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Low-Cost Audio/Video Distribution Amplifier Do you need to distribute video and audio signals to a bunch of monitors or VCRs, without any loss in quality? This easy-tobuild Audio/Video Distribution Amplifier can split normal composite video (plus stereo audio) signals six ways, or you can use it to split S-video signals three ways. By JIM ROWE T HERE ARE LOT’S of situations where an AV Distribution Amplifier is necessary. For example, let’s say that you want to set up an audio-video (AV) system for a college classroom, where half-a-dozen monitors are to be fed with video and audio signals from a single VCR. Or perhaps you want to set up a small video duplication facility, with a “master” VCR or VCD (video CD) player feeding up to five recording VCRs plus a video 54  Silicon Chip monitor (so that you can keep an eye on recording quality). Another possibility is that you want to set up a stand at a trade show, with the output from a DVD player fed to a video projector, three or four monitors and a sound system. In situations like these, there’s more to it than just hooking everything up with the necessary cables and some multi-way connectors. Distributing video and audio signals to multiple destinations has to be done properly, or signal losses and cor­ruption can make the results very disappointing. Blurred pictures with “ringing” and colour “bleed”, together with weak and muffled sound, are inevitable unless the system is properly set up. In general, the way to prevent these problems is to use an “audio-video distribution amplifier”. This provides enough gain (or amplification) to compensate for the losses involved in “splitting” the video and audio signals to feed multiple loads. It also ensures that the video cables can all be terminated in the right impedance, to prevent ringing and other distortion. Commercial AV distribution amplifiers are available but they’re not exactly cheap. That’s why you might like to consider this design. The kit costs less than half as much as a comparable commercial unit. What it does Our AV Distribution Amplifier accepts a standard composite video www.siliconchip.com.au All the parts are mounted on a large double-sided PC board, so building the unit is really easy. This is the composite video version – only a few minor changes are required to distribute S-video signals. signal (PAL or NTSC) from a VCR, camcorder, VCD or DVD player and provides six “clone” signals to drive the same number of monitors, projectors, VCRs or whatever. The video output signals are all virtually identical to the input signal, because the video amplifier stages inside the unit have a frequency response flat to within 0.1dB to over 100MHz. They also operate with very low distortion, noise and phase shift. Any line-level mono or stereo audio signals which accompany the video can also be split six ways, again without any signifi­cant reduction in frequency response or signal-to-noise ratio. The audio stages also operate with very low distortion and chan­nel crosstalk. What’s more, the unit is very easy to build. It’s built inside a standard low-profile plastic instrument case, with all parts mounted directly on a PC board so there’s no off-board wiring to worry about. And it all runs from a nominal 12V DC supply, which can be either a low-cost plugpack supply www.siliconchip.com.au or a battery. The total current drain is less than 60mA. By the way, although the video side of the unit is mainly intended for distributing normal composite video to six loads, the circuit and PC board pattern also allow it to be wired to distribute S-video or “Y/C” signals to three loads, instead. So if you need an S-video distribution amplifier, we’ll explain how this can be done later in the article. Amplifier chip The video side of the project is based on a very impressive wideband buffer amplifier chip made by Maxim Integrated Products. Designated the MAX­497, it includes four closed-loop buffer ampli­fiers, each with a voltage gain of 2.0 and the kind of perfor­ mance we could only dream about a few years ago. As you can see from Fig.1, its buffers have a rated fre­quency response for small signals of about 120MHz (-0.1dB) and the response is still rated to extend to around 215MHz (-3dB) at full power output. Each buffer amplifier inside the MAX497 offers a typical input impedance of 1MΩ shunted by 2pF, an output impedance of only 1.5Ω at 10MHz, an output slew rate of better than 1100V/ Where To Buy The Kit The copyright on this project is owned by Jaycar Electronics who will have complete kits available shortly after publication. These kits will include pre-punched front and rear panels with screened lettering. Prices are as follows: (1) Complete kit for composite video version ..........................................$139.95 (2) Complete kit for S-video version.........................................................$139.95 Kits can be purchased from you nearest Jaycar store or via mail order. November 2001  55 Parts List 1 PC board, code 02111011, 198 x 158mm 1 plastic instrument case, 225 x 165 x 40mm 3 RCA sockets, PC-mounting 9 dual RCA sockets, vertical PC-mount 1 2.5mm PC-mount DC power connector (J22) 8 PC board terminal pins, 1mm diameter 3 10mm x M3 machine screws with M3 nuts 15 small self-tapping screws, 6mm-long Semiconductors 1 MAX497 quad video amp (IC1) 1 LM833 dual low-noise audio amplifier (IC2) 1 LM555 timer (IC3) 1 7809 +9V regulator (REG1) 1 7805 +5V regulator (REG2) 1 7905 -5V regulator (REG3) 1 3mm red LED (LED1) 1 zener diode, 9V 400mW (ZD1) 3 1N4001 diodes (D1-D3) Capacitors 1 1000µF 25VW RB electrolytic 2 220µF 25VW RB electrolytic 2 100µF 16VW RB electrolytic 2 10µF 16VW TAG tantalum 2 2.2µF 16VW TAG tantalum 2 0.39µF MKT polyester 2 0.22µF MKT polyester 4 0.1µF monolithic ceramic 1 0.1µF MKT polyester 1 0.01µF MKT polyester Resistors (0.25W, 1%) 4 100kΩ 4 150Ω 14 47kΩ 5 75Ω 1 10kΩ 1 22Ω 1 2.2kΩ 2 10Ω Changes for S-video version 6 dual RCA sockets, PC board mounting (not 9) 2 single RCA sockets, PC board mounting (not 3) 4 4-pin mini DIN sockets 9 additional 1mm PC board terminal pins 6 75Ω resistors (not 5) 8 additional 10mm x M3 machine screws with M3 nuts WHERE TO BUY A KIT Kits for this project will be available from Jaycar Electronics – see panel. 56  Silicon Chip µs for a 4V step and an output THD (total harmonic distortion) of better than -58dBc for a 2Vp-p output swing at 10MHz. The overall device also offers adjacent-channel crosstalk of better than -72dB, “all hostile” crosstalk of better than -65dB, and differ­ ential gain/phase errors of less than .01%. So it’s a very im­pressive device. But why a gain of 2.0? Simply because the MAX497 is de­signed specifically for driving “back terminated” coaxial cables – where the source end of the cable is presented with its match­ing impedance as well as the load end. This is done by using a series resistor, normally 75Ω for driving video cables. However this means that a 2:1 voltage divider is formed by the back termina­ tion resistor and the cable’s termination resistor at the load end. So by giving the buffer amplifier a gain of 2.0, we restore the overall gain to unity and ensure that each load receives a full-amplitude replica of the input signal. Circuit details As shown on Fig.2, a single MAX497 (IC1) forms the heart of the distribution amplifier’s video circuitry. To use the chip’s four internal amplifiers to provide six output channels, we pull a small trick. This relies on the fact that each of the four channels in the MAX497 can actually drive a total load as small as 100Ω (ie, a back-terminated 50Ω cable and load), rather than the 150Ω presented by a back-terminated 75Ω cable and load. The inputs of all the amplifiers are tied together, so that they produce exactly the same output signal. Then as well as using each output to drive its own specific load via a 75Ω back-termination resistor, we also use each output to provide half the drive to a third output, via 150Ω resistors. This means that the third output from each pair is still driven with an identical signal to the other two and with the same effective back termina­tion resistance (150Ω/2 = 75Ω). Thus, when the inputs of the two pairs are also tied togeth­er, this provides a total of six buffered outputs from a single composite video input. By separating the two pairs we’re also able to use them for handling the separate Y (luminance) and C (chrom­ inance) signals of S-video and split each of them three ways. That’s how Fig.1: the gain of each amplifier in the MAX497 chip is 2.0 (6dB), and remains virtually ruler flat until beyond 100MHz! the video side of the unit is changed from 1:6 distribution for composite video to 1:3 distribution for S-video. The audio side is just as straightforward as the video side, being based on a single LM833 low-noise dual op amp IC (IC2). The two op amps in IC2 are used in identical cir­cuits, one for each of the stereo audio channels. Each op amp is configured as a buffer, with the 100kΩ resistor pairs applying negative feedback to give a voltage gain of (you guessed it) 2.0. This allows their inputs to be provided with the usual “line level” terminating impedance of 47kΩ and their outputs to be connected to splitter resistors of the same value – so that the overall gain is unity when the outputs are connected to audio equipment inputs with an impedance of 47kΩ. In this case coupling resistors are used to protect both the inputs and outputs of the op amps from any possible DC levels, but the capacitor values are chosen to minimise any change in frequency response. Power supply That’s really all there is to it in terms of the distribu­tion amplifiers. The rest of the circuitry is for the unit’s power supply, to provide the correct supply voltages which are derived from a nominal 12V DC source. The MAX497 operates from supply rails of ±5V, while the LM833 needs rails of at least ±9V to handle typical line level audio signals. So the power supply circuitry is designed to provide all four of these voltages from the www.siliconchip.com.au S-VIDEO IN *LINK 150 15 11 13 9 2 16 4 14 6 12 8 10 75 75 75 75 150 75 1 5 3 7 150 10 1k 2.2F 16V 0.22F LEFT AUDIO INPUT 1k 3 47k 2 1k 8 IC2a LM833 1 1F 1k 100k 1k -9V 0.1F LEFT AUDIO OUTPUTS +9V 1k 0.1F LED K 10 -9V 1k A 2.2F 16V 0.22F RIGHT AUDIO INPUT 1k 5 47k 6 IC2b LM833 1k 4 7 1F 1k 100k 1k LED1 POWER  A K 1k 2.2k D1 1N4001 12V DC INPUT DC SOCKET + 1000F 25V _ +9V REG1 7809 IN OUT GND REG2 7805 IN OUT GND 10F 16V +5V 2x 0.1F 100F 16V 10k 7 0.1F 6 www.siliconchip.com.au 150 VIDEO OUTPUTS 75 2 Fig.2 (right): the circuit for the video distribution amplifier. IC1 handles the video signals, while IC2 handles the stereo audio. The circuit can be wired to output six composite video channels or three S-video channels. IC1 MAX497 VIDEO IN Construction Fig.3 shows the wiring details for the Video Distribution Amplifier. Note that this is for the composite video version – we’ll cover the S-video version later. All the parts are mounted on a single PC board coded 02111011 and measuring 198 x 158mm. This board needs to be dou­ ble-sided to allow correct configuration of the MAX497 video chip but there’s no need for plated-through holes. Instead, the con­ nections between the top and bottom copper layers are made via PC terminal pins and by soldering the component leads on both sides of the board where necessary. We also make use of the top copper layer to provide signal shielding. Why does the PC board have to be the size it is, when there’s not all that much circuitry inside? Well, it’s simply to allow all of those output connectors to be fitted directly to the board, along its back edge. This reduces the off-board wiring to zero, making -5V +5V *INSTALL LINK FOR COMPOSITE VIDEO RIGHT AUDIO OUTPUTS incoming “raw” 12V DC input, at the modest current levels needed. As shown in Fig.2, series diode D1 is used to protect the supply from reverse-polarity damage. The 1000µF capacitor provides smoothing before the input is fed to REG1, a 7809 regu­ lator which provides the +9V rail. This, in turn, also feeds REG2, a 7805 regulator which provides the +5V rail. We generate the negative supply rails by using a 555 timer (IC3). This is wired as a high-speed commutator switch and drives a charge-pump voltage inverter circuit based on diodes D2 & D3 and two 220µF capacitors. This produces an output voltage of about -10V across the second capacitor, which drops to around -9V across zener diode ZD1. The zener thus establishes the -9V supply rail and also drives REG3 to produce the -5V rail. Finally, we also use the raw DC input to operate the pilot LED, via a 2.2kΩ series resistor. SC 2001 8 4 VCC RES TRIG OUT DIS IC3 555 CV THR GND 1 3 5 220F 25V D2 1N4001 D3 1N4001 220F 25V -9V 22 100F 16V -5V REG3 7905 IN OUT GND ZD1 9V 10F 16V .01F 6 CHANNEL A/V DISTRIBUTION AMP November 2001  57 This is the completed PC board assembly. Note that some component leads must be soldered on both sides of the board – see text. the unit easy to build and eliminating potential wiring errors. RCA connectors The video and audio outputs are all made via nine double board-mounting Capacitor Codes      Value   IEC Code EIA Code 0.39µF   390n  394 0.22µF   220n  224 0.1µF   100n  104 .01µF    10n  103 RCA connectors. The three directly behind the MAX497 chip provide the six video outputs, while the remaining six each provide one pair of stereo audio outputs. At the front, the inputs are made via three single board-mounting RCA sockets. A 2.5mm concentric power connector is used for the 12V DC input and this too is at the front because there was no room at the rear. The only other item on the front panel is the pilot LED, at the lefthand end. As shown in Fig.3, everything is laid out fairly spacious­ly, with the signal flow from front to rear for both video and audio signals. PC board assembly Before fitting any components to the PC board, position it in the bottom half of the case and check that it has been trimmed to the correct size. The board has to fit quite snugly between the front and rear panels and if it’s too large, you won’t be able to assemble everything later. It’s better to check this out now and if necessary file it down to size, as it’s much harder to do if all the components have been mounted Resistor Colour Codes   No.    4  14    1    1    4    5    1    2 58  Silicon Chip Value 100kΩ 47kΩ 10kΩ 2.2kΩ 150Ω 75Ω 22Ω 10Ω 4-Band Code (1%) brown black yellow brown yellow violet orange brown brown black orange brown red red red brown brown green brown brown violet green black brown red red black brown brown black black brown 5-Band Code (1%) brown black black orange brown yellow violet black red brown brown black black red brown red red black brown brown brown green black black brown violet green black gold brown red red black gold brown brown black black gold brown www.siliconchip.com.au K 1k AUDIO OUT LED1 A 1k AUDIO OUT 1k 1k 220F 10k 1F +9V 10 ZD1 10 D2 1k 1F P2 100F 75 COMPONENTS MARKED WITH ( ) MOUNT ON BOTTOM SIDE OF PC BOARD * 0.1F IC1 MAX497 AUDIO OUT 150 * 75 0.1F 1 * -5V 75 *+ RIGHT AUDIO IN 10F + P5 -5V REG3 7905 INSTALL FOR COMPOSITE VIDEO ONLY 75 + 10F LINK 100k 1k VIDEO OUT REG2 7805 100F +5V VIDEO IN 47k 100k P4 47k +9V 100k -9V 1k 0.1F LEFT AUDIO IN P1 1k + 0.22F P3 + + 12V DC INPUT 2.2F DC SOCKET IC2 LM833 + D1 1k 2.2F 1 220F 100k + 1000F 22 AUDIO OUT .01F 0.22F AUDIO OUT 1k D3 0.1F 0.1F INSTALL FOR S-VIDEO ONLY 150 VIDEO OUT IC3 555 1k REG1 7809 150 150 75 VIDEO OUT 1 AUDIO OUT 1k + 2.2k 75 P6 GND = PC BOARD PIN = TOP SIDE Fig.3: install the parts on the PC board as shown here to build the composite video version. Note that the capacitors shown in blue are installed on the underside of the PC board – see text and photo. on the board. Next, it’s a good idea to check the board’s patterns (top and bottom) for any possible defects – bridges or hairline cracks, etc. Fix these if you find them, then fit the six PC terminal pins www.siliconchip.com.au (P1-P6) used to make the layer-to-layer connections and the video input linking. P6 is in the bottom righthand corner of the board, while pins P1-P5 are fitted near REG2 and the two 100µF = BOTTOM SIDE capacitors. These pins also provide convenient places to check the +9V, -9V and -5V supply rails. Note that all six pins (P1-P6) should be soldered to both the top and bottom copper pads, to ensure they act as through-hole vias. Install the link near the video input socket if you’re building the unit to distribute normal composite video but November 2001  59 The composite video version has no less than 18 RCA output sockets – six for the video outputs and 12 for the stereo audio output pairs. The S-video version substitutes three 4-pin mini DIN sockets for the video outputs, plus another 4-pin mini DIN socket for the video input. leave it out for S-video. The next step is to fit the double RCA connectors along the rear of the board and the other connectors along the front. The double RCA connectors have plastic locating lugs on each side, which mate with matching 3mm holes in the board. You push each connector’s three connection pins through their holes until the barbs on the plastic lugs clip into position, then you solder the pins to the pads below. With the single RCA connectors, the connection tails them­selves hold the connectors in place but you may need to enlarge the holes in the board to take them because the tails are rec­ tangular in cross-section (about 0.5 x 2.5mm). This is also true for the DC power connector. A round jeweller’s file can be used to convert the drilled holes into slots. This view shows the mounting details for the two 0.1µF monolithic ceramic capacitors and the 10µF tantalum capacitor on the underside of the PC board. 60  Silicon Chip Once the connectors are all fitted and their leads soldered to the board underneath, you’re ready for the final stages of the assembly. This simply involves fitting the resistors, capacitors, diodes, ICs and regulators, mainly in that order. Most of this wiring is very straightforward and shouldn’t pose any problems. But be especially careful around the video chip (IC1), because many of its pins have to be soldered to pads on both the top and bottom of the board. Two of the 0.1µF mono­lithic ceramic bypass capacitors for this chip (the “centre” pair) also have to be mounted on the bottom of the board and soldered on the top. By contrast, the “end” pair are mounted on the top of the board. It’s best to leave all four of these monolithic capacitors until after you have fitted IC1, because they’re easier to fit afterwards without damaging them. When you fit IC1, all of its pins should first be soldered to the pads on the bottom of the board. Then after allowing the chip to cool down for a minute or so, turn the board over and solder pins 1, 3, 5, 7, 9, 11, 13 and 15 to their top pads as well. This done you can then fit the two end bypass capaci­ tors, soldering their leads underneath, and finally the two centre capacitors from below with their leads soldered on the top. Note that it will be necessary to bend the leads of these two “underneath” capacitors by about 45°, so that they don’t foul the bottom of the case later www.siliconchip.com.au on – see photograph. By the way, DO NOT use an IC socket for the MAX497 chip. While it’s a fairly pricey chip, it needs to be soldered directly to the board to minimise lead inductance (otherwise it won’t operate properly). It’s a fairly rugged device though, so don’t be too nervous. Just use a clean, well-tinned fine bit on your soldering iron and make the soldered joints quickly to avoid overheating. The only other part that’s mounted underneath the board is one of the two 10µF tantalum bypass capacitors adjacent to IC1. This capacitor bypasses the -5V rail. Again, it may be necessary to bend its leads by about 45° before you fit it, so its body won’t hit the bottom of the case when the board is mounted. Note that all three TO-220 regulator chips are mounted flat on the top of the board. This means that their leads all need to be bent down by 90° about 5mm from the body, before each device is fitted. This done, it’s a good idea to fit an M3 ma­chine screw and nut to hold each device down to the board, before soldering the leads on the bottom. Don’t fit the second 75Ω input terminating resistor (near the link) if you’re building the unit for distributing normal com­posite video. This resistor is only needed when the board is used for S-video. The power indicator LED (LED1) should be fitted with its body about 15mm above the board. Make sure Fig.4: if you want to build the project to distribute S-video signals, here’s how to modify the 4-pin mini DIN sockets to fit inside the low profile case. that it’s correctly orientated before soldering its leads – the anode lead is the longer of the two. Once it’s in, bend its leads at right angles about 8mm above the board, so that it will later mate with its hole on the front panel. Final assembly If you’re assembling the unit from a complete kit, the front and rear panels will be supplied punched and silk-screened. Alternatively, if you’re building from scratch, you’ll have to drill and ream the various holes for the connectors, using photo­copies of the front and rear panel artworks as templates. Note that most of the RCA socket clearance holes should be 10mm or 10.5mm in diameter, while that for the DC input socket should be 7.5mm and that for the LED 3mm. As you can see from the rear panel artwork, there’s also a 3mm hole alongside each double RCA connector position. These holes are used for the small self-tapping screws which attach each connector to the rear panel, to strength­en the complete assembly. Once all the holes have been drilled in the panels, you’re ready for the final assembly. This is easiest if you first offer both panels up to the board assembly so that they mate with the connectors and attach the rear panel using the small self-tappers. The complete assembly can then be fitted into the bottom half of the case and secured by fitting self-tapping screws into the matching integral plastic standoffs – one in each corner, and two more along the front to add strength and rigidity. Quick checkout Now for the smoke test. Connect the board to a 12V DC supply (eg, a plugpack) and check the various supply rail voltag­es using a multimeter (or Fig.5: here’s how to wire the mini DIN sockets to the Distribution Amplifier PC board for the S-video version. www.siliconchip.com.au November 2001  61 eo Distribution Ampli­fier is probably working correctly. If you don’t get the correct voltages, remove power immediately and look for the problem. It’ll probably be a faulty solder joint or a component mounted incorrectly. Assuming everything checks out correctly, you can com­plete the assembly by attaching the top of the case. The unit is now ready for business. S-video version Now let’s look at building the unit for 1:3 distribution of S-video. Figs.4 & 5 show the details. First of all, you don’t fit the video input RCA socket to the front of the board. Nor do you link the two PC terminal pins just behind the 75Ω input resistor (ie, near the video input socket). However, you do fit the second 75Ω resistor, just behind these pins. Also, at the rear of the board, you leave off the three double RCA sockets used for the video outputs. Instead, you fit PC terminal pins into the holes where the three pins of each socket normally go and solder them to the pads underneath – ie, nine more terminal pins in all. These changes allow you to fit the unit with the usual 4-pin “mini DIN” sockets used for S-video connections, with the sockets mounted directly on the front and rear panels and the board connections made via very short wires to the terminal pins – see Fig.5. Modifying the S-video sockets Fig.6: here are the full-size artworks for the front panel and the two rear panel versions (composite video and S-video). DMM). P1 and P2 should both be at +9V (relative to board earth, eg, P6). Similarly you should find -9V at P3 or P4 and -5V at P5. 62  Silicon Chip You should also be able to measure +5V at the output termi­nal of REG2 – ie, the lead nearest REG3. If all four voltages are OK, your Vid- Because of the limited panel space of this low-profile case, the commonly available S-video sockets have be modified to fit in – see Fig.4. This involves drilling new mounting holes in their flang­es and then cutting the flanges somewhat shorter – about 24mm end-to-end. Note that although the unit has only three video output channels in the S-video version, there are still six audio output RCA socket pairs on the rear panel. Only three stereo pairs are required, of course, but you can still fit all six so that there are no holes in the rear panel. Alternatively, you can leave out three of these double RCA output connectors and their matching 47kΩ resistors and cover the holes with a SC blanking panel. www.siliconchip.com.au