Cambridge Audio Evo 150 Streaming Integrated Amplifier Measurements

Link: reviewed by Dennis Burger on SoundStage! Access on September, 2022

General Information

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

The Evo 150 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 Cambridge Audio Evo 150 offers one unbalanced line-level analog input (RCA), one unbalanced moving-magnet (MM) phono input (RCA), one balanced line-level analog input (XLR), one coaxial S/PDIF digital (RCA), two optical S/PDIF digital inputs (TosLink), one USB digital input, an HDMI ARC digital input, an ethernet digital connection for streaming, one pair of analog line-level pre-outs (RCA), one line-level sub-out (RCA), and two sets (A and B) of speaker level outputs. On the front of the unit is a 1/8″ TRS headphone output. For the purposes of these measurements, the following inputs were evaluated: digital coaxial, and the analog line-level (RCA) and MM unbalanced inputs.

Most measurements were made with a standard 2Vrms line-level analog input, 5mVrms MM input, and 0dBFS digital input. For the analog inputs, the tone-control bypass function was enabled. 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 150W (8 ohms). For comparison, on the line-level input, an SNR measurement was also made with the volume at maximum, where only 0.7Vrms was required to achieve 150W into 8ohms.

Based on the accuracy and non-repeatable results at various volume levels of the left/right channel matching (see table below), the Evo 150 volume control is likely operating in the analog domain but with digital control. The volume control offers a total range of 0 to 100 on the display, which measured from -46.2dB (position 1) to +33.9dB between the line-level analog input and the speaker outputs, in increments of 2dB steps below 10, 1dB from 10 to 20, then 0.5dB steps above 20 to 100.

Because the Evo 150 uses switching amplifier technology that exhibits 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 Cambridge Audio for the Evo 150 compared directly against our own. The published specifications are sourced from Cambridge’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 500kHz, 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 sine wave at 2Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the Evo 150 was able to sustain only 216W into 4 ohms (~1% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (13.3W) for 5 seconds, for 5 continuous minutes without the protection circuit shutting down the unit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the Evo 150 was slightly warm to the touch. It should be noted that 216W is well below the measured 292W at 1kHz. This is because the Evo 150 exhibits much higher levels of distortion at lower frequencies at high power levels (see THD Versus Frequency Versus Output Power graph below).

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz 5mVrms sine-wave 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 sine-wave input, 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth):

Of note in the chart above are the unusually high THD ratios measured at the headphone output at 2Vrms into 300 ohms. This was explored further with a THD vs Measured Output Level sweep (shown below). There is a distinct jump in THD ratios right around the 1Vrms mark, well within the linear range of the amp, and well below the 1% THD mark of nearly 10Vrms. This behavior is unusual, but was repeatable. Into 600 ohms, the THD behavior was similar; however, into a high-impedance load (100k ohms), as well as a low impedance load (32 ohms), this behavior was not seen. We are currently exploring the issue with Cambridge Audio.

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

In our measured frequency-response chart above, the Evo 150 is nearly flat within the audioband (20Hz to 20kHz). At the extremes, the Evo 150 is 0dB at 5Hz and -0.5dB at 20kHz. This corroborates Cambridge’s claim of 20Hz-20kHz, 0/-3dB. The -3dB point was measured around 70kHz. 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 frequency-response plots measured at the speaker-level outputs into 8 ohms, with the bass and treble controls set to maximum (blue/red plots) and minimum (purple/green plots). We see that for the bass and treble controls, roughly +/-10dB of gain/cut are available at 20Hz and 20kHz.

Frequency response (sub-out line-level analog output)

Above is the frequency response plot measured at the line-level sub output. We see that this output does not implement a useable low-pass filter, but would rely on a device upstream to implement filtering (e.g., control within an active subwoofer). The -3dB point is at roughly 2.5kHz.

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 Evo 150 does not invert polarity and exhibits, at worst, less than 20 degrees (at 20Hz) of phase shift within the audioband.

Frequency response vs. input type (analog and 16/44.1, 24/96, and 24/192 inputs with 8-ohm loading, left channel only)

The chart above shows the Evo 150’s frequency response as a function of input type measured across the speaker outputs at 10W into 8 ohms. The green trace is the same analog input data from the previous graph. The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial digital input, and the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, while the pink trace is for a 24/192 dithered digital input signal from 5Hz to 96kHz. The 16/44.1 data exhibits brick-wall-type filtering, with a -3dB point at 20.9kHz. The 24/96 and 24/192 kHz data yielded -3dB points at 44.9kHz and 49.5kHz, respectively. All frequency-response curves are down 0.5dB at roughly 20kHz. The analog input shows more extended response at high frequencies than the 24/192 input. There’s no evidence that the Evo 150 digitizes incoming analog signals.

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 maximum deviation of about -1dB at 50Hz and +0.5dB at 300Hz, from 20Hz to 20kHz.

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 Evo 150 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 -60 degrees at 300Hz and 6kHz, and +80 degrees at 20Hz.

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 Evo 150 for a 2Vrms (at 0dBFS) output signal. 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 up to 0dBFS. At -120dBFS, the 16/44.1 data were about +4/2dB (left/right) above reference, while the 24/96 data were within -1dB of reference. This is a good linearity test result.  

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 pre-outs of the Evo 150. We can see that the Evo 150 DAC utilizes a typical sinc function reconstruction filter.

J-Test (coaxial)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level pre-outs of the Evo 150. The J-test was developed by Julian Dunn in the 1990s. It is a test signal—specifically a -3dBFS undithered 12kHz square wave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz square wave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sine wave (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 sine-wave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial input (the optical input behaved identically) exhibits a few low-level peaks in the left channel at low frequencies within the audioband, at -120dBrA and below. This is a good J-test result, indicating that the Evo 150 DAC likely has good jitter immunity.

J-Test with 500ns of injected jitter (coaxial)

Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sine-wave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved excellent, with visible sidebands at only -125dBrA despite a very high 500ns of jitter level.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input, Filter 1)

The plot above shows a fast Fourier transform (FFT) of the Evo 150’s line-level pre-outs with white noise at -4dBFS (blue/red) and a 19.1 kHz sine wave at 0dBFS (1Vrms) 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 brick-wall type reconstruction filter. There are no aliased image peaks in the audioband above the -130dBrA noise floor. The main 25kHz alias peak is at -115dBrA. The second, third, and fourth distortion harmonics (38.2, 57.3, 76.4kHz) are at -100dBrA and below. Here again, like in the J-Test result, the left channel is exhibiting low-level (-120dBrA and below) peaks at low frequencies within the audioband in the 19.1kHz sinewave FFT.

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 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 can see a maximum deviation within the audioband of about 0.1dB 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 even less, deviating by about 0.05dB within the flat portion of the curve (20Hz to 10kHz). Note that the dip in RMS level at higher frequencies is a result of the frequency response of the Evo 150, and not a damping factor issue, as all four plots show the same dip, at roughly the same rate.

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 sine-wave stimulus at the analog line-level 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 124W. The power was varied using the volume control. The Evo 150 manages to maintain consistently flat THD ratios from 20Hz to 6kHz at both 1W and 10W, hovering around 0.002-0.003%. At 124W, THD ratios were much higher, at around 0.3-0.4% from 20 to 100Hz, then a steady decline down to nearly 0.003% at 5kHz.

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. The input sweep is EQ’d with an inverted RIAA curve. The THD values for the MM configuration vary from around 0.005% (20/30Hz) down to 0.001% (5kHz).

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 Evo 150 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). Both 8- and 4-ohm data sets track fairly closely, with THD ratios from about 0.002% down to as low as 0.0005% (8-ohm at 0.5W) and 0.007% (4-ohm at 1W). The “knee” for the 8-ohm data is roughly at 120W and nearly 0.01% THD. The 4-ohm data “knee” is just shy of 200W, but at a lower 0.002% THD.  The 1% THD values are reached at about 152W (8-ohm) and 292W (4-ohm).

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 Evo 150 as a function of output power for the 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). Overall, THD+N values for both loads were similar but slightly lower for the 8-ohm load, ranging from about 0.01%, down to 0.002%.

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 Evo 150  as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 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 nearly identical THD ratios, from 0.005% to roughly 0.002% across all three loads—an exceptional result, which showcases how impervious the Evo 150 is to even very low speaker impedances.

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 Evo 150 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). Above 50Hz or so, all three plots show similar THD ratios from 0.005% to about 0.002%. At very low frequencies, THD ratios were the highest with the two-way speaker, measuring 0.05% at 20Hz, over 20dB higher than the 0.002% measured acoss the dummy load. This again showacses that across most of the audioband, the Evo 150 is mostly impervious to speaker-load variations.

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 Evo 150 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 is 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, or 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 two-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are very similar, with relatively constant IMD ratios between 0.002% to 0.003%. This is an excellent result.

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 Evo 150 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 is 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 results are nearly identical, with flat IMD results just below 0.01%.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input. We find the third signal harmonic (3kHz) dominating at -90dBrA, or 0.003%, compared to the other signal harmonics which are below -110dBrA, or 0.0003%. On the left side of the main signal peak, we find the odd harmonics (180/300/420Hz) of the power-supply fundamental dominating with levels around -105dBrA, or 0.0006%, for the right channel, and -115dBrA, or 0.0002%, for the left channel.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 16/44.1. We find the same signal- and noise-related harmonics dominating and at the same levels as seen in the analog FFT above.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the coaxial digital input, sampled at 24/96. We find the same signal- and noise-related harmonics dominating and at the same levels as seen in the 16/44.1 FFT above, but with a lower noise floor due to the 24-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 sine-wave 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 noise harmonics between -100dBrA, or 0.001%, and -140dBrA, or 0.00001%. The noise floor of the left channel (blue) is slightly better throughout the audioband, though the noise floor of the right channel (red) is still commendably low.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sine-wave 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 noise harmonics between -100dBrA, or 0.001%, and -140dBrA, or 0.00001%. Once again, the noise floor of the left channel (blue) is lower than that of the right channel (red), though not above about 6kHz.

FFT spectrum – 1kHz (MM phono input)

Shown above is the FFT for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the MM phono input. The second (2kHz) and third (3kHz) signal harmonics dominate at around -100dBrA, or 0.001%. On the left side of the main signal peak, we find the odd harmonics (180/300/420Hz) of the power-supply fundamental dominating with levels from -90dBrA, or 0.003%, down to -100dBrA, or 0.001%. This is a clean, low-noise MM phono FFT for an integrated amplifier.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sine-wave 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 second (100Hz) and third (150Hz) signal harmonics dominate at -100dBrA, or 0.001%, and -90dBrA, or 0.003%, respectively. We also see the odd 60Hz power-supply related harmonics at -105dBrA, or 0.0006%, and below.

FFT spectrum – 50Hz (MM phono input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the MM phono 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) peak is from the 60Hz power-supply third harmonic (180Hz) at -90dBrA, or 0.003%. The second-order (100Hz) and third-order (150Hz) signal harmonic peaks are below this level at around -100dBrA, or 0.001%.

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 sine-wave 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 just below -100dBrA, or 0.001%. The third-order modulation products, at 17kHz and 20kHz are around -105dBrA, or 0.0006%.

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 sine-wave 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 product (i.e., the difference signal of 1kHz) is just below -100dBrA, or 0.001%. The third-order modulation products, at 17kHz and 20kHz, are at around the same 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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 24/96. The FFT spectrum is nearly identical within the audioband to the 16/44.1 FFT above, except for the lower noise floor due to the increased bit depth.

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 sine-wave 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 right around -100dBrA, or 0.001%. The third-order modulation products, at 17kHz and 20kHz, are around -105dBrA, or 0.0006%.

Square-wave response (10kHz)

Above is the 10kHz square-wave 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 Evo 150’s slew-rate performance. Rather, it should be seen as a qualitative representation of the Evo 150’s extended bandwidth for a switching amplifier. An ideal square wave can be represented as the sum of a sine wave 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. Here we can see the 400kHz switching oscillator frequency used in the digital amplifier section clearly visible modulating the waveform.

Square-wave response (10kHz)

Above is the 10kHz square-wave 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 can see a relatively clean square-wave reproduction for a switching amplifier.

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

The Evo 150’s class-D amplifier relies on a switching oscillator to convert the input signal to a pulse-width-modulated (PWM) square wave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The Evo 150 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 sine wave. We can see that the 400kHz peak is quite evident, and at -35dBrA. There is also a peak at 800kHz and 1200kHz, (the second and third harmonic of the 400kHz peak), at -65/-75dBrA. Those three peaks—the fundamental and its second and third harmonics—are direct results of the switching oscillators in the Evo 150 amp modules. 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. Both channels show a relatively constant and high damping factor of 230 (right) and 250 (left) from 20Hz to around 7kHz, then a small rise to 250 (right) and 300 (left) at 20kHz.

Diego Estan
Electronics Measurement Specialist