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Engineering Projects: VTB AudioTron Hacking : Audio Stage

An Introduction...

It should be noted that there are some fairly harsh criticisms of VTB's product on this page. Many might argue that such points don't make any sense given the context of the product, which is playing back lossily-compressed audio. However, I hate to see compromises in such an important place in an otherwise good product. Take the gripes on this page with an appropriate grain of salt. If you are displeased with the quality of audio output on your unit, it's probably worth considering an external DAC, or at least a digital connection into your receiver. While modifications of VTB's design are certainly possible, anything that would seriously improve the audio quality will be beyond the skills of most folks. Making changes to high-density surface-mount components, like the AudioTron's DAC, is not a task to be taken lightly. Not to mention, it's close to impossible to do properly without the right (i.e. expensive) tools such as hot air rework stations, etc.

DAC and Digital Output Transmitter

[audio chips image] [audio chips image]
Crystal Semiconductor CS43L43 Stereo DAC with Headphone Amplifier. My complaint about this part is that it does not offer a non-amplified output. This means that the analog outputs on the rear of the unit are compromised from the start by a DAC with a presumably marginal amplifier on-board. This Crystal part is targeted at portable devices, not home stereo components!
Crystal Semiconductor CS8405A 96kHz Digital Audio Interface Transmitter. No complaints here. This is a decent part with solid performance. The "problems" people have been seeing where the S/PDIF output will not play streams of certain sampling frequencies is an inherent limitation of S/PDIF. Other than re-sampling the output stream (which introduces its own artifacts), there's no easy way around this one.
I believe the SOT-23 package labeled "78N03" is a 3V linear regulator. Perhaps this is a dedicated non-switched supply for the output DAC. If that's the case, it's one of the first plusses of the audio design I've found.

Audio Is Routed Off-Board

[audio lines routed off-board] [audio lines routed off-board] [headphone jack wiring]

My next big complaint about the audio output section is that the audio is routed off of the board via JP1, to the headphone jack in the front of the unit. VTB chose to use the "automatic" switching type 1/4" stereo jack for the headphone plug. When you plug in headphones, it disconnects the RCA outputs. This is strange behavior indeed for a piece of gear intended to replace/complement your CD player. It would seem that the VTB engineers did this because they make the DAC do double-duty - it drives the headphones as well as the line outputs on the rear of the unit. If they hadn't chosen this switch approach, it would have sounded even worse, due to all sorts of strange impedence interplay between the gear connected to the line outs and the headphones plugged into the front of the unit.

What gets me even more is that, even though the cables going off-board are braid-shielded (not 100% foil, as far as I saw), the cables pass directly under the 110VAC power inputs on their way to the headphone jack! I'm sure some amount of 60Hz hum is being induced along the way there.

My suggestion would be to build a small shunting jumper that eliminates the run of cable going to the front panel of the unit. This should improve audio quality at least slightly, at the expense of the marginal quality headphone output.

Unpopulated Feature - Coaxial Digital Out

coaxial S/PDIF]
In addition to the optical TOSlink output provided for S/PDIF, it appears that VTB designed in (likely verbatim from the CS8405A evaluation board) the option for coaxial S/PDIF. The unpopulated components T1, R6, R7, C13, J3 (and perhaps others in the vicinity) would appear to make up the coaxial digital output circuit. Look at page 12 of the CS8405A evaluation board data sheet for the schematic of the output stage. It seems likely that VTB copied this, save for the AES/EBU output option.

Voyetra, Turtle Beach, AudioTron, and likely a whole bunch of other things are probably trademarks of their respective owners. I do not try to lay claim to any of them!

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