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Upgrading OPAMPS on the X-Mystique

Gimmick or Necessity?

by Josh Walrath

This digital information from the source file is taken by the sound chip and sent to the DAC (Digital to Analog Converter).  The higher the quality of DAC that is used, the more accurate the conversion is.  A low quality DAC can handle CD audio and make it sound much like it is supposed to, but if given a higher quality data file it will downsample that information and it will provide analog playback that can be fairly inaccurate.  After this conversion the newly formed analog signal is transferred from the sound card to the amplifier in the external speakers.

Digital passthrough puts the onus of accurate digital to analog conversion on an external receiver.  The original file is taken from the source material and literally passed through the sound card without any modification done.  This does not always happen with some sound card hardware, as some conversion is done and the playback is not “bit accurate”.  An example of this is if CD audio is taken by the sound card and upsampled to 16 bit/48 kHz or 24 bit/96 kHz.  This introduces errors into the playback.  If the data sent through the card is bit accurate though, the digital signal is taken by the receiver and converted to an analog signal for the speakers to reproduce.  If the receiver is of sufficient quality, then it will do a better job with the conversion than the sound card.  If the receiver is of medium or low quality, then the conversion might in fact be better performed by the soundcard.

For quite some time the source material and the speakers had more to do with audio quality than the circuitry in between.  As time has passed though, this quality equation has shifted.  This is exceptionally true for PC audio.  PC users are no longer hampered by $5 beige speakers and low quality DAC’s.  Not to mention that DVD Audio and other “lossless” forms of audio playback are now available. 

The X-Mystique Issues

            This card throws some really interesting issues into the mix by its feature set.  Because it can encode an audio file into a Dolby Digital 5.1 stream, it bypasses any issues with its onboard DAC that may come up.  This also allows the signal to be transferred in a pure digital form along a coax or optical cable.  The problem with Dolby Digital 5.1 is that it is a “lossy” encoding process which throws away a lot of information.  DD 5.1 does not sound necessarily bad, but for the purists out there it is not acceptable.  Considering that DD 5.1 has a bitstream of 384 kbps in 6 discrete channels, there can be a lot of information dropped by the encoding process even for a two channel 16 bit/44.1 kHz file.  For the audio enthusiast, keeping things bit accurate is a must, which is why this soundcard also features PCM passthrough in a variety of formats (16 bit/44.1 kHz, 16 bit/48 kHz, and 24 bit/96 kHz).

            This could lead to a problem though.  If the receiver that this digital signal is sent to has low quality DACs, then it might be better for the soundcard to handle the conversion.  Many desktop PC speakers that do feature a built-in decoder often rely on inexpensive DACs and decoding hardware.  For example, the Logitech Z-680’s that I use for testing have fairly mediocre quality DACs and a low end decoder chip.  While the speakers themselves are great, and the amplifier is pretty outstanding, the decoding properties of this product are not all that great.  When using the pure analog inputs from a high quality sound card, these speakers really come alive.

In this rather unoriginal shot, we see the old OPAMPS still sitting quietly in their sockets, while the new OPAMPS are anxiously awaiting their installation.

            The CMI-8768+ chip that the X-Mystique uses features on onboard DAC that can handle up to a 24 bit/96 kHz signal.  This is not exactly a high quality DAC as it cannot handle a 24 bit/192 kHz signal.  For most applications though, it is clearly enough.  The only problem is that it would not be bit accurate in playback of DVD Audio at 24 bit/192 kHz.  There is a lot of debate about the overall quality of this built in DAC, but for the scope of this article this point is moot.

            The X-Mystique uses 6 fairly low quality OPAMPS, which take the signal from the DAC and amplify it in an analog form.  Operational Amplifiers are actually quite complex for such a small chip, and the better quality OPAMPS that are used, the better the external amplified signal will be.  Typically the signal sent from a DAC is not strong enough to get a clean signal to an external amplifier, so it has to be sent through OPAMPS.  If we were to merely amplify this signal, often we would introduce errors and artifacts (noise).  Higher quality OPAMPS achieve a greater signal to noise ratio (SNR) by the use of negative feedback as well as increased gain (the higher the gain, the better the SNR because the noise will be a lower percentage of the overall signal).

            Since the main thrust of the X-Mystique is to provide a comprehensive set of digital outputs, the decision to use cheaper and lower quality OPAMPS is a simple one.  Since the majority of users of this card will simply plug a coax or optical cable into the card and use the PCM and DD 5.1 settings, analog circuitry took a backseat to price.  The analog outputs on the X-Mystique are really not that great, and I noticed a pretty severe degradation of audio quality when using them vs. PCM or DD 5.1.  Many users of this card have noticed the same thing, and are often quick to blame the integrated C-Media DAC.

Something to recognize here is that C-Media has not yet released an ASIO driver for their sound chips.  This means that there is signal manipulation being done through the Windows Mixer.  When using an ASIO enabled sound card, the signal goes directly from the source to the sound card’s outputs, and no mixing is done at all.

            Being the curious person I am, I decided to splurge and pay for a full set of higher quality OPAMPS.  The ones in question are the Burr Brown/Texas Instruments OPA2134PA.  These are 2 channel, ultra low distortion, ultra low noise OPAMPS designed for high end audio applications.  I was unable to discover the performance characteristics of the stock OPAMPS, but needless to say they are probably quite poor as compared to the Burr Brown/TI chips.

            Using higher quality OPAMPS will also allow for greater consistency across 2+ channels.  This is not only good for 6 channel playback, but it will also improve 2 channel playback.  The standard OPAMPS for this card appear pretty sloppy even across 2 channels.

Next: Impressions

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