NuPrime Evolution Two Mono Amplifier Measurements

Link: reviewed by Roger Kanno on SoundStage! Hi-Fi on October 1, 2023

General Information

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

The NuPrime Evolution Two was conditioned for one hour at 1/8th full rated power (~35W 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 Evolution Two is a monoblock (i.e., single channel) amplifier with one unbalanced (RCA) and one balanced (XLR) input, and one speaker-level output. 400mVrms was required at the input to achieve the reference 10W into 8 ohms. For the purposes of these measurements, unless otherwise specified, the balanced input was used.

Because the Evolution Two uses a digital amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz–90kHz for frequency sweeps was necessarily changed to 10Hz–22.4kHz, and limited to 6kHz to adequately capture the second and third signal harmonics with the restricted-bandwidth setting.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by NuPrime for the Evolution Two compared directly against our own. The published specifications are sourced from NuPrime’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 from DC 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 channesl.

* protection circuit engages after a few seconds at THD = 0.04%

Our primary measurements revealed the following using the Line 2 unbalanced analog input (unless specified, assume a 1kHz sinewave at 400mVrms at the input, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

* protection circuit engages after a few seconds at THD = 0.04%

For the continuous dynamic power test, the Evolution Two was able to sustain about 630W into 4 ohms (~1% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (63W) for five seconds, for five continuous minutes. However, during the test, the initiation of a protection circuit did occur several times. The protection circuit engages and disengages very quickly, allowing the Evolution Two to run through the test mostly uninterrupted. Therefore, we are calling a conditional pass on this test. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the Evolution Two was cool to the touch. Note that the Evolution Two is not able to sustain 630W into 4 ohms continuously for more than about one second before a protection circuit is triggered.

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

In our frequency-response plot above, measured across the speaker outputs at 10W into 8 ohms, the Evolution Two exhibits a clear rise at high frequencies. We have confirmed with NuPrime that this is intentional and not due to a defective unit. At low frequencies, the Evolution Two is essentially flat down to 5Hz. The rise at high frequencies was measured at about 0.6dB at 10kHz and 2dB at 20kHz. The rise peaks around 80–90kHz, at +6.5dB. Whether or not this deviation from a flat response would be audible would depend on the speakers used, musical content, and most importantly, the age of the listener. The -2dB point is at 200kHz, which is the maximum allowable frequency using the AP analyzer.

Phase response (8-ohm loading)

Above is the phase response plot from 20Hz to 20kHz for the balanced line-level input, measured across the speaker outputs at 10W into 8 ohms. The Evolution Two does not invert polarity and exhibits, at worst, about +5 degrees of phase shift within the audioband between 10kHz and 20kHz.

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 5Hz 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 around 0.08dB. This is an indication of a very high damping factor, or low output impedance. With a real speaker, the deviations are much lower, at about 0.02dB between 20Hz and 1kHz.

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 plot is at 1W output into 8 ohms, purple at 10W, and pink at 265W. The 1W data yielded fairly constant THD figures, at 0.003% from 20Hz to 3kHz, then a rise to 0.01% at 6kHz. At 10W, THD data ranged from 0.001% from 20Hz to 100Hz, then a steady rise to 0.03% at 6kHz. At 265W, THD data ranged from 0.002% from 20Hz to 100Hz, then a steady rise to 0.1% at 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 Evolution Two as a function of output power for the analog line-level input for an 8-ohm load (blue) and a 4-ohm load (purple). The 8-ohm data ranged from about 0.1% to 0.05% from 50 to 400mW, then a dip down to 0.003% to 0.005% from 500mW to 20W, a rise to 0.03% at the “knee” at just shy of 300W, then reaching the 1% THD mark at roughly 350W. Please note that each measurement in this sweep is only two seconds long, and by contrast, we were not able to sustain more than 294W into 8 ohms continuously. The 4-ohm data ranged from about 0.1% from 50 to 100mW, then a dip down to 0.003% to 0.01% from 150mW to 100W, a rise to 0.03% at the “knee” at 300W, then the Evolution Two protection circuit was triggered, precluding the collection of reliable data points above 300W.

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 Evolution Two as a function of output power for the analog line-level input, for an 8-ohm load (blue) and a 4-ohm load (purple). The 8-ohm data ranged from about 0.1% to 0.05% from 50 to 400mW, then a dip down to 0.005% from 500mW to 20W, a rise to 0.05% at the “knee” at just shy of 300W, then reaching the 1% THD mark at roughly 350W. Please note that each measurement in this sweep is only two seconds long, and by contrast, we were not able to sustain more than 294W into 8 ohms continuously. The 4-ohm data ranged from about 2% from 50 to 100mW, then a dip down to 0.01% from 150mW to 100W, a rise to 0.03% at the “knee” at 300W, then the Evolution Two protection circuit was triggered, precluding the collection of reliable data points above 300W.

THD ratio (unweighted) vs. frequency vs. load

The chart above shows THD ratios measured at the output of the Evolution Two as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields roughly 30W at the output into 8 ohms (blue), 60W into 4 ohms (purple), and 120W into 2 ohms (pink). The 8-ohm data ranged from 0.001% from 20Hz to 100Hz, then a steady rise to 0.03% at 6kHz. The 4-ohm THD data were only about 5dB higher compared to the 8-ohm data, but converging at the same 0.03% at 6kHz. The 2-ohm data were considerably higher from 20Hz to 500Hz, yielding a fairly constant 0.005% to 0.008%, then a rise to 0.06% at 6kHz. The maximum achieved continuous power into 2 ohms was about 190W, where the protection circuit engaged after about four seconds. Nevertheless, the Evolution Two proved to be stable into 2 ohms with continuous power in the 60W to 100W range.

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

The chart above shows THD ratios measured at the output of the Evolution Two 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 freqencies, the two-way speaker yielded the highest THD ratios (0.02%), which is typical with many amps. From 30Hz to 500Hz, all three THD plots are very close to one another, around the 0.003% mark. From 500Hz to 6kHz, the Evolution Two yielded lower THD ratios with real speakers compared to the dummy resistive load, ranging from 0.001 to 0.002%.

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

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the Evolution Two 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). Here the three-way speaker yielded the highest IMD results, from 0.001% to 0.02%. The dummy load was in between, while the two-way speaker yielded the lowest IMD results, between 0.0005% and 0.003%.

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

The chart above shows IMD ratios measured at the output of the Evolution Two 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). The resistive dummy load was fairly constant at 0.008%, while the speaker IMD results fluctuated above and below this value by as much as 10dB.

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 third (3kHz) harmonic dominates at -90dBrA, or 0.003%. The second, fourth, and fifth harmonics are a little lower, around -100dBrA, or 0.001%. Power-supply-related noise peaks can be seen but at low levels: -120dBrA, or 0.0001%, and below.

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 dominant (non-signal) peaks are the second (100Hz) and third (150Hz) signal harmonics at roughly -100dBrA, or 0.001%. Power-supply-related noise peaks can be seen but at low levels: -120dBrA, or 0.0001%, 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 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 -100dBrA, or 0.001%, and the third-order modulation products, at 17kHz and 20kHz, are higher at below -75dBrA, or 0.02%.

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

Shown above is the FFT of the speaker-level output of the Evolution Two 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 below the -100dBrA, or 0.001%, level. Higher-amplitude distortion products are seen at higher frequencies.  

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 Evolution Two’s slew-rate performance. Rather, it should be seen as a qualitative representation of the Evolution Two’s mid-tier 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 the case of the Evolution Two, however, what dominates the plateaus of the squarewave in the top graph is a 600kHz sinewave, the frequency at which the switching oscillator in the class-D amp is operating (see FFT below).

Square-wave response (10Hz–250kHz bandwidth)

Above is the 10kHz squarewave response using the analog input, at roughly 10W into 8 ohms, with a 250kHz restricted bandwidth to remove the modulation from the 600kHz oscillator. Here we find a squarewave with significant over-shoot in the corners, likely due to the Evolution Two’s rise in frequency response at high frequencies.

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

The Evolution Two’s 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 Evolution TWo oscillator switches at a rate of about 600kHz, 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 600kHz peak is quite evident, and at -30dBrA. There is also a peak at 1.2MHz (the second harmonic of the 600kHz peak), at -75dBrA. Those two peaks—the fundamental and its second harmonics—are direct results of the switching oscillators in the Evolution Two amp module. 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 graph above is the damping factor as a function of frequency. We find very high damping factor values, from 500 up to over 1000 at higher frequencies. We were unable to reliably measure damping factor above around 6–7kHz, where we actually measured negative output impedances. This is impossible, and we can only speculate as to what the amplifier is doing. Based on the measurements, it seems as though the Evolution Two increases gain slightly into small loads at high frequencies.

Diego Estan
Electronics Measurement Specialist