INTRO: This is the first in a series of inexpensive portable USB DAC reviews I’ll be publishing in the next week or two. The idea is to test the DACs with a high impedance load (such as the O2 Headphone Amp) and a typical headphone load. The DACs are all small, easily portable, and USB powered. The $25 Turtle Beach Audio Advantage Micro II is just such a DAC. According to Turtle Beach, it’s supposed to provide “higher quality sound” compared to internal PC audio. I also have updated how I test and present DAC results.
TURTLE BEACH MICRO II: The Micro II is a small “dongle” with an attached 2 inch “pigtail” USB cable. It has a single 3.5mm jack which serves as line out, headphone out, and an optical digital out (using a supplied 3.5mm-to-Toslink adapter). There’s no volume control, other controls, or inputs of any kind--just a blue LED.
WINDOWS INSTALLATION: The Micro II installed smoothly in both XP and Windows 7 without needing any drivers. Windows reported it as a “USB Sound Device”. The only sample rates and bit depths available are 16/44 and 16/48 as shown to the right in Windows 7.
SUBJECTIVE SOUND QUALITY: There was moderate hiss with my Ultimate Ears IEMs but the Micro II was fairly quiet with less sensitive headphones. The sound quality, however, was seriously odd. Playing familiar well recorded audiophile tracks the Micro II made them sound shrill, glaring, and harsh regardless of what headphones I used. I was really curious to measure the Micro II and find out why it sounded so obviously bad.
POOR DRIVER DESIGN (updated): I first checked the Micro II’s frequency response and it was reasonably flat out to 15 Khz so the poor sound was still a mystery. Then I checked the 1 Khz THD and while it wasn’t great it also wasn’t bad enough to explain the poor sound quality. When I dropped the level to see if the distortion would drop, I found the problem. The Micro II was displaying a horrible linearity problem. Dropping the input from 0 dBFS to –20 dBFS should drop the output by 20 dB as well. But it only drops 8 dB! That’s a massive 12 dB error. The net effect is the Micro II was heavily compressing music—making softer sounds much louder than they should be.
BAD SOUND EXPLAINED: It turns out, as described in the comments to this article, the C-Media CM102 integrated USB sound chip apparently used in the Micro II has a “feature” called Dynamic Range Control (DRC) that defaults to on. Confusingly, there’s an advanced option in the Windows 7 sound options for the Micro II simply labeled “Loudness”. And, worse, it’s enabled by default. You have to uncheck the box to stop the Micro II from heavily compressing anything you play through it.
DRC vs LOUDNESS: The Turtle Beach choice of calling the C-Media’s DRC option “Loudness” is very misleading. In audio, Loudness Compensation involves changing the frequency response at low listening levels to compensate for human hearing. It’s generally based on the Fletcher-Munson Equal Loudness Curves. In this case, however, it has nothing to do with changing the frequency response—only the overall dynamic range. I don’t know what Turtle Beach’s driver calls this feature as it’s an unsigned driver so I didn’t install it on my test bench PC that runs the dScope software. The whole idea is to not need proprietary drivers.
WHAT WERE THEY THINKING (updated)? The “loudness wars” are already out of control without any further help from Turtle Beach. A lot of pop music has a peak to average volume difference of only around 8 – 12 dB as the labels keep compressing music ever further in an effort to have it stand out as being louder. The last thing pop music needs is another approximately 12 da lot more compression but that’s exactly what you get, by default, with the Micro II. For anyone unaware of the option, or who knowingly leaves it on, the Micro II is likely to sound significantly worse than the internal audio of just about any computer its plugged into.
MEASUREMENT SUMMARY: The overall results, even with DRC disabled, are not terribly impressive. The high frequency distortion, in particular, is poor. Here are the results compared to the more expensive FiiO E7 (some tests were run slightly differently but I’ve tried to adjust for that in the E7 numbers):
|Measurement||TB Micro II||Fiio E7|
|Frequency Response 20hz-15Khz 33 ohms||+/- 1.8 dB Fair||+/- 0.1 dB Excellent|
|THD 0 dBFS USB 10K||0.20% Fair||0.05% Good|
|THD 1 Khz 10K Ohms -3 dBFS||0.022% Good||0.03% Good|
|THD 1 Khz 33 Ohms -3 dBFS||0.12% Fair||0.03% Good|
|IMD CCIF USB||0.28% Poor||0.03% Good|
|IMD SMPTE||0.03% Fair||0.008% Excellent|
|Noise A-Weighted||-93.8 dBu Fair||-96.7 dBu Good|
|Max Output 33 Ohms Vrms/mW||1.26v 52mW Good||57 mW Good|
|Max Output 10K Ohms Vrms||1.34v Good||1.4v Good|
|Output Impedance 100hz||0.95 Ohms||0.13 Ohms Excellent|
|Jitter USB 16/44 Jtest||Fair||Very Good|
BOTTOM LINE: In my opinion the “Loudness” feature enabled by default is an epic fail. Someone either got sloppy or they have very odd priorities for a “high quality” USB DAC. Putting that aside, the rest of the performance of the Micro II still isn’t very impressive. The next several reviews of low priced USB DACs will help put the Micro II’s performance in perspective.
FREQUENCY RESPONSE: The frequency response with a 10K load (such as a headphone amp) at 16/44 is acceptable but not great. There’s a fraction of a dB of variation below about 30 hz and it’s down –1 dB at about 15 Khz. The steep roll off above 12 Khz is typical of a cheap DAC running at 44 Khz and is due to cost savings in the digital and analog filters. The slight peak around 8 Khz is also disturbing as it indicates either poor DAC filtering and/or potential instability in the headphone amp. Into 33 ohms you can see a slight drop due to the output impedance and a low frequency roll off of about –1.7 dB at 20 hz. That’s borderline audible. Into 16 ohms it would be even worse and more likely to be audible. This indicates less than ideal capacitor coupling in the output:
THD+N vs OUTPUT: This test starts at 10 mV and the rise at below 250 mV is more likely due to more quantization error than noise. The unweighted noise should be below 0.004% but the distortion is more than ten times higher. The lower blue plot is into 10K and the distortion is around 0.025% which is under the worst case guideline of 0.05%. Into 33 ohms, however, it’s over 0.1% above about 700 mV which could be audible under some circumstances. In both cases, the maximum output level is around 1.3 Vrms. This works out to 52 mW into 32 ohms, 104 mW into 16 ohms and only 6 mW into 300 ohms:
THD+N 100 hz 0 dBFS & OUTPUT IMPEDANCE: I now run this test at 100 hz as that’s where output impedance is usually most critical due to the resonance frequency of many headphones. A low 100 hz output impedance keeps the frequency response accurate and provides electrical damping of the driver which can improve the quality of the bass performance. The test is run at 0 dBFS input to reveal any digital overload problems such as the NuForce uDAC-2 exhibits. The Micro II does not reach clipping even at full volume into 15 ohms. The distortion here is mostly in the DAC itself and remains similar into 100K even at lower volume settings. The resulting output voltages at 100K and 15 ohms are used to calculate the output impedance. The Micro II’s distortion is relatively poor at 0 dBFS. It hit 0.14% into 100K and 0.21% into 15 ohms. The output impedance was 0.95 ohms which is acceptably low and it’s slightly lower at 1 khz due to less impact by the output capacitors:
THD+N vs FREQUENCY: Here’s the 1 Khz THD+Noise plotted from 20 hz to 20 Khz into 10K (yellow) and 33 ohms (blue) at 775 mV (0 dBu). The input is –3 dBFS to prevent any digital overload of the DAC. The increase in low frequency distortion into 33 Ohms is another sign of a capacitor coupled output. The rise from 0.05% to 0.15% is likely the output capacitor’s non-linearity. The drop above about 6 Khz is related to the bandwidth limit of 22 Khz as the harmonics move past the audible band. The sharp rise again above 10 Khz is due to very poor high frequency performance in the DAC (and/or filter) despite the fact the harmonics are cut off:
SMPTE IMD: The result here is marginal but acceptable at this price. Ideally all distortion products should be below –80 dB but that’s not the case. The spread (or “mountain”) at the base of the 7 Khz signal is another bad sign. This test is run below the DAC’s digital limit and also well below the maximum output (at 0 dBu) into 33 ohms. It’s somewhat better into 10K but not a lot:
CCIF IMD BENCHMARK DAC1 PRE: This is a more challenging test, and again, the goal is to have everything except the 19 and 20 Khz signals below –80 dB. To show how it can look, here’s the result with the Benchmark DAC1 Pre:
CCIF IMD MICRO II 44 Khz 33 Ohms: By comparison, here’s the same test as above from the Micro II. There’s an entire “forest” of distortion products above –80 dB with the 1 khz difference signal at –52 dB which is very likely audible. The digital/analog filters in the Micro II are in real trouble here as might be the DAC itself. The two spikes around 15 Khz exceed –40 dB and may also be audible. This is admittedly a tough test for a cheap DAC running at 16/44 but this is still a much worse than average result made worse by the headphone amp struggling with a 33 ohm load:
CCIF IMD MICRO II 44 Khz 10K: Removing the load results in the 1 Khz difference signal improving significantly from –52 dB to about –72 dB dropping the reading by a factor of ten. But note there are still a lot of spikes above –80 dB and even above –60 dB within the audio band. Worst of all, the spikes at 15 Khz are still crossing –40 dB:
CCIF IMD MICRO II 48 Khz 10K: DACs will typically do better on this test running at 48 Khz but it depends on their filtering and design. In this case, things get significantly better but are still not great with the 10/11 Khz signals still around –40 dB and several other spikes still above –80 dB. If your operating system lets you run a DAC at 48 Khz, 99% of digital music will be re-sampled from 44 Khz up to 48 Khz by the operating system with mixed results. In this case, it’s hard to say which would yield the better result. In XP it’s not an option as the DAC is forced into 44 or 48 Khz depending on the source sampling rate:
NOISE & LINEARITY: I’ve changed this test slightly to use units of dBu rather than my previous dBr referenced to 400mV. 0 dBu is 775 mV. The Micro II’s –93.8 dBu A-Weighted noise referenced to 400 mV would be 88 dBr (it’s always a difference of 5.7 dB). That’s decent noise performance for a USB powered DAC but falls well short short of what’s required for reasonable silence with the most sensitive IEMs. The goal is –103 dBu. I’m also now showing the absolute (unweighted) noise in microvolts. The linearity was fairly good with an error of only 0.8 dB at –90 dBFS:
-20 dBFS LINEARITY WITH “DRC”: With the “Loudness” option enabled (which is on by default) here’s what happens with a –20 dBFS 1 Khz signal. It’s played back at –8 dB instead of –20 dB. The Micro II is raising the level by a whopping 12 dB as part of its “Dynamic Range Control” feature. This is why, by default, it sounds relatively awful:
JITTER: Here’s the spectrum from the dScope’s J-Test for jitter. The side bands are average at –110 dB but the “spread” of the signal is relatively poor indicating significant low frequency jitter. The frequency accuracy (clock accuracy) is very good as shown by the frequency reading on the left:
RMAA RESULTS: Out of curiosity I tested the Micro II with RMAA with the DRC/Loudness option enabled (10K load). While the frequency response was very similar, the THD spectrum showed some significant differences although the 2nd and 3rd harmonics were similar. The IMD was even worse than the dScope measured and could have been a clipping/level problem. The noise measured a relatively poor –77.5 dB for which I have no explanation. It presumably limited the dynamic range to a similar value. Interestingly, there’s no solid indication of the DRC compression. RMAA missed the huge linearity problem. The results are shown below along with the sound hardware by itself (2nd column) in loopback. For more, see my RMAA article:
TECH COMMENTS: The good news is the output impedance is below 2 ohms, the noise/linearity is decent, and the frequency response and midrange distortion are semi-acceptable into a 10K load like a headphone amp. Into 32 ohm headphones, however, the distortion rises to unacceptable levels and the high frequency distortion into any load is even worse.