Fullscreen HTML5Med Res (??MB)HTML4

By JOHN CLARKE 48V Dual Phantom Power Supply for DI boxes & Condenser Microphones Lots of audio equipment needs phantom power. As well as condenser mikes, it’s also required for active DI boxes, preamplifiers and effects units. This phantom power supply runs from a 24VAC plugpack transformer and delivers a regulated 48V DC via XLR sockets. P UBLIC ADDRESS systems in theatres, churches and halls all require microphones, preamplifiers and possibly powered DI (direct injection) boxes and effects pedals for musicians. Many microphones are dynamic types that do not require a power source but the more sensitive condenser microphones require power and the same goes for other items of equipment. While these can often be run from batteries, it is far more convenient to have a “phantom supply”. This avoids the need to check batteries that can go flat at the most inconvenient times, ie, when you need ‘em! So what’s a phantom supply? Well, 40  Silicon Chip it’s a way of providing power to equipment via balanced signal leads. “Phantom” refers to the apparently invisible manner in which power is applied. 48V DC is the favoured phantom supply standard in the commercial sound industry. 24V and 12V are also used but these are not popular. Fig.1 shows how it’s done. The 48V DC supply is applied via 6.8kΩ resistors to the hot (non-inverted) and cold (inverted) signal leads for the device being powered. The output signal leads from the unit are capacitively coupled to the following device, so that the DC voltage is removed from the signal. Phantom supplies should not be confused with the bias voltage applied to electret microphones. A bias supply is applied to an unbalanced lead comprising a shield and signal wire, rather than to a balanced signal line with a shield. Additionally, such a bias supply is typically around 1.5V and has a current of 1mA or less. More information about this can be found at: http://blog.shure.com/shure-notes/ shure-tech-tip-phantom-power-vsbias-voltage/ So what happens if a phantom supply is connected to a dynamic microphone? Will it be damaged by current flow? The answer is “no”. Fig.2A shows this connection. A dynamic microphone employs a coil siliconchip.com.au that’s floating and not connected to the grounded shield. With 48V applied to both sides of the coil, no DC current flows through it. Problems can arise when a dynamic microphone or the connecting lead is wired incorrectly, with one side of the microphone coil connected to ground as shown in Fig.2B. Current would then flow in the coil, leading to magnetisation that may permanently affect the microphone. Problems also occur if a centretapped microphone coil is incorrectly connected to ground at the centre tap (Fig.2C). In this case, a different current may flow in each half of the coil due to slight differences in the values of the 6.8kΩ resistors and imbalances in the coil windings. Getting back to Fig.2A, matching the 6.8kΩ resistors will also improve noise rejection due to more equal impedance matching in the two signal leads. For those unfamiliar with balanced audio leads, the twisted pair wires in the balanced lead carry out-of-phase signals. At the receiving end, the out-of-phase signals are “subtracted” and this has the effect of “adding” the two signals. In addition, because they are twisted, the signal wires each tend to pick up the same level of hum and this is cancelled by the subtraction at the receiving end. A shield wire that’s connected to ground further minimises hum and noise pick-up. Some mixing desks do include a phantom supply for microphones, etc. However, even if you do have such a mixing desk, it may not have sufficient capacity. This new 48V Dual Phantom Power Supply can be used with two phantom-powered devices. As shown in the photos, the 48V Dual Phantom Power Supply is housed in a diecast box, for ruggedness and for shielding. It has two female XLR Fig.1: how phantom power is applied. The 48V DC supply is applied via 6.8kΩ resistors to the balanced signal leads of the device being powered. 2 6.8k 2 POWERED APPLIANCE sockets and two male XLR sockets. The 48V supply is applied to the female XLR sockets, while the male sockets provide the signal output with the DC voltage blocked by 22µF electrolytic capacitors. Circuit details Now take a look at Fig.3 for the full circuit details. As shown, the incoming 24VAC from the plugpack transformer is connected to a half-wave voltage doubler rectifier comprising diodes D1 & D2 and two 470µF 63V electrolytic capacitors. This will result in a nominal DC voltage of about 67V but will typically be much higher at around 75V DC, depending on the incoming mains voltage and the plugpack’s voltage regulation. REG1, an LM317 3-terminal adjustable regulator, is used to derive the 48V DC supply. This device is rated for a maximum differential of 40V between its input and output. With a 75V input and a 48V output, the input to output difference is a comfortable 27V but when power is initially applied, the regulator circuit’s input can be 75V or more while the output can be as low as 1.3V. This is due to REG1’s adjust terminal being initially held at 0V via a 1µF bypass capacitor. Since the LM317 cannot cope with this admittedly brief overload, a pre- SIGNAL OUTPUT 3 regulator comprising Darlington transistor Q1 and 33V zener diode ZD1 is used to protect it from over-voltage. Q1 acts as an emitter follower, while ZD1 has its anode connected to REG1’s output, thereby limiting the voltage across the regulator to about 31.7V (after allowing for the voltage drop across the two base-emitter junctions in Darlington transistor Q1). Following the pre-regulator, the circuit involving REG1 is fairly standard. REG1’s supply input is decoupled using a 100nF MKT capacitor, while the output and adjust terminals are bypassed using 1µF 63V electrolytic capacitors. The minimum load current for REG1 to give its specified performance is 12mA. Since the voltage between the output and adjust terminals could be as low as 1.2V, we would normally connect a 100Ω resistor between these two terminals to provide this minimum current. However, this wouldn’t allow us to use convenient standard resistor values for the adjust-to-ground resistors and so we have used a 150Ω resistor instead. This provides a minimum load of 8mA, with the remaining 4mA required being added by the current through power indicator LED1. In fact, assuming a 48V output and 2V across LED1, the LED current will actually be 4.6mA. +48V DC 6.8k 6.8k DYNAMIC MICROPHONE 6.8k 1 +48V DC 6.8k +48V DC 2 = ‘HOT’ (IN PHASE) 3 = ‘COLD’ (OUT OF PHASE) 1 = GROUND 2 +48V DC 6.8k 6.8k DYNAMIC MICROPHONE 6.8k DYNAMIC MICROPHONE 2 CT 3 3 1 A 3 B C Fig.2: a correctly wired dynamic microphone coil is shown at (A) but problems occur if the microphone is incorrectly wired as shown at (B) and (C) due to current flowing in the coil. siliconchip.com.au November 2014  41 D3 1N4004 SCREW TERMINALS D1 1N4004 A K Q1 TIP122 C K E IN K CON1 24V AC INPUT A D2 1N4004 470 µF B 0.5W ZD1 K OUT K ADJ 100nF 4.7k 63V A REG1 LM317T A A λ K 63V 2.4k SOCKET 63V A POWER LED1 33V 1W 470 µF 1 µF D4 1N4004 150Ω 0.5W 1 µF 63V TP1 10k GND 0.5W 2.7k 0.5W LED * MATCH EACH PAIR OF THESE VR1 1k RESISTORS TO WITHIN 27Ω OUTPUT ADJUST K A +48V 6.8k* 6.8k* 0.5W 6.8k* 0.5W 6.8k* 0.5W 22 µF 63V 0.5W 22 µF 63V 22 µF 63V 3 2 22 µF 63V 3 XLR FEMALE SOCKET1 1 2 1 SHIELD XLR MALE SOCKET1 SHIELD 10Ω 100k 3 2 SHIELD 10Ω 100k 100k 1N4004 20 1 4 48V DUAL PHANTOM POWER SUPPLY XLR MALE SOCKET2 1 SHIELD A SC  2 3 XLR FEMALE SOCKET2 1 B K LM317T TIP122 K ZD1 A 100k C C E OUT ADJ OUT IN Fig.3: the circuit of the 48V Dual Phantom Power Supply. The 24VAC supply input is rectified by voltage doubler D1 & D2 and fed to an LM317T adjustable regulator (REG1) via a pre-regulator consisting of Darlington transistor Q1 & ZD1. The resulting 48V DC output from REG1 is then fed to pins 2 & 3 of the female XLR sockets via 6.8kΩ resistors. Trimpot VR1 (1kΩ) allows the output voltage to be adjusted from 40.8V to 48.8V if the output-to-adjust terminal voltage is at its 1.2V minimum. If the output-to-adjust terminal voltage is at its 1.3V maximum, the current through the adjust resistors is 8.66mA and the output voltage can be adjusted using VR1 from 44.2V to 52.8V. These calculations do not include the current flowing from the adjust terminal itself. This is typically 45µA but can be as high as 100µA. For the adjust terminal to ground resistance used, this can add an extra 0.61V to the output. Note that the output voltage is required to be between 44-52V in order for the phantom supply to comply with the DIN EN 61938 standard. 42  Silicon Chip Diodes D3 & D4 are included as standard protection. D3 allows current flow from the output back to the input if the regulator’s input is shorted. Similarly, D4 allows current to flow from the 1µF bypass capacitor at the adjust terminal if the output is shorted. XLR sockets The 48V DC supply is fed to the XLR sockets via 6.8kΩ resistors. These limit the short-circuit current to a nominal 7mA (6.5-7.7mA range) for each supply pin (pins 2 & 3 on the XLR sockets). Ideally, each 6.8kΩ resistor pair should be matched to within 27Ω to comply with the 0.4% tolerance allowed by the phantom power specifications. That can be easily achieved by using resistors that are from the same manufacturer’s batch. In practice, the resistor values are checked with a digital multimeter before installation. The resistors in each pair do not need to be within 27Ω of 6.8kΩ; just within 27Ω of each other. As mentioned previously, 22µF electrolytic capacitors are used to block the 48V DC on the balanced signal lines from being fed to the XLR male output sockets and these work in company with 100kΩ bias resistors from the outputs to ground. The 22µF capacitors ensure a low-frequency roll-off that’s well below 20Hz for a typical sound mixer or amplifier input impedance of 10kΩ. The 10Ω resistors isolate the ground connections between each pair of female and male XLR sockets to prevent high-level ground loop currents. siliconchip.com.au The 48V Dual Phantom Supply is built on a double-sided plated-through PCB coded 18112141 and measuring 56.5 x 113mm. This is housed in a diecast box measuring 122 x 66.5 x 39mm and a panel label (113 x 56mm) is affixed to the lid. Fig.4 shows the parts layout on the PCB. Begin by installing the resistors, zener diode ZD1 and diodes D1-D4. A digital multimeter should be used to check the resistor values before they are installed. As mentioned above, you will need to select two pairs of 6.8kΩ resistors that are within 27Ω of each other. Make sure the diodes are all installed with the correct polarity. The banded end of each diode must be orientated as shown on the layout diagram. The PC stakes for TP1 and GND can go in next, followed by REG1 and Q1 (don’t get these latter two parts mixed up). As shown, REG1 & Q1 are mounted horizontally, with their leads bent down by 90° so that they go through their respective PCB holes. Be sure to secure the metal tab of each device to the PCB using an M3 x 6mm machine screw and nut before soldering their leads. Trimpot VR1 can now be installed, TP1 22 µF 63V 2.7k 1 µF 63V 6.8k 6.8k 6.8k 10Ω REG1 LM317T 10Ω 22 µF 63V 22 µF 63V A 100nF D4 2.4k 1k 1 µF 63V C 2014 GND VR1 4004 4004 150Ω CON1 Q1 TIP122 ZD1 22 µF 63V 2 3 1 1 3 2 SHIELD 2 XLR MALE SOCKET2 3 SHIELD 100k 1 100k XLR FEAMALE SOCKET2 1 SHIELD 3 XLR MALE SOCKET1 XLR FEMALE SOCKET1 10k 2 SHIELD 100k LED1 D3 100k T NA HP V 8 4 YLPPUS M OSUPPLY 1 4 1 2 1 1 8 148V PHANTOM 4004 4004 33V 470 µF 63V 470 µF 63V 6.8k 24VAC 4.7k + Construction D2 D1 + By contrast, the shield connections of each XLR socket pair are connected together (but not to each other). In other words, the Female Socket1 shield connects to the Male Socket1 shield and the Female Socket2 shield connects to Male Socket2 shield. There is no interconnection between the two sets of shields. In practice, the Socket1 pair shield is also connected to the metal case used to house the circuit. This connection is made via one of the mounting screws that’s used to secure the XLR female socket to the case. Fig.4: follow this diagram to install the parts on the PCB. LED1 should be mounted with the top of its lens 30mm above the board so that it will later protrude through the lid of the case. This view shows the completed PCB assembly. Note that The top of each 470μF capacitor must be covered with insulating tape (12mm-diameter) to ensure that they cannot later short to the case lid. followed by the capacitors. Make sure the electrolytics are installed with the correct polarity. The 2-way screw terminal block (CON1) is next on the list (wire entry holes towards the adjacent edge of the PCB). Alternatively, a DC socket can be fitted instead. A screw terminal block would normally be used, since AC plugpacks are usually supplied with bare leads. Next, install the XLR sockets, making sure that they all sit flush against the PCB before soldering their leads. LED1 can then go in; it must be fitted with the correct polarity and with the top of its lens 30mm above the PCB so that it later just protrudes through the Table 1: Resistor Colour Codes o o o o o o o o o siliconchip.com.au No.   4   1   4   1   1   1   1   2 Value 100kΩ 10kΩ 6.8kΩ 4.7kΩ 2.7kΩ 2.4kΩ 150Ω 10Ω 4-Band Code (1%) brown black yellow brown brown black orange brown blue grey red brown yellow violet red brown red violet red brown red yellow red brown brown green brown brown brown black black brown 5-Band Code (1%) brown black black orange brown brown black black red brown blue grey black brown brown yellow violet black brown brown red violet black brown brown red yellow black brown brown brown green black black brown brown black black gold brown November 2014  43 The PCB is installed in the case by mounting it on two M3 x 6mm tapped spacers (secured with M3 x 12mm countersink screws) at the rear and by fitting six M3 x 12mm screws (one Nylon, the rest metal) into the XLR sockets at the front. + + XLR socket holes: 22mm diameter + + + + + + Before installing the PCB in the case, it’s necessary to drill mounting holes for the XLR sockets and the two rear PCB mounting points. A hole is also required is the lefhand side of the case to accept a cable gland (for the 48VAC supply leads) or a DC plug, while a 3mm hole must be drilled in the lid for the power indicator LED. Fig.5 shows the drilling template for the XLR sockets, while Fig.6 (the front panel artwork) shows the location of the LED (these can also be downloaded in PDF format from the SILICON CHIP website). As shown, the XLR sockets require 22mm-diameter clearance holes, with 3mm-diameter holes for the mounting screws. The 22mm holes can be easily cut using an Irwin Speedbor or similar drill. These are intended for + + + Before going any further, check that all components are orientated correctly and that you haven’t missed any solder joints. That done, apply 24VAC power and check that LED1 lights. If it does, connect your multimeter between TP1 and GND and adjust trimpot VR1 for a reading of 48V. Check that 48V is also present at pins 2 & 3 of the XLR female sockets; ie, by measuring between each pin and GND. Now check pins 2 & 3 of the XLR male sockets. They should each be at a low voltage and this should continue dropping over time as the 22µF capacitors fully charge. In fact, they may take several minutes to drop below 50mV, Preparing the box 13mm Test & adjustment due to capacitor leakage current. If the voltage on one or more pins remains higher than 100mV, change the relevant capacitor. You can use lowleakage 50V capacitors if necessary. + lid of the case. Note that the longer lead is the anode. The PCB assembly can now be completed by covering the top of each 470µF capacitor with a circular piece of insulating tape cut to a diameter of 12mm. This is necessary to ensure that the capacitors cannot later short to the lid of the case. XLR mounting holes: 3mm diameter Fig.5: the drilling and cutout template for the front side of the case. This can also be downloaded in PDF format from the SILICON CHIP website. 44  Silicon Chip use in timber but they also work well on aluminium. Drill just down to a depth where the internal ribs of the box begin; any further and the drill will begin to wobble. The inside piece can then be forced sideways in several directions by inserting a screwdriver in the centre hole and applying leverage. Do this until the inside piece eventually gives way and falls out, then clean up the hole with a round file. If you don’t have a Speedbor drill, drill a series of small holes around the inside perimeter of the hole, then knock out the centre piece and file the job to a smooth finish. Each XLR female socket also requires a cut-out between the top of its 22mm hole and the top edge of the box (see Fig.5). This cut-out is necessary to allow the ‘push to release’ lever on each XLR female socket to be inserted. It’s just a matter of making these cutouts using a hacksaw after the 22mm holes have been drilled. The two mounting holes for the rear of the PCB are marked out after the XLR cut-outs have been made. It’s just a matter of temporarily fitting the PCB assembly into the case, marking out the two holes, the removing the PCB and drilling them to 3mm. Deburr the holes using an oversize drill, then countersink them from the outside to suit countersink-head M3 screws. Next, drill the hole in the side of the case for the power cable (either to accept a cable gland or a DC power plug). This hole should be positioned siliconchip.com.au XLR Female Socket2 (see photos). This is necessary to prevent the screw from making a connection between this socket’s shield and the case, thereby creating an earth loop (and causing hum). That’s because the lower mounting hole of each female socket connects the shield to the case when a metal screw is used. By the way, you will have to cut a thread in the plastic of XLR Female Socket2 with one of the M3 metal screws before replacing this with the Nylon screw. Do all the screws up so that there is a gap of about 2.5-3mm between the socket and the case, so that the lip on the inside of the lid will fit between them. Front panel label NYLON SCREW towards the rear of the case must be directly in-line with the DC socket (if used). Final assembly Once all the holes have been drilled, the PCB assembly can be mounted in the case. The first step is to install two M3 x 6mm tapped spacers to support the rear edge of the PCB. Secure these using M3 x 12mm countersink head screws inserted up through the base of the case, then drop the PCB into position and fit nuts to hold the assembly in place. The PCB assembly is secured to the front of case by fitting M3 x 12mm mounting screws to the XLR sockets. Seven of these screws are metal but a Nylon screw must be used for the lower (righthand) mounting hole of The front-panel label can be produced by printing it onto photo paper. This is then affixed to the case lid using a suitable glue or neutral-cure silicone and the hole cut out for the LED. For a more rugged label, print a mirror image onto clear overhead projector film, so the print side will be on the back of the film when the label is affixed to the lid (eg, using silicone sealant). Alternatively, you can print onto an A4 sized synthetic ‘Dataflex’ self-adhesive label if you have an inkjet printer or onto a ‘Datapol’ self-adhesive label if you have a laser printer. Further information on where to buy these labels is in the panel in the Mains Switch Timer article published elsewhere in this issue. Once the label is in place, it’s then just a matter of attaching the lid using the four countersunk M3 screws provided and the 48V Dual Phantom SC Power Supply is ready for use. 24VAC SILICON CHIP 48V Dual Phantom Power Supply + Power Input 1 Output 1 Input 2 Output 2 Fig.6: the front-panel artwork is also available on the SILICON CHIP website. You will need to drill a hole in the case lid for the power LED. siliconchip.com.au Parts List 1 PCB, code 18112141, 113 x 56.5mm 1 panel label, 113 x 56mm 1 diecast box 122 x 66.5 x 39mm (Altronics H 0453) 1 24VAC plugpack (50mA minimum rating) 2 XLR female 3-pin connectors (compact, PCB mount, 90°) (Altronics P 0875) (Female Socket1, Female Socket2) 2 XLR male 3-pin connectors (PCB-mount, 90°) (Altronics P 0874) (Male Socket1, Socket2) 1 2-way PCB-mount screw terminal block with 5.08mm spacings (CON1) 1 cable gland (3-6.5mm dia. cable) 1 PCB-mount DC socket, 2.1mm or 2.5mm (Jaycar PS-0520, Altronics P0620, P0621A – optional) 2 M3 x 6mm spacers 2 M3 x 10mm machine screws (to secure REG1 and Q1) 7 M3 x 12mm machine screws (for XLR socket mounting) 1 M3 x 12mm Nylon or polycarb­ onate screw (lower right female XLR socket mounting) 2 M3 x 12mm countersink-head screws (rear PCB mounting) 4 M3 nuts 2 PC stakes 1 25mm length of insulation tape 1 1kΩ mini horizontal trimpot (VR1) Semiconductors 1 LM317T adjustable regulator (REG1) 1 TIP122 NPN Darlington transistor (Q1) 4 1N4004 1A diodes (D1-D4) 1 33V 1W zener diode (ZD1) 1 green 5mm LED (LED1) Capacitors 2 470µF 63V PC electrolytic (26.5mm height maximum) 4 22µF 63V PC electrolytic 2 1µF 63V PC electrolytic 1 100nF MKT polyester Resistors (0.25W, 1%) 4 100kΩ 1 2.7kΩ 0.5W 1 10kΩ 0.5W 1 2.4kΩ 0.5W 4 6.8kΩ 0.5W* 1 150Ω 1 4.7kΩ 0.5W 2 10Ω * Select each pair to be within 27Ω of each other November 2014  45