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This versatile Direct Injection (DI) Box incorporates a 3-band equaliser (EQ) and can be powered using battery, plugpack or phantom power. You can use it for DI-ing your instruments and as an in-line equaliser. A DI BOX FOR MUSICIANS By JOHN CLARKE “WOTSA DIRECT injection box?” we hear you ask, so let’s cut straight to the main chase. Basically, a DI Box is a device that accepts an unbalanced mono or stereo input signal from a musical instrument and converts it into a balanced output signal. This signal is then fed into a balanced microphone input on a mixing desk. This has lots of advantages when it comes to minimising hum and noise, especially where long cable runs are involved. We’ll have more to say on this shortly. The “direct injection” bit is a musician’s term. It refers to signals that are directly coupled (or injected) into the audio chain from a musical instrument, rather than picked up by a microphone. The signal can come from an outlet socket on the instrument itself, from a pickup (eg, on an electric guitar), or from any other source such as a CD player or tape player. In a nutshell, a DI-Box allows musical instruments to be coupled to the balanced microphone inputs of a mixing desk. It has a high-impedance input so that it doesn’t load (or 12  Silicon Chip degrade) the signal source and a low output impedance, similar to that provided by a balanced microphone. In fact, it’s fair enough to say that a DI-Box “looks” just like a microphone as far as the mixing desk is concerned. Do you really need it? So do you really a DI-Box? You “betcha” – if you’re into serious sound reinforcement, you generally need one for each instrument. But why use a DI-box? Why not connect the output from the instrument directly into the mixer? The answer is that you’ll almost certainly run into serious hum problems and signal losses if you do. The big advantage of using a DI-box is the balanced output it provides for connection to the mixing desk (all high-quality mixers have balanced inputs). This balanced output has two signal lines and a ground return and these connect to standard 3-pin XLR sockets. Pins 2 & 3 of the XLR socket carry the signal and these operate in anti­ phase to each other. In other words, when one line goes positive, the other line swings negative by the same amount. At the mixing end, the two signals are subtracted to recov­er the original signal. Any hum signal which is picked up along the line is effectively cancelled because the same amount of hum will be present in both signal lines. As a result, the subtrac­ tion process attenuates the hum to very low levels. This hum rejection ability using balanced lines is the main reason for using a DI box. Similarly, other forms of interference (eg, from a lighting control desk) are also rejected, since the interference signal will be common to both lines. Another good reason for using a DIBox is that its high-impedance input prevents loading of guitar pickups. By contrast, if the pickup was to be excessively loaded, the high-frequency response would suffer. Don’t be unbalanced In some cases, the output from an instrument can be con­nected directly www.siliconchip.com.au to a mixer using an unbalanced signal. This involves using either one of the mixer’s unbalanced inputs or by using a specially wired lead which connects the inverted signal line to ground. There will no longer be any hum cancellation but this may not be a problem if lead lengths are kept short or if the output impedance of the signal source is very low. That said, unbalanced coupling is seldom used and there are several reasons for this apart from the lack of noise cancella­ t ion. First, some mixers cannot cope with the line-level outputs from musical instruments, since they are usually set up for amplifying low-level microphone signals (usually only tens of millivolts). As a result, the mixer will overload and the sound will be badly distorted. In addition, “hum loops” can be a real problem, especially when a stage amplifier is also connected to the instrument. In this case, there will be a continuous earth loop because the amplifier and mixing desk are connected together via their mains earths and also via the shield connections in the signal cables. These problems can all be solved by using a DI-Box which provides for signal attenuation and includes a so-called “ground lift” circuit. This “ground lift” circuit is simply a switch which disconnects pin 3 on the XLR socket from ground – ie, it disconnects the signal earth at the DI-Box output to break the earth loop. A ground lift switch can literally make the difference bet­ween a very loud audible hum in the system and virtually no audible hum. The SILICON CHIP DI-Box boasts all the above necessary features, including high input impedance, a low-impedance bal­anced output, an atten­uator control (to prevent signal overload) and a ground lift switch. As a bonus, it also includes a 3-band equaliser (consisting of bass, mid and treble controls), so that you can adjust the sound to suit the venue. Another worthwhile feature is the provision of a stereo input so that it can be used with signal sources such as stereo keyboards, CD players and MP-3 players. Note that this stereo input is mixed internally to provide a mono signal. Genuine stereo operwww.siliconchip.com.au The circuit is built into a rugged diecast case to prevent damage during transport. ation will require two DI boxes – one for each channel – and a stereo mixer. Other uses The DI-Box can be used for other purposes as well. For example, it could be connected in-line between the mixer’s fold­back output and the input to a foldback amplifier. That way, you can adjust the EQ (equalisation) of the foldback signal as op­posed to equalising the sound before the signal is sent to the mixer. Alternatively, you could use it to equalise the effects output from the mixer. Power for the SILICON CHIP DI-Box can come from a 12VDC plugpack, a 9V battery or via phantom power from the mixer. All three supplies are isolated from each other so that no harm can occur, even if all three power sources are connected simultane­ously. A separate power switch is used to turn the unit on and off and there’s also a battery test switch so that you can quick­ly check the condition of the battery. Circuit details With all that magic, you might think that the circuit has to be complicated but it’s not. All the details for our DI-Box are shown in Fig.1. It uses two low-cost op amp packages, four potentiometers, two jack sockets, several switches, an XLR panel plug for the balanced output and a handful Main Features •  High-impedance mono input (for guitar pickup) •  Stereo input mixing for tape, CD or other stereo signals •  Input level control, allowing optimum signal level before over­load •  Balanced output •  Three-band equaliser (EQ) •  Can run from battery, plugpack or phantom power •  Battery check function •  Ground lift switch for hum loop control •  Housed in a rugged metal diecast case August 2001  13 BALANCED OUT 2 10mF BP 620W VCC/2 VCC/2 PHANTOM POWER 680W X Y 27k 680W VR5 VCC/2 10k OFFSET 4 1 100mF 16V OUT A ZD2 12V 1W IN K LED 100mF 16V GND 100k 10mF 16V 7812 100k CUT BOOST TREBLE +9V (12V) 10k VR4 100k LIN .0015mF 10k MID VR3 100k LIN .012mF 12k BATTERY 9V 10mF 16V D4 1N4004 SC Ó 2001 S1 POWER 12V DC INPUT LOOP OUT + _ TIP RING TIP RING MONO/ STEREO IN DIRECT INJECTION BOX 10mF 16V OUT GND IN REG1 7812 220W ZD1 5.1V 1W D1 1N5819 D2 1N4004 1M 10pF 4 2 IC1a TL072 8 10mF BP LED1 l A K 10k S3 BATTERY TEST 10mF BP VR1 1M LOG LEVEL 10mF BP S4 MONO/ STEREO 10k 10pF 1M 3 VCC/2 +9V (12V) 1 2.2mF BP 12k VR2 100k LIN 18k .0027mF 18k BASS D3 1N4004 1k 560pF 2 3 7 IC2 TL071 +9V (12V) 5 6 10k 4.7k 5 6 IC1b TL072 10k 100pF 7 0.47mF Y SHELL XLR PLUG 10mF BP S2 LIFT/ GROUND X 1 3 COLD 620W HOT .015mF Fig.1 (left): the complete circuit for the DI Box. IC1a buffers the incoming signal and drives a 3-band tone control stage (bass, mid & treble). This stage then drives op amps IC2 and IC1b to produce the balanced output signals. 14  Silicon Chip of minor parts. As shown, the incoming mono signal is fed in via the tip connection of a 6.35mm jack socket. This signal is then applied to potentiometer VR1 via a 10kΩ resistor and series 10µF bipolar capacitor. A 10pF capacitor is wired across VR1 and acts with the 10kΩ input resistor to reject RF (radio frequency) signals. The associated “Loop Out” socket is simply wired in parallel with the input socket so that the unprocessed signal can fed to other audio equipment; eg, to a stage amplifier. In the case of stereo input signals, the second channel is fed to the ring terminal on the input socket and then applied to VR1 via mono/stereo switch S4 and a second 10kΩ resistor and series 10µF capacitor combination. The two channels are then mixed together at the top of VR1, to form a mono signal. The 10µF bipolar capacitors are included to prevent DC from being applied to VR1, so that it isn’t noisy in use. VR1 acts as the level control. Its output is AC-coupled via another 10µF bipolar capacitor to the non-inverting input (pin 3) of op amp IC1a. This input is biased to the half-supply rail (Vcc/2) via a 1MΩ resistor. Because of this, a second 1MΩ feed­back resistor is connected to the inverting input (pin 2), to minimise the output offset due to input bias currents. The 10pF capacitor across the 1MΩ feedback resistor prev­ents IC1a from oscillating. In operation, IC1a acts as a unity-gain buffer amplifier. It drives the following equaliser (or tone control) stage via a 2.2µF bipolar capacitor. EQ controls The tone controls are based on op amp IC2 and potentiome­ters VR2, VR3 & VR4. These pots and their associated resistors and capacitors are in the feedback path between IC2’s output at pin 6 and its inverting input (pin 2). Each of the bass, mid and treble stages can be considered separately www.siliconchip.com.au since they are connected in parallel between the signal output of IC1a and the inverting input (pin 2) of IC2. Note that pin 2 of IC2 is a virtual ground. Let’s first look at the bass control (VR2). When VR2 is centred, the resistance between pin 1 of IC1a and pin 2 of IC2 is equal to the resistance between pin 6 of IC2 and pin 2 of IC2 – ie, the input and feedback resistances are equal. As a result, IC2 operates with a gain of -1 (the .015µF capacitor has no effect since it is equally balanced across the potentiometer). Now let’s see what happens when we wind VR2’s wiper fully towards the output of IC1a. The input resistance for IC2 now decreases to 18kΩ, while the feedback resistance increases to 118kΩ. At the same time, the .015µF capacitor is now completely included in the feedback circuit. Without the capacitor, the gain would be -118kΩ/18kΩ = -6.5 (16dB) at all frequencies. In practice, though, the .015µF ca­pacitor rolls off the response above 100Hz, so that the gain quickly reduces towards -1 as the frequency increases. As a result, we have maximum bass boost below 100Hz. Conversely, when the wiper is wound towards IC2, the gain without the capacitor is 18kΩ/118kΩ = -0.15 (-16dB). The capaci­tor is now on the input side so the gain rapidly increases to -1 at frequencies above 100Hz. Thus the maximum bass cut is below 100Hz. Intermediate settings of VR2 between these two extremes provide lesser amounts of bass boost or cut. The midrange section (VR3) works in a similar manner except that there is now a .012µF capacitor in series with the input. This combines with the .0027µF capacitor across VR3 to give a bandpass filter. Finally, the treble control (VR4) operates with only a .0015µF input capacitor; ie, there’s no capacitor across VR4 in the feedback path. As a result, this control produces a high frequency boost or cut at 10kHz. Fig.2 shows the response of the tone controls. Note that the maximum bass boost is 12dB at 100Hz. The maximum boost and cut is lower for the midrange and treble controls. The 560pF feedback capacitor across IC2 provides high fre­quency rolloff to prevent instability. Similarly, the 1kΩ resis­tor at the inverting input acts as a stopper for RF signals to prewww.siliconchip.com.au AUDIO PRECISION FREQRESP AMPL(dBr) vs FREQ(Hz) 20.000 05 MAY 100 23:27:05 15.000 BASS 10.000 MID TREBLE 5.0000 0.0 -5.000 -10.00 -15.00 -20.00 20 100 1k 10k 20k Fig.2: this graph shows the responses generated by the bass, mid-range and treble controls. The maximum bass boost is 12dB at 100Hz, while maximum mid-range boost is about 9dB at 850Hz. The treble boost is limited to about 7dB at 11kHz. vent radio pickup. Trimpot VR5 acts an offset adjustment for IC2 – it allows the DC output of IC2 to be nulled to prevent DC current from flowing in bass control VR2. This is necessary since any DC current flowing in VR2 would make the pot noisy to operate. IC2’s output appears at pin 6 and drives pin 3 (cold) of the XLR plug via a 10µF bipolar capacitor and series 620Ω resis­tor. The resistor provides the requisite 600Ω output impedance while the capacitor prevents the phan- tom supply voltage (if present) from being loaded by IC2’s output. It also prevents the Vcc/2 voltage on IC2’s output from being applied to the XLR plug. As well as driving pin 3 of the XLR plug, IC2 also drives op amp IC1b via a 10kΩ resistor. This stage is wired as an in­verting amplifier with a gain of -1 to derive the in-phase signal. Its output appears at pin 7 and drives pin 2 (hot) of the XLR plug. The remaining pin on the XLR plug is the ground pin (pin 1). This is Specifications Signal Handling: 2.42V RMS at maximum level and equaliser at flat settings with 12V supply (greater at lower level control settings); 1.74V RMS with 9V supply Input Impedance: 470kΩ mono; 10kΩ for stereo Total Harmonic Distortion: .009% at 100Hz and 200mV; .02% at 1kHz; .05% at 10kHz Frequency response: -3dB at 13Hz; -2dB at 20kHz Equaliser response: see graphs Signal-to-noise ratio: 93dB with respect to 1V 20Hz-20kHz filter (96dB A weighted) Phase difference between pin 2 & pin 3 XLR output: 180° at 1kHz; 160° at 20kHz Battery test: LED dims for low battery voltages Battery current: 8.8mA <at>9V August 2001  15 Fig.3: install the parts on the PC board and complete the wiring as shown here. Note that the component shown in purple should not be installed until after the four pots have been soldered to their respective PC stakes. Take care with component orientation. either directly connected to ground via S2 or AC-coupled to ground via a 0.47µF capacitor when this switch is open. Open­ing the Ground Lift switch prevents hum loops if the input to the DI-Box is separately grounded to earth (eg, via a foldback amplifier). Power supply As mentioned earlier, power for the circuit can come from a DC plugpack, a 9V battery or via phantom power. Diode D4 provides reverse polarity protection for external DC power sources such as plugpacks. The DC supply rail is then filtered and applied to 3-terminal regulator REG1 to derive a +12V rail. This is then applied to the op amps IC1 & IC2 via diode D2. The internal 9V battery supply (if present) is fed to the op amps via Schottky diode D1. A Schottky diode has been used here because it has a much lower voltage drop across it than a standard diode and this extends the 16  Silicon Chip useful battery life. Note that the negative return of the battery goes via the DC power socket as well as via power switch S1. As a result, the battery is automatically disconnected when ever a plug is insert­ed into the DC power socket. Phantom power is delivered via pins 2 & 3 of the XLR plug and is applied via two 680Ω resistors to diode D3. Zener diode ZD2 regulates the voltage to 12V before it is applied to the rest Table 1: Capacitor Codes  Value IEC Code EIA Code  0.47µF   470n   474  .015µF   15n  153  .012µF   12n  123  .0027µF   2n7  272  .0015µF   1n5  152  560pF   560p   561  100pF   100p   101  10pF   10p   10 of the circuit. Note: phantom power is usually produced from a source of either 48V with a 3.4kΩ impedance or from 24V and a 600Ω im­pedance. This means that we can draw up to about 9mA from each supply, or 18mA in total at 12V. Diodes D1, D2 & D3 isolate each supply so that only one source can deliver power to the circuit. Essentially, where more than one supply is connected, it is the highest voltage source that powers the unit. The half-supply rail (Vcc/2) is obtained using two 100kΩ resistors connected in series across the power supply. The half-supply point is de­ coupl­ed using a 100µF capacitor to prevent any supply ripple. S3, LED1, ZD1 and the series 220Ω resistor form a simple battery test circuit. If the battery voltage is 9V, the voltage across the 220Ω resistor will be 9V - 5.1V - 1.8V (the voltage across the LED), or about 2.1V. As a result, www.siliconchip.com.au about 9.5mA will flow through LED1 when S3 is closed and the LED will glow brightly. As the battery voltage goes down, the current through the LED drops accordingly and so its brightness also decreases. For example, a battery voltage of 7.5V will leave about 0.6V across the 220Ω resistor and so just 2.7mA will flow through the LED which will now be quite dim. Putting it together Building it is easy because most of the parts are mounted on a PC board coded 01108011 (102 x 84mm). This is housed in a metal diecast box measuring 119 x 94 x 57mm. The diecast case serves two purposes: (1) it provides the necessary shielding for the audio circuitry; and (2) it makes the unit extremely rugged – a necessary requirement for stage work. Fig.3 shows the PC board assembly and wiring details. Begin by checking the PC board for any shorts or breaks in the copper tracks. Check also that the PC board fits neatly into the case. If it doesn’t, file the corners and edges of the board, so that it fits when seated on 9mm standoffs (these can be temporarily attached for testing the board fit). Note that the case tapers in slightly towards the base. The board doesn’t have to go all the way down – just to within 9mm. Now for the board assembly. Install the three wire links first, then fit the resistors. Table 2 shows the resistor colour codes but it’s also a good idea to check each one using a digital multimeter, as the colours can be hard to recognise. The diodes can go in next but make sure that D1 is the 1N5819. Be careful not to mix up the two zener diodes – Inside the completed prototype. The two 6.5mm jack sockets (at left) have to be wired before they are attached to the side of the case. Similarly, you will have to complete the wiring to the PC board before fitting the other hardware items. ZD1 is the 5.1V zener, while ZD2 is the 12V zener. The 5.1V zener will probably be marked “1N4732”, while the 12V zener can carry a “1N4742” marking. The two ICs can now be installed, taking care to ensure that IC1 is the TL072 (or LF353). This done, install the capaci­tors, using Table 1 to identify the low-value units. The bipolar electrolytic capacitors can be installed either way around but make sure that the “normal” electrolytic capacitors (ie, the polarised types) are installed with the correct polarity. The capacitors marked in purple should be left out for the time being – see Fig.3. VR5, REG1 and the DC power socket can go in next, followed by the PC stakes. You will need PC stakes at all the external wiring points, including three stakes for each of the pots. LED1 should be installed with its body about 20mm above the board. It is later bent over and pushed into a bezel mounted on the side of the case. Table 2: Resistor Colour Codes  No.   2   2   1   2   2   6   2   2   1 www.siliconchip.com.au Value 1MΩ 100kΩ 27kΩ 18kΩ 12kΩ 10kΩ 680Ω 620Ω 220Ω 4-Band Code (1%) brown black green brown brown black yellow brown red violet orange brown brown grey orange brown brown red orange brown brown black orange brown blue grey brown brown blue red brown brown red red brown brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown red violet black red brown brown grey black red brown brown red black red brown brown black black red brown blue grey black black brown blue red black black brown red red black black brown August 2001  17 These two views shows the locations of the RCA input sockets, the 3-pin panel-mount XLR socket and the Ground-Lift and Stereo/Mono rocker switches. The Power and Battery Test rocker switches are mounted on the rear panel, along with the battery test indicator LED. Finally, complete the board assembly by securing the battery holder using three M2.5 screws. Don’t forget to solder its leads. Final assembly OK, now for the final assembly. First, position the PC board inside the case and mark out the four corner mounting holes. This done, drill these holes to 3mm and countersink the holes on the underside of the box. Next, attach the four 9mm tapped spacers to the underside of the PC board using M3 screws and secure these into the box using countersunk M3 screws. Now mark out the po- sitions for the pot shafts – these are mounted directly above their corresponding stakes on the PC board, with the shaft centres about 28mm above the base. Once the centres have been marked, remove the board and drill the holes for the pots. It’s best to start with a small pilot drill and then carefully enlarge the holes to size using a tapered reamer. Once this has been done, use a rat-tail file to elongate the holes vertically – this will make it easier to insert the pots through the holes when they are later attached to the PC board. Now mark out and drill mounting holes for the 6.35mm jack sockets, the XLR panel plug, the DC socket entry, the LED and the switches. You can use the front panel artwork and the photographs to guide you in positioning these holes. The switch cutouts can be made by first drilling a series of small holes around the inside perimeters, then knocking out the centre-pieces and carefully filing the edges. Note that all three switches must be a snug fit, so that they are held in position by their plastic retaining lugs. Don’t make the holes too big, otherwise the switches will fall out. The four pots can now be attached to the PC board by sol­ dering their leads to the front of the PC stakes (make sure that VR1 is the 1MΩ pot). Install them so that their shaft centres are about 17mm above the top of the board. It’s best to lightly tack solder one of the pots first, then test the assembly to make sure it fits in the case before finally installing the remaining pots. This done, install the capacitors marked on the overlay (Fig.3) in purple, then reinstall the board and secure the pots to the case by doing up the nuts. Internal wiring Fig.4: this full-size front panel artwork can be used as a guide when positioning the switches and sockets. 18  Silicon Chip All that remains now is to fit the remaining hardware items and complete the wiring. You will find that it’s easier to run the wiring from the PC board to several of these items before they are attached to the case (eg, to the XLR plug, the 6.35mm jack sockets and the power switch). The panel-mounting XLR plug is secured using M3 x 9mm screws, star www.siliconchip.com.au washers and nuts. The lower nut can initially be held in place using some adhesive tape, to make it easy to attach the screw. The LED is inserted into its adjacent bezel on the side of the case by bending its leads over and clipping it into position. Finally, complete the assembly by fitting the front panel label to the lid of the case and sliding the knobs onto the pots. Testing Now for the smoke test. Apply power using a 9V battery or 12VDC plugpack (or a DC power supply set to about 15VDC) and check that the LED lights when the Battery Test switch is on. This done, check that for +9V (or +12V) on pin 8 of IC1 and on pin 7 of IC2. The voltage should be around +9V when a fresh battery is used and +12V for a plugpack supply. Now connect your DMM across bass pot VR2 and adjust VR5 for 0V DC. This stops DC current flowing through VR2 which might make it noisy. Further testing can be made using your DI source. This can range from a guitar pickup through to high-level inputs such as keyboards. Set the input level control to maximum when using low-level sources such as guitar. Conversely, it may be necessary to wind the input level control down for high-level sources to prevent clipping, particularly when equaliser boost is applied. Make sure that you select mono for high output impedance sources such as a guitar pickup. This is because the input im­pedance of the DI box in mono is 470kΩ but only 10kΩ for stereo. The stereo selection is used only with stereo sources and, as explained previously, mixes the signal to a mono output. Parts List 1 PC board, code 01108011, 102 x 84mm 1 diecast box, 119 x 94 x 57mm 1 front panel label, 100 x 87 1 XLR metal panel plug 2 6.35mm stereo jack panel sockets 3 SPST mini rocker switches (S1-S3) 1 1MΩ 16mm log pot (VR1) 3 100kΩ 16mm linear pots (VR2-VR4) 1 10kΩ 16mm linear pot (VR5) 4 knobs to suit pots 1 DC socket (PC-mount) 1 216 9V battery or 12VDC 200mA plugpack 1 9V battery holder 1 5mm LED bezel 4 9mm long M3 tapped spacers 4 M3 x 6mm screws 4 M3 x 6mm countersunk screws 4 M3 x 9mm countersunk screws 2 M3 nuts and star washers 3 M2.5 x 9mm screws 23 PC stakes 1 400mm length of green hookup wire 1 300mm length of black hookup wire 1 200mm length of blue hookup wire 1 200mm length of yellow hookup wire 1 200mm length of 0.8mm tinned copper wire Semiconductors 1 TL072 dual op amp (IC1) 1 TL071 op amp (IC2) 1 7812 12V 3-terminal regulator (REG1) 1 5.1V 1W zener diode (ZD1) 1 12V 1W zener diode (ZD2) 1 5mm red LED (LED1) 1 1N5819 Schottky diode (D1) 3 1N4004 diodes (D2-D4) Capacitors 2 100µF 16VW PC electrolytic 3 10µF 16VW PC electrolytic 5 10µF bipolar electrolytic 1 2.2µF bipolar electrolytic 1 0.47µF MKT polyester 1 .015 MKT polyester 1 .012 MKT polyester 1 .0027 MKT polyester 1 .0015 MKT polyester 1 560pF ceramic 1 100pF ceramic 1 10pF ceramic Resistors (0.25W 1%) 2 1MΩ 6 10kΩ 2 100kΩ 2 680Ω 1 27kΩ 2 620Ω 2 18kΩ 1 220Ω 2 12kΩ XLR-to-jack plug lead If you are using the DI Box as an inline equaliser, you may need to make up an unbalanced XLR line socket to jack plug lead. It is wired with the pin 3 connection open, the signal connected to pin 2 and the lead shield connected to pin 1. Ground lift (S2) should only be selected if there is a ground loop that’s causing hum. The hum should cease when S2 is opened. Finally, make sure that the DI Box is switched off when not in use to conserve battery life. You can test the battery with the Battery Test switch at any time when the power is on. SC www.siliconchip.com.au Fig.5: this is the full-size etching pattern for the PC board. Check your board for defects before installing any of the parts. August 2001  19