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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). Issues Getting Dog-Eared? Keep your copies of SILICON CHIP safe with these handy binders REAL VALUE AT $14.95 PLUS P & P Available Aust, only. Price: $A14.95 plus $10.00 p&p per order (includes GST). Just fill in and mail the handy order form in this issue; or fax (02) 9939 2648; or call (02) 9939 3295 and quote your credit card number. 48  Silicon Chip siliconchip.com.au