gallery/2

ecpsoftware

articles

diy
audio

links

contact

HPDAC v0.1b2

This is the project page for the
HPDAC v0.1b2. (Updated 12/24/2006)

Background:

The problem, for me, with the available portable amps is that they
aren't really that portable. They all need batteries, and, even worse,
they all need sources which also need batteries as well as cables to
connect them. I wanted something that I could take to a coffee shop (or
the sofa) connect to my computer, and listen without a pile of boxes and
wires next to me. That is, I wanted a usb dac with a built in headphone
amp that was all powered via USB, and I wanted it to sound good. As there
seemed to be nothing of the sort available, I made it myself.

The HPDAC is a non-oversampled (NOS) USB digital to analog converter
(DAC) with a built-in headphone amp. The DAC is based on earlier work of
mine, though it turns out to be a very similar circuit to several
other NOS USB
DAC's. The amplifier section is essentially the same circuit as the Mini3'ified
version of the now discontinued Pint.
The entire system is powered via the USB supply, so there are no
batteries or power cords necessary. The project is also very small and
will fit comfortably into the smallest Hammond extruded case.

Not only are the amplifier and DAC sections of the HPDAC of very high
quality, great care has been taken to ensure that the power derived from
the USB source is clean and free of noise. The power supply utilizes a
highly efficient DC-DC boost converter to deliver enough voltage to the
amplifier and DAC. Further, this boosted power is regulated with high
quality regulators and further cleaned with ferrite beads. This
arrangement delivers clean strong power which translates into clean strong
sound.

Compared to similar projects, the HPDAC provides extraordinarily high
quality sound in a small inexpensive package.

Technical Background:

In order to power the entire project via the USB supply, the HPDAC uses
a DC-DC boost converter to convert the approvimate 5V input to a higher
voltage which is then filtered and reregulated. The exact voltage is
determined by the builder. This power supply scheme presents several
challenges which must be considered while building. Primarily, since the
DC-DC converter is about 70% efficient, and USB can supply about 500mA at
5V, this means that at 10V you have about 170mA at most to work with.
Practicality means that 100 to 120mA is probably generous, and since the
TDA1543 draws up to 60mA, this is a real limit. Second, the USB specs
limit the amount of capacitance on the USB rail unless there is current
limiting. The DC-DC converter does have a slow start feature, and the
voltage regulators do limit current, but this is still worth being careful
with. It is not worth damaging your computer. That said, I have had no
problems, but prudence suggests being aware. As a DIY project, you are the
responsible party. Finally, it should be mentioned, this is a fairly
challenging DIY project. It involves a considerable amount of SMD
soldering and is intened for advanced DIY'ers. As such, the only build
instructions are to select the parts, and assemble from shortest to
tallest. I like to do the tricky stuff first (the PCM2707, the DC-DC
converter, and the REG101) but YMMV.

At the request of some car audio guys, I added one last option: the
HPDAC can be used as a source/preamp in a speaker amp setup. To do this,
you need to remove the option for the buffered ground by not populating
L12 (which is on the bottom of the board) and adding a small jumper
between J1 and J2. To reduce noise a bit, you might try using a ferrite
instead of a wire jumper, though I have not tried this myself to know if
it is a good option -- try it and see. I should warn you that there are no
output coupling caps from the amplifier, so obtaining a vanishingly low DC
offset or using a power amp with input coupling caps is essential. Using
OPA2132 opamps mine had an offset of 0.4mV on each channel. Note that you
should still be able to drive headphones with this setup, but perhaps not
quite as well, though the difference is minimal.

View the
schematic
View the top of the
board
View
the bottom of the board

Late Breaking News

There appears to be a small bug in the schematic and hence in the
board. To date, there have been no reports of problems being caused by it,
but if you are concerned, it is an easy fix. Pin 11 (SSPND) on the PCM2707
is powered while it should not be (it actually provides voltage to turn on
some DAC chips.) To disable this, you can either cut the small trace that
connects pin 10 to pin 11 (it is right between the two pads -- this is
most easily done before the board is built) or, once assembled, use an
X-acto knife to cut pin 11 off of the chip.

Other late breaking news is that if a short occurs, the DC-DC converter
is susceptible to damage. The most likely cause of this is a short at the
headphone jack during insertion or removal of the headphones. As such, it
is recommended that you only plug or unplug the headphones while the unit
is powered off, or at least the volume is turned down. It might be that
this only occurs if the computer does not have a built-in current limiter
to the USB port, but I am not sure about that.

Ordering

Boards are sold out. As of now, there are no plans to order another
run. It is possible that another revision may appear, but not any time
soon. Thanks to everyone who ordered one and participated in this project.

Samgotit's
comments at Head-Fi (regarding the v0.1a board with NJR4556
opamps)...
The first revision was a huge surprise considering
its size and power supply. I found Bass and "Weight" I have never heard
before out of the K701. This includes through all of the non-portable
gear I own! There were moments when the bass was... "concentric rings in
the water cup"... spectacular. Understand, I'm not a huge bass head, but
the new sound was very addictive.

Parts List

Part

Value/Notes

Size/#

C1

This is the main power decoupling
cap for the DAC chip. 47uF is probably a decent value to use though
some people like to use larger. Quality counts in this spot, so an
Os-Con, Cerafine, or Blackgate is a good idea. Digikey has the
Silmic II or the Panasonic FM series which are also good choices.

3.5mm pin, 8mm width

C2 (A, B, C,
D)

These are the main power supply
caps. They provide some filtering, but mainly seem to provide
instantaneous power for the amplifier section. There is a dilemma
here where higher capacitance will offer better sound, more bass,
etc., but the USB standard limits the total amount of capacitance
that can be used safely The HPDAC does limit the inrush current both
with a slow starting DC-DC converter, and a voltage regulator that
limits the current. Exactly what value capacitance to use is the
responsibility of the builder to determine. 470uF is probably on the
high end, 4x47uF high quality caps is perhaps on the low end? I
would say that 4x 47uF Silmic II caps from Digikey (part
#604-1054-ND) is probably a decent place to start, but this might
also be a good place to experiment. You only need to populate one
position but can do all 4 if you want.

up to 4 caps with 3.5mm pin, 8mm
width

C3, C4

C3 is the input cap for the DAC's
regulator. C4 is the regulator's output cap as well as the
decoupling cap for the DAC. Wima MKP2 0.1uF/63V caps (or their
equivalent from other manufacturers) are recommended, but polyesters
will do in a pinch. The Mouser part number for these caps is
505-MKP20.1/100/5

5mm pin, 5.5mm x 7.5mm

C5, C6

33pF ceramic

1206

C7-C19,
C26

0.1uF (100nF) ceramic (but see
the note about C8 below in the VR3 section as in some cases it needs
to be significantly higher in value.)

1206

C20

4.7uF Tantalum
Assembly
Note: Tantalum caps are polarized, and unlike electrolytics have
their positive side marked. In this case, the positive side is the
one furthest from the USB jack side of the board.

1206

C21

.01uF (10nF) ceramic, but see the
note about this cap below in the VR3 section.

1206

C25

4.7uF Tantalum
Assembly
Note: The positive side is the one nearer the board's edge (furthest
from the label.)

1206

C27, C28

Optional, 10pF Silver Mica or
Film. In general, these should only be used if there are oscillation
problems, and then only as a last resort. The amp sounds better
without them.

1210

CR, CL, CG

These are the AC coupling caps
that connect the DAC and the amplifier sections. High quality is
essential as the entire signal passes through them. The cheapest I
would use would be 3 x 4.7uF Nichicon ES bipolar caps which you can
get from me. You could also use 4.7uF Blackgate N series caps here.
The Blackgates cost significantly more. They do sound better (more
detailed, less grain, maybe a little less bass but more balance) but
not necessarilly a ton better. (You can see my comments about these
caps at http://www.ecp.cc/cap-notes.html.)
My sense is if you have $200 + headphones, go with the blackgates,
otherwise don't worry about it. If you do use the Blackgate caps
here, give them minimum 100 hours to break in. I know this sounds
silly, but it took mine in this design at least that to develop any
bass at all. Both Panasonic and Nichicon make cheaper bi-polar
electrolytics that are available from Digikey/Mouser, but they are
not recommended as they sound awful. If you choose to use polar
caps, the "+" on the board is meaningless, so be sure to get the
direction right -- either by measuring or by following the
schematic.

2,5mm pin 6mm case

L1

There is no L1.

.

L3-L12

These are ferrite beads used to
help clean up the power supply. Specific parts are not essential.
MI1206K260R-10 works fine.

1206

The
following parts (DC1, C22, C23, C24, R5, R6, L2, D1) make up the
boost switching power supply. I have included a suggestion for parts
which provides about 9.5V and which has proved to be stable.
However, there is no substitute for reading the DC-DC converter's
datasheet.

DC1

This is the DC-DC converter. Use
a TPS61040. As this is an important element of the project, it is a
very good idea to read over the datasheet.
Also note that Digikey shows this as out of stock, but if you add
it to your cart you will see that they have several thousand, so no
worries.

TPS61040

C22 (Cff on
the datasheet)

100pF ceramic

0805

C23 (Co on
the datasheet)

1uF tantalum or
ceramic.
Assembly Note: If you use a polarized tant here, the
positive side is the one furthest from the board's edge. I have only
used ceramic caps, so I can't comment on the difference.

1206

C24 (Cin on
the datasheet)

4.7uF Tantalum
Assembly
Note: This is a very small part. The positive side is the one
furthest from the "C24" label.

0805

L2 (L1 on the
datasheet)

ELJ-FA100KF

1206/1210

R5 (R2 on the
datasheet)

R5 and R6 set the ouput voltage
if the DC-DC converter. The chosen values will give you about 9.5V.
If you want more or less, read the datasheet for how to determine
these resistors' values.
100K

R1206

R6 (R1 on the
datasheet)

634K (see note above)

R1206

D1 (D1 on the
datasheet)

MBR0530T1G, or perhaps
MBR0530TPMSCT
Assembly Note: D1 is polarized. The cathode (the
marked end) is the one furthest from the USB jack side of the board.

SOD123

The
following parts (R1L and R to R4L and R and Pot) are the resistors
in the amplifier section. If you are using an unstable "cranky"
opamp, several things need to be balanced:

The gain of the amp section is determined by R3* and R4*
(R4/(R3 + 1))

R1* in parallel with R2* should equal R3* in parallel with R4*

R2* should be 10X the value if the pot

The suggested
values below are simply one such configuration. If you are using a
stable opamp (usually a FET input part) then think of the amp
section as a basic CMoy to determine part values.

Pot

This is the potentiometer. It
sets the input impedence of the amplifier section as well as
adjusting volume. 10K is the default value here. Increasing the size
of the pot means also increasing R2L and R2R which could increase
the noise level a little. Probably not enough to worry about, so 20K
or 50K is fine but 10K is preferred. EVJ-C20F02D14 is the default
recommendation.
Assembly Note: The pot has very small spacers
built in to it. Using a wire cutter, you should trim these off as it
will fit the case a little better.

Panasonic EVJ

R1L, R1R

1K This may be jumpered in some
configurations -- if in doubt, leave it in.

1/8W Axial

R2L, R2R

100K for a 10K pot, 200K for a
20K pot, etc. (this may be optional in some configurations, but if
used it should be 10X the value of the volume pot.)

1/8W Axial

R3L, R3R

1K2

1206

R4L, R4R

6K2

1206

R1, R2

1M

1206

R3, R4

These form the voltage divider
for the amp's virtual ground. 47K is appropriate. A little higher
or lower should be fine.

1206

R7, R8

22R

1206

R9, R10

1K5

1206

RBCK, RD,
RWS

These connect the USB receiver
chip to the DAC chip and are used to help clean the digital signal.
Anything from 22R to 100R should be fine with 22R being the default
recommended value.

1206

RIVL, RIVR,
REF

These are the so-called I/V
resistors that convert the current output from the DAC to voltage.
Quality counts here more than usual with different resistors
offering very different sound. If you use metal film, use high
quality resistors like PRP or older Holco. Dale's are fine though
they are not my favorite as are Draloric. You can also use Allen
Bradley or Stackpole for a fuller sound. Rikens are a great choice,
though they will be very tight so must be flush with the board --
Kiwame's won't fit. There is also space on the bottom of the board
for 1206 sized resistors which might be a decent option.
For what
it's worth, my impression of various resistors follows: I find the
PRPs to be very clean and sweet sounding, but to lack a little bass
and be a little cold. They make the HPDAC sound a little more
solid-statey with a lot of crisp detail. The PRPs (and really any
metal film resistors) will sound a litle bit sheeny in the way that
a lot of high end solid state gear does. If you like that sort of
sound, then this is what you want here. Steel string acoustic
guitars sound very real to me with these. Other metal films like
Dales or Panasonics, or other generic resistors are similar but lack
the sweetness.
Allen Bradleys (and other vintage carbon
composition resistors) are the polar opposite. They are warm and
pillowey and very euphonic. They offer a thick warm sound. They are
a little noisier and you might hear it as background hiss at high
volumes, though there is some hiss anyway due to the PS, so the
addition is probably not noticable.. You can get these at various
places, including Handmade electronics, Nebraska Surplus, and a few
other places.
Riken Ohms are in the middle. While perhaps closer
to the ABs in tonal balance, they have all the detail of the PRPs
and also do not have the noise problems of the vintage resistors.
They are, however, expensive.
One last resistor to think about
are Tantalum resistors. These are my favorite I/V resistor offering
a very natural organic sound. I use them in my personal HPDAC
(though I use the ABs in my secondary one.) They are really
expensive, though and are available from Parts Connexion and
Angela.
Also, the REF resistor is considerably less important to
the overall sound, so a generic metal film is okay there.

Optimal value is tricky here. This is in part because it depends
upon what voltage you give the DAC chip (I'll assume 5V here) but
also because they set the output impedance of the DAC section and
influence the amplifier section. If you are using a cranky high
current opamp, then you will probably be using a 10K pot which means
that you want the output impedence of the DAC to be 1K or lower.
Thus, in this case, 1K is recommended. If you are using a less
cranky opamp, then you can use a higher value pot and thus a higher
value I/V resistor. If this is the case, then you will need to do
some reading and or experimenting to find the optimal value. 2.7K
for the I/V's and 1.4K for the Ref is probably a decent place to
start, though you may want 8V for this option. Anyhow, as noted, 3 x
1K work well with a 5V supply.

up to 1/2W Axial (lower W rating
is fine) or 1206 SMD

RLED

Sets the LED current. Anything
from 470R to 1K is fine, lower will lead to a brighter LED and will
drain more power.

1206 SMD

PCM

This is the chip that converts
from USB to something the DAC chip can use (I2S). The board was
designed with the PCM2707 in mind, but the PCM2706 also works just
fine.

PCM2707

TDA1543

This is the DAC chip and is
supplied with the board. In theory, you could stack these*, but if
you decide to do this be very cautious about current draw.
Personally, I wouldn't do it. Also, don't put the chip in a socket.
Considerable attention has been paid to layout such that the
digital signal to the chip is as clean as possible. The socket is
counterproductive and will audibly degrate the sound quality.
*
Some people think this sounds better, though I don't. It seems to
trade some midrange magic for greater extension at the frequency
extremes. The idea is that on any given bit a DAC can make an
error. By stacking chips you increase the chance that there is some
error on any given bit, but decrease the amount of that error.

8 pin DIP

OP1, OP2

These are the opamps for the
amplifier section. OP2 provides voltage gain while one half of OP1
creates a virtual ground while the other half buffers the ground. In
general, since there are no buffers, at least for low impedence
headphones, you want high current opamps. This typically means
something that has some amount of instability.* The amp section is
designed to handle this instability in all but the worst cases.
Thus, opamps like the NJR4556 of the LM6172 are appropriate. (For
what it is worth, I have found the NJR4556 to have very good bass
and dynamics but to be a bit harsh on the top end. Conversely, the
LM6172 is very smooth and open on the top end but does not have
quite as much bass slam, though there is plenty for most purposes.)
The default recomendation is thus the LM6172. It is cheap and
sounds very very good. My sense is you should have a good reason for
using something else, but then, it is your amp so you maybe
shouldn't listen to me. For headphones that do not need as much
current, less cranky opamps (OPA2132, AD8620) may work well. Also,
if you plan to set this up as a preamp (this is a note to the car
audio guys!), using a less cranky fet input opamp like the OPA2132
or AD8620 might be a good idea as the offset should be slightly
lower.
* A full vetting of this topic is well beyond the scope
of this parts list. The amp section is based upon the Mini3'ified
version of the now discontinued Pint amp. However, while those amps
concentrate in the AD8397, that particular part draws enough current
that its use here might be problematic. If you want to use it, be
sure you know what you are doing. I have not tried it, so I can't
really offer any advice. I have, however, tried the AD8397 in other
circuits and to my ear the LM6172 is the better sounding opamp
anyway. The high current opamps (the ones mentioned above) that are
perhaps more appropriate here are a bit more stable than the 8397
making them a little easier to work with.

2x Dual 8 pin SMD

VR1

This is the voltage regulator for
the amplifier section. My suggestion is that you want something that
limits current to avoid damaging the USB bus in the computer either
due to the inrush of C2 or due to a short somewhere. You also want a
LDO regulator so that it can be as close to the output of the DC-DC
converter as possible to avoid wasting voltage. So, if you used the
suggested values for R5 and R6, then an 8V LDO regulator with 150mA
current capability is perfect here. TL750L08CLP works quite nicely.

TL750L08CLP

VR2

This is the voltage regulator for
the TDA1543. The part you use will depend on a number of factors.
First, you must give it a high enough voltage on the input to work
and it's output voltage must fall within the range specified by the
TDA1543. This means that the output must be between 5 and 8 volts.
It also means that, depending upon where you set the DC-DC
converter's output you might need a LDO regulator here. While high
voltage does change the sound of the TDA1543, it is not necessarilly
for the better, though it is not necessarilly for the worse either.
The default choice would be the LM78L05 as it is stable and easy to
use and cheap. However, some people swear by the AN8008 and AN8005
Vregs for the position next to the 1543. My own experience is that
these do indeed sound pretty good. However, they are only rated for
50mA and the 1543 datasheet says that it can draw up to 60mA, so
they might not be the best choice. On the other hand, they might be
worth considering.

LM78L05ACZFS-ND

VR3

This is the voltage regulator for
the PCM2707. There are a couple of ways to set this up. The
PCM2707/2706 has built in internal voltage regulators. They are
enabled by supplying the Vbus pin with 5V. When this is done, then
the other power supply pins need to be decoupled externally, and
have a measured 3.3V on them whic his internally supplied.
Alternatively, you can diasable the the internal regulators by
supply 3.3V to the Vbus pin and then supply 3.3V to all of the power
pins youself.

So, there are 3 ways to set this up:

The simplest (and cheapest) is to use the PCM's internal
voltage regulator. To do this, do not populate C7, C8, C21, L6,
and L10. Instead of using a regulator for VR3, simply jumper from
the input to the output pins.

To supply 3.3V externally, use a 3.3V REG101 for VR3 and
populate C7, C8, C21, L6, and L10 as per the normal instructions.

If you like to live on the edge and want to use dual
regulation of the PCM, you can use a low dropout voltage
regulator for VR3 to supply something on the order of 4.5 to 4.75V
(the choice may depend on what voltage your USB supplies). In this
case you should populate C7, C8, and C21 according to the voltage
regulator's datasheet, but you should not populate L6 and L10 as
you will be using the PCM's internal regulators.

The
$20,000 question here is, does this make any difference, and the
answer is, I have no idea. I have been using the 3.3V REG101 option
as my default, but the simple cheap option seems fine even though it
is theoretically noisier. I have not tried the 3rd option, so I have
no opinion there.

SOT23-5 foot print

LED1

Any 3mm LED should be fine. This
part can be left out if you don't like LED's. It only indicates
that usb power is connected, not that anything is
working.
Assembly Note: The anode (the longer lead) goes
closer to the "RLED" label.

3MM

Q1

The 12MHz crystal oscillator for
the PCM2707. Part number MP120 (CTX058-ND) from Digikey is fine
here.

HC49

USB

Part #571-7877801 from Mouser is
suggested. I have tried a lot of USB jacks and these have become my
favorite as the USB plug clicks in and stays stable.

571-7877801

OUT

CP1-3513-ND (SJ1-3513) from
Digikey is the best option. However, 161-3507 from Mouser also fits,
though it it a little crooked. I was also at Radio Shack the other
day and noticed that they have a jack that should fit -- it looked
the same as the Digikey jack to me.

CP1-3513-ND

Case

The board was designed to fit in
a Hammond 1455C802. Digikey seems to have some of these in stock,
but if they run out Mouser stocks them as well.

1455C802

Knob

You will probably want a knob of
some sort. Smaller is better here as otherwise it will extend beyond
the edge of the case. Digikey has a bunch to choose from.
(226-1025-ND, 226-1029-ND, 226-1033-ND, 226-1041-ND)

.

Some build notes and options:

In general, this project like most PCB based projects should be
built from shortest to tallest parts. In addition, I have found that a
good order is to first add the most difficult parts (the PCM2707, the
REG101, the DC-DC converter, and the opamps), then add the smd parts on
the top of the board, and then add the rest of the surface mount parts
to the bottom, then do the tall stuff.

If desired, one can build this project as only a NOS USB DAC leaving
off the headphone section. In addition, CG should be left out and the
output ground should be from true ground. There will be no cap bleeding
resistors on the output, so the next stage will need resistors (10K to
100K) from input to ground (usually a volume pot.)

Special thanks go to Tangent without whose Pint this project would not
have happened, at least not in the form it happened in. Similarly, Amb and
Morsel's modification of the Pint and work on the Mini3 proved very
helpful. It is probably worth looking at the Pint
project page for understanding the issues with the amplifier section
though since I suggest you stay away from the AD8397, some of those
issues are not that important.