Link: reviewed by Roger Kanno on SoundStage! Simplifi on February 1, 2025

General Information

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

The Caspian 4G Streaming Amplifier was conditioned for 1 hour at 1/8th full rated power (~13W 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 Caspian offers three set of line-level analog inputs (two RCA, one XLR), one moving-magnet (MM) phono input, two digital coaxial (RCA) S/PDIF inputs, two digital optical (TosLink) S/PDIF inputs, one HDMI eARC connection, left/right analog pre-outs (RCA and XLR), and one set of speaker level outputs a. A Bluetooth input is also offered. For the purposes of these measurements, the following inputs were evaluated: digital coaxial, analog (XLR) line-level and phono.

Most measurements were made with a 4Vrms line-level analog input and 0dBFS digital input. The RCA line-level input offers 6dB more gain than the XLR input (i.e., 2Vrms in over RCA yields the same speaker-level output as 4Vrms in over XLR). The signal-to-noise ratio (SNR) measurements were made with the default input signal values but with the volume set to achieve the achievable output power of 105W into 8 ohms. For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum.

Based on the accuracy and repeatability of the left/right volume channel matching (see table below), the Caspian volume control is digital through the lower range (steps 1 through 40). Above 40 up to the maximum step (100), the volume appears to be digitally controlled but operating in the analog domain. To achieve this, the Caspian must digitize incoming analog signals, including the phono input. The Caspian overall volume range is from -70dB to +29dB (line-level XLR input, speaker output). It offers 1dB increments throughout the range.

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.

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

Volume position Channel deviation
1 0.061dB
10 0.048dB
20 0.043dB
30 0.043dB
40 0.043dB
50 0.068dB
60 0.081dB
70 0.059dB
80 0.037dB
90 0.001dB
100 0.001dB

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Roksan for the Caspian 4G compared directly against our own. The published specifications are sourced from Roksan’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.

Parameter Manufacturer SoundStage! Lab
Amplifier rated output power into 8 ohms* 105W 107W
Amplifier rated output power into 4 ohms* 250W 182W
Frequency response (digital in, 24/96) 20Hz-20kHz (±0.25dB) 20Hz-20kHz (-0.03,-0.13dB)
Frequency response (analog in) 20Hz-20kHz (±0.25dB) 20Hz-20kHz (+0.07,-0.77dB)
THD+N (1W at 1kHz) <0.007% <0.008%
Signal-to-noise ratio (1W 8 ohms, 2Vrms in, A-wgt) >85dB 85dB
Pre-amp volume control matching <0.25dB <0.08dB
Pre-amp signal-to-noise ratio (3.9Vrms out, A-wgt, XLR) >108dB 101dB
Pre-amp signal-to-noise ratio (3.9Vrms out, A-wgt, RCA) >108dB 102dB
Pre-amp THD+N (3.9Vrms out, A-wgt, XLR) <0.0008% <0.0015%
Pre-amp THD+N (3.9Vrms out, A-wgt, RCA) <0.0008% <0.0019%
Phono MM gain (with preamp) 53dB 53.3dB
Phono MM input impedance 47k ohms 52.6k ohms
Phono MM RIAA response accuracy 20Hz-20kHz (±0.3dB) 20Hz-20kHz (-1,-0.75dB)

* Roksan specifics power output with 235VAC mains level, whereas our measurements reflect the North American standard of 120VAC

Our primary measurements revealed the following using the line-level XLR analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 4Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

Parameter Left channel Right channel
Maximum output power into 8 ohms (1% THD+N, unweighted) 107W 107W
Maximum output power into 4 ohms (1% THD+N, unweighted) 182W 182W
Maximum burst output power (IHF, 8 ohms) 115W 115W
Maximum burst output power (IHF, 4 ohms) 210W 210W
Continuous dynamic power test (5 minutes, both channels driven) passed passed
Crosstalk, one channel driven (10kHz) -71dB -73dB
Damping factor 203 200
DC offset <-5mV <3.3mV
Gain (pre-out, XLR in/out) 5.7dB 5.7dB
Gain (pre-out, RCA in/out) 6.5dB 6.5dB
Gain (maximum volume, XLR in) 28.9dB 28.9dB
Gain (maximum volume, RCA in) 35.0dB 34.9dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-77dB <-72dB
IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) <-87dB <-87dB
Input impedance (line input, XLR) 24.3k ohms 24.4k ohms
Input impedance (line input, RCA) 21.0k ohms 21.1k ohms
Input sensitivity (105W 8 ohms, maximum volume) 1.04Vrms 1.04Vrms
Noise level (with signal, A-weighted) <215uVrms <220uVrms
Noise level (with signal, 20Hz to 20kHz) <293uVrms <302uVrms
Noise level (no signal, A-weighted, volume min) <155uVrms <153uVrms
Noise level (no signal, 20Hz to 20kHz, volume min) <202uVrms <189uVrms
Output Impedance (pre-out, XLR) 112 ohms 112 ohms
Output Impedance (pre-out, RCA) 23.4 ohms 23.8 ohms
Signal-to-noise ratio (105W 8 ohms, A-weighted, 4Vrms in) 96dB 96dB
Signal-to-noise ratio (105W 8 ohms, 20Hz to 20kHz, 4Vrms in) 94dB 93dB
Signal-to-noise ratio (140W 8 ohms, A-weighted, max volume) 95dB 95dB
Dynamic Range (105W 8 ohms, A-weighted, digital 24/96) 96dB 96dB
Dynamic Range (105W 8 ohms, A-weighted, digital 16/44.1) 93dB 93dB
THD ratio (unweighted) <0.0013% <0.0011%
THD ratio (unweighted, digital 24/96) <0.0012% <0.0011%
THD ratio (unweighted, digital 16/44.1) <0.0012% <0.0011%
THD+N ratio (A-weighted) <0.0027% <0.0027%
THD+N ratio (A-weighted, digital 24/96) <0.0026% <0.0026%
THD+N ratio (A-weighted, digital 16/44.1) <0.0031% <0.0031%
THD+N ratio (unweighted) <0.0037% <0.0038%
Minimum observed line AC voltage 125VAC 125VAC

For the continuous dynamic power test, the Caspian 4G was able to sustain 210W into 4 ohms (~6% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (21W) for 5 seconds, for 5 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 Caspian 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):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -52dB -67dB
DC offset <-5mV <3.5mV
Gain (default phono preamplifier) 48.8dB 48.8dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-66dB <-66dB
IMD ratio (CCIF, 3kHz + 4kHz stimulus tones, 1:1) <-80dB <-82dB
Input impedance 52.6k ohms 52.0k ohms
Input sensitivity (to 105W with max volume) 1.74mVrms 1.74mVrms
Noise level (with signal, A-weighted) <2.5mVrms <1.7mVrms
Noise level (with signal, 20Hz to 20kHz) <10mVrms <7mVrms
Noise level (no signal, A-weighted, volume min) <156uVrms <152uVrms
Noise level (no signal, 20Hz to 20kHz, volume min) <203uVrms <188uVrms
Overload margin (relative 5mVrms input, 1kHz) 20.7dB 20.7dB
Signal-to-noise ratio (105W, A-weighted, 0.5mVrms in) 75dB 77dB
Signal-to-noise ratio (105W, 20Hz to 20kHz, 0.5mVrms in) 62dB 64dB
THD (unweighted) <0.0025% <0.0022%
THD+N (A-weighted) <0.022% <0.016%
THD+N (unweighted) <0.11% <0.08%

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

frequency response

In our frequency-response plots above (relative to 1kHz), measured across the speaker outputs at 10W into 8 ohms, the Caspian 4G is near flat within the audioband (20Hz to 20kHz, +0.07,-0.77dB). The -3dB point is at roughly 40kHz, and it is +1.7dB at 5Hz. The Caspian appears to digitize the incoming analog signal as we see high-frequency brickwall-type filtering just below 50kHz. 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)

phase response

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 Caspian yields significant phase shift, more than -9000 degrees at 20kHz. The phase shift is so significant because the plot shows total phase shift, including any timing delays from input to output due to digitization. As a point of comparison, the plot below . . .

phase response

. . .shows only the excess phase shift (beyond the frequency independent timing delays), where we find +20 degrees at 20Hz and 0 degrees at 20kHz.

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

frequency response phono mm

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 very flat response from 50Hz to 10kHz. Below 30Hz, there is steep attenuation (-1dB at 20Hz then a brickwall below 15Hz), as Roksan appears to have implemented an anti-rumble filter on their phono input, likely in the digital domain. The adherence to the RIAA curve is so strict between 50Hz and 10kHz that we may conclude that RIAA EQ is likely implemented in the digital domain as well.

Phase response (MM input)

phase response phono mm

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 Caspian does not invert polarity. Again, because the input is digitized, which introduces significant timing delays, phase shift is -9500 degrees at 20kHz. Below is . . .

phase response phono mm

. . . the same phase-shift plot but only showing excess phase shift (beyond the frequency independent timing delays). With it displayed this way, we see a fairly typical phono RIAA phase shift response.

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

frequency response vs input type

The chart above shows the Caspian 4G’s frequency response (relative to 1kHz) as a function of input type measured across the speaker outputs at 10W into 8 ohms. The two dark 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 near-flat response down to 5Hz (-0.1dB), which differs from the +1.7dB at 5Hz response seen with the analog response. The -3dB points are roughly: 21kHz for the 16/44.1 data, 46kHz for the 24/96, 90kHz for the 24/192 data, and 40kHz for the analog input. Also of note, the 16/44.1 and 24/96 input data showed brick-wall-type high frequency filtering, while the 24/192 does not. The analog input frequency response, which is digitized, oddly, does not look like the 24/96 response.

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

digital linearity

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 XLR pre-outputs of the Caspian, where 0dBFS was set to yield 2Vrms. 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. Both data were essentially perfect as of -90dBFS down to 0dBFS. The 24/96 data were above +10dB at -120dBFS, while the 16/44.1 data were +3dB at -120. This is a poor result.

Impulse response (24/44.1 data)

impulse response 2444 1

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 XLR pre-outs of Caspian 4G. We see a typical, symmetrical sinc-function response.

J-Test (coaxial)

jtest coax 2448

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level XLR pre-outputs of the Caspian where 0dBFS is set to 2Vrms. The J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (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 strong J-Test result, with some lower frequency peaks likely due to power-supply noise, and two very small peaks below -130dBFS flanking the 12kHz fundamental. This is an indication that the Caspian DAC should have strong jitter immunity.

J-Test (optical)

jtest optical 2448

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 Caspian 4G. The optical input yielded effectively the same result as the coax input.

J-Test (coaxial, 100ns jitter)

jtest coax 2448 100ns

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the Caspian, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The result is identical to the original J-Test result without the added jitter, with no signs of the tell-tale peaks at 10kHz and 14 kHz due to the added jitter. This is further evidence of the strong jitter immunity in the Caspian 4G DAC. The optical input yielded the same result.

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

wideband fft noise plus 19 1khz 1644 1kHz

The chart above shows a fast Fourier transform (FFT) of the Caspian 4G’s line-level XLR 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. The steep roll-off around 20kHz in the white-noise spectrum shows the implementation of a brickwall-type reconstruction filter. There are no low-level aliased image peaks within the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is highly suppressed at -130dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are at -110dBrA and below.

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

rms level vs frequency vs load impedance

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 . . .

rms level vs frequency vs load impedance

. . . 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 0.08dB. This is a strong result and an indication of a low output impedance, or high damping factor. With a real speaker load, deviations measured lower at roughly 0.04dB.

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

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 at roughly 95W. The power was varied using the Caspian 4G’s volume control. Between 20Hz and 3kHz, all three THD results are closely clustered together, ranging from roughly 0.0006% to 0.005%. Above 3kHz we find clear deviations between the three power levels. At 20kHz, the 1W data was at 0.01%, the 10W data at 0.02%%, and the 95W data at 1.5%.

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

thd ratio unweighted vs frequency vs output power

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. THD ratios below 400Hz vary wildly (40dB swings); however, this is likely due to sporadic and significant rises in the noise floor that were observed during the measurements. From 400Hz to 20kHz, THD ratios ranged from roughly 0.002% up to 0.02%.

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

thd ratio unweighted vs output power at 4 8 ohms

The chart above shows THD ratios measured at the speaker-level outputs of the Caspian 4G 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 ratios into 4 and 8 ohms are close (within 3-5dB, the 8-ohm data outperformed the 4-ohm data). They range from 0.01% at 50mW, down to 0.001% in the 10 to 90W range for the 8-ohm data. The “knee” into 8 ohms can be found at roughly 95W, while the 4-ohm knee can be seen around 170W. The 1% THD marks were hit at 107W and 182W into 8 and 4 ohms.

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

thd n ratio unweighted vs output power at 4 8 ohms

The chart above shows THD+N ratios measured at the speaker-level outputs of the Caspian 4G 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 remarkably close (within 2-3dB). They range from 0.1% at 50mW, down to 0.003% at the “knee” for the 8-ohm data, and 0.2% down to 0.005% at the “knee” for the 4-ohm data.

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

thd vs frequency load

The chart above shows THD ratios measured at the output of the Caspian 4G 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 and the 4-ohm load the purple trace and the 2-ohm load the pink trace. We find a roughly 5dB increase in THD from 8 to 4 ohms, and a 5-10dB increase from 4 to 2 ohms. The 2-ohm load ranged from 0.01% at 20Hz down to 0.003% at 50-200Hz, then up to 0.07% at 20kHz.

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

thd vs frequency vs speakers

The chart above shows THD ratios measured at the output of the Caspian 4G as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Generally, THD ratios into the real speakers were higher than those measured across the resistive dummy load. The differences ranged from 0.06% at 20Hz for the two-way speaker versus 0.004% for the resistive load, and 0.02% at 20kHz into the 3-way speaker versus 0.01% for the resistive load. Between the important frequencies of 300Hz to 6kHz, all three THD traces were close (within 5dB), around the 0.001-0.002% mark. This is a strong result.

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

IMD CCIF vs frequency vs speakers

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the Caspian 4G as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). Generally, IMD ratios into the real speakers were higher than those measured across the resistive dummy load. IMD results into the dummy load ranged from 0.001% to 0.005%, while the highest IMD ratios came from the three-way speaker, from 0.003% to 0.008%.

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

IMD SMPTE vs frequency vs speakers

The chart above shows IMD ratios measured at the output of the Caspian 4G as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). We find very similar IMD ratios into all three loads, a constant 0.007%.

FFT spectrum – 1kHz (XLR analog line-level input)

FFT spectrum 1khz XLR

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 XLR line-level input. We see that the signal’s second (2kHz) and third (3kHz) harmonics dominate at -100dBrA, or 0.001%, and just above -110dBrA, or 0.0003%. On the right and left side of the signal peak, we find power-supply-related noise peaks, ranging in amplitude from -100dBrA, or 0.001%, down to -120dBrA, or 0.0001%. We also see strong peaks around 96kHz, likely due to the ADC’s sampling frequency of the analog signals.

FFT spectrum – 1kHz (XLR analog line-level input)

FFT spectrum 1khz RCA

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 RCA line-level input. We see essentially the same result as the FFT above for the XLR input.

FFT spectrum – 1kHz (MM phono input)

FFT spectrum 1khz phono mm

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the analog phono MM input. We see that the signal’s second (2kHz) and third (3kHz) harmonics, while difficult to discern amongst the power supply related noise peaks, are around the -100dBrA, or 0.001%, level. On the right and left side of the signal peak, we find power-supply related noise peaks, ranging in amplitude from -65dBrA, or 0.06%, down to -120dBrA, or 0.0001%. 

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

fft spectrum 1khz 1644 1 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 a very similar result as the FFT above for the XLR analog input, except for the 96kHz peaks due to the ADC sampling not occurring here.

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

fft spectrum 1khz 2496 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)

fft spectrum 1khz 2444 1 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 just below the correct amplitude, with no obvious signal harmonics amongst the numerous power-supply noise-related harmonics.

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

fft spectrum 1khz 2496 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 just below the correct amplitude, with no obvious signal harmonics amongst the numerous power-supply noise related harmonics.

FFT spectrum – 50Hz (line-level input)

fft spectrum 50hz

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 many power-supply noise-related harmonics at -100dBrA, or 0.001%, and below. The second (100Hz) and third (150Hz) signal-related harmonics are just above and below -110dBrA, or 0.0003%.

FFT spectrum – 50Hz (MM phono input)

fft spectrum 50hz phono mm

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 60Hz fundamental power-supply noise peak and its third (180Hz) harmonic at -65dBrA, or 0.06%. The highest signal harmonic is at 100Hz, at -90dBrA (left channel), or 0.003%.  

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

intermodulation distortion fft 18khz 19khz summed stimulus

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 XLR 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 -80dBrA, or 0.01%, while the third-order modulation products, at 17kHz and 20kHz are at -100dBrA, or 0.001%, level. This is a mediocre IMD result.

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

fft spectrum 32 tone

Shown above is the FFT of the speaker-level output of the Caspian 4G 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 very low -120dBrA, or 0.0001%, level. The other peaks are related to power-supply noise harmonics.

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

intermodulation distortion fft 18khz 19khz summed stimulus

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 -90dBrA, or 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are at -105dBrA, or 0.0006%.

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

intermodulation distortion fft 18khz 19khz summed stimulus

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 -90dBrA, or 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are at -105dBrA, or 0.0006%.

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

intermodulation distortion fft 18khz 19khz summed stimulus phono mm

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. Here we experienced the same issue as seen in our THD vs Frequency sweep for the phono input—sporadic rises in the noise floor. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at around -80dBrA, or 0.01%, while the third-order modulation products, at 17kHz and 20kHz, cannot be seen above the rise in the noise floor up to -90dBrA.

Squarewave response (10kHz)

square wave 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 Capsian 4G’s slew-rate performance. Rather, it should be seen as a qualitative representation of the Caspian 4G’s limited bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. In this case, due to the digitization of the analog input and brickwall behaviour above 50kHz, we find a very poor 10kHz square wave reproduction.

Squarewave response (1kHz)

square wave response 1kHz

Above is the 1kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms. At this lower frequency, the squarewave reproduction is much cleaner than at 10kHz; however, significant ringing can be seen in the corners.

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

damping factor vs frequency

The final graph above is the damping factor as a function of frequency. We can see here a constant damping factor of roughly from 20Hz to 3kHz, then down to 80 at 20kHz. This is a strong result for a medium-powered solid-state integrated amplifier.

Diego Estan
Electronics Measurement Specialist