth

Above is the 1kHz squarewave response using the analog input, at roughly 10W into 8 ohms, with a 250kHz restricted bandwidth to remove the modulation from the 400kHz oscillator. We see more evidence here, in the over and undershoot at the squarewave corners, of the C 700 V2’s limited bandwidth with an analog input.

FFT spectrum of 400kHz switching frequency relative to a 1kHz tone

The C 700 V2’s class-D amplifier relies on a switching oscillator to convert the input signal to a pulse-width-modulated (PWM) squarewave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The C 700 V2 oscillator switches at a rate of about 400kHz, and this graph plots a wide bandwidth FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sinewave. We can see that the 400kHz peak is quite evident, and at -40dBrA. There is also a peak at 800kHz and 1200kHz (the second and third harmonics of the 400kHz peak) at -65/-90dBrA. Those three peaks—the fundamental and its second/third harmonics—are direct results of the switching oscillators in the C 700 V2 amp modules. Also seen are the 43.1/44.1/45.1kHz peaks due to the ADC sampling the incoming signal at 44.1kHz. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audioband—and therefore inaudible—and so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

Damping factor vs. frequency (20Hz to 20kHz)

The final plot above is the damping factor as a function of frequency. Both channels show a relatively constant damping factor from 20Hz to 20kHz. The damping factor for the left and right channels are just above and below 100 from 20Hz to 20kHz.

Diego Estan
Electronics Measurement Specialist