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A balanced output board
for the Stereo DAC
By NICHOLAS VINEN
This add-on board is designed to provide a pair of balanced
audio outputs for the High-Quality Stereo DAC (Sept-Nov
2009). Two 3-pin male XLR connectors are used for the new
outputs and they can either replace or augment the existing
unbalanced outputs without affecting their performance.
B
ALANCED AUDIO is used in
recording studios and on stage because of its improved noise immunity.
This is due to the fact that the signal
is sent differentially (ie, as two signals
180° out of phase) and then converted
to a single-ended voltage signal at the
far end.
If any noise is picked up in the cable,
it affects the two out-of-phase signals
equally so that when the signals are
subsequently subtracted, most of the
noise is eliminated.
In addition, the DAC’s performance
at the balanced outputs generally exceeds that of the unbalanced outputs,
although only by a small margin.
The signal-to-noise ratio, frequency
Table 1: Balanced/Unbalanced Output Performance Comparison
Measurement
THD+N, 1kHz
SNR (unweighted)
SNR (A-weighted)
Frequency Response, 20Hz-20kHz
Channel Separation <at> 1kHz
Channel Separation <at> 20kHz
42 Silicon Chip
Unbalanced
0.00090%
-108dB
-114dB
+0,-0.15dB
-105dB
-73dB
Balanced
0.00095%
-112dB
-116dB
+0.02,-0.05dB
-115dB
-111dB
response and channel separation are
all better, although we measured a tiny
bit more distortion from the balanced
outputs. However, both levels are so
low as to be almost negligible.
Performance
At this point, it is worth mentioning
that during the development of this
board, we used a new source of digital
sinewave data for distortion measurements. This revealed that the DAC
is capable of lower distortion than
originally quoted. With a 44.1kHz 16bit computer-generated sinewave, the
THD+N at 1kHz is 0.0012% and with
a 48kHz 20-bit sinewave the THD+N
is 0.0009%.
These measurements are only
slightly higher than the distortion
siliconchip.com.au
V+
10nF
22pF
1 F
560
910
820
3
BP
2
100k
5.6nF
7
8
IC1
1nF
100nF
5
100
6
4
2.2nF
LEFT IN
0
L+
V–
LEFT
OUT
CON1
XLR
1
3
L-
2
V+
10nF
22pF
1 F
560
910
820
3
BP
2
100k
5.6nF
7
8
IC2
1nF
100nF
5
100
6
4
2.2nF
V+
0
V–
IC1–IC4:
NE5534 OR OPA134
V+
910
820
3
BP
2
100k
5.6nF
7
8
IC3
1nF
1
V–
100nF
5
100
6
4
2.2nF
RIGHT IN
0
R+
POWER
IN
GND
–
100 F
22pF
560
+
2
10nF
1 F
3
100 F
V–
1
3
R-
2
RIGHT
OUT
CON2
XLR
V+
10nF
22pF
1 F
560
910
820
BP
3
2
100k
5.6nF
1nF
7
8
IC4
100nF
5
6
4
V–
SC
2010
100
2.2nF
0
BALANCED OUTPUTS FOR THE STEREO DAC
Fig.1: the incoming differential signals from the the DAC Board are fed to separate passive high-pass filter stages and
then to four active low-pass filter stages based on op amps IC1-IC4. These op amps then drive pins 2 & 3 of the XLR
output sockets via passive low-pass filters based on 100Ω resistors and 2.2nF capacitors.
measured directly from the Audio
Precision System One’s internal sinewave generator (0.0006%) so it’s hard
to say exactly what the actual level of
siliconchip.com.au
distortion is. However, we can safely
say it is very low indeed.
Table 1 shows a performance comparison between the balanced and
unbalanced outputs, measured with
the new 48kHz 20-bit sinewave source.
Note that while the channel separation from the balanced outputs is
January 2010 43
BALANCED RIGHT OUTPUT
100 F
CON2 RLX
XLR
–
CON1
XLR
0
TFEL
-
2.2nF
820
910
NE5534
OPA134
100nF
1nF
RIGHT
IN
L
L–
L+
IC4
100k
560
1nF
IC3
R
1 F BP
10nF
22pF
5.6nF
100nF
5.6nF
5.6nF
1nF
LEFT
IN
10nF
22pF
NE5534
OPA134
IC2
1nF
100nF
IC1
100k
560
100nF
NE5534
OPA134
100k
560
NE5534
OPA134
2.2nF
10nF
100
100k
560
2.2nF
22pF
10nF
820
910
22pF
2.2nF
100
100
820
910
100
5.6nF
T H GIR
RLX
r e w oP
POWER
+
+
00000000
100 F
820
910
BALANCED LEFT OUTPUT
1 F BP
1 F BP
R–
R+
1 F BP
TO
POWER
SUPPLY
BOARD
SHIELDED STEREO
CABLES FROM DAC BOARD
(CONNECT SHIELDS AT THIS END ONLY)
Fig.2: follow this parts layout diagram to assemble the Balanced Output Board.
The L+, L-, R+ and R- inputs are derived from the DAC Board (see below).
STEREO AUDIO OUT
RIGHT
(RED)
LEFT
(WHITE)
22pF
L
R
TUO
100nF
8.2nF
100nF
180
180
200
27nF
220
220
22pF
200
R+
100nF
IC11
OPA134
NE5534
820
2.7nF
100nF
820
100
8.2nF
27nF
220
L+
220
22pF
22pF
IC10
OPA134
NE5534
8.2nF
180
180
200
8.2nF
R-
2.2nF
200
100
22pF
IC9
OPA134
NE5534
IC12
OPA134
NE5534
2.2nF
100nF
100nF
IC7
OPA134
NE5534
IC8
OPA134
NE5534
820
820
2.7nF
2.7nF
L22pF
2.7nF
Fig.3: the L+, L-, R+ and R- points on the DAC Board are marked here in red
and drive the inputs of the Balanced Output Board. Note that the parts on the
47 Foutput socket and vice versa.
righthandside of this board drive the left channel
47 F
10k
47 F
+
44 Silicon Chip
–
+
D15
100nF
100nF
much better than from the unbalanced
balanced outputs. These
are converted
REG5
IC6
LM7805T
outputs, in practice 73dB is more than (UNDER)
to single-ended signals
on the DAC
10 F
adequate. In29fact,
board via
47 Fa pair of differential ampli09011it’s
0 very unlikely that
100F
F Fig.3,
anybody can hear the difference under 100nF
fiers (IC9 & IC12100on
September
+15V
0V -15V
normal circumstances.
2009).
This
means
that
the
simplest
16
2
1
way to15provide balanced outputs is to
Deriving balanced
TPOWER
UPNsignal
I V5IN
1-/+ going to these
tap the
O/I outputs
LATIGID
DIGITALdirectly
I/O
In practice, providing balanced differential amplifiers.
outputs from the DAC is relatively
Theoretically, the outputs from the
straightforward since the DAC chip current-to-voltage (I/V) converter stagwe used – the DSD1796 – itself has es (ICs7, 8, 10 & 11) could be connected
directly to the XLR socket outputs via
100Ω isolating resistors. However, we
have come up with a more complicated
design for a couple of reasons.
First, making a direct connection
from the existing DAC board to the
XLR sockets would bypass some of
the low-pass filtering. This filtering
is important because it’s designed
to remove high-frequency switching
artefacts.
Second, a direct connection would
load the I/V converter stages even
more than they already are. In view
of this, asking the op amps to drive an
additional, unknown amount of cable
capacitance seems unwise.
As a result, we feed the signal at
the outputs of the I/V converter stages
to an interface board to provide the
balanced outputs. This board also includes four active low-pass filter stages
based on NE5534 op amps.
Note that because the DAC’s outputs are asymmetric (they only sink
current), the outputs of the I/V converters (ICs 7, 8, 10 & 11) are always
above 0V. As a result, these outputs
are AC-coupled to the op amps in the
balanced output stages to remove the
DC component of the signal, so that it
is centred around 0V.
Circuit details
Refer now to Fig.1 for the circuit
details. It consists of two identical
sections, one for each channel.
As mentioned, the incoming differential signals are AC-coupled via 1µF
bipolar capacitors. These capacitors
and the following RC components also
form 6dB/octave high-pass filters. We
have set the corner frequency of this
filter low enough (1.6Hz) so that it has
minimal effect on the 20Hz-20kHz
frequency response (-0.046dB).
The remainder of the circuit consists
mainly of the four active low-pass
filter (LPF) stages and these are based
on op amps IC1-IC4. Each filter is an
active third-order LPF with a -3dB
point (corner frequency) of 52kHz and
a slope of -18dB per octave. These are
then followed by passive first-order
720kHz low-pass filters, each based on
a 100Ω current-limiting resistor and a
2.2nF capacitor.
These are identical to those used at
the outputs of IC9 & IC12 on the DAC
board and attenuate the 60MHz (approx.) switching spikes that the DAC
generates.
In addition, since these are passive
siliconchip.com.au
from the ±15V outputs of the Power
Supply Board. The supply rails are fed
in via another 3-way screw terminal
block on the Balanced Output Board,
with two 100µF capacitors providing
additional filtering.
Construction
Refer now to Fig.2 for the parts layout on the PC board. As can be seen,
the assembly is straightforward.
Begin by checking the PC board for
defects, then start the assembly by
installing the resistors and wire links.
You can either use 0.71mm tinned
copper wire for the links or you can
use 0Ω resistors (as in the prototype).
Next, install the IC sockets, ensuring they are correctly oriented. Follow
these with the terminal blocks, ensuring that the openings point towards the
edge of the board in all cases. Be sure
to seat them properly on the PC board
before soldering their pins.
The capacitors can go in next. The
two 100µF filter capacitors are polarised, so watch their orientation. Follow them with the XLR connectors,
then install the four ICs (again, make
sure they are correctly oriented).
Finally, complete the board assembly by fitting M3 x 6mm tapped Nylon
spacers to the mounting points. You
will need at least four of these (one
in each corner) and they should be
secured using M3 x 4mm screws.
It’s also a good idea to fit an extra
spacer between the two XLR sockets,
to ensure extra rigidity when plugging
in external leads. An extra mounting
This view shows the fully assembled PC board. Be
careful with the orientation of the ICs.
filters, they are effective at filtering
any high-frequency noise which the
active filter stages may allow through.
The third order active LPFs only
require a single op amp each (ICs14). However, unlike the DAC board,
there is no performance advantage to
be gained by using OPA134 op amps
over NE5534s. Instead, testing has
revealed that it is the I/V converter
stages on the DAC board that benefit
from the improved performance of the
OPA134s.
By contrast, on the Balanced Output
Board, the op amps only act as unity
gain voltage buffers and the NE5534
performs admirably in this role. However, you can use OPA134s if you wish.
For example, if you are not going to be
installing the unbalanced outputs, you
will have two spare OPA134s from the
DAC board, so you only need to buy
two more for the Balanced Output
Board.
Note that the board has pads for
the 22pF compensation capacitors
required for the NE5534s and if you
are purchasing op amps specifically
for this board, NE5534s are recommended. Alternatively, if you decide
to use OPA134s, you can leave out the
22pF capacitors (although installing
them does not hurt).
The output of each op amp appears at pin 6. IC1 & IC2 provide the
differential output signals for the left
channel and these respectively drive
pins 2 & 3 of the left-channel XLR
socket via the low-pass passive filter
stages. Similarly, IC3 & IC4 drive the
right-channel XLR socket.
The XLR outputs are mounted
directly on the PC board, while the
input signals from the DAC board are
fed in via 3-way screw terminal blocks.
The latter provide a 0V connection
for shielding purposes but the shield
should only be connected at one end.
Power for the Balanced Output
Board circuitry is derived directly
Table 3: Capacitor Codes
Value
100nF
10nF
5.6nF
2.2nF
1nF
22pF
µF Value
0.1µF
0.01µF
.0056µF
.0022µF
.001µF
N/A
IEC Code
100n
10n
5n6
2n2
1n0
22p
EIA Code
104
103
562
222
102
221
Table 2: Resistor Colour Codes
o
o
o
o
o
o
siliconchip.com.au
No.
4
4
4
4
4
Value
100kΩ
910Ω
820Ω
560Ω
100Ω
4-Band Code (1%)
brown black yellow brown
white brown brown brown
grey red brown brown
green blue brown brown
brown black brown brown
5-Band Code (1%)
brown black black orange brown
white brown black black brown
grey red black black brown
green blue black black brown
brown black black black brown
January 2010 45
Here’s one way of installing the Balanced Output Board in the chassis. In this case, the new board has been
mounted in the rear righthand corner of the chassis, while the DAC Board has been moved to a new position
in the front righthand corner. The left & right channel outputs from the DAC Board are then connected via
shielded figure-8 cable to RCA sockets mounted on the rear panel. Be sure to mount the DAC Board
far enough to the left to leave room for the RCA plugs.
NOTE: THE SUPPLY LEADS TO THE FINAL VERSION OF
THE INPUT BOARD ARE REVERSED AT THE TERMINAL
BLOCK COMPARED TO THOSE SHOWN HERE.
point is also provided along the opposite edge of the board but its use is
optional.
Installation
There are a couple of options when
it comes to installing the Balanced
Output Board into a case.
First, if you are starting from scratch
and drilling your own case, then the
board can be mounted with its XLR
connectors protruding through the
front panel, on the righthand side. This
would mean moving the Switch Board
further towards the centre of the front
panel than in the prototype, to allow
room for the Balanced Output Board.
Alternatively, if you are installing
the new board into an Altronics kit
46 Silicon Chip
chassis, it will have to be mounted in
the rear righthand corner of the chassis, in place of the DAC board – see
photo. The DAC board is moved to
the location shown in the photo and
installed with its RCA output connectors facing towards the righthand
side panel. The RCA outputs are then
connected via figure-8 shielded cable
to a pair of RCA sockets mounted on
the rear panel between the Input Board
and the Balanced Output Board.
Which ever method you choose, you
will have to drill the necessary mounting holes for the boards and cut holes
in the front or rear panel to match the
XLR sockets.
The XLR socket holes are the first
on the list. These are holes best made
by initially drilling two pilot holes
35.5mm apart at the correct height.
They are then reamed out to 22mm
to allow the socket centre sections to
protrude through.
That done, mark out and drill the
four 2.5mm holes around the outside
edge of each cutout. The XLR connectors can then be firmly secured to the
panel using the supplied self-tapping
screws.
Having secured the assembly in this
manner, the next step is to remove the
Nylon spacers so that you can mark out
the mounting holes for the Balanced
Output Board in the base of the chassis. The PC board is then removed so
that the holes can be drilled (to 3mm).
Once these holes have been drilled,
siliconchip.com.au
place. This step is vital because they
are subject to quite a bit of force during
cable insertion and removal.
Wiring
mark out and drill the two holes for
the panel-mount RCA sockets. Again,
use a pilot drill to start the holes, then
carefully ream them to size (9.5mm)
using a tapered reamer.
If you are modifying an Altronics
kit, then the DAC Board can be installed in the location shown in the
photo. Once again, you will have to
mark out and drill a new set of mounting holes. Note that the edge of the
board should be at least 55mm from
the righthand chassis panel, to ensure
sufficient clearance for the RCA plugs.
Next, deburr all the mounting holes
using an oversize drill before installing
the boards in the chassis. Don’t forget
to refit the four screws through the
panel to hold the XLR connectors in
siliconchip.com.au
It’s now just a matter of completing
the wiring as shown in Figs.2 & 3 and
the photo.
First, you will need to run three
power supply leads (+15V 0V -15V)
to the Balanced Output Board. These
supply rails are derived from the
output terminal block on the Power
Supply Board.
Unfortunately, it can be difficult to
fit two wires into the terminal block
entries (due to the leads already running to the DAC board) but there is a
way around this – splice the wires into
a “Y” shape with heatshrink insulation applied to the join. You can then
connect one end to the power supply,
the middle to the DAC board and the
remaining end to the Balanced Output
Board. Make sure you don’t get any
of these +15V 0V -15V connections
mixed up.
It’s a good idea to twist the supply
leads together as shown in the photo.
This not only minimises noise pickup but also ensures that a lead cannot
wander if it comes adrift. You should
also use cable ties to additionally secure the supply leads at the terminal
blocks.
The connections between the DAC
Board and the rear-panel RCA sockets
are run using figure-8 shielded cable
(ie, two cores with separate shields
– do not use 2-core cable with a common shield for these connections). As
shown, the leads are directly soldered
to the rear-panel RCA sockets at one
end and are terminated in RCA plugs
at the DAC Board end.
Alternatively, if you don’t intend
ever using the unbalanced outputs,
then this wiring can be left out.
Two lengths of twin-core shielded
cable are used for the signal connections between the DAC Board and
the Balanced Output Board. Begin
by stripping back 20mm of the outer
insulation from one end of each cable
and about 40mm from each of the
other ends. Then, at the 40mm ends,
trim the shield wires back completely
so that they do not project out of the
outer insulation.
Now, at the 20mm end of each cable,
twist the shield wires together tightly
and tin them with solder. That done,
remove 10mm of insulation from the
Parts List
1 PC board, code 01101101,
110 x 67mm
2 PC-mount male 3-pin XLR
connectors plus self-tapping
screws (Altronics P-0874)
3 3-way screw terminal block
(5.08mm pitch)
4 8-pin machined IC socket
6 10mm tapped Nylon spacers
6 M3 x 6mm machine screws
1 500mm length twin-core
shielded cable
Semiconductors
4 NE5534 op amps (IC1-IC4)
Capacitors
2 100µF 25V electrolytic
4 1µF bipolar electrolytic
4 100nF MKT
4 10nF MKT
4 5.6nF MKT
4 2.2nF MKT
4 1nF MKT
4 22pF ceramic
Resistors
4 100kΩ
4 910Ω
4 820Ω
4 560Ω
4 100Ω
Miscellaneous
The following parts are
necessary to complete the
chassis wiring:
2 RCA plugs, 1 red, 1 black
2 panel-mount RCA sockets
1 500mm length figure-8
shielded cable
8 cable ties
1 600mm-length heavy-duty red
hook-up wire
1 600mm-length heavy-duty blue
hook-up wire
1 600mm-length heavy-duty
black hook-up wire
inner wires at both ends, then double
the exposed wires back and tin them.
Finally, trim the shield wires back
to about 10mm and attach the signal
cables to the input terminal blocks on
the Balanced Output Board - see Fig.2.
As shown, the shield wire goes to the
centre terminal of each block, the red
wire to the “+” terminal and the white
wire to the “-” terminal.
The red & white wires at the other
end of each cable are connected to
the pin 6 outputs of ICs 7, 8, 10 & 11
January 2010 47
ning to the 3-terminal input blocks.
That way, the lefthand XLR socket
(when looking at the front panel)
will really be the left channel, while
the righthand socket will be the right
channel.
Testing
Another view of the completed Balanced Output Board, this time looking
at the XLR connectors. The latter are secured to the rear panel using the
self-tapping screws supplied. This ensures that the solder joints on the
board don’t crack due to stress as cables are plugged in and removed.
on the DAC Board. The best place to
make these connections is at the 220Ω
resistors that connect to these pins, as
shown in Fig.3.
You can either make the connections
to the top of the DAC Board or you can
solder the wires to the pads on the
underside of the board (as in the prototype). If you are attaching the wires
to the top of the board, simply melt a
little solder onto the exposed resistor
legs, then solder each wire in turn.
Alternatively, if you are not installing the unbalanced outputs, you can
leave out the 220Ω resistors and simply feed the wires down through the
board holes before soldering them to
the pads. Either way, you must protect
the board so that the trimmed shield
wires can’t short against anything.
That can be done either by using heatshrink sleeving or a blob of hot melt
glue, or even insulating tape.
Once all the wiring has been completed, secure it in place using cable
ties as shown in the chassis photo.
This not only helps prevent leads
from flexing and coming adrift but
also ensures that a wire cannot move
and contact other parts of the circuit
(including the mains terminals on the
back of the IEC socket) if its connection is broken.
Don’t get the channels mixed
Be sure to connect the leads exactly
as shown in Figs.2 & 3, so as not to
transpose the left and right channels.
In particular, note that the components on the righthand side of the
DAC Board are actually for the left
channel, ie, they drive the left audio
output socket. Similarly, the parts on
the lefthand side of the board drive
the right channel audio output socket.
This was done to simplify the layout
of the PC tracks running from the DAC
chip (IC6).
All you have to do is run the signal
leads as shown in Figs.2 & 3 and all
will be correct. There’s just one wrinkle here – if you mount the Balanced
Output Board on the front panel, then
you should swap the signal leads run-
Once the power supply and signal
wiring are complete, power the Stereo
DAC up and check that the +15V and
-15V inputs to the Balanced Output
Board are correct. If these are OK, uou
are then ready to connect the balanced
outputs to your external equipment
and check that they are functioning
correctly.
If there is a problem, switch off
immediately and use a multimeter
to confirm that all power and signal
connections are correct. If that checks
out but it still doesn’t work properly,
you will need to remove the Balanced
Output Board and check it for short
circuits, missed solder joints and incorrect parts placement.
If you have not tested the rest of
the DAC yet, then it’s a good idea to
temporarily disconnect the Balanced
Output Board while you make the necessary checks. That way, you’ll at least
know that the rest of the DAC works
correctly before looking for problems
on the Balanced Output Board.
That’s it – once wired up, the balanced outputs should provide a very
clean output signal from the DAC, even
with long cable runs.
Phantom power
Finally, note that phantom power
should not be applied to the XLR sockets on the Balanced Output Board (ie,
phantom power should be switched
off). Alternatively, cut the tracks
between the 100Ω resistors and the
XLR sockets and install 10µF bipolar
(BP) electrolytic capacitors across the
gaps (ie, in series with the pin 2 & pin
SC
3 outputs).
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48 Silicon Chip
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