Introduction
Reviewing these two devices started out from a simple motivation for replacing my dead E-MU 0404 USB. I am a long-time speaker DIY'er and the E-MU 0404 USB, a state-of-the-art device at its time, had served my needs for 16 years admirably. But then it suddenly died. After studying all audio interfaces tested by Julian Krause at his YouTube channel, I made a short list: Solid State Logic SSL 2 MKII and Topping Pro E2x2 OTG. Ordered the SSL. Loved its design and it seemed to be working fine. But I noticed distortion levels in acoustic and DAC measurements somewhat high---not great. So I ordered the Topping, too. Right after receiving it, however, I discovered the E1DA Cosmos ADCiso, and found it had been a hit in small DIY world. After seeing its amazing performance, I couldn't help getting it along with the Cosmos Scaler. I also needed a DAC for measurement tasks, so I ordered the Topping D50 III based on the ASR review.
To cut a long story short, I decided to measure these two audio interfaces to satisfy my own curiosity. But then noticed that the SSL 2 MKII, being relatively new on the market, has not been tested extensively except by Julian Kraus. Showing only the dynamic range and noise level, Julian's tests are not adequate. As you will see below, distortion performance turns out to be a key factor in a proper assessment of the SSL.
The primary focus of this review is on the devices' ADC performance (i.e., recording quality) though their DAC sections are briefly examined as well. The reason is that I believe recording/measuring must be the main reason for purchasing a USB audio interface. Note that Topping Pro E2x2 has also been reviewed by Amir here at ASR, the test results of which may not precisely match my reported ones---they are very well aligned with each other, though.
Measurement Setup
- Signal generator: Topping D50 III as a USB DAC.
- AD converter: E1DA Cosmos ADCiso Grade A in Stereo Mode, and Cosmos Scaler as a buffer.
- Software: Paul Kane's Multitone Loopback Analyzer for most of the tests, and Room EQ Wizard. Whenever possible, results were cross-checked b/w two software packages.
- External, analog filters were not used in this test. I could have used a low pass filter to remove harmonics from test tones, or a notch filter to avoid ADC-induced harmonics.
- Nonetheless, I believe the comparison of the two devices should be informative because they were measured under exactly the same conditions. That is, cabling, input/output levels, and software settings such as sample rates, FFT sizes, FFT window types, averaging, etc. were all matched.
- All tested input/output connections were balanced.
- I made sure of using a powered USB hub since SSL 2 MKII is solely bus-powered.
- The "4K" effect of SSL 2 MKII was not tested. This function is just for fun experiments; it adds a huge amount of harmonic distortions to the input.
I could get better results (by 3 dB) from ADCiso's Mono Mode, but did not bother to use it---too lazy for cable swapping and additional measurements. Note that the results (20 Hz - 20 kHz) are still very close to Amir's or Rja4000's D50 III measurements, except that slightly higher harmonics must be due to no analog notch filter being employed.
Solid State Logic SSL 2 MKII
Solid State Logic (SSL) is a British pro audio company. Though not as popular as Focusrite, Motu, RME, etc., it is a well-known brand in pro audio. The SSL 2 MKII is the company's entry-level 2-channel audio interface, an update of its predecessor SSL 2. There is a 2x4 variant, SSL 2+ MKII (update of SSL 2+), which adds 2 output channels and MIDI in/out. The SSL 2 MKII retails at US $230 and SSL 2+ MKII at US $300.
Intended for use on studio desks, its console-style design is very attractive. All switches, knobs, indicators, and input/output jacks are positioned well. The parts seemed to be of high quality and their operation feels great. Its case is a mixture of metal and plastic, and I loved its overall premium feel. Also very nice is a weighted base of its case giving stability on a desk. One important design choice is that it is to be entirely USB bus-powered.
Its double-sided PCB has ES9821Q as its ADC chip on the front side and ES9016K2M as its DAC chip on the back side. I had high hopes for its ADC performance as ES9821Q is ESS's 2nd best 2-ch ADC chipset next to the venerable ES9822PRO adopted in the RME ADI-2 Pro line and E1DA Cosmos ADCiso. On paper ES9821Q's DNR and THD+N numbers are only 5 dB worse than ES9822PRO's.
Topping Pro E2x2 OTG
This is one of the audio interface models offered as Topping's first pro audio products. This OTG model is essentially the sams as the non-OTG variant that was reviewed at ASR, except it has an additional USB-C port that can be connected to a smartphone simultaneously with a computer, and an optical out as well as an additional analog AUX out on the back panel. The E2x2 OTG retails at US $199 and the non-OTG model at US $159.
Pictures are from the Topping Pro website---didn't have a chance to take pictures of its exterior (mine is the black version). Its build quality is good though I personally like the SSL's build more. The case is made of full metal (aluminum and steel) but sharp edges and corners do not feel sophisticated. Although it can be bus-powered, a separate power supply through its dedicated USB-C power port is recommended.
But I did take a picture of its interior:
Unfortunately, it is impossible to know which ADC and DAC chipsets this audio interfance is based on, because all the chips suspected to function as such have no markings, or markings masked. Why?
ADC Measurements
First up is SSL 2 MKII's ADC results of a 1 kHz tone from D50 III fed into SSL's
The noise floor must be largely limited by ADC as we know D50 III's noise floor is much lower and noise must be uncorrelated b/w DAC and ADC. What mainly affects SINAD should be harmonic distortions contributed by both DAC and ADC. Let's compare this to the results from Topping E2x2:
In terms of SINAD, the two devices are neck and neck though E2x2 has cleaner higher-order harmonics. It is interesting to compare the above results to amir's E2x2 measurements: they are very, very similar---even the higher H2 of Ch2 and higher H3 of Ch1---except harmonics are a bit higher in mine. It is easy to conjecture that the test tone from D50 III is contributing some harmonics.
Dynamic range is great for both devices:
As expected, DNR measurements are unaffected by the test tone because the signal must be basically free of harmonics at this low amplitude.
Frequency response was measured as a sanity check:
Also measured was their wideband ADC quality of a 10 kHz tone:
One may think a 10 kHz tone test is meaningless because harmonics being excited are beyond the audible range. But I consider it still informative. These help us better understand the following results of a sine tone sweep over frequencies.
These results, combined with the wideband FFT results, suggest that SSL 2 MKII's THD+N performance is dominated by THD whereas E2x2's THD+N is masked by N. In particular, the THD performance of SSL 2 MKII's ADC is not great at higher frequencies. Sure, fundamentals at 10 kHz and above may be fine, but how about 3k-7kHz tones? Undesirable. On the other hand, E2x2 exhibits substantially lower THD across all fundamental frequencies. Its THD+N for the 96 kHz bandwidth is masked by its ultrasonic noise (> 50 kHz).
SSL 2 MKII's relatively high distortion levels are further revealed in the following IMD vs. level sweep:
EDIT: Ignore the "IMD+N" curves---just look at IMD results. There seems to be something incorrect with the calculation of IMD+N in the software.
It turns out that the distortion of SSL 2 MKII's ADC rises greatly when the signals approach the amplitude of input sensitivity whereas E2x2 shows much milder effects of test tone levels. Note, here, that the input sensitivity includes the effect of the input gain which is set to -1.5 dBFS when a 1 kHz sine tone at 4.2 Vrms is fed (i.e., generator 0 dBFS = 4.2 Vrms sine tone = ADC -1.5 dBFS). In fact, THD vs. level sweeps told a similar story (not shown here as I forgot to save them).
Testing their microphone input was tricky because I was trying to mimic low-level microphone signals which resulted in noisy test tones. But the results are still interpretable:
It turns out that E2x2's ADC THD from its mic input is much lower than SSL 2 MKII's across all testing frequencies (20 Hz-10kHz fundamentals). E2x2's THD+N is completely masked by noise due to the compromised test tone quality. In hindsight I should've done this test differently to obtain a cleaner noise floor. Instead of lowering the test tones with external preamp attenuation, I could've simply used stronger test tones, increased the input sensitivity with the gain knob, and accepted some digital noise with attenuation on D50 III. Anyway, because we know both devices' noise performance belongs in the top tier according to Julian Krause's tests (w/ dScope M1), we can focus on their distortion performance.
DAC Measurements
Let's look at their DAC performance:
On SSL 2 MKII I used a volume setting a little lower than its max volume to match the output voltage of E2x2 on its max volume. The higher THD from SSL doesn't look great.
How about IMD sweeps?
Topping is much better. No contest.
THD vs. frequency sweep:
Topping wins again.
Ultrasonic filtering at 44.1 kHz sample rate:
SSL wins here, but this test is not that critical.
Jitter tests:
Both are fine.
I did not test their headphone outputs. According to Julian Krause's tests, Topping has an excellent headphone amp, which is also backed by Amir's measurements, and SSL's headphone out is also nice within its limit of being bus-powered.
Conclusion
I had high hopes for SSL 2 MKII because it adopts the ES9821 ADC chipset which is ESS's relatively new ADC solution. Not the level of ES9822PRO, but still excellent on paper. SSL 2 MKII's ADC performance, however, is not impressive. It shows problematic distortion behavior especially when it meets with strong input signals, which worsens with higher-frequency tones. The same goes for its DAC which is based on ES9016K2M. Distortion increases with output amplitude and frequency. In contrast, Topping E2x2 OTG does not exhibit such problems. I can easily recommend the Topping but can't SSL.
Then, why do we see this undesirable distortion behavior from SSL? I don't believe it extracts the full potential of its employed chipsets. My strong suspicion is that designers at SSL made some compromises to make it stably operate entirely bus-powered. It has to provide everything, including mic preamp, phantom power, over-4-volt line out, and relatively powerful headphone out. It does all of these solely bus-powered---I made sure of using a quality powered USB hub for this testing. I'm not sure if it's possible to achieve all these without making a compromise.
There are some quirks I do not like with the Topping, though. Its input gain control knobs are very sensitive at some positions, making it difficult to set levels. Its design feels less sophisticated than the SSL. I love the SSL's appearance and ergonomics. I would have preferred it if its performance had been on par with the Topping's. Practically, however, it is possible to use SSL 2 MKII within its limitation. If one makes sure of setting its gain to a lower position to keep recorded signals well away from the peak level, the recording quality should be fine with DNR somewhat compromised.
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I've absolutely no idea, but I wonder how their higher-tier products compare and at what level of sophistication is 'mastering grade' for release, if you get my meaning?
