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Non Over Sampling USB
DAC

The standard arrangement for a non
oversampling DAC has been to use a CS8412 or a CS8414 as a receiving chip
to convert S/PDIF into I2S which is fed into a DAC chip like a
TDA1543. However, to use this setup with USB audio, one would need to use
something like a PCM2902 to
convert USB to S/PDIF to
then be fed into a CS8412 to convert the signal to I2S. While
digital conversions should be lossless, this clearly offers too many
opportunities for jitter and noise to creap into the signal. A better
solution, it would seem, would be to use a USB audio chip that could
output I2S directly. And indeed TI offers a pair of chips, the
PCM2706 and
PCM2707, that can do just that.

A quick search of the DIYAudio.com forums dug up a thread
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
well as a small power supply and cram it onto a ExpressPCB
sized circuit board.

passive
I/V

For the power supply, I copied the AC filtration from Tangent's STEPS, but I left off
some of the filtering stages to save space. One of the transformer's
secondaries is dedicated to the PCM2707 and is regulated by a LM7805 and
reregulated by a higher quality 3.3V regulator. The other secondary is for
the DAC chip, and is first regulated by a LM7809 and reregulated by a high
quality 8V regulator. Additionally, every power connection to the PCM2707
is run through a ferrite bead and decoupled with a 0.1uF ceramic cap. The
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
with a small inductor.

Below is a picture of the circuit board file (click it for a larger
version.) The actual size is 3.5"x2.8".

Below are several images of completed and partially completed boards.
These are an earlier version of the PCB design and are thus slightly
different.

Okay, here are the rules for the use of these
board files...
The board files are licensed by
me to you under a creative
commons license. The gist of the license is that you may only use
these files and/or boards produced from these files for non-commercial
use, and any use or derivative use must give attribution. You may not make
boards from these files, or from derivative files, and sell them for any
more than you paid nor may you sell a completed board for any more than
the cost of materials (i.e., the cost of a board plus the cost of the
parts to populate it.) Further and in addition to the creative commons
license, these boards may not be produced in quantities greater than 3 at
a time without contacting me for permission. This means no large "group
buys" are allowed. Any boards produced with these files, or with
derivative files, must contain "http://diyaudio.ecp.cc" and a copyright
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
silkscreen and not covered by parts.) Finally, these files are
distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.

To download the board files, click
here

Assembly Tips
There are a number
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
suggest finding some sort of insulating material (insulating pads for
mounting components to heatsinks work well) and placing them between the
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
one.

The parts listed for C1, C2, and C3 are .082uF caps. The .1uF cap in
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
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
in the rectifier pads. If you do them second, cut the leads of D1 and D2
first.

Many of the parts were chosen because they are low profile. In
particular, C4 and C5 are the short versions of these caps. If you use
other caps, be sure that the lead spacing is correct as the boards are
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
lull you into a sense of safety. There are places on it that are not.
They are at wall voltage and could kill you if you touch them. Be very
careful with this project, and only build it if you are experienced with
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
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
helped reduce it a bit, but not as much as might be hoped. Also, the
small Blackgate output caps are quieter than large film caps hanging
underneith. My sense is that, unless space is a big concern, that one
might to better using an Amveco 62001 transformer. This allows you to
mount it off the board by soldering the leads to the pads provided for
the 70001. This preserves the Baby Steps (tm) circuitry while reducing
the noise induced by the transformer (just don't forbget to twist those
leads.) You might even move up to a 62011 or 62021 as the larger VA
rating might help out the sound a little.

For informatrion on how to calculate the resistor values in the I/V,
take a look at this
post at DIYAudio.com.

Other tips and tweaks for TDA1543 based DACs can be found here.

Parts List
Here is the parts list.
I believe it to be complete and correct, but you should check it
carefully. If you are in doubt which part goes where on the board, use the
board file to check.

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
TDA1543A does not work.

T1

Transformer

70001 (you might also want to try a 62001, 62011,
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
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
series, but see my
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
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 = 1K, 1/4W)
(configuration 2 = 2.7K, 1/4W) (Different values and types will
change the sound, sometimes for better, sometimes for worse. Be sure
you know what you are doing if you change these.)

.

2

You can get these from DigiKey, but high quality
Metal Films (PRP, Draloric) or Carbon Comps (AB, Riken, Kiwame) are
recommended and sound better.

R10

Bias Resistor (configuration 1 = 1K, 1/4W)
(configuration 2 = 1.4K, 1/4W) (Different values and types will
change the sound, sometimes for better, sometimes for worse. Be sure
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
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
Audio