Rogue Audio Sphinx integrated amplifier Measurements

Sidebar 3: Measurements

I measured the Rogue Audio Sphinx using my top-of-the-line Audio Precision SYS2722 system (see, and the January 2008 "As We See It"). Before performing any measurements, I ran the Sphinx for an hour at one-third its specified maximum power of 100Wpc into 8 ohms. Thermally, this is the worst case for an amplifier with a class-B or -AB output stage, but it's irrelevant with an efficient class-D amplifier like the Rogue, which uses the well-regarded Hypex modules. By the end of that period the Sphinx's chassis was only mildly warm, but at least I could be confident that the circuits were all nicely toasted for testing.

Looking first at the line inputs, the maximum gain into 8 ohms was 31.16dB, with excellent matching of the two channels. The Sphinx preserved absolute polarity (ie, was non-inverting), and its input impedance was usefully high, at well above 100k ohms at almost all audio frequencies, and still 103k ohms at 20kHz. The output impedance was very low for a class-D design, at around 0.02 ohm at 20Hz and 1kHz (including 6' of speaker cable), rising slightly to 0.11 ohm at 20kHz. As a result, the variation in frequency response with the Sphinx driving our standard simulated loudspeaker (fig.1, gray trace) was negligible. Unusually for a class-D design, the Rogue's ultrasonic rolloff was identical into loads ranging from 2 to 8 ohms. There is usually a passive low-pass filter between the class-D output stage and the speaker terminals to reduce the level of ultrasonic switching noise, and this filter will misbehave into loads other than the one it was optimized for. The Sphinx offered no such misbehavior.


Fig.1 Rogue Sphinx, volume control set to maximum, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green) (2dB/vertical div.).

Present in the Rogue's output, however, was around 225mV of ultrasonic noise with a center frequency of 415.5kHz in the left channel and 418.5kHz in the right. This can be graphically seen in the waveform of a 10kHz squarewave (fig.2). For all the measurements other than this one, I used a precision passive low-pass filter from Audio Precision to eliminate noise above 200kHz that would otherwise contaminate the analyzer's reading. Repeating the squarewave measurement with this filter gave the waveform shown in fig.3—the risetimes are slowed by the amount you would expect from the response in fig.1, and though there is a small degree of overshoot on the waveform's leading edges, this is critically damped, with no ringing.


Fig.2 Rogue Sphinx, small-signal 10kHz squarewave into 8 ohms.


Fig.3 Rogue Sphinx, small-signal 10kHz squarewave into 8 ohms with Audio Precision low-pass filter.

Channel separation (not shown) was good rather than great, at 60dB in both directions below 1kHz, and around 40dB at the top of the audioband. The unweighted signal/noise ratio in the audioband, taken with the input shorted but the volume control at its maximum, was moderately good, at 67.2dB left and 63.3dB right. This was primarily due to full-wave–rectified, supply-related spuriae, which were a little higher in the right channel (fig.4). I experimented with the grounding between the amplifier and the analyzer; this was the spectrum I took with the optimal arrangement.


Fig.4 Rogue Sphinx, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms with Audio Precision low-pass filter (linear frequency scale).

The Sphinx precisely met its specification of maximum power into 8 ohms: 100Wpc, or 20dBW at 1% THD+noise (fig.5), with very low distortion between 1 and 20W. Into 4 ohms, the amplifier clipped at 155Wpc or 18.9dBW (fig.6), but with a little higher distortion at lower powers. Plotting the THD+N percentage at 8.7V into 8 and 4 ohms (fig.7), a voltage where I could be sure I was looking at actual distortion rather than noise, indicated that the left channel (blue and cyan traces) was a little more linear than the right (red, magenta). Neither channel changed its behavior with frequency, other than above 10kHz, where I assume the limited ultrasonic bandwidth is affecting the result.


Fig.5 Rogue Sphinx, distortion (%) vs 1kHz continuous output power into 8 ohms with Audio Precision low-pass filter.


Fig.6 Rogue Sphinx, distortion (%) vs 1kHz continuous output power into 4 ohms with Audio Precision low-pass filter.


Fig.7 Rogue Sphinx, THD+N (%) vs frequency with Audio Precision low-pass filter at 8.9V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta).

The distortion result is heavily second-harmonic in nature, though there must be higher-order products present into lower impedances, giving rise to low-level peaks that are slightly displaced in time from the waveform's zero crossings (fig.8). This can also be seen in fig.9, which shows the spectrum of the Sphinx's output with a 50Hz tone driven at 40% power into 4 ohms. However, intermodulation distortion with an equal mix of 19 and 20kHz tones at the same power into 4 ohms is respectably low (fig.10). The second-order product at 1kHz lies at –64dB (0.06%).


Fig.8 Rogue Sphinx, 1kHz waveform at 1W into 8 ohms with Audio Precision low-pass filter, 0.058% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).


Fig.9 Rogue Sphinx, spectrum of 50Hz sinewave with Audio Precision low-pass filter, DC–1kHz, at 60W into 4 ohms (linear frequency scale).


Fig.10 Rogue Sphinx, HF intermodulation spectrum with Audio Precision low-pass filter, DC–30kHz, 19+20kHz at 60W peak into 4 ohms (linear frequency scale).

I measured the phono input at the fixed-output jacks with the volume control set to its minimum so that I could keep the output stage quiescent. These outputs operate at unity gain for line-level inputs; for the phono input I measured a voltage gain of 37.7dB, which is appropriate for moving-magnet cartridges and high-output moving-coils. The input preserved absolute polarity, and the input impedance was 41k ohms at 20Hz and 20kHz, rising slightly to 46k ohms at 1kHz. Again, these figures are appropriate for MMs and high-output MCs.

The RIAA correction was superbly accurate for what must be a relatively inexpensive circuit, with very little error and the two channels matching to within 0.1dB (fig.11). The bass rolls off early, conforming to the IEC modification of the RIAA specification and reaching –3dB at 11Hz. Channel separation via the phono input was excellent, at 80dB in both directions at 1kHz. The phono input's noise performance was also excellent, with unweighted audioband signal/noise ratios (ref. 1kHz at 5mV input signal) of 77.3dB left and 79.8dB right. The ratios improved by 6dB when A-weighted.


Fig.11 Rogue Sphinx, phono input, response with RIAA correction (left channel blue, right red) (0.25dB/vertical div.).

With its relatively low gain, the Sphinx's phono input offered superb overload margins, ranging from 30.6dB at 20Hz to 28.7dB at 1kHz, and still 21.7dB at 20kHz. Distortion at typical recorded levels was very low, at <0.005% at all frequencies, and all that can be seen in the spectrum of a 1kHz signal at 5mV input are some low-level, supply-related spuriae (fig.12). Intermodulation distortion via the phono input (fig.13) was also superbly low, the 1kHz difference component lying at –86dB (0.005%).


Fig.12 Rogue Sphinx, phono input, spectrum of 1kHz sinewave, DC–10kHz, at 384mV (5mV input) into 100k ohms (linear frequency scale).


Fig.13 Rogue Sphinx, phono input, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 2V peak into 100k ohms (linear frequency scale; left channel blue, right red).

Even without taking into account its affordable price, Rogue Audio's Sphinx offers excellent measured performance with little sign of the usual compromises made in class-D designs. It also has an excellent, moving-magnet–compatible phono stage.—John Atkinson

Rogue Audio Inc.
PO Box 1076
Brodheadsville, PA 18322
(570) 992-9901

xsipower's picture

It’s nice to see that another manufacturer using Hypex’s OEM class-D amplifier modules perform so well. The last one Stereophile tested was the MBL Corona C15 monoblock power amplifier and it also performed very nicely (

Visually the 415 kHz ripple on the output appears incongruent to high quality audio, but apparently it doesn’t seem to impact the listening experience Mr. Reichert’s and Mr. Atkinson had. Makes you wonder if having a linear amplifier with radio frequency bandwidths (200khz and above) is really necessary or just a marketing ploy. The best tweeter rolls off sharply above 40 Khz and there is the question whether any audio information in any source goes beyond a few tens of kilohertz.

Hypex is demonstrating that Class-D has made it's place in the audiophile world when implemented properly.

Thank you for the review Stereophile.

BradleyP's picture

I'd love to hear a Sphynx! Class D has come of age. When I read reviews like these and see what kind of sound quality can be had for such do-able sums, I am convinced that these are the good old days of audio. A swell integrated that doubles power into four ohms--who ever heard of such at this price? As technology makes quality audio less and less expensive, it's possible in the future that an "audiophile" by today's standards will be anyone who owns a stereo. I just bought a $260 pair of new JBL studio monitors sporting four on-board class D amps for my desk, and they are almost a religious experience. (The good kind.)

BradleyP's picture

Oh, and thank you for the listening tip. The Guillaume De Machaut Motets are a real treat. Right up my alley. With the advent of Spotify, I devour every issue of Stereophile--I'm a paid subscriber--looking for music tips in the equipment reviews and always finding some keepers. Confession: on my desktop rig, which is no slouch, I am HARD pressed to tell the difference between 44.1/16 and a Spotify stream of the same material. I really ought to buy the stuff I like, but I like so much more than I can afford to buy. A trip to the online catalog (with audio samples!) which features the Zig Zag Territories and related labels is alike a trip to an art museum. Seriously do check it out:

remlab's picture

Nicely done. I'm really looking forward to your inteligent, thought provoking reviews. JA made a great move by bringing you in. It's just what the magazine needed.

commsysman's picture

The comment that the Sphinx is "all analog' is obviously incorrect, because any Class D output stage is a digital output stage.

Its output only becomes analog after the duty-cycle-modulated DIGITAL output of the switching stage is filtered back into an analog waveform by a filter.

deckeda's picture

That's a common misconception.

The modulated signal and filtering have constructs and methodology that superficially resemble digital signal sampling and filtering, and you could have an amplifier employ digital controls to the operation of the output devices but that's as far as it goes.

Transistors and MOSFETs are very much analog devices, and there's no ADC nor DAC at work here.

russ_777's picture

Agree, it's always been difficult to characterize because it has features that are both analog and digital in nature. At its core PWM is a non-linear analog modulation (similar to FM in that sense) as the width of the pulse within any period of the sawtooth carrier WF represents a continuously variable characteristic of the input waveform - that being the difference between the amplitude of the input waveform and the sawtooth carrier waveform where they intersect.

It "appears" to be a digital waveform because the output of the comparator and the output switching stage before the LPF has only two amplitude values - full on or full off. But the information from the input signal is carried in the width of the pulses, which is not discretized.

rimu's picture

Hey, I'm a bit surprised noone owning a unit comments anything. Probably thay are so much into listening, that they have no time to write comments anymore.

I've bought a used unit a few months ago after listening and comparing it with a few other candidates. I could definitely say that the Sphinx is a very strong contender in its price range. Nothing I heard of comparable price came even close. Musical Fidelity M6si felt sounding in about the same league as the Sphinx despite MF being twice more expensive. The point I'm trying to make is that Rogue sounds a class higher than it costs.

What may be not completely clear from the start is that despite being a hybrid, it's still a device with tubes inside. This means that the sound quality would be heavily dependent on the tubes used. The best choice to upgrade the tubes, in my opinion, would be vintage Telefunken ECC82. The amp truely sings with these tubes. The opposite is also true: using low grade tubes would kill all the beauty of its sound. Unfortunately I got my unit with already upgraded tubes and never had a chance to audition it with stock tubes.

Another thing one considering to choose the Sphinx should be aware of, is that this amp has various kinds of noise problems. First of all - the tubes matter in this regard - they give a pretty high noise floor in comparison to most solid state amps. Verious tubes have various problems with noise floor and microphonics. Another issue is specific for the Sphinx - it's humming. The power supply unit of the amp is not screened from the audio components under the hood. This is really disappointing. As a result you may hear humming with high volume and no input signal. It's not catastrophic at all and doesn't affect auditioning way too much. One should be prepared however that between the tracks you are likely to note the unit's noise.

All in all this amp sounds really sweet, with very high resolution throughout the spectrum. It a worthy choise and unless you are not willing to spend more money there are no other options with comparable fidelity. I had a great time with it and would definitely reccomend it as a great buy!

mrvco's picture

I replaced an MF M3i with the Sphinx v2 and couldn't be happier with it running a pair of Omega single-driver, high-efficiency speakers. The noise floor of the v2 is supposed to be lower than the v1 models. I did end up adding a Blue Circle Audio power conditioner since the Sphinx did let through some intermittent electrical noise that I hadn't heard in the past.

audioguy85's picture

Could the noise you mention be caused by the aftermarket tubes installed? also is this a v1 or v2? The review, at least for the v2 version made no mention of any appreciable noises, and the cv2 is supposedly more quiet, lower noise floor.

audioguy85's picture

I own the black version, the build quality looks to be all there. Very minimalist design which I like. Have not had the time to hook it up just yet, but will add a future comment as to how it sounds. For me it was a bit pricey as I normally go for the 500-700 range of integrated's, but I think I made a significant upgrade buying this. Time will tell. I appreciate that it Is a made in US product and I find the Class D combined with tubes to be interesting. I will be driving my wharfedale 225's with this amp, as I do feel they need a good push to sound their best. This Amp should do the trick, and if Stereophile's review is on the money, I think I will be very happy!

brad maestas's picture

I just received my Sphinx v2 in black yesterday and am having a ton of fun running it in. I am already very impressed, especially in its resolution throughout its gain range. I spend a good deal of time listening at lower levels late in the night and it still gives me a great sense of space and presence even at low levels. Granted, my previous amplifier was most certainly nothing special, a Harman/Kardon HK 3480, so it should be no surprise that it's quite a big step up however I've been able to audition and borrow quite a few nice separates and integrated amps over the last decade and the Sphinx is really quite something for the price. Right now it's powering Mission 703s that I'm soon upgrading to Wilson Benesch Square Twos.