Article Updated: 22 Mar 2022
C-MEDIA CM6206 is a cheap and solid performance audio codec chip supporting 16 bit / 48 kHz audio, stereo* MIC and LINE-IN inputs. Works out of the box on Windows 10 operating system with stock drivers. Great for hobby recordings & measurements (e.g. using Virtins Multi-Instrument / Oscilloscope, ARTA / LIMP, etc.), despite the fact it has only 16 bit A/D converters and 48 kHz sampling rate! 99.999% of us humans can’t hear the difference between standard 16 bit / 44.1 kHz and 24 bit / 192 kHz audio anyway thanks to low-pass filtering, interpolation and human hearing characteristics.
PART 1: C-MEDIA CM6206 5.1 External USB Audio Sound Card Line-In Bass Extension Frequency Response Modification
C-MEDIA CM6206 – THE PROBLEM
If you wish to start a YouTube or Twitch career, transfer your old audio cassette recordings or vinyl collection to your computer, C-MEDIA CM6206 External USB Sound Card is a very nice solution in case you use a modern laptop without integrated multi-channel audio card and dedicated Line-In or MIC inputs. This card can be bought really cheap on Amazon, AliExpress, BangGood and similar online stores, and have it delivered for free in few days or weeks.
Paired with a cheap studio-like condenser microphones such as BM-800 or Neewer NW-700 you can really step-up the quality and get yourself going in the right direction for well under $50 budget.
Of course, we cannot possibly compare it to the more advanced and higher-quality Realtek, Intel, Nvidia or AMD HD Audio codecs (supporting up to 5.1 / 7.1 / 24-32 bit / 192-384 kHz with absolutely flat frequency response in 20-20 kHz range and beyond), but for this price we can’t and don’t complain. Plus, unless it’s a desktop computer, current laptop computers using them only have stereo headphones output and (maybe) single mono DC-biased MIC input (sometimes hidden in a single 4-wire 3.5 mm socket), and do not support analog stereo LINE or MIC inputs that we need!
However, engineers who designed and manufactured this sound board crippled LINE-IN analog recording inputs frequency response in low / bass section by putting very small coupling capacitors! Why oh why ???
CM6206 hardware is perfectly capable going all the way down to a sub-hertz (less than 1 Hz) flat frequency response if desired / required. You can even bypass input AC coupling capacitors to enable DC input, with some precaution (useful for audio oscilloscope applications and DC signal measurements e.g. using Daqarta software).
C-MEDIA CM6206 audio codec is used in many different sound cards, including 7.1 surround models. It supports separate Headphone output, mute, rec mute, digital volume controls, dual mono MICROPHONE inputs and more! Of course, in this 5.1 model not all the features are used (obvious cost reduction saving on external parts and components), but what can you expect from a $5 or $10 sound card? More expensive $20 models come with 7.1 audio, dedicated headphones output, dual MIC and LINE-IN inputs, Volume and Mute controls, but those are not really essential, as you can achieve the same in software, although they can be convenient as dedicated hardware buttons.
In essence, this is a very solid audio card for the money!
C-MEDIA CM6206 5.1 External USB Audio Sound Card – Line-In Frequency Response Measurements
[1] Right-Mark RMAA without modification (loopback audio Line-In Line-Out)
RMAA C-MEDIA CM6206 5.1 USB Audio Card 16-bit, 48 kHz Loopback Line-In Frequency Response Spectrum + THD
[2] Right-Mark RMAA without modification (loopback audio Line-In Line-Out)
RMAA C-MEDIA CM6206 5.1 USB Audio Card 16-bit, 48 kHz Loopback Line-In Frequency Response Spectrum TEST OVERVIEW
[3] without modification (ARTA FR1 CH loopback audio Line-In Line-Out)
[4] with 4.7 uF input coupling capacitors modification (ARTA FR1 CH loopback audio Line-In Line-Out)
[5] with 1 uF input coupling capacitors modification (ARTA FR1 CH loopback audio Line-In Line-Out)
C-MEDIA CM6206 5.1 External USB Audio Card Line-In Modification Video Tutorial
In this video I have upgraded input coupling capacitors with larger values and improved frequency response considerably! In the end of the video is a line-in recording example using C-MEDIA CM6206 5.1 External USB Sound card with and without this modification compared to the original audio, along with the frequency response charts.
Watch Video Tutorial:
Used SMD capacitors: 1608 A (0603) XR7 type
C-MEDIA CM6206 5.1 External USB Audio Sound Card – High Resolution PCB Board Photos with Annotations
Note: click on photos to load full resolution images
C-MEDIA CM6206 5.1 USB Audio Card – TOP & BOTTOM PCB Layers Overlay – Bottom Side (Horizontally Mirrored Photo)
I used solid capacitors with 2nd unit as they were much easier to solder than SMDs and have better electrical characteristics (performance)
Note: Outer “metal” case shell on solid (polymer) electrolytic capacitors is electrically non-conductive (in audio frequency range, at least — confirmed this with my own measurement), so it’s safe if they touch each other! If you still don’t have a piece of mind, you can very gently put a tiny sheet of paper or translucent plastic foil between them later after soldering (but it’s not really necessary) — just be careful not to rip apart SMDs from the PCB with too much force!
IMPORTANT NOTES & TIPS
MIC INPUT Frequency response without modification (ARTA FR1 CH loopback audio Mic-In Line-Out)
Update: Well, it is not possible to measure MIC IN coupling capacitors capacity directly, but I have measured frequency response instead! And, yes, just as I suspected, undervalued 0.1 uF (100 nF) input coupling capacitors are used which results at approx. -3 dB @ 80 Hz point. In another words, you need to modify MIC input coupling capacitors as well if you plan to use them with higher quality microphone or simply wish to achieve better low frequency (bass) response with existing one.
DC offset can be compensated using Sound Forge or similar recording software with DC offset adjust compensation option enabled. Calibrated offset before each recording will help, but the drifting offset over time will still be present in some cases. Using some smart VST plug-in for post-processing should also help. FL Studio has a nice built-in factory DC remove effect called Fruity Center that works dynamically but it’s not enabled by default in newer versions, you must click on More plugins… in the Mixer channel FX slots menu and activate it in the list to make it visible/available.
C-MEDIA CM6206 5.1 USB Audio Card – DC Offset Calibration in Sound Forge 11 Recording Panel
Another related issue which I discovered recently is that setting Windows 10 device slider for Line gain to 0 will not mute C-Media CM6206-LX USB sound card, only reduce gain to default predefined internal value! This means that for some devices such as CD players and audio cassette decks, which inherently do not have integrated output volume (gain) control knob, recording levels (= gain) may still be too high and you’ll need a passive attenuation resistor network in between your recording cables and C-MEDIA CM6206-LX USB audio card line input. See below recording for demonstration of such scenario where positive half-periods in recorded signal were clipped because of present DC offset, and input device (DENON cassette deck) does not have output level control knob.
C-MEDIA CM6206 5.1 USB Audio Card – Windows 10 Generic Drivers Line Input Gain Settings
C-MEDIA CM6206 5.1 USB Audio Card – Asymmetrical Recording Clipping with DC Offset and Strong Input Signal
This isn’t something I’m particularly happy about to be completely honest and you should be extremely careful when making recordings via Line input port to avoid strong signals and potential waveform single-side or double-side uneven clipping and distortion because of the present DC offset. Using -3 dB safety margin should be OK, -6 dB if you wish to be extra safe and avoid unexpected peaks.
You should avoid recording at high levels near 0 dB (+/- 1.40 Volts) anyway, to avoid clipping. Remember, DC offset in audio is bad because it effectively reduces available dynamic range and maximum signal input voltage span.
CM6206-LX Volume Gain Control Resolution
Consulting official datasheet, volume controls and gain levels are:
- DAC Volume Control / Master Output (Playback) [Front/Surround/Headphones/SPDIFO]: from -34.5 dB to 0 dB plus MUTE state (38 linear steps / lin scale)
- ADC Volume Control [Line / Mic Input Gain] (Recording): from -6 dB to +33 dB plus MUTE state (26 x 1.5 dB steps / log scale)
- ADC Volume Control [Line / Mic Monitor Control Gain] (Playback): from -33 dB to +12 dB plus MUTE state (31 x 1.5 dB steps / log scale)
- ADC Analog Input Gain [Line / Mic]: from -36 dB to +33 dB (48 x 1.5 dB steps / log scale)
I have yet to test official C-MEDIA USB sound card drivers, which may be the key answer to solving the input gain control problem.
PART 2: C-MEDIA CM6206 5.1 USB Audio Card Headphones Line-Out Bass Frequency Extension Modification
So far, we have only modified Line-In inputs used to record/monitor analog electronic musical instruments or pair of high quality microphones (via preamplifier — not directly!). What about Line-Out (marked as FRONT OUT) outputs?
If you take a closer look at the output stage on the PCB board you will notice six 100 uF electrolytic coupling capacitors connected to their respective color-coded 3.5 mm output sockets. Each capacitor pair is dedicated for L+R main output (C5/C6), L+R rear surround output (C1/C2), and finally c/sub CENTER + SUBWOOFER output (C3/C4), respectively.
C-MEDIA CM6206 5.1 USB Audio Card – TOP & BOTTOM PCB Layers Overlay – Bottom – Headphones Line-Out Mod (NOTE: Corrected Output Coupling Capacitors Polarity!)
⚠️ If you take a closer look at the PCB board, you will notice that the output coupling electrolytic capacitors have their negative minus “-” terminals connected to the CM6206 chip’s side (pins). However, this is very wrong and illogical, because chip operates at single-ended USB +5.0 V power supply and output DC offset is biased at positive +1.40 volts, which allows AC audio signal to swing between 0 V and +2.80 V.
According to official CM6206 datasheet, output coupling capacitors should be connected with their positive “+” terminal at the CM6206 chip’s side, and this is exactly why we marked it as such in our annotated photo! The factory PCB version is technically wrong, and this negative -1.40 voltage will negatively impact capacitor’s life expectancy and performance. Because outputs are ended with 47 kOhms resistors to the ground, capacitors are constantly polarized with reverse voltage and current, even without any external load connected to the output sockets!
Now, if your setup uses external audio amplifier or some other medium-to-high impedance equipment (e.g. 1k ~ 100k Ohms input impedance; CM6206 outputs are ended with 47 kOhm resistors), those 100 uF factory coupling capacitors will be more than adequate. Headphones with 64 Ohms impedance will be fine, and even most 32 Ohms models. Why? Well, most home headphones have issues reproducing frequencies lower than 40 Hz anyway (yes, they can reproduce them in Signal Generators, but in the real music mix those frequencies get lost because of poor frequency response and attenuation), so you will definitely not miss much without this modification.
Headphones with 32 Ohms nominal impedance and decent bass response such as EDIFIER H840 (they go all the way down to the useful 35 Hz in our tests) will be just fine with a -3 dB point at ~ 50 Hz (54 Hz measured in ARTA), but technically still far from Hi-Fi “on paper”. On the other hand, headphones such as SENNHEISER HD 201 won’t gain much because they cannot reproduce anything below 40 Hz properly (forget that 21 Hz from the official specification!). Still, if you test them with pure sine waves with signal generator, there is no doubt that 20 ~ 50 Hz range gets louder with larger coupling capacitors. And let’s not forget that your ears’ sensitivity to low frequencies, head physical profile and headphones anatomy equally play important role in low frequency response.
However, with ultra linear and low-impedance 16~24 Ohms headphones you will definitely notice some bass improvement because -3 dB point will be at 100 Hz and 65 Hz, respectively. Again, we stress out that your headphones must be real subwoofer monsters and linear all the way down to 20-25 Hz to notice any real difference, aside the measurement graphs! Otherwise, the effect of this Line-Out modification will be extremely subtle and barely noticeable if your headphones drivers cannot deliver.
How To Fix Line-Out Low Frequency Response with Low-Impedance Headphones?
Simply replace capacitors C5 and C6 with a higher value one. For 32 Ohms headphones use 1000 ~ 2200 uF / 6.3V, and for 16-24 Ohms headphones use 2200 ~ 4700 uF / 6.3 V electrolytic type. You can even use low-ESR types (usually used in switching power supplies) and solid aluminum electrolytic capacitors (usually more compact than ordinary wet electrolyte types; so you can easily squeeze larger capacity in a constrained space) for best possible audio performance, and low distortion.
NOTE: Capacitors height must not exceed ~ 13 mm (1/2″ inch) or you’ll have to position them horizontally in order to be able to put aluminum enclosure back. Distance (pitch) between capacitor’s leads (pins or “legs”) is 2.54 mm ideally, but you can bend larger 3.5 mm / 5.0 mm types easily.
C-MEDIA CM6206 Line-Out Modification Measurements
[1] without modification + EDIFIER H840 (32 Ohms) headphones load connected @ output
(ARTA FR1 CH loopback audio Line-In Line-Out) (Line-In 1 uF input capacitor mod applied)
ARTA #4-1 C-MEDIA CM6206 5.1 USB Audio Card Line-Out + EDIFIER H840 + 100 uF Capacitor Factory -3dB Marker
[2] with 1000 uF capacitors modification + EDIFIER H840 (32 Ohms) headphones load connected @ output
(ARTA FR1 CH loopback audio Line-In Line-Out) (Line-In 1 uF input capacitor mod applied)
