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
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!
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
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
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
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!