Non Over =
Sampling USB=20
DAC
The standard arrangement =
for a non=20
oversampling DAC has been to use a CS8412 or a CS8414 as a =
receiving chip=20
to convert S/PDIF into I2S which is fed into a DAC chip =
like a=20
TDA1543. However, to use this setup with USB audio, one would need =
to use=20
something like a PCM2902 to=20
convert USB to =
S/PDIF to=20
then be fed into a CS8412 to convert the signal to I2S. =
While=20
digital conversions should be lossless, this clearly offers too =
many=20
opportunities for jitter and noise to creap into the signal. A =
better=20
solution, it would seem, would be to use a USB audio chip that =
could=20
output I2S directly. And indeed TI offers a pair of =
chips, the=20
PCM2706 =
and=20
PCM2707, that can do just that.=20
A quick search of the DIYAudio.com forums dug up a =
thread=20
with a basic schematic of how to implement the PCM2707.
I decided to simply couple this to a non-filtered passive I/V =
stage as=20
well as a small power supply and cram it onto a ExpressPCB=20
sized circuit board.
 passive=20
I/V For the power supply, I copied the AC filtration from Tangent's =
STEPS, but I =
left off=20
some of the filtering stages to save space. One of the =
transformer's=20
secondaries is dedicated to the PCM2707 and is regulated by a =
LM7805 and=20
reregulated by a higher quality 3.3V regulator. The other =
secondary is for=20
the DAC chip, and is first regulated by a LM7809 and reregulated =
by a high=20
quality 8V regulator. Additionally, every power connection to the =
PCM2707=20
is run through a ferrite bead and decoupled with a 0.1uF ceramic =
cap. The=20
TDA1543 is decoupled with a 0.1uF film cap.
Digital connections from the PCM2707 to the TDA1543 go through =
a small
resistor and a small inductor. Last, the USB host connection is =
filtered=20
with a small inductor.
Below is a picture of the circuit board file (click it for a =
larger=20
version.) The actual size is 3.5"x2.8".
Below are several images of completed and partially completed =
boards. =20
These are an earlier version of the PCB design and are thus =
slightly=20
different.
Okay, here are the rules for the use =
of these=20
board files...
The board files are licensed by=20
me to you under a creative=20
commons license. The gist of the license is that you may only =
use=20
these files and/or boards produced from these files for =
non-commercial=20
use, and any use or derivative use must give attribution. You may =
not make=20
boards from these files, or from derivative files, and sell them =
for any=20
more than you paid nor may you sell a completed board for any more =
than=20
the cost of materials (i.e., the cost of a board plus the cost of =
the=20
parts to populate it.) Further and in addition to the creative =
commons=20
license, these boards may not be produced in quantities greater =
than 3 at=20
a time without contacting me for permission. This means no large =
"group=20
buys" are allowed. Any boards produced with these files, or with=20
derivative files, must contain "http://diyaudio.ecp.cc" and a =
copyright=20
notice. This must be etched on the board such that it is readable =
on a
completed board (i.e., in copper on the ExpressPCB boards if =
there is no=20
silkscreen and not covered by parts.) Finally, these files are=20
distributed in the hope that it will be useful, but WITHOUT ANY =
WARRANTY;=20
without even the implied warranty of MERCHANTABILITY or FITNESS =
FOR A=20
PARTICULAR PURPOSE.
To download the board files, click=20
here
Assembly Tips
There are a =
number=20
of tips for assembly that might prove helpful.
- There are a couple of components (the USB jack and the =
crystal in
particular) with metal bodies that can short traces on the =
board due to
there not being a soldermask on the boards. To prevent a =
problem, I=20
suggest finding some sort of insulating material (insulating =
pads for=20
mounting components to heatsinks work well) and placing them =
between the=20
component and the board. The leads will easily pierce the =
pads.
- There are two suggested 8v regulators. The AN8008-ND seems =
to sound
better, but is only rated at 50mA. The TDA1543 draws on the =
order of
60mA. I have not had a problem, but if you are worried, use the =
other=20
one.
- The parts listed for C1, C2, and C3 are .082uF caps. The =
.1uF cap in=20
the same series is what you really want, but DigiKey was out of =
them
when I built this.
- C17-24 are a pain. They are also optional. I tried soldering =
them on=20
both before and after D1 and D2, and both methods presented some =
difficulty. If you do the caps first, be careful to not let =
solder fill=20
in the rectifier pads. If you do them second, cut the leads of =
D1 and D2=20
first.
- Many of the parts were chosen because they are low profile. =
In=20
particular, C4 and C5 are the short versions of these caps. If =
you use=20
other caps, be sure that the lead spacing is correct as the =
boards are=20
pretty tight, and a fatter cap will not fit.
- To add the fuse clips, put the fuse into two clips, then =
solder the
clips to the board. If you simply solder the clips, the fuse =
may not
later fit.
- While the bulk of the board is low voltage, do not let =
this=20
lull you into a sense of safety. There are places on it that =
are not. =20
They are at wall voltage and could kill you if you touch them. =
Be very=20
careful with this project, and only build it if you are =
experienced with=20
high voltage. Electricity is dangerous.
- Metal standoffs (such as shown in the pictures) may bridge =
the case
to ground or live as the standoffs are wide enough to extend =
beyond the
mounting holes.. The standoffs shown are insulated with plastic =
washers. Similarly, this project will not slide into a case. It =
needs to=20
be mounted via the mounting holes.
- I have found that there is some noise somewhere, most likely =
from
the onboard transformer. At low listening levels, this is not =
really a
problem. However, as you turn things up really loud, the noise =
does
creap in. Using metal film I/V resistors instead of Allen =
Bradleys=20
helped reduce it a bit, but not as much as might be hoped. Also, =
the=20
small Blackgate output caps are quieter than large film caps =
hanging=20
underneith. My sense is that, unless space is a big concern, =
that one=20
might to better using an Amveco 62001 transformer. This allows =
you to=20
mount it off the board by soldering the leads to the pads =
provided for=20
the 70001. This preserves the Baby Steps (tm) circuitry while =
reducing=20
the noise induced by the transformer (just don't forbget to =
twist those=20
leads.) You might even move up to a 62011 or 62021 as the larger =
VA=20
rating might help out the sound a little.
- For informatrion on how to calculate the resistor values in =
the I/V,=20
take a look at this=20
post at DIYAudio.com.
- Other tips and tweaks for TDA1543 based DACs can be found here.
Parts List
Here is the =
parts list. =20
I believe it to be complete and correct, but you should check it=20
carefully. If you are in doubt which part goes where on the board, =
use the=20
board file to check.=20
| Position |
Part Type |
Part # |
Quantity |
Supplier |
| U1 |
USB Receiver |
PCM2707 (296-15276-5-ND) |
1 |
DigiKey |
| U2 |
DAC Chip |
TDA1543 |
1 |
You can usually find these on ebay. Note =
that the=20
TDA1543A does not work. |
| T1 |
Transformer |
70001 (you might also want to try a 62001, =
62011,=20
or 62021 -- see the note above.) |
1 |
DigiKey |
| L1 |
AC Filter Line Choke |
PLK1156-ND |
1 |
DigiKey |
| D1, D2 |
Bridge Rectifier |
W01GDI-ND |
2 |
DigiKey |
| C4, C5 |
Capacitor Electrolytic (1800uF/25V) |
P12382-ND |
2 |
DigiKey |
| C6, C7 |
Capacitor Electrolytic (680uF/10V) |
P12365-ND |
2 |
DigiKey |
| C8, C9, C10 |
Capacitor Film - MKP (0.1uF/63V) |
BC2054-ND |
3 |
DigiKey |
| C11 |
Capacitor Film (0.1uF/63V) This is a =
polyester cap.
The MKP used in C8-10 arguably sounds better (though =
probably not
noticably,) but does not fit the board. You can use it =
instead, but=20
it has to be mounted underneith. |
495-1103-ND |
1 |
DigiKey |
| C1, C2, C3 |
Capacitor XY Type (.082uF/275VAC) |
P11005-ND |
3 |
DigiKey |
| C17, C18, C19, C20, C21, C22, C23, C24 |
Capacitor Ceramic (SMD 0805, =
100pF/50V) |
BC1268CT-ND |
8 |
DigiKey |
| C15, C16 |
Capacitor Ceramic (SMD 0805, 33pF/50V) |
BC1262CT-ND |
2 |
DigiKey |
| C12, C13, 14, C27 |
Capacitor Ceramic (SMD 0805, =
0.1uF/16V) |
BC1300CT-ND |
4 |
DigiKey |
| C25, C26 |
Capacitor (Output, 2.2uF-4.7uF) |
The board is spaced for Blackgate 4.7uF/50V =
N=20
series, but see my=20
capacitor page for some other options. |
2 |
PartsConnexion, Michael Percy =
Audio |
| U3 |
Voltage Regulator (9V) |
497-1448-5-ND |
1 |
DigiKey |
| U4 |
Voltage Regulator (5V) |
LM78M05CTNS-ND |
1 |
DigiKey |
| U5 |
Voltage Regulator (8V) |
AN8008-ND (or AN77L08-ND) |
1 |
DigiKey |
| U6 |
Voltage Regulator (3.3V) |
LP2950CZ-3.3-ND |
1 |
DigiKey |
| X1 |
Crystal 12.000MHz |
CTX058-ND |
1 |
DigiKey |
| X1a (optional) |
Crystal Insulator. If you don't use one of =
these,
you'll need to find some way to keep the crystal from =
shorting=20
traces on the board. |
HC49INS-ND |
1 |
DigiKey |
| L2, L3, L4 |
Inductor (1uH, SMD 0805) |
PCD1184CT-ND |
3 |
DigiKey |
| L5, L6, L7, L8, L9, L10 |
Ferrite Bead (SMD 0805) |
445-1567-1-ND |
6 |
DigiKey |
| R1 |
AC Filter Resistor 1M, 1/4W |
. |
1 |
. |
| R2, R3 |
AC Filter Resistor 100R, 1/4W |
. |
2 |
. |
| R4 |
LED Resistor 680R 1/8W (or Panasonic =
1/4W) |
. |
1 |
. |
| R5 |
LED Resistor 480R 1/8W (or Panasonic =
1/4W) |
. |
1 |
. |
| R6, R7 |
Output Resistor 47K, 1/4W |
. |
2 |
. |
| R8, R9 |
I/V Resistors. (configuration 1 =3D 1K, =
1/4W)
(configuration 2 =3D 2.7K, 1/4W) (Different values and =
types will=20
change the sound, sometimes for better, sometimes for worse. =
Be sure=20
you know what you are doing if you change these.) |
. |
2 |
You can get these from DigiKey, but high =
quality=20
Metal Films (PRP, Draloric) or Carbon Comps (AB, Riken, =
Kiwame) are
recommended and sound better. |
| R10 |
Bias Resistor (configuration 1 =3D 1K, =
1/4W)
(configuration 2 =3D 1.4K, 1/4W) (Different values and =
types will=20
change the sound, sometimes for better, sometimes for worse. =
Be sure=20
you know what you are doing if you change this.) |
. |
1 |
DigiKey |
| R11, R12, R13 |
100R (0805) |
. |
3 |
. |
| R14, R15 |
1M (0805) |
. |
2 |
. |
| R16, R17 |
22R (0805) |
. |
2 |
. |
| R18, R19 |
1K5 (0805) |
. |
2 |
. |
| J1 |
USB Jack |
154-2442 |
1 |
Mouser (There do not seem to be any =
available at=20
DigiKey that will work) |
| J2 |
Terminal Block |
ED1623-ND |
1 |
DigiKey |
| F1 |
Fuse Clip |
F058-ND |
2 |
DigiKey |
| F1a |
Fuse 5x20mm (100mA/250V) |
WK5027-ND |
1 |
DigiKey |
| J3 |
IEC |
486-1003-ND |
1 |
DigiKey |
| J4 |
RCA Jacks |
. |
pair |
PartsConnecion, Michael Percy=20
Audio |
|