Peachtree Audio Carina 300 Integrated Amplifier Measurements

Link: reviewed by Dennis Burger on SoundStage! Access on August 1, 2024

General Information

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

The Peachtree Audio Carina 300 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 Carina 300 offers one set of line-level and phono analog inputs (RCA, switchable), left/right pre-outs, two coaxial (RCA) digital inputs, one optical (TosLink) digital input, one USB digital input, one set of speaker-level outputs, and two headphone outputs over 1/4″ TRS and 4.4mm balanced TRRRS connectors. A Bluetooth input is also offered. For the purposes of these measurements, the following inputs were evaluated: digital coaxial, analog line-level and phono, and the 4.4mm balanced headphone output.

Most measurements were made with a 2Vrms line-level analog input and 0dBFS digital input. The signal-to-noise ratio (SNR) measurements were made with the default input signal values but with the volume set to achieve the rated output power of 300W into 8 ohms. For comparison, on the line-level input, an SNR measurement was also made with the volume at maximum. The Carina 300 DAC offers three different reconstruction filters: Linear Phase Fast, Hybrid Fast, and Minimum Phase Slow. Unless otherwise stated, the Linear Phase Fast filter was used for the digital measurements.

The Carina 300 offers two types of volume controls: digital and hybrid. Unless otherwise stated, the hybrid volume control was used for all measurements. Based on the accuracy and repeatability of the left/right volume channel matching (see table below), the Carina 300 hybrid volume control is operating partially in the digital domain, partially in the analog domain. The Carina 300 overall volume range is from -56dB to +32.3dB (line-level input, speaker output). It offers 1dB increments throughout the volume range.

Our typical input bandwidth filter setting of 10Hz to 22.4kHz was used for all measurements except FFTs and where a bandwidth of 10Hz to 90kHz was used. Frequency response measurements utilize a DC to 1MHz input bandwidth. Because the Carina 300 uses digital amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), the 22.4kHz bandwidth setting was maintained for THD versus frequency sweeps as well. For these sweeps, the highest frequency was 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 Peachtree for the Carina 300 compared directly against our own. The published specifications are sourced from Peachtree’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 8ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channel.

* protection circuit engages at 5Vrms output into 30 or 32 ohms

Our primary measurements revealed the following using the line-level analog and digital coaxial inputs (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 Carina 300 was able to sustain 550W into 4 ohms (~1.8% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (17W) for five seconds, for five continuous minutes without inducing a fault protection circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the Carina 300 was 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 balanced headphone output (unless specified, assume a 1kHz sinewave, 24/96 0dBFS input/2Vrms output, 300 ohms loading, 10Hz to 22.4kHz bandwidth):

* protection circuit engages at 5Vrms output into 30 or 32 ohms

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 Carina 300 is near flat within the audioband (-0.1dB at 20Hz, -0.6dB at 20kHz). The -3dB high frequency point is at roughly 40kHz. The Carina 300 is at roughly -0.4dB at 5Hz.

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 Carina 300 appears to invert polarity, as we see -180 degrees of phase shift at 20Hz, and more than -1400 degrees at 10kHz.

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). Here we see maximum deviations within ±0.5dB or so from 20Hz to 20kHz, and worst-case channel-to-channel deviations of roughly 0.3dB.

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. What is shown is the excess phase shift, to show the phase shift differences between the MM input and the line-level input. We find +40 degrees at 20Hz, and +20 degrees at 1kHz.

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

The chart above shows the Carina 300’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. 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 a flat response down to 5Hz, whereas the analog input is -0.4dB at 5Hz. The 16/44.1 data yields a brick-wall type response just past 20kHz. The 24/96 and 24/192 data yielded very similar high-frequency responses compared to the analog input, but with slightly higher -3dB points (roughly 45kHz).

Frequency response vs. filter type (16/44.1, left channel only)

The plots above show frequency response for a 0dBFS input signal sampled at 16/44.1kHz for the Linear Phase Fast filter (blue), the Hybrid Fast filter (purple), and the Minimum Phase Slow filter (pink) into an 8-ohm load for the left channel only. The graph is zoomed in from 1kHz to 22kHz to highlight the various responses of the three filters. We can see that the Linear Phase Fast filter provides the most “brick-wall” type of response, the Minimum Phase Slow filter shows the gentlest attenuation around the corner frequency (-1dB at 17.1kHz), and the Hybrid Fast filter is very similar to the Minimum Phase Slow filter. The -3dB points for are 20.9kHz for Linear Phase Fast and 18.8kHz for Hybrid Fast and Minimum Phase Slow.

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-outputs of the Carina 300, where 0dBFS was set to yield 1Vrms at the line-level pre-outs. The digital input was swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output was analyzed by the APx555. The ideal response would be a straight flat line at 0dB. The 16/44.1 data were essentially perfect as of -110dBFS down to 0dBFS, while the 24/96 data were near perfect down to -120dBFS. We all also extended the sweep down to -140dBFS, to . . .

. . . see how well the 24/96 would perform. We can see here that below -120dBFS, the 16/44.1 data significantly over-responds (over 10dB), while the 24/96  significantly under-responds (less than -10dB).

Impulse response (24/44.1 data)

The graph above shows the impulse responses for the Carina 300, fed to the coaxial digital input, measured at the line-level pre-outputs, using the Transfer Function measurement in the APx500 software. We find that the Linear Phase Fast filter (blue) yields a sinc function response with symmetrical pre- and post-ringing, the Hybrid Fast filter (purple) yields little pre-ringing but exhibits post-ringing, while the Minimum Phase Slow filter (pink) yields virtually no pre-ringing and more significant post-ringing.

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-outputs of the Carina 300 where 0dBFS was set to 1Vrms. 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 an above average test result, with low-level peaks below the -130dBrA level. This is an indication that the Carina 300 DAC should have relatively strong 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 Carina 300. The optical input yielded essentially the same result as the coax 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 Carina 300, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. The results are strong, with no visible peaks at the 10kHz and 14kHz positions.

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 Carina 300, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The results are strong again, this time with the tell-tale peaks at 10kHz and 14kHz, but highly suppressed at the -140dBrA level. This is further indication that the DAC in the Carina 300 has strong jitter immunity.

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 Carina 300, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The results are strong again, but slightly worse than with the coaxial input. Here the 10/14kHz peaks are higher, at the -125dBrA level.

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 Carina 300’s line-level pre-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 using the Linear Phase Fast filter. The steep roll-off around 20kHz in the white-noise spectrum shows that the Linear Phase Fast filter is of the brick-wall-type variety. There is only one clear low-level aliased image peak within the audioband at -115dBrA at roughly 18kHz. The primary aliasing signal at 25kHz is highly suppressed at -120dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are at roughly the same level.

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 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 around 0.1dB. This is a strong result and an indication of a very low output impedance, or high damping factor. With a real speaker load, deviations measured lower at roughly the 0.04dB level.

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 channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange just at 285W (near the rated output power). The power was varied using the Carina 300 volume control. The 1W THD ratios were the lowest, with a constant 0.001% from 20Hz to 2kHz, then down to 0.0003% up to 6kHz. At 10W, the right channel outperformed the left by as much as 10dB, yielding THD ratios from 0.002% at 20Hz, down to 0.0005% from 100Hz to 5kHz. At 285W, THD ratios were fairly constant at around 0.3-0.5%.

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 was EQ’d with an inverted RIAA curve. The THD values for the MM configuration vary from around 0.01% (20Hz) down to 0.0003% from 3kHz to 5kHz.

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 Carina 300 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). Into 8 ohms, THD ratios ranged from 0.004% at 50mW down to 0.0005% at 10W, then up to 0.004% at the “knee” at roughly 200W, then up to the 1% THD mark at 314W. Into 4 ohms, THD ratios for the right channel ranged from 0.005% at 50mW down to 0.002%  from 1W to 10W, then a rise to 0.005% from 20W to 50W, a dip down to 0.001% from 100W to the “knee” at roughly 400W, then up to the 1% THD mark at 573W. The left channel performed considerably worse. This test was one of the last performed, and we began to see intermittent high THD and noise behavior from the left channel. For example, at 200W, there was almost a 40dB difference in THD between channels. We feel that this result for the left channel should not be considered representative for a typical Carina 300 sample since something might’ve gone wrong with our unit during these bench tests.

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 Carina 300 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). Into 8 ohms, THD+N ratios ranged from 0.02% at 50mW down to 0.002% at 10-30W, then up to 0.005% at the “knee” at roughly 200W. Into 4 ohms, THD+N ratios for the right channel ranged from 0.04% at 50mW down to 0.001% at 200-300W.

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 Carina 300 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. We find a roughly 15dB increase (0.001% to 0.005%) in THD ratios from 8 to 4 ohms across most of the sweep. From 4 to 2 ohms, there is a roughly 20dB increase in THD, with the 2-ohm data yielding THD ratios from 0.02% to 0.3%.

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

The chart above shows THD ratios measured at the output of the Carina 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). Generally, THD ratios into the real speakers were close to those measured across the resistive dummy load. The differences ranged from 0.02% at 20Hz for the two-way speaker versus 0.002% for the resistive load, and 0.0003% at 1kHz-4kHz for both speakers compared to 0.0007% for the resistive load. This is a very strong result and shows that the Carina 300 will maintain low THD into real-speaker loads.

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

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the Carina 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). We find that the IMD ratios are all similar (within 5dB), ranging from 0.0003% to 0.001%. Another strong result.

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

The chart above shows IMD ratios measured at the output of the Carina 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). We find very similar IMD ratios into all three loads, between 0.002 and 0.003%.

FFT spectrum – 1kHz (line-level input, hybrid volume control)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the analog line-level input. We see that the signal’s second (2kHz) and third (3kHz) harmonics dominate at a relatively low -110dBrA, or 0.0003% (the left channel at 2kHz measured -130dBrA, or 0.00003%). Subsequent signal harmonics were at the -120dBrA, or 0.0001%, and below level. On the right side of the signal peak, we see power-supply-related noise peaks (60/180/300/420Hz, etc.) around the relatively low -120dBrA, or 0.0001%, level.

FFT spectrum – 1kHz (line-level input, digital volume control)

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, this time with the volume control set to digital. The differences here between the FFT above where the hybrid volume control was used are a slightly higher noise floor (-140dBrA as opposed to -150dBrA) and higher signal harmonics at 5kHz and 7kHz, measuring -110dBrA, or 0.0003%.

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 MM phono input.  We see that the signal’s second (2kHz) and third (3kHz) harmonics are difficult to distinguish amongst the noise-realted peaks, but are below the -110dBrA, or 0.0003%, level (although the left channel reached -105dBrA at 3kHz). On the right side of the signal peak, we see power-supply-related noise peaks (60/180/300/420Hz, etc.) at the -85dBrA, or 0.006%, and below 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. We see essentially the same result as with the analog FFT above using the digital volume control, but with a higher noise floor (-135dBrA) due to the restricted bit depth.

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 analog FFT above using the digital volume control.

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 noise peaks at the -110dBrA and below level. The left channel noise floor and signal harmonic peaks are nearly 10dB higher than the right channel.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, with noise peaks at the -110dBrA and below level. The left channel noise floor and signal harmonic peaks are 5-10dB higher than the right channel.

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 a -110dBrA, or 0.0003%. Other peaks can be seen at -120dBrA, or 0.0001%, 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 noise related at 60/180/300/420Hz, etc., at the -85dBrA, or 0.006%, level. The second (100Hz) signal harmonic can be seen at -105dBrA, or 0.0006%.

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 -110dBRa, or 0.0003%, while the third-order modulation products, at 17kHz and 20kHz, are at -100dBrA, or 0.001%.

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

Shown above is the FFT of the speaker-level output of the Carina 300 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 -120dBrA, or 0.0001%, 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 (0dBFS). We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -105dBrA, or 0.0006%, while the third-order modulation products, at 17kHz and 20kHz, are at -100dBrA, or 0.001%.

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 (0dBFS). We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -105dBrA, or 0.0006%, while the third-order modulation products, at 17kHz and 20kHz, are at -100dBrA, or 0.001%.

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

Shown above is an FFT of the intermodulation (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 just above the noise floor at around -105dBrA, or 0.0006%, while the third-order modulation products, at 17kHz and 20kHz, are at -100dBrA, or 0.001%.

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 Carina 300’s slew-rate performance. Rather, it should be seen as a qualitative representation of the Carina 300’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 this case, we see the 400Hz switching frequency from the digital amplifier riding on top of a distorted 10kHz fundamental sinewave.

Square-wave response (10kHz)—250kHz bandwidth

Above is the 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms, this time with a 250kHz input bandwidth on the analyzer to filter out the 400kHz switching frequency. Here we see an average squarewave response with soft corners.

FFT spectrum of 500kHz switching frequency relative to a 1kHz tone

The Carina 300’s class-D amplifier relies on a switching oscillator to convert the input signal to a pulse-width-modulated (PWM) squarewave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The Carina 300 oscillator switches at a rate of about 400kHz, and this graph plots a wide bandwidth FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sinewave. We can see that the 400kHz peak is quite evident, and at -35dBrA. There are also two peaks at 800kHz and 1.2MHz (the second and third harmonic of the 400kHz peak), at -75/-85dBrA. In addition, there is a rise in the noise floor above 30kHz, peaking at -80dBrA. Those peaks—the fundamental and its harmonics—are direct results of the switching oscillators in the Carina 300 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. We can see here a constant damping factor right around 320 (left channel) and 560 (right channel), from 20Hz to 20kHz. Although there is a significant difference between channels, this is nonetheless a very strong damping factor result.

Diego Estan
Electronics Measurement Specialist