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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 April 2025

Link: reviewed by Matt Bonaccio on SoundStage! Hi-Fi on April 1, 2025

General Information

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

The Hegel Music Systems H190v was conditioned for 1 hour at 1/8th full rated power (~18W 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 H190v offers three set of line-level analog inputs (two single-ended RCA, one balanced XLR), one MM phono input (single-ended RCA), one digital coaxial (RCA) S/PDIF input, three digital optical (TosLink) S/PDIF inputs, one USB input, left/right line-level pre-outs (single-ended RCA) and fixed-outs (single-ended RCA), one set of speaker level outputs, and on the front panel, one headphone output over 1/4″ TRS connector. An ethernet network input is also offered. For the purposes of these measurements, the following inputs were evaluated: digital coaxial, analog (XLR) line-level and phono, as well as the headphone output. There were no appreciable differences between the XLR and RCA line-level inputs, nonetheless, 1kHz FFTs for each are included in this report.

Most measurements were made with a 2Vrms line-level analog input, 5mVrms phono-level input, and 0dBFS digital input. The signal-to-noise (SNR) measurements were made with the default input signal values but with the volume set to achieve the achievable output power of 150W into 8 ohms. For comparison, on the line-level input, a signal-to-noise ratio (SNR) measurement was also made with the volume at maximum.

Based on the accuracy and randomness of the left/right volume channel matching (see table below), the H190v volume control is digitally controlled but operating in the analog domain. The H190v overall volume range is from -70dB to +38dB (line-level input, speaker output). It offers 2-3dB increments from position 0 to 9, and 1dB increments from positions 9 to 100. Also noteworthy is that several step positions do not actually change the volume (e.g., steps 84 and 85 yield the same volume level).

Our typical input bandwidth filter setting of 10Hz-22.4kHz was used for all measurements except FFTs and THD versus frequency, where a bandwidth of 10Hz-90kHz was used. Frequency response measurements utilize a DC to 1 MHz input bandwidth.

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 Hegel for the H190v compared directly against our own. The published specifications are sourced from Hegel’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 extended to 1MHz, assume, unless otherwise stated, 10W into 8 ohms and 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 line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the H190v was able to sustain 250W into 4 ohms (~2% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (16.1W) for 5 seconds, for 5 continuous minutes without inducing the fault protection circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the H190v was very warm to the touch.

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

Our primary measurements revealed the following using the analog input at the headphone output (unless specified, assume a 1kHz sinewave, 2Vrms input/output, 300 ohms loading, 10Hz to 22.4kHz bandwidth):

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

In our frequency-response plots above (relative to 1kHz), measured across the speaker outputs at 10W into 8 ohms, the H190v is near flat within the audioband (20Hz to 20kHz, -0.2/-0.1dB). The -3dB point is at roughly 120-130kHz, and -2dB at 5Hz. The H190v appears to be AC-coupled. 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.

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

Above are the phase-response plots from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The H190v does not invert polarity and yields only about +15 degrees of phase shift at 20Hz and -20 degrees at 20kHz.

Frequency response (8-ohm loading, MM phono input)

The chart above shows the frequency response 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 relatively flat (+/-0.5dB) response from 35Hz to 20kHz and a worst-case channel-to-channel deviations of roughly 0.1dB at 100 to 300Hz. Below 35Hz, there is steep attenuation (-3dB at ~17Hz), as Hegel appears to have implemented an anti-rumble filter on their phono input.

Phase response (MM input)

Above is the phase response plot from 20Hz to 20kHz for the MM phono input, measured across the speaker outputs at 10W into 8 ohms. The H190v does not invert polarity. For the phono input, since the RIAA equalization curve must be implemented, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst case of about +40 degrees at 20Hz and -100 degrees at 20kHz.

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

The chart above shows the H190v’s frequency response (relative to 1kHz) as a function of input type measured across the speaker outputs at 10W into 8 ohms. The two green traces are the same analog input data from the speaker-level frequency-response graph above (but limited to 80kHz). The blue and red traces are for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial input, the purple and green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and the pink and orange traces are for a 24/192 dithered digital input signal. At low frequencies, the digital signals yielded the same response down to 5Hz (-2dB) as the analog response. The -3dB points are: 21kHz for the 16/44.1 data, 46kHz for the 24/96, 50.5kHz for the 24/192 data, and 130kHz for the analog input. Also of note, all three digital input data showed brick-wall-type high-frequency filtering and a rise in output (up to +0.5dB) past 20kHz.

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 fixed line-level outputs of the H190v, where 0dBFS yielded approximately 2Vrms. For this measurement, 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 data were essentially perfect as of -100dBFS down to 0dBFS. The 24/96 data were at +2/3dB at -120dBFS, while the 16/44.1 data were +4/5dB at -120dBFS.

Impulse response (24/44.1 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 fixed outputs of H190v. We see a typical symmetrical sinc-function response.

J-Test (coaxial)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the fixed line-level outputs of the H190v where 0dBFS is just over 2Vrms. J-Test was developed by Julian Dunn in 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 (the main peak). 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 -144dBrA 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.

Here we see a relatively strong J-test result, with several peaks in the audioband but at low levels, just above and below -130dBFS. This is an indication that the H190v DAC may have good jitter immunity.

J-Test (optical)

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level pre-outputs of the H190v. The optical input yielded essentially the same result compared to the coaxial input.

J-Test (coaxial, 10ns jitter)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the H190v, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. The telltale peaks at 10kHz and 14kHz cannot be seen in the FFT. The performance of the optical input with 10ns of jitter was similar.

J-Test (coaxial, 100ns jitter)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the H190v, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The telltale peaks at 10kHz and 14kHz can be seen, but are at the relatively low -100dBFS level.

J-Test (optical, 100ns jitter)

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the H190v, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The results are essentially the same as with the coaxial input.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (Linear Phase Fast filter, coaxial input)

The chart above shows a fast Fourier transform (FFT) of the H190v’s line-level outputs with white noise at -4dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1. The steep roll-off around 20kHz in the white-noise spectrum shows that the filter is of the brickwall-type variety. There are a few low-level aliased image peaks within the audioband at the -120dBrA and below level. The primary aliasing signal at 25kHz is highly suppressed and buried in the noise floor, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are around -100dBrA.

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

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 50kHz. 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. Here we see that the deviations between no load and 4 ohms are very small at roughly 0.04dB. This is a strong result and an indication of a low output impedance, or very high damping factor. With a real speaker load, deviations measured at roughly the same level from 60Hz to 8kHz.

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

The chart above shows THD ratios at the speaker-level outputs into 8 ohms as a function of frequency for a sinewave stimulus at the analog line level input. The blue and red plots are for left and right at 1W output into 8 ohms, purple/green at 10W, and pink/orange just at 103W. The power was varied using the H190v volume control. The 1W and 10W THD ratios were close, hovering around the 0.01% level from 20Hz to 4kHz, then up to 0.03% at 20kHz. The 103W THD ratios were higher and relatively flat across the audioband at 0.03% to 4Khz, then up to 0.05% at 20kHz.

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

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 measurement, the input sweep was EQ’d with an inverted RIAA curve. The THD values for the MM configuration vary from around 1% (20Hz) down to 0.02% at 4Khz to 20kHz. The limiting factor in the measured THD values is the noise floor (the analyzer cannot assign a THD value to a harmonic peak it cannot see below the noise floor), and since the RIAA curve applies more gain at low frequencies than high frequencies, we find the THD plots above roughly following the shape of the noise floor (higher to lower from low to high frequencies).

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

The chart above shows THD ratios measured at the speaker-level outputs of the H190v 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). THD ratios into 4 and 8 ohms are close (within 5dB). The 8-ohm THD ratios are relatively constant to the “knee,” ranging from 0.005% to 0.01%. The “knee” into 8 ohms can be found just past 100W, while the 4-ohm knee can be seen around 200W. The 1% THD marks were hit at 149W and 240W into 8 and 4 ohms.

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

The chart above shows THD+N ratios measured at the speaker-level outputs of the H190v 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). THD+N ratios into 4 and 8 ohms are close (within 5dB). The 8-ohm data range from 0.02% at 50mW, down to 0.005% in the 50 to 50W range.

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 H190v as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 20W at the output into 8 ohms (and roughly 40W into 4 ohms, and 80W 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. Here again we see the 8 and 4-ohm THD data are close together (within 5dB). Below 1kHz, the 4-ohm data yielded lower THD ratios whereas above 1kHz, the 8-ohm data were lower. These ranged from 0.006% at 20Hz, down to 0.003% from 60Hz to 200Hz, then up to 0.02-0.03% at 20kHz. The 2-ohm load ranged from 0.01% at 20Hz, down to 0.004% at 50-100Hz, then up to 0.06% at 20kHz.

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 H190v 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 all three loads were close (mostly within 5dB), which is a strong result. As is typical with this test, the worst results were at 20Hz into the two-way speaker (0.04%), and at 20kHz into the 3-way speaker (0.04%). Generally, most of the measured THD ratios hovered around the 0.005% to 0.01% range below 4kHz.

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 H190v 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). We find that all three IMD traces are close to one another, with the real speakers yielding 4-5dB lower results in the 2.5-5kHz range, and 2dB higher results from 10kHz to 20kHz. Most of the IMD results are hovering around the 0.01%-0.02% level.

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 H190v 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, 0.03% from 40Hz to 250Hz, and 0.006% from 300Hz to 1kHz. Another strong result.

FFT spectrum – 1kHz (XLR analog 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 balanced analog line-level input. We see that the signal’s second (2kHz) and third (3kHz) harmonic dominate at -80dBrA, or 0.01%. There are subsequent signal harmonics visible at and below -100dBrA, or 0.001%. On the right side of the signal peak, we find power-supply-related noise peaks, with the fundamental (60Hz) and second harmonic (120Hz) dominating at -105/-110dBrA (left/right), or 0.0006/0.0003%. Other noise peaks can be seen below the -110dBrA level.

FFT spectrum – 1kHz (RCA analog 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 unbalanced analog line-level input. We see essentially the same FFT as with the analog balanced input above.

FFT spectrum – 1kHz (MM phono 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 phono (MM) input. We see that the signal’s second (2kHz) and fourth (4kHz) harmonics dominate at -65dBrA, or 0.06%, and -85dBrA, or 0.006%, respectively. There are subsequent signal harmonics visible at and below the -90dBrA, or 0.003%, level. On the right side of the signal peak, we find significant power-supply-related noise peaks, with the second harmonic (120Hz) dominating at -45dBrA, or 0.6%, and the fourth (240Hz) harmonic reaching -50dBrA, or 0.3%. Other noise peaks can be seen throughout the audioband, down to the -120dBrA level.

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. The results are very similar to the analog line-level FFTs above.

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 result as with the 16/44.1 FFT above.

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, with no signal harmonics above the -130dBrA noise floor, and power-supply noise-related peaks at the sub -100dBrA level.

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 effectively the same FFT as with the 16/44.1 sampled data above.

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 line-level input. The X axis is zoomed in from 40Hz 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 second (100Hz) and third (150Hz) signal harmonics at just below -80dBrA, or 0.01%. Other peaks (both signal harmonics and power-supply noise related harmonics) can be seen at -100dBrA and below.

FFT spectrum – 50Hz (MM phono 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 MM phono 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 second (120Hz) and fourth (240Hz) power-supply noise peaks at roughly -45/-50dBrA, or 0.6/0.3%, and the second (100Hz) signal harmonic at -50dBrA, or 0.3%.

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 line-level 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) is at -90dBRa, or 0.003%, while the third-order modulation products, at 17kHz and 20kHz are at the -85dBrA, or 0.006%, level.

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

Shown above is the FFT of the speaker-level output of the H190v 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 -105dBrA, or 0.0006%, level.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 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 (-1dBFS). We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -90dBRa, or 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are at the -85dBrA, or 0.006%, level.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 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 (-1dBFS). We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -90dBRa, or 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are at the -85dBrA, or 0.006%, level.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, MM phono 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 MM phono input. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -70dBRa, or 0.03%, while the third-order modulation products, at 17kHz and 20kHz, are at the -90dBrA, or 0.003%, level.

Squarewave response (10kHz)

Above is the 10kHz squarewave response using the analog line-level 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 H190v’s slew-rate performance. Rather, it should be seen as a qualitative representation of the H190v’s mid-to-high 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. In this case, we find relatively clean corners, with only mild softening in the corners.

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

The final graph above is the damping factor as a function of frequency. We can see here very high damping factor ranging from roughly 500 to 250 (left) and 400 to 185 (right). This is a strong result for a medium-powered solid-state integrated amplifier.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Amplifier Measurements

Created: 01 March 2025

Link: reviewed by Jason Thorpe on SoundStage! Ultra on March 1, 2025

General information

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

The Balanced Audio Technology (BAT) REX 300 was conditioned for 1 hour at 1/8th full rated power (~25W 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 REX 300 is a two-channel amplifier with two balanced (XLR) inputs and two sets of speaker level outputs. An input of 500mVrms was required to achieve the reference 10W into 8 ohms.

Our typical input bandwidth filter setting of 10Hz-22.4kHz was used for all measurements except FFTs and THD versus frequency, where a bandwidth of 10Hz-90kHz was used. Frequency response measurements utilize a DC to 1MHz input bandwidth.

Of note is that BAT claims no global negative feedback for the REX 300. Our measurements corroborate this claim, as two clear consequences of this design can be seen: a low damping factor (high output impedance) and high THD.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by BAT for the REX 300 compared directly against our own. The published specifications are sourced from BAT’s website, either directly or from the manual available for download, or a combination thereof. Assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz:

Our primary measurements revealed the following (unless specified, assume a 1kHz sinewave at 500mVrms at the input, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the REX 300 was able to sustain about 250W into 4 ohms (~1.5% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (25W) 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 sides and top of the REX 300 were hot to the touch. Of note, just above the 250W mark, the REX 300’s protection circuit was engaging almost immediately.

Frequency response (8-ohm loading)

In our frequency response (relative to 1kHz) plots above, measured across the speaker outputs at 10W into 8 ohms, the REX 300 exhibits a near-flat frequency response across the audioband (0/-0.1dB at 20Hz/20kHz). The REX 300 appears to be DC-coupled, as it is perfectly flat down to 5Hz. The -3dB point is at 150kHz.  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 perfectly overlap, indicating that the two channels are ideally matched.

Phase response (8-ohm loading)

Above are the phase response plots from 20Hz to 20kHz for the balanced line-level input, measured across the speaker outputs at 10W into 8 ohms. The REX 300 does not invert polarity and exhibits, at worst, only -10 degrees of phase shift at 20kHz, due to its extended bandwidth.

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 line-level 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. Here we find the maximum deviation between an 8-ohm load and no-load to be nearly 1dB. This is an indication of a very low damping factor, or high output impedance. With a real speaker, the maximum deviation from 20Hz to 20kHz was roughly 0.6dB, which may be audible.

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 for a sinewave stimulus at the analog line-level input. The blue and red plots are at 1W output into 8 ohms, purple and green at 10W, and pink and orange at 150W. At 1W and 10W, the left channel outperformed the right by as much as 10dB from 300Hz to 20kHz. The 1W left channel data ranged from 0.05% at 20Hz, down to 0.02% from 100Hz to 20kHz. The 10W left channel data ranged from 0.06% from 20Hz to 4kHz, then up to 0.15% at 20kHz. The 150W THD data are higher, ranging from 0.2% from 20Hz to 100Hz, then up to a very high 7% at 20kHz.

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 REX 300 as a function of output power for the analog line level-input, for an 8-ohm load (blue/red) and a 4-ohm load (purple/green). The left channel outperformed the right up to the “knee” for both loads by as much as 10dB. The 8-ohm data for the left channel ranged from 0.003% at 50mW, with a steady climb to 0.2% at the “knee,” at roughly 100W. The 1% THD mark was hit at 165W, and at the rated output of 200W, THD ratios measured around 3%. The 4-ohm data for the left channel ranged from 0.005% at 50mW, with a steady climb to 0.4% at the “knee,” at roughly 200W. The 1% THD mark was hit at 284W, and at the rated output of 400W, THD ratios would have (the plot stops just shy of 400W) measured around 4%.

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 REX 300 as a function of output power for the analog line level-input, for an 8-ohm load (blue/red) and a 4-ohm load (purple/green). The left channel outperformed the right up to the “knee” for both loads by as much as 5dB. The 8-ohm data for the left channel ranged from 0.05% from 50mW to 2W, then a steady climb to 0.2% at the “knee,” at roughly 100W. The 4-ohm data for the left channel ranged from 0.1% from 50mW to 5W, then a steady climb to 0.4% at the “knee,” at roughly 200W.

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 REX 300 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yielded roughly 50W at the output into 8 ohms (blue), 100W into 4 ohms (purple), and 200W into 2 ohms (pink). The 2-ohm data is not present because the protection circuit was initiated almost immediately after the start of the sweep. The 8-ohm data ranged from 0.1% at 20-2kHz, then up to 0.4% at 20kHz. The 4-ohm THD data ranged from 0.2% at 20-2kHz, then up to 2% at 20kHz.

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 REX 300 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). At very low frequencies, the two-way speaker yielded the highest THD ratios (0.6%). Between 40Hz and 10kHz, the THD ratios into all three loads were within roughly 5dB of one another, ranging from roughly 0.01% to 0.1%. At the highest frequencies, the three-way speaker yielded the highest THD ratios (0.08% at 20kHz), nearly 15dB higher than the two-way speaker and the resistive load.

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 REX 300 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). The IMD results into the resistive load range from 0.03% at low frequencies, down to 0.002% at high frequencies. The two-way speaker IMD results were flatter, ranging from 0.01% to 0.006%, while the three-way speaker ranged from 0.02% to 0.005%.

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 REX 300 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). All three plots are essentially identical and constant right around 0.1%.

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 balanced analog line-level input. We see that the signal’s second (2kHz) and third (3kHz) harmonics dominate at a very high -60dBrA and -80dBrA, or 0.1% and 0.01%. Power-supply noise-related harmonics, and what are likely IMD products between those noise peaks and the signal and its harmonics, are significant and can be seen throughout the FFT at levels of -70dBrA, or 0.03%, and below. This is an extremely poor FFT result. It should be noted, however, that the noise floor and noise peaks are far more significant with the REX 300 when a signal is present. This can be seen in our main measurement table where signal-to-noise ratios are respectable because the noise is measured separately from the signal. It can also be seen by comparing the noise levels in the table with and without a signal. Noise levels with the signal present are very high for a modern solid-state amplifier using a line-level input, and even higher that an average noise level from a solid-state amplifier’s phono input. Noise levels with a signal are 20 to 30 times higher than the noise measured without a signal present.

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 balanced analog line-level input. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most dominant (non-signal) peak is the signal’s second (100Hz) harmonic at -70dBrA, or 0.03%. Again, power-supply noise-related harmonics and IMD peaks can be seen throughout the FFT at 10Hz intervals at -80dBrA, or 0.01%, and below.

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 balanced analog line-level 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) is at -70dBrA, or 0.03%, level, while the third-order modulation products, at 17kHz and 20kHz, cannot be distinguished from the densely packed noise peaks rising up to -70dBrA, or 0.03%. This is an extremely poor IMD result.

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

Shown above is the FFT of the speaker-level output of the REX 300 with the APx 32-tone signal applied to the 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 around the -80dBrA, or 0.01%, level and below.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the balanced analog line-level 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 REX 300’s slew-rate performance. Rather, it should be seen as a qualitative representation of the REX 300’s wide 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. In this case, we see a very clean result, with no ringing in the corners and only very mild softening.

Damping factor vs. frequency (20Hz to 20kHz, two-channel mode)

The final graph above is the damping factor as a function of frequency. We find very low damping factor values, hovering around 18 to 19 from 20Hz to 15kHz, and 17 at 20kHz. This is a poor damping factor result for a solid-state amplifier.

Diego Estan
Electronics Measurement Specialist