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Passive Direct Injection (DI) Box Add this vital piece of equipment to your musician’s or roadie’s toolbox Hum and noise plaguing your performance? A DI box that converts unbalanced signals from a musical instrument into a balanced output signal is the answer. This Passive DI Box performs as well as a powered unit in many applications and doesn’t require batteries. By JOHN CLARKE D IRECT INJECTION BOXES are used to connect musical instruments into a sound system, whether they are electric, electronic or fitted with a microphone. Many such instruments have a 6.35mm (1/4-inch) ‘jack’ output socket whereas a PA system or mixing board will typically have XLR inputs. The term ‘Direct Injection’ (DI) refers to a physical (wired) connection, rather than using a microphone to pick up the instrument’s sound output. A cable with different connectors on the ends won’t do the job; a DI Box is required to convert the signal from the musical instrument so that it’s suitable to feed to a sound system. Specifically, 66  Silicon Chip the unbalanced signal from the instrument must be converted to a balanced signal for the sound system, to avoid an inordinate amount of hum injection. Commercial passive DI boxes can be heavy and bulky and many units have extruded finned aluminium heatsinkstyle cases. This SILICON CHIP DI Box is compact, looks and sound good, and requires no external power or batteries. Balanced signals A DI box is usually connected to a musical instrument via a relatively short lead. That’s because an unbalanced input is not as good at rejecting hum pick-up as the balanced output on   Main Features •  Suitable for use with powered instruments •  No battery or power supply required •  Compact size and rugged construction •  Balanced XLR output •  6.35mm jack socket for mono input •  3.5mm jack socket for stereo input (mixed to mono) •  20Hz-20kHz frequency response •  Ground lift switch Note: not suitable for direct connection to high-impedance stringed instrument pick-ups. the DI box. A balanced output lead can be quite long and is typically run to a remote sound system (eg, an amplifier or mixer). A balanced cable has good hum rejection because it has two signal conductors, one with the original signal and the other with an inverted version of the same signal. These conductors are normally twisted together and any hum pick-up (or other interference) should be coupled into both conductors almost equally. At the far end, the signals from the two conductors are subtracted, which reinforces the original signal but cancels out any extraneous signals which may have siliconchip.com.au been picked up along the way. Converting between unbalanced and balanced signals can be done using either electronic circuitry in an Active DI Box or by using a transformer in a Passive DI Box. Active DI boxes have the advantage that their input impedance can be very high (over 1MΩ), making them suitable for the pick-ups used in guitars, violins and other stringed instruments. Their disadvantage is that if powered from a battery, the battery ultimately goes flat, causing distortion in the sound. And they often stop working at the most crucial time – right in the middle of a session! This Passive DI Box does not need power and can be relied on to perform faultlessly with no maintenance. It does not have such a high input impedance but it is suitable for many instruments that have a low output impedance. This includes electronic keyboards, computer audio outputs and A/V system sound outputs. In the case of stringed instruments, it is increasingly common for these to include an in-built preamplifier for the pick-up and this can easily drive a Passive DI Box. Another feature of a DI Box is that it provides impedance matching and has a reasonably high input impedance that’s suitable for powered instrument signals. The output impedance of the DI box is much lower, at 600Ω or less, and this is better for driving balanced leads. A lower output impedance further reduces the effects of hum and noise pick-up. Hum loops As with any sound system, the way that the cable shielding is grounded can have a huge impact on the amount of hum induced in the leads and in the sound output. Unbalanced leads need to have the shield earthed to minimise the hum pick-up. That normally means that in a DI Box, the shield of the unbalanced lead will be connected to the shield of the balanced lead which is ultimately grounded at the mixer (or sound system, etc). However, some unbalanced leads are already grounded at the signal source and so this connection will produce an earth loop due to circulating currents in the shield wire, thus injecting hum. As a result, many DI Boxes include a ‘ground lift’ switch. This is used to disconnect the two shields siliconchip.com.au MONO IN RING TIP CON1 6.35mm STEREO JACK SOCKET T1 YELLOW SLEEVE RED 10k 600Ω BLK 2 3 GREEN BLUE STEREO IN RING SC 20 1 4 SHELL 2x 2.2k TIP CON2 3.5mm STEREO JACK SOCKET 1 XLR MALE CONNECTOR (PIN VIEW) CON3 GROUND LIFT S1 CHASSIS SLEEVE PASSIVE DIRECT INJECTION (DI) BOX Fig.1: the circuit uses just two input jack sockets (one for mono signals and one for stereo), a 10kΩ:600Ω transformer, an XLR output connector and a rocker switch to provide ground lift. No power supply is required. when they are separately earthed. DI Boxes can carry very low signal levels and in use, are often surrounded by many other leads. Some of these nearby leads will likely carry mains power. Thus, a DI Box must be well shielded from 50Hz fields. It must also be robust as it will typically be on the floor and is liable to be trodden on, kicked or tripped over. Connectors Our Passive DI (Direct Injection) Box is built into a metal diecast case with both 6.35mm and 3.5mm jack sockets at one end and a male XLR connector at the other. The input impedance is 10kΩ for the 6.35mm jack socket and about 3kΩ for each input of the 3.5mm socket. This is sufficiently high for virtually all powered instruments. That includes keyboards and stringed instruments such as guitars that have an internal preamplifier. It is not suitable for high impedance pickups unless these are connected via a preamplifier or effects box. The 3.5mm jack socket can be used to connect a computer, MP3 player or other stereo source. The Passive DI Box mixes the incoming stereo signal into mono. Performance The performance of the SILICON CHIP Passive DI Box is impressive. It easily outperformed one commercially available unit we compared it against, both in terms of audio sound quality and frequency response. While the frequency response of the commercial unit was very restricted in the bass region (-3dB at 250Hz, -6dB at 125Hz and more than -12dB at 60Hz!), our unit could pass signals well below 20Hz without any appreciable attenuation. In addition, the unit does not add any noticeable noise to the signal. Our tests for signal-to-noise ratio do not do the unit justice as the tests results are below the noise floor of our Audio Precision measuring equipment. The distortion is very low and is typically below 0.02% for a 1V signal above 100Hz. It has even lower distortion with lower signal levels. The exceptional sound quality from the Passive DI Box is due to use of a high-quality audio transformer from Altronics. It incorporates a Mu metal shield for extra low noise and hum. Several musicians who tested our Passive DI Box remarked that it has better quality sound than many commercial units. Circuit details Fig.1 shows the complete circuit, which is based around the Altronics M-0705 transformer with a 10kΩ primary and 600Ω secondary. 6.35mm stereo jack socket CON1 is used for mono input signals while 3.5mm jack socket CON2 is used for stereo signals. The tip (left) and ring (right) signals from the latter are mixed by a pair of 2.2kΩ resistors and fed to T1’s primary. Since CON1 is for mono signals, only its tip contact is connected and this also goes to T1’s primary winding. October 2014  67 Yellow 2.2k CON3 Red 2 1231 4 1 909 0 1141 32 3 C 2014 1 S1 2.2k T1 Blue CON1 Chassis Green SHIELD (VIEW FROM ABOVE) DI BOX CABLE TIE HEATSHRINK SLEEVES SOLDER LUG ATTACHED TO SIDE OF BOX USING 10mm M4 SCREW WITH NUT & LOCKWASHER SIDE OF BOX S1 (VIEW FROM BELOW) S T Fig.2: install the parts on the PCB and complete the wiring as shown in this diagram and the accompanying photograph. 231 09 141 CON2 R (T1 ABOVE) The other end of the primary winding connects to the input signal ground, ie, the two connector sleeves. The balanced output signal appears across the 600Ω secondary of T1, so pins 2 & 3 of XLR plug CON3 are connected directly across this winding. The centre tap of the secondary winding isn’t used so the output ‘floats’. Pin 1 of the XLR connector is the ground pin and this is connected to the DI Box case and the shell of the plug housing. Even though the XLR socket is a plastic connector, there is a metal contact that connects to provide shielding for the XLR plug when it is inserted into this socket. The input and output grounds are joined only by switch S1. Opening this switch provides the ground lift. Normally S1 is left closed unless there is a hum loop. neatly into a 111 x 60 x 30mm diecast aluminium box (with a little coaxing). Fig.2 shows the parts layout on the PCB and the external wiring. The 3.5mm jack socket is installed on the underside of the PCB while the 6.35mm jack socket, two resistors, transformer T1 and the XLR plug all mount on the top side. Solder the 3.5mm jack socket first because when the 6.35mm jack socket is installed, the 3.5mm socket pins will be inaccessible. Both sockets should be pushed as far down onto the PCB as they will go. Construction The SILICON CHIP Passive Direct Injection Box is assembled onto a double-sided PCB coded 23111141 and measuring 105 x 24mm. This fits TOP EDGE OF CASE (WITHOUT LID) TOP EDGE OF CASE (WITHOUT LID) A 6.5mm DIAM. 10.5mm DIAM. 10 CL CL 22mm DIAM. 10 HOLES A: 3mm DIAM. HOLE B: 4mm DIAM. 12.5 A 10 CL 24.5 TOP EDGE OF CASE (WITHOUT LID) 13 10 B CL 19.5 30 50 ALL DIMENSIONS IN MILLIMETRES Fig.3: this diagram can be copied and used the three sections cut out and used as drilling templates for the metal case. It can also be downloaded (in PDF format) from the SILICON CHIP website and printed out. 68  Silicon Chip siliconchip.com.au   Specifications Input signal handling: 2V RMS Input impedance: ~3kΩ (stereo source), 10kΩ (mono source) Output impedance: 600Ω Output level: typically 250mV RMS (balanced) for a 1V RMS mono input Frequency response: ±0.5dB, 20Hz-20kHz Signal-to-noise ratio: -98dB unweighted (22Hz-22kHz); -101dB A-weighted, both with respect to 1V RMS input Total harmonic distortion: <0.02%, 100Hz-20kHz (0.1% <at> 30Hz), 1V RMS input Phase shift between input & output: 7° at 20Hz, 3° at 100Hz, ~0° above 1kHz Dimensions: 115 x 65 x 33mm   Mass: 175g Parts List: Passive DI Box With the sockets fitted, follow with the 2.2kΩ resistors. The transformer is then secured to the PCB using M3 x 6mm screws and star washers. The screws (fitted with the washers) are fed up from the underside of the PCB and go into M3 tapped holes on either side of the transformer. That done, feed the transformer wires up through the adjacent holes in the PCB (see Fig.2), then back down again and solder them to the indicated pads (ie, with the solder joints on the top). The wires are colour coded and must be connected as shown to correctly preserve the signal phase (ie, in-phase output to XLR pin 2 [hot]). We have marked the colours of the wires that correspond to the 10kΩ and 600Ω windings on both the PCB itself and the parts layout diagram. The leads for the switch and chassis earth can now be stripped and soldered to the PCB. Solder the other end of the earth wire to the solder lug. The soldered section of this terminal should be covered with heatshrink tubing, to prevent the lead from breaking at the solder joint. The XLR socket is held down using a cable tie that straps around the body and around the PCB at the recessed cutouts on either side. The positioning of the cable tie joiner is important. It must be positioned as shown in the photos, so it does not foul the case or lid. The case is used upside-down, with siliconchip.com.au 1 double-sided PCB, code 23111141, 105 x 24mm 1 panel label, 51 x 102mm 1 diecast box, 111 x 60 x 30mm (Jaycar HB-5064 or Altronics H 0432) 1 6.35mm stereo switched PCBmount jack socket (CON1) (Jaycar PS-0190, Altronics P 0073 or PA0073) 1 3.5mm stereo PCB-mount jack socket (CON2) (Jaycar PS0133, Altronics P 0092) 1 right-angle PCB-mount XLR male connector (CON3) (Altronics P 0874) 1 10kΩ to 600Ω Mu metal shielded transformer (T1) (Altronics M 0705) 1 SPST rocker switch (S1) (Jaycar SK-0984, Altronics S 3210) the lid as the base. Note that the Altronics version has a flanged lid; if you don’t want the flanges, it’s just a matter of cutting them off. Cut-out and drilling templates are provided for the various holes required in the base – see Fig.3. These templates can also be downloaded (no charge) from the SILICON CHIP website and printed out (browse to www. siliconchip.com.au then mouseover ‘Shop’, click ‘by Year/Month’ and select October/2014). The diecast aluminium is very easy to drill and file. The 6.5mm and 10.5mm-diameter holes are best made by first drilling small pilot holes, then carefully enlarging them to size using a tapered reamer. By contrast, the 22mm hole for the XLR connector is 2 2.2kΩ 0.25W or 0.5W 1% resistors 2 M3 x 6mm pan head machine screws 2 M3 x 10mm countersink head machine screws 2 3mm star washers 1 M4 x 10mm countersink head machine screw 4 M4 x 10mm Nylon pan head screws (optional, for feet) 1 M4 nut 1 4mm star washer 1 solder lug 1 60mm length of green medium duty (24 x 0.2mm) hookup wire 1 120mm length of black medium duty (24 x 0.2mm) hookup wire 1 100mm cable tie 1 100mm length of 6mm heatshrink tubing best made using a 22mm speed bore drill. Alternatively, it can be made by drilling around the inside perimeter with a small drill, then knocking out the centre piece and filing to shape. This same method can be used for the rectangular switch cut-out You also have to drill holes for the XLR mounting screws and the earth screw. If using countersunk screws (a good idea), countersink the holes to suit. The mounting holes on the XLR connector are untapped however threads can be formed by simply forcing the M3 screws into the holes. Once all the holes have been drilled, cover the threaded ferrule on the 3.5mm socket with a short length of 6mm-diameter heatshrink tubing. This is necessary to insulate it from October 2014  69 si In ve (D je I) ct B ox ion ct Pa s D ir e Ground Lift (Change Switch Position When Hum Is Present) This view shows the PCB assembly prior to installation in the case. the case. The heatshrink tubing must be shrunk down so that it cannot fall off and it should later fit snugly into the 6.5mm hole for this socket. We tapped the box corner holes to an M4 thread so that M4 Nylon screws could be used to secure the lid. The heads of these screws then act as feet. Alternatively, you can skip this step and use the original metal screws. You can then elect to either not have feet or you could attach separate screw-on/ stick-on feet to the lid (although these may not last long if the unit is treated roughly). Final assembly The PCB assembly is inserted into the box in a special way. First, the 6.35mm and 3.5mm jack socket ferrules are inserted into their respec- tive holes. The PCB is then bent in the middle by pushing down on the transformer and pushing the XLR connector inwards. This then allows the XLR socket to be slid into the box, after which the PCB is released so the socket fits into its 22mm-diameter hole. Fig.5 shows the details. If necessary, the PCB assembly can be removed from the case using the reverse procedure. The rocker switch can now be clipped into position and the earth lug secured to the side of the case using an M4 x 10mm screw, star washer and nut. Finally, complete the wiring to the switch lugs and secure the XLR socket to the case using two M3 x 10mm countersink head screws. As with the earth lug connection, it’s a good idea to fit heatshrink tubing over CO N3 CO N1 T1 PCB CO N2 Fig.5: the PCB is installed in the case by inserting the jack socket ferrules into their holes at one end and then bending the PCB by pushing against transformer T1 and the XLR socket (CON3) as shown here. 70  Silicon Chip CHIP SILICON www.siliconchip.com.au Fig.4: the full-size front panel label. It’s also available (in PDF format) from the SILICON CHIP website. the connections to the switch lugs to prevent the leads from breaking at the solder joints. Testing The Passive DI Box is tested by feeding in a signal from an instrument or signal generator and measuring the output across pins 2 & 3 of the XLR socket. To do this, set your multimeter to read ‘mV AC’, plug the instrument or generator into the DI box and play the instrument. You should get a signal reading of about 250mV on the meter for an input of around 1V RMS. If that checks out, set your multimeter to read ohms and connect it between the ground (sleeve) connection of the instrument’s jack plug in the DI Box and pin 1 of the XLR plug. Now check that this connection can be opened and closed using switch S1. Assuming it all works as expected, you can now fit the lid and affix the front-panel label to the base (not the lid). This label can be downloaded from the SILICON CHIP Chip website in PDF format and printed out on a colour printer. You have several options here. First, you can just use paper or photo-paper siliconchip.com.au KEEP YOUR COPIES OF AS GOOD AS THE DAY THEY WERE BORN! Above: once the PCB is in the case, it’s secured by fitting a nut to the 6.35mm jack socket at one end and two M3 x 6mm countersink-head machine screws to the XLR socket at the other end. Magazines are sneaky things: left to themselves, they’ll hide, they’ll get crushed, folded, spindled, dogeared, pages will disappear . . . not good when you want to refer to an article in the future. ONLY 14 95 $ INC GST PLUS p&p A SILICON CHIP binder will keep your copies in pristine condition – and you’ll always be able to find them! * Each binder holds up to 14 issues * Made from heavy duty vinyl * Easy-fit wire inserts ORDER NOW AT www.siliconchip.com.au/shop It’s a good idea to fit heatshrink tubing over the wiring connections to the switch and the earth lug to prevent the wires breaking at the solder joints. and affix the resulting label using a suitable glue or neutral cure silicone. However, this type of label is easily damaged. Alternatively, for a more rugged label, print a mirror image label onto clear overhead projector film (be sure to use a film that’s suitable for your printer). This is then attached using clear or coloured silicone sealant, with the image on the inside. Another approach is to print onto an A4-size synthetic ‘Dataflex’ sticky label if using an inkjet printer, or onto a ‘Datapol’ sticky label if using a laser printer. This can then be trimmed to size and affixed to the base of the case using the label’s self-adhesive backing. Dataflex and Datapol labels are available from www.blanklabels.com.au siliconchip.com.au  Dataflex & Datapol Labels (1) For Dataflex labels, go to http:// www.blanklabels.com.au/index. php?main_page=product_ info&cPath=49_60&products_ id=335 (2) For Datapol labels go to http:// www.blanklabels.com.au/index. php?main_page=product_ info&cPath=49_55&products_ id=326 and sample sheets are available on request to test in your printer – see panel. Your Passive DI Box is now ready for use. Just remember that you can’t plug the high-impedance output from an electric guitar directly into it. SC Where do you get those HARD-TO-GET PARTS? Many of the components used in SILICON CHIP projects are cutting-edge technology and not worth your normal parts suppliers either sourcing or stocking in relatively low quantities. Where we can, the SILICON CHIP PartShop stocks those hard-to-get parts, along with PC boards, programmed micros, panels and all the other bits and pieces to enable you to complete your SILICON CHIP project. SILICON CHIP PARTSHOP www.siliconchip.com.au/shop October 2014  71