AK103E USB sound
Cheap (~$12) USB sound card using CM6206 LX. Very likely the cheapest sound card with stereo line input and SPDIF optical input and output.
This is a pretty mature and mass produced sound card, sold probably in hundreds of thousands, and it still has obvious flaws that could be corrected at no cost.
This won't win any spelling contest.
Extruded aluminum case with plastic front and back panels.
This looks like at least third PCB version. The first one had THT capacitors and mini USB, second: SMD capacitors, this one switched to USB-C port.
Personally I would prefer mini-USB version as USB-C in the one I have seems to require more than comfortable force and probably would be more prone to damage.
These 6 electrolytic capacitors at audio outputs are mounted with incorrect polarity. Previous PCB versions had the same issue and since
footprint are matching silkscreen, this is design issue, not assembly issue.
Let's test it using RightMark Audio Analyzer and external LINE OUT - LINE IN loopback. Here are results before any hardware modification (and full report):
Summary
| Frequency response (from 40 Hz to 15 kHz), dB | +1.02, -7.27 |
Poor |
| Noise level, dB (A) | -87.0 |
Good |
| Dynamic range, dB (A) | 87.2 |
Good |
| THD, % | 0.026 |
Good |
| THD + Noise, dB (A) | -69.0 |
Average |
| IMD + Noise, % | 0.261 |
Average |
| Stereo crosstalk, dB | -83.3 |
Very good |
| IMD at 10 kHz, % | 0.024 |
Good |
| General performance | Good |
The first and foremost issue is poor response at low frequencies, -7.3 dB at 40 Hz, -12.8 dB at 20 Hz. This is caused by insufficient capacitor values at line input (C30, C31 - originally 100 nF). I've changed these capacitors to ceramic 1 uF (slightly larger package - 0805 - as this what I had on hand, but still fits). RMAA results (full report):
Summary
| Frequency response (from 40 Hz to 15 kHz), dB | +0.37, -0.19 |
Good |
| Noise level, dB (A) | -86.9 |
Good |
| Dynamic range, dB (A) | 87.1 |
Good |
| THD, % | 0.021 |
Good |
| THD + Noise, dB (A) | -70.8 |
Average |
| IMD + Noise, % | 0.026 |
Good |
| Stereo crosstalk, dB | -83.7 |
Very good |
| IMD at 10 kHz, % | 0.024 |
Good |
| General performance | Good |
Suprisingly this not only improved frequency response but also significantly reduced measured IMD + Noise.
Second modification was fixing reversed polarity of 6 electrolytic capacitors at analog outputs. It did not changed sound card parameters (full report), but it might prolong life of these capacitors.
Testing with LINE OUT to MIC IN loopback also show poor response at low frequencies (full report).
Note: ignore create crosstalk, this is a mono microphone input.
This might be less of an issue than LINE IN (perhaps higher cut off would be prefered sometimes),
but I've also replaced capacitor here, from 0.1 uF to this time 0.68 uF 0603. RMAA full report.
PCB after modifications:
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