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uses . . . e h t f o e m Jus t so s to balanced gnal ixers. unbalanced si • Connecting ss Amplifier inputs and m re d d A ents. Public wered instrum o p to g n ti ec n n pickups • Ideal for co pedance guitar c.) m -i h ig h r fo et • (Not suited preamp or effects unit, unless via a Hum and noise plaguing your performance? Longing for a DI Box that performs as well as a powered unit but does not require batteries? Here it is: SILICON CHIP Passive DI Box by JOHN CLARKE 62  Silicon Chip siliconchip.com.au M usicians and performers often have to connect to an existing PA system in a pub or club, hall, auditorium, church, etc. You might think that’s as simple as plugging a lead from the output socket of an instrument, effects unit, preamp, etc into an appropriate input on the PA system. But it’s not usually quite that simple. For a start, most musical equipment has 6.35mm phono jacks, while most “pro” mixers and amplifiers use XLR connectors. Even if the incompatible leads problem can be solved (why is it that your lead always has male plugs and their equipment has male sockets?), even using a special interconnecting lead, there is usually a far greater problem. Hummmmmmmmmmmm . . . . . There will sometimes, even often, be a large amount of mains hum and noise introduced into the long leads generally associated with these installations. So by the time the signal reaches the mixer, which is the heart of a typical Public Address System, hum and noise will mar the performance. If you’re lucky, it’s barely tolerable; usually it’s not! That’s where the DI Box comes in. Just in case you were wondering, the term DI stands for Direct Injection and refers to the direct coupling-by-wire of a musical instrument to a sound system rather than using a microphone to pickup the sounds. Apart from converting from the 6.35mm jack connection to the XLR type, the DI Box produces a balanced signal output. The balanced signal is then applied to the balanced input of the audio mixer. Balanced signal lines can greatly reduce injected hum and noise. The balanced signal cable has two signal lines and a ground return. Pins 2 and 3 of the XLR connector carry the signal and pin 1 is the ground. The signal lines operate in antiphase to one another so that as one line goes positive, the other line swings negative. At the mixing end, the balanced signal is applied to a differential amplifier which amplifies the “difference” between the two signal lines. This process means that any siliconchip.com.au Features . hum and noise picked up along • No power re the balanced lines is effectively quired • Small size cancelled. • Rugged hous One final feature of the ing • Balanced XLR DI Box allows isolating the output • Unbalanced grounds of the musical instru“thr u” output (stereo or mon • Stereo inpu ment and the Public Address o) tm • Wide freque ixing for mono out system. As every hifi buff ncy response • Ground lif t knows, all equipment needs switching to be earthed back to the same point. Otherwise circulating currents can occur in the earth system In order to provide this high input and large amounts of hum can be impedance, a DI Box generally reintroduced to the system. quires electronic circuitry to perform For safety’s sake, both the PA system the impedance transformation and amplifier and the instrument need to to drive the balanced lines. These be grounded (or earthed – it means the DI Boxes require power by way of a same thing) – but these grounds can battery, plugpack or phantom power be many (sometimes many, many!) from the mixer. metres apart: a classic recipe for hum. Our new DI Box is not powered. It’s called a hum loop – and forIt uses a transformer to provide the tunately can be easily remedied by balanced output and the input impedlifting or breaking the two earth conance is not high enough for a guitar nections. pickup on its own. Of course, it can be used with a guitar if you use a preamplifier or suitable effects box ahead of the DI Box. These units will provide the drive required. Otherwise, for stand-alone guitar use, we recommend the powered DI Box published in the August 2001 issue of SILICON CHIP. The Passive DI Box can be driven from any source that is capable of driving a 9.4kW load at up to 1V rms. This would include virtually any output from keyboards, preamps and effects units, mixers, etc. Speaking of mixers, the Passive DI Box has its own stereo mixing feature. If you insert a stereo jack with stereo signals, the DI Box will mix the two channels before A DI Box generally does this with a providing a balanced mono output. switch that opens or closes the earth This feature is ideal for connecting connection. Some DI Boxes have a stereo sources, such as from audioseparate ground lift input instead of visual equipment and multimedia the switch. In most cases, the DI box players. can be used with the ground lift switch Note that the signal produced open or closed. through the PA system will be mono. The SILICON CHIP Passive DI Box To provide a stereo sound, you’ll need provides all the above features. Howtwo DI Boxes, one for the left channel ever, it does not provide for all DI and one for the right channel . applications – for instance, it cannot be used directly with a guitar. Many DI Performance Boxes provide high input impedance Performance of the SILICON CHIP Passo that the unit will not load down sive DI Box is exceptional. During tests, and degrade the signal source from a it far outperformed one commonly guitar pickup. .. May 2006  63 Just input and output sockets, a couple of resistors, a transformer, switch and an output plug make up the circuit of the Passive DI Box. available commercial unit we were using for comparison, in terms of audio sound quality and frequency response. In fact, our tests for signal-to-noise ratio do not do the unit justice. The tests results are below the noise floor of our equipment. While the frequency response of the commercial unit was very restricted in the bass region (reducing signal by -3dB at 250Hz, -6dB at 125Hz and more than -12dB at 60Hz), our unit could pass signals well below 20Hz without any appreciable attenuation. Musicians who have played with our Di Box also remarked that it had a higher quality sound than the commercial unit. So you’d be much better off building the SILICON CHIP Passive DI Box than buying some commercial units at many, many times the price. If you must buy a DI box (for example, to use with a guitar), be sure that the specifications are suitable for your application. In most cases, the 12dB reduction in signal at 60Hz for the commercial unit would not be satisfactory unless the response is tailored with an equaliser. But why lose frequency response in the first place? Having said that, many higher quality commercial DI boxes do use a professional audio transformer that has a wide frequency response. So shop around and read the specs! The circuit The exceptional sound quality from the Passive DI Box is because the circuit is based around a high-quality audio transformer from Altronics. While the frequency response of the Passive DI Box may not appear flat in this graph, take a look at the vertical scale. The full scale vertical axis is only 1dB above and below the central 0dB level! The DI Box response is within 0.5dB from 20Hz to 20kHz when driven with a 50W source. 64  Silicon Chip Designated the M 0705, this particular transformer has a 10kW winding and a 600W centre-tapped winding that makes it ideal for our DI Box design. The transformer also uses a Mu-metal shield to prevent hum fields entering the windings. Two stereo 6.35mm jack sockets connect in parallel. Ordinarily, one is for the input signal while the second provide a “thru” output for daisy chaining the signal to another input. This “thru” output can be connected to an amplifier. As we mentioned, the DI box will also mix a stereo signal to mono. The tip and ring connections on the socket are coupled using 4.7kW resistors for this purpose. A stereo source will not be shorted because of the use of a stereo socket and the isolation of the left and right channels via a 4.7kW resistors required for stereo mixing. This would not be the case if a mono socket were used instead. Note that the stereo source will be shorted at the ring terminal if a mono jack pluf is inserted into the ‘thru’ socket. The resulting signal is applied to the 10kW primary winding of T1. The input socket grounds connect to the ground end of this winding. The 600W secondary of T1 becomes the balanced output, with pins 2 and 3 of the XLR plug connecting directly to this winding. The centre tap of the secondary winding is not used. Pin 1 of the XLR is the ground pin and is connected to the shield (shell) of the plug housing. This provides shielding for the XLR plug when it is inserted into this socket. As shown in this graph, the total harmonic distortion (THD) for the Passive DI Box is less than 0.02% over the frequency range from 80Hz to 20kHz when supplied with a 1V RMS signal. The distortion is even lower with less input signal level. siliconchip.com.au Here’s how to wire up the DI Box – compare this with the photo below. No PC board is used because the wiring is so simple. The common ground connection between the input ground and the pin 1 ground on the output plug is passed through switch S1. This provides the ground lift (when open) or a ground connection when closed. Normally S1 is left closed unless there is a hum loop. Construction There is no PC board for this project. This is because there is no need to use one, with all connections being made with short lengths of wire. Before wiring, the box should be drilled out to suit all the components. The two 6.35mm jack sockets are spaced 22mm apart and require 11mm diameter holes. We placed our sockets 9mm down from the top edge of the box. This makes them sufficiently high in the box so that there is room for the terminals but not so high that they foul the lid. The XLR plug is located in the centre of the opposite end of the box. Its 19mm mounting hole is made by drilling a series of holes close to the inside of this circumference and then knocking out the unwanted piece and filing to shape. The XLR plug is then secured using two M3 x 10mm countersunk screws, star washers and nuts. And this is what the inside of the SILICON CHIP Passive DI Box looks like: 6.35mm sockets on the left, transformer and ground lift switch in the middle and XLR output socket on the right. We covered all exposed wiring with heatshrink tubing. siliconchip.com.au May 2006  65 Parts List – Passive DI Box 1 diecast aluminium box, 111 x 60 x 30mm 1 front panel label, 100 x 50mm 1 10kW to 600W audio transformer with Mu metal shield (Altronics M-0705) (T1) 2 6.35mm insulated stereo jack sockets (Altronics P-0073, Jaycar PS-0190) 1 SPST rocker switch (S1) 2 4.7kW 0.25W 1% metal film resistors 1 crimp eyelet or solder lug with 3mm eyelet hole 4 M3 Nylon washers (or 2 x 3mm spacers) 2 M3 x 6mm countersunk screws 3 M3 x 10mm countersunk screws 3 M3 nuts 3 M3 star washers 1 50mm green hookup wire 1 20mm length of black hookup wire 1 20mm length of red hookup wire 1 100mm cable tie 1 50mm length of 5mm diameter heatshrink tubing The Ground Lift switch is mounted centrally on the side of the box. Its 19 x 13mm cutout is made in a similar way to the XLR socket. The earthing screw is also mounted on this side of the box. Countersink this hole for the countersunk screw. The transformer is mounted on 3mm-high spacers and secured with two M3 x 6mm countersunk screws from the underside of the box. The spacers allow the wires to exit from beneath the transformer body. Countersink these holes. If you don’t use countersunk screws here, you will need to use some rubber feet on the underside of the box. The wiring diagram shows how to connect the parts. Fit heat­shrink tubing over the exposed terminals on the jack sockets and the switch terminals to prevent the leads coming adrift. The jack socket terminals will have to be bent over to provide clearance between the terminals and the base of the box. The switched terminals on the sockets are unused and can be bent over at 90° against the socket body. The main terminals on the sockets can be bent about 45° inwards. The two 4.7kW resistors are only needed for stereo mixing but at just a few cents each, you might as well include them for both mono or stereo. Wire the resistors to the tip and ring terminals and then join them to make the mono connection to T1’s primary (yellow wire). The earth lug is secured to the side of the case and wired to the switch and shield on the XLR socket. Use an M3 x 10mm countersunk screw, star washer and M3 nut to secure the lug in place. Testing The Passive DI Box is best tested using a signal from an instrument (or signal generator) and measuring the output across pins 2 and 3 of the XLR plug. Set your multimeter to read AC mV, plug the instrument/generator into the DI box and play the instrument. You should get a signal reading on the meter at about 100mV if the input Same-size artwork for the DI Box front panel. There is no front panel/lid drilling required – all holes are on the sides of the box. is around 1V. Set your multimeter to read ohms and check that the ground connection between the instrument’s jack plug in the DI Box and the XLR plug at pin 1 can be opened (a high ohms reading) and closed (a low ohms reading) using switch S1. So there it is: a very simple and even simpler-to-build DI Box which you’ll wonder how you ever got along without. Just remember, you can’t plug SC your guitar in directly! Specifications: Input signal handling: ........................................... Input impedance: ................................................... Output Impedance: ................................................ Signal Level: ........................................................... Frequency Response:............................................ Signal-to-Noise Ratio: ........................................... 3.5V RMS 9.4kW (stereo source); 14.7kW mono source 600W (nominal) typically 120mV out (balanced) for 1V input within ±0.5dB from 20Hz to 20kHz -98dB unweighted (22Hz to 22kHz); -101dB “A” weighted, both with respect to 1V input (see text) Total Harmonic Distortion:..................................... < 0.1% 30Hz to 20kHz at 1V RMS input Phase Shift Between Input And Output: ............... -7° at 20Hz, <3° at 100Hz, 0° above 1kHz Dimensions:............................................................ 120 x 65 x 32mm overall Mass: ....................................................................... 200g 66  Silicon Chip siliconchip.com.au Direct Injection from a 70V/100V speaker line How do you connect a signal from a 100V (or 70V) speaker line into an amplifier line input? The question arose while we were discussing roles for the DI Box in the office. But most people would ask “why would you want to connect a 100V speaker line into an amplifier line input?” Consider two scenarios: in the first, you’re installing a large-area PA system which, of course, would use 100V speaker lines to minimise losses in those lines. But you’ve reached the power output limit of the PA amplifier and still need more speakers to cover the area, possibly quite some distance from the amplifier. So you have to put in a second (remote) system – amplifier and speakers. But how can you supply it with signal? Here’s an example of a second scenario: at last year’s NSW Surf Lifesaving Championships, the carnival was spread out for more than a kilometre along the beach. The surfboat area had one PA system while the rest of the carnival had another, with two panels of announcers used to call the various races. But the referee wanted the race finals in one area to be heard in the other – in other words, two independent systems with the capability of being linked together. What to do? Of course, you could run a coax lead all the way back to the first amplifier and use a “line out” (if it has one!). Apart from the cost and inconvenience of such a lead, that can cause problems of its own – not the least noise pickup and line losses over such a length. When you think about it, you already have signal close by – on the speaker lines themselves – it’s just that the signal is at too-high a level to use in its present state. The level of the signal on the speaker lines depends on the volume set back at the amplifier so effectively, any signal you extract will also be remote-volumecontrolled. But how do you stop the 100V signal destroying your amplifier input (which it no doubt would if you connected it direct!)? Fortunately, the solution is delightfully simple. Before we look at our approach, we should mention that one of these devices is commercially available from Altronics. Their A-4902 100V Line Adaptor has a lot more bells and whistles than we are siliconchip.com.au giving, including a volume control and both mic and auxlevel outputs. It uses a slightly different (though technically correct) approach as well. So if you don’t want to build a 100V line adaptor, have a look at the Altronics version – (www.altronics.com.au). (In fact, this unit was precisely the solution used for the surf carnival problem). Commercial alternative: the Altronics A4902 100V Line – Mic/Aux Adaptor. Anyway, back to our design: all we do is attenuate the signal from the speaker lines with a nominal 100:1 resistive divider, leaving us about 1V maximum at the junction. To isolate the speaker line from the amplifier (and so eliminate the possibility of hum loops) we use a 1:1 audio transformer. Like the DI Box, we use one of the highquality Altronics audio coupling transformers, in this case the M0706 10kW:10kW (the 5kW centre tap is not used). The circuit is housed in a metal box (the same as used in the DI box) with a 2-way screw terminal block on one end for speaker line connection (Altronics P2072A) and a by Ross Tester 3-pin male XLR output socket on the other (Altronics P 0807). The only other “refinement” you could add would be a signal on/off switch – this could be just about any small switch – eg, the rocker switch used in the DI box (Altronics S3220) and the best place to connect it would be across the transformer primary (ie, shorting out the 1kW resistor, thus killing amplifier input when the switch was turned on). If you don’t mind a bit of double-dutch, that means when the switch is turned off, the line adaptor is on, and vice versa. Construction Like the DI Box, there is no PC board – it is too simple! We mounted the resistors point-to-point from the input terminals across to the switch and wired the transformer leads direct as well. Note that the centre tap (black) lead is not used. Again as in the DI Box, connect pin 1 of the XLR socket to the chassis (ground). And that’s it. You can add a suitable label if you wish (we did!) just to make it look professional but that is up to you. There’s many a gizmo in the average roadie’s box of tricks that has Texta marker or even pencil labelling! SC Point-to-point wiring is again used for our 100V-line to line-level adaptor. May 2006  67