Integrated Amp Reviews

I performed a full set of measurements on Schiit Audio's Ragnarok 2 using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). I preconditioned the amplifier by running it at one-third the specified power into 8 ohms for 45 minutes. At the end of this period, the temperature of the top panel had stabilized at 106.0°F (41.1°C), while the heatsinks measured 126.9°F (52.8°C). This amplifier should be placed in a well-ventilated location.

Looking first at the small-signal measurements with line-level input signals, the Schiit amplifier's maximum voltage gain, measured at the loudspeaker outputs, was 29dB into 8 ohms with the gain set to "25," 18.6dB with it set to "8," and 5.9dB with it set to "2." (Schiit specifies gain in multiples—2×, 8×, 25×—instead of the usual dB units.) These gains were identical for balanced and single-ended input signals, and, on the 8× and 25× settings, match up well with the specifications. On the 2× setting, the gain was (5.9dB) instead.

The amplifier preserved absolute polarity (ie, was noninverting) for all inputs and outputs. The maximum gain, measured at the unbalanced headphone jack on the front panel, was 29dB (balanced input) and 23.1dB (unbalanced input) at the highest gain setting; the output impedance from this jack was a very low 1.3 ohms from 20Hz to 20kHz. The maximum gain from the balanced preamplifier output was 29dB sourced from 600 ohms. The single-ended preamplifier gain was 23.1dB, with an output impedance of 75 ohms.

The unbalanced input impedance was close to the specified 10k ohms at all audio frequencies and twice that value for the balanced inputs, as expected. The output impedance from the speaker terminals was a low 0.08 ohms at 20Hz and 1kHz, rising to a still-low 0.12 ohms at 20kHz. As a result, the amplifier's response driving our standard simulated loudspeaker varied by just ±0.05dB (fig.1, gray trace). The responses into 8 and 4 ohm resistive loads (blue, red, cyan, and magenta traces) were flat up to 30kHz, with excellent matching between the channels and a gentle ultrasonic rolloff. Into 2 ohms (green trace), the output rolled more rapidly above the audioband, reaching –7dB at 200kHz. Commendably, the amplifier's frequency response was not affected by the gain and volume-control settings and was identical when measured from the headphone and preamplifier jacks. With its wide small-signal bandwidth, the Ragnarok 2's reproduction of a 10kHz squarewave into 8 ohms (fig.2) was very good, with no overshoot or ringing visible.

The channel separation was excellent, at >100dB in both directions below 2kHz and still 75dB at 20kHz. The unweighted, wideband signal/ noise ratio, ref. 2.83V into 8 ohms and taken with the gain set to 2×, the unbalanced input shorted to ground, and the volume control set to its maximum—the worst case—was very good, at 88.4dB. Restricting the measurement bandwidth to the audioband improved the ratio to 96.7dB, while an A-weighting filter improved it further, to 108.5dB. (All ratios are the average of the two channels.) Setting the gain to 8× reduced the S/N ratios by 1–5dB; setting it to 25× further reduced the ratios by another 6–9dB. But even in the worst case—right channel, high gain, unweighted, wideband—the ratio was a still-good 81dB. This is a quiet amplifier, and as shown in fig.3, in the low-frequency spectrum of the Ragnarok 2's output while it drove a 1kHz tone at 1W into 8 ohms, the power supply-related spuriae all lie at or below –105dB.

The Ragnarok 2's maximum output power is specified as 60W into 8 ohms (17.8dBW) and 100W into 4 ohms (17dBW). Figs. 4 and 5 plot the percentage of THD+N against power with both channels driven into 8 ohms and 4 ohms. With clipping defined as when the THD+noise reaches 1%, this sample of the Ragnarok 2 clipped at 78W into 8 ohms (18.9dBW) and 93W into 4 ohms (16.7dBW). Below 10W or so in these graphs, the actual distortion lies below the noise floor, so I plotted how the THD+N percentage changed with frequency at 8.95V, which is equivalent to 10W into 8 ohms, 20W into 4 ohms, and 40W into 2 ohms. The result is shown in fig.6. The THD is extremely low, especially in the lower midrange, but is a little higher in the right channel into 8 and 4 ohms (red and magenta traces) than it is in the left channel (blue, cyan). Though the THD into these loads rises in the treble, it remains low in absolute terms. Only with the left channel driving 2 ohms (gray trace) does the distortion reach 0.1%, but then only in the top audio octave.

The waveform of the residual distortion and noise (fig.7) suggests that the subjectively benign second harmonic is dominant. Spectral analysis (fig.8) in dicates that the third harmonic is 10dB lower in level than the second, with the fourth harmonic about 5dB lower still. Higher-order spuriae are all very low in level. Tested with an equal mix of 19 and 20kHz tones at moderate power into 8 ohms, the second-order difference product lay at just –93dB (0.006%) in both channels (fig.9). However, higher-order intermodulation products can be seen, due to the amplifier's reduced linearity at high frequencies.

Turning to the USB digital input, I tested this with data sourced from my MacBook Pro running on battery power, with Pure Music 3.0 playing WAV and AIFF test-tone files. Apple's USB Prober utility identified the Schiit amplifier as "Schiit USB Multibit\000" from "Schiit Audio\000." The Ragnarok 2's USB port operated in the optimal isochronous asynchronous mode, and Apple's AudioMIDI utility revealed that the amplifier accepted 16-, 24-, and 32-bit integer data sampled at all rates from 44.1 to 192kHz.

The USB input preserved absolute polarity, and with the volume control set to its maximum and the Ragnarok's gain set to 2×, a 1kHz tone at 0dBFS resulted in a level at both the balanced preamplifier and loudspeaker outputs of 4.5V. Setting the amplifier's gain to 8× increased the maximum level with digital data by 12.7dB, which suggests that setting it to 25× will clip the Ragnarok 2 with full-scale data unless the volume control is backed off. Even so, the digital input's gain architecture seems sensibly organized.

The following measurements were all made at the balanced preamplifier output. The Schiit's impulse response with 44.1kHz data (fig.10) was typical of a time-symmetrical, linear-phase reconstruction filter. The frequency response with 44.1kHz-sampled white noise (fig.11, red and magenta traces) reached full stop-band suppression just above half the sample rate, indicated by the vertical green line in this graph. The aliased image at 25kHz of a full-scale tone at 19.1kHz (blue and cyan traces) is suppressed by >90dB. The harmonics of the 19.1kHz tone can be seen at –61dB (second) and –80dB (third), though the noise floor with this high-frequency tone is more ragged-looking than I usually see with this test.

When I increased the bit depth from 16 to 24 with a dithered 1kHz tone at –90dBFS (fig.12), the noise floor dropped by 10dB, meaning that the Ragnarok 2's USB input offers just below 18 bits' worth of resolution. The Ragnaol 2 readily resolves a dithered tone at –120dBFS (fig.13). With undithered data representing a tone at exactly –90.31dBFS, the three DC voltage levels described by the data were well resolved and the waveform was symmetrical, though overlaid with high-frequency noise (fig.14).

Intermodulation distortion via the USB input was low (fig.15), the difference product at 1kHz generated by high-level tones at 19kHz and 20kHz lying at –74dB (0.02%), though the noise floor is not as clean as I would have liked to see. Tested for its rejection of word-clock jitter with 16-bit data, most of the odd-order harmonics of the LSB-level, low-frequency squarewave were reproduced at the correct levels (fig.16, sloping green line). However, there was some accentuation of the sideband pair closest to the primary tone, and a low-level spurious tone at 14.35kHz can be seen. This tone was also present with 24-bit J-Test data (not shown).

As well as providing a digital USB input, the Fully Loaded option adds a moving-magnet phono stage. With the amplifier gain set to 2×, a 1kHz tone at the standard MM reference level of 5mV resulted in maximum levels of 2V from the loudspeaker and balanced preamplifier outputs, equivalent to a gain of 52.1dB. The phono input didn't invert absolute polarity, and the input impedance measured an appropriate 45k ohms at 20Hz and 1kHz, dropping inconsequentially to 40k ohms at 20kHz.

Measured at the preamplifier outputs, the Ragnarok 2 offered superbly accurate RIAA equalization (fig.17); any error remained below ±0.1dB. Channel separation via the phono inputs was also excellent, at >80dB in both directions across most of the audioband. As with the Schiit's line inputs, the phono input's S/N ratios depended on the amplifier's gain setting. The unweighted, wideband ratio, ref. 1kHz at 5mV, was an excellent 76dB with the gain set to 2×, respectively falling to 63.6dB and 53dB with the gain set to 8× and 25×. This is still good performance, however.

The overload margin, again ref. 1kHz at 5mV, was excellent, at 22.7dB across the audioband, and harmonic distortion was extremely low. Fig.18, for example, was taken with a 1kHz tone at 20mV, 12dB higher than the nominal MM reference level, yet the second and third harmonics lie at just –114dB (0.0002%) and –119dB (0.0001%). The phono input's intermodulation was also extremely low in level (fig.19).

I had problems measuring the behavior of Schiit's original Ragnarok amplifier, due to the unique biasing scheme, where the output stage bias was modulated by the signal history. But I had no such issues with the Ragnarok 2. It is a well-engineered, apparently conventional design offering low noise and distortion coupled with usefully high power. The options include an excellent, low-noise, MM-compatible phono preamplifier, though the USB digital input will be outclassed by standalone digital processors.—John Atkinson