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Get big sound from your computer with this . . . Multimedia Sound System Most computer sound systems are wimpy little boxes with poor quality sound. This system is compact, has plenty of power and produces high quality wide range sound with oodles of bass. Design by RICK WALTERS 66  Silicon Chip Let’s face it, while today’s computers may be superfast, with millions of colours on their monitors and connected to the whole world via the Internet, their sound quality is strictly yesterday’s fodder. Not only are the amplifiers on sound cards puny by comparison to any home music system, most Multimedia speakers are just a joke – you could get better quality out of an old 6 x 9-inch car radio speaker! So what do you want from your Left: the neat speakers flanking this monitor are only part of our new Multimedia Sound System. The other part is the power amplifier and electronic crossover board (above) which plugs into a spare slot in your computer. It puts out better sound than you have ever heard from a computer. Multimedia sound system? For a start, if you are into games, you want the sound effects to be at least halfway realistic. If you’ve just blown up the robber kingdom with a 5-megaton bomb, the sound effect should be a little more notable than a sneeze from a guinea pig. And if you’ve just crashed out of the Monte Carlo rally, you expect to hear a little more than a few coins rattling inside a drink can. Well, don’t you? We certainly do. And if you are listening to CDs or sound tracks via your CD-ROM player, you have every right to expect clean wide-range sound, every bit as good as from a home music system with a CD player. Our new Multimedia Sound System will deliver the goods. It has decent power output, high quality low distortion amplifiers and decent loudspeakers which cover the full audio spectrum from 50Hz to 20kHz. They will blow existing commercial computer sound systems and speakers into the weeds! Total power output of the audio amplifier system is in excess of 20 watts, distortion is less than 0.2% and signal to noise ratio is better than 65dB with respect to full power. And as the photos show, you don’t need a massive amount of electronics to get this power. It all fits on a standard half-size card which slips into any vacant slot in your PC. There is no power supply required because the card makes use of the 12V supply inside the computer. Apart from slotting the amplifier PC board into your com­puter, there is no other modification required to your machine. You will need to connect cables from your computer’s sound card to the amplifier PC board and there are also the connecting cables to the two speakers. But once you have the amplifier and speakers connected, your computer will function exactly Performance Output power �������������������������������1.5 watts per channel into 8Ω (tweeter); 9 watts per channel into 4Ω (woofer) Frequency response ��������������������-4dB at 20Hz and -0.2dB at 50kHz (see Fig.3 & Fig.4) Input sensitivity ����������������������������32mV for tweeter amplifier; 40mV for woofer amplifier Harmonic distortion ���������������������0.2% (see text) Signal-to-noise ratio ��������������������65dB with respect to 1.5 watts (tweeter); 59dB with respect to 8 watts (woofer) October 1996  67 AUDIO PRECISION SCTHD-W THD+N(%) vs measured 10 LEVEL(W) 20 AUG 96 15:22:48 1 0.1 0.1 1 3 Fig.1: power output of the tweeter drive amplifier. Maximum power is about 1.5 watts before clipping. Note that the true harmonic distortion is less than 0.2%. AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 15.000 20 AUG 96 14:11:44 10.000 5.0000 0.0 -5.000 -10.00 -15.00 3k 10k 50k Fig.2: this graph shows the frequency response of the tweeter amplifier, over the range from 3-50kHz. as it did before, except that you will have big sound to match. Nor is there any need for massive loudspeaker boxes that would dwarf your computer system. While they are bigger than typical Multimedia speakers, they are still quite compact – the volume of each enclosure is a mere five litres. So they will sit quite comfortably on either side of your computer 68  Silicon Chip monitor. Naturally, the woof­ers and tweeters in the enclosures have full magnetic shielding so there will be no adverse effects on your monitor. Power amplifier features As already noted, the power amplifiers for this new Multi­media system are all on one PC board which is the size of the standard half-size card for a PC-compatible computer. On board are three Philips TDA1519A stereo amplifier ICs which are specif­i cally designed for use in car radios. Why three ICs? We’ll tell you about that later. Only four connections are made via the PC board edge con­nector to the computer’s motherboard: two for the earth or 0V connection and two for the +12V and -12V supply rails. There are two 9-pin female D sockets on the metal mounting bracket, together with a 3.5mm stereo jack socket and two screw­driver-adjustable multi-turn volume controls. These are set when you first connect the system up but after that they are not touched – you will normally set the volume by using your mouse and on-screen controls. The 9-pin D sockets are used for making the loudspeaker connections. The two enclosures each have a 5-inch woofer and a 1-inch tweeter. The enclosures are ported, to give an extended bass response down to 50Hz. Each loudspeaker is connected to the amplifier PC board via a 4-way cable; two wires for the woofer and two for the tweeter. There are no crossover networks inside the loudspeaker enclosures since the tweeters and woofers are separately powered. Now let’s have a look at the electronics on the amplifier card. Fig.5 shows the circuit of the whole Multimedia Sound System. Both channels are shown. If you look at the righthand side of the diagram you will see that each tweeter is driven by its own power amplifier while each woofer is driven by two power amplifiers in “bridge” mode. In effect, this doubles the power delivered to the woofer and makes best use of the power available from the 12V supply in the computer. There are several reasons for this unusual amplifier setup. First, the specified tweeter is an 8Ω type and has an efficiency of 94dB at 1 watt and 1 metre (usually expressed as 94dB/1W/1m). By contrast, the woofer is a 4Ω type and has an efficiency of only 86dB. In other words, the tweeter is twice as efficient. Therefore, we need to deliver four times as much power to the woofer as to the tweeter. This is why the woofer is driven in bridge mode. Using a 12V supply rail, the TDA­ 1519 can typically deliver a maximum of 1.5 watts into an 8Ω load before clipping, from each channel. This is AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 15.000 20 AUG 96 14:05:54 10.000 5.0000 0.0 is loafing along, using the regulated 12V supply in the computer. That is just as well, because we have mounted the three TDA1519As on quite small heatsinks, bearing in mind that most of the time they will be delivering little or no power at all. And just in case the chips do get too hot, they are thermally pro­tected and will shut down safely if the going gets too tough. By the way, they are also protected against short-circuited outputs. Performance graphs -5.000 -10.00 -15.00 20 100 1k 5k Fig.3: frequency response of the woofer amplifier from 20Hz to 5kHz. Note the 3dB boost in the region of 35Hz. AUDIO PRECISION SCTHD-W THD+N(%) vs measured 10 LEVEL(W) 20 AUG 96 13:27:36 1 0.1 0.2 1 10 20 Fig.4: power output of the woofer amplifier. Maximum power from the bridged amplifiers is about 9 watts before clipping. what the tweeters get. In bridge mode, the two amplifiers in the TDA1519A are driven out of phase so that their output voltages add across the speaker. Under this condition, the TDA1519A can deliver 9 watts. In practice, this gives a safety margin – we don’t need to drive the woofers at a power level six times that of the tweeter but it is good to have a little in hand. By the way, if you come across the specs for the TDA1519A you will see that it is rated for a maximum power output of 22 watts in bridge mode into a 4Ω load. However, this is for a supply of 14.4V and a harmonic distortion level of 10% – hardly what you would regard as hifi specs. By comparison, under our design conditions the chip The performance of the Multimedia Sound System is summar­ised in the accompanying panel and we have included a number of graphs which need a little explanation. Fig.1 shows the power output of the tweeter drive amplifier and as you can see, it delivers about 1.5 watts before clipping, at which point the harmonic distortion suddenly rises. The minimum distortion on this graph is 0.4%, which is double what we claimed above for this parameter. What the graph doesn’t show is that the distor­tion measured is mostly due to the 54kHz hash superimposed on the computer’s power supply. This is completely inaudible and does not affect the sound quality; as stated above, the true harmonic distortion is less than 0.2%. Fig.2 shows the frequency response of the tweeter amplifi­er, over the range from 3kHz to 50kHz. Similarly, Fig.3 shows the frequency response of the woofer amplifier from 20Hz to 5kHz. Note the 3dB boost in the region of 35Hz. Fig.4 shows the power output of the woofer amplifier and the above remarks about power supply hash also apply here. The power supply hash also affects the signal-to-noise ratio, so while we have quoted a figure of -65dB for the tweeter amplifier and -59dB for the woofer amplifi­er, the true figures are considerably better. In any case, there is little point in having signal-to-noise ratio figures much in excess of -60dB in a Multimedia sound system since the computer itself generates so much noise from its fan and disc drives. Now let’s have a closer look at the circuit details shown in Fig.5. At the lefthand side of the circuit are the stereo inputs, at the jack socket SK1. These are fed via 2.2µF non-polarised October 1996  69 70  Silicon Chip Fig.5: the two tweeters are driven from single power amplifiers (IC5), while each woofer is driven in bridge mode by a pair of power amplifiers. IC1 and IC2 provide slight bass boost and the electronic crossover at 3.5kHz. The amplifier board is the same size as a typical half-size PC card and plugs directly into a slot on the motherboard. The edge connector makes contact with the ±12V rails of the computer and the 0V line. (NP) capacitors to the 10kΩ multiturn trimpots, VR1 & VR2. Now let’s talk about the left channel only, since both channels are identical. VR1 feeds an op amp buffer, IC1d, and then the signal is split into two paths. The first is via a bass boost stage involving op amp IC1a. This is really a high pass filter which gives a 3dB boost to frequencies in the region from 35Hz to 50Hz. Following the bass boost stage, the signal is fed to a low pass filter employing op amp IC2d. This is a Linkwitz-Riley filter which rolls off signals above 3.5kHz and drives a voltage divider comprising resistors R1 & R2. These are used to adjust the drive signal to the amplifier stage so that the woofer signal level can match that of the tweeter. The equivalent resistors in the right channel are R3 & R4. The Linkwitz-Riley filter configu­ration is used here because it gives the flattest response from the two speakers in the crossover region. Woofer drive The signal is then coupled via a 2.2µF capacitor to the inputs of bridged amplifiers IC3a & IC3b, the TDA1519A. Note that the signal drives the non-inverting input of IC3a (pin 1) and the inverting input of IC3b (pin 9). Note also that pin 3 is the inverting input of IC3a and the non-inverting input of IC3b (internally con­nected). This automatically gives the condition whereby the outputs of the two amplifiers are out of phase; ie, when the output at pin 4 is swinging positive, the output at pin 6 is swinging negative. This means that the total voltage across the speaker is the sum of the two amplifier outputs. Hence This shot shows how the audio cables from a sound card are plugged into the power amplifier PC board. The two female D sockets for the four-way cables to the speaker boxes. the output power delivered to the speaker is about four times what it would be if a single amplifier was employed. As both inputs of IC3 are biased to the same DC potential (half the 12V supply), there is negligible DC voltage across the 4Ω woofer and so no large coupling capacitor is required. Howev­er, Zobel networks, consisting of a 4.7Ω resistor and 0.1µF, are used at the output of each amplifier to ensure stability at high frequencies. Tweeter drive Going back the buffer stage IC1d, it also feeds a Linkwitz-Riley high-pass filter based on op amp IC2a. This rolls off frequencies below 3.5kHz. The output of IC2a feeds one power amplifier, IC5a. As the output voltage at pin 4 of IC5a is close to +6V the tweeter must be AC-coupled and a 100µF electrolytic capacitor is used to do this. A Zobel network is also connected at the out­put. Due to phase inversion in the filter (IC2a) at the crosso­ver frequency, the tweeter polarity must be the reverse of the woofer. This is why the positive terminal of the tweeter is grounded. On the right channel tweeter, you will notice that the tweeter connection is different. In this case, the positive terminal is driven by the output of October 1996  71 Fig.6: this is the component layout for the PC board. Note the two long jumpers underneath the board. Fig.7 (right): this diagram shows the dimensions of the two heatsink brackets. IC5b and the negative terminal is grounded. The reason for this is that because of the internal connection of the amplifier inputs at pin 3, IC5a must be driven via its inverting input at pin 9. Therefore it inverts the signal and the tweeter connections must therefore be reversed. This reversing of tweeter connections is automatically taken care of by the PC board and so each speaker box is wired identically via its respective cable and 9-pin D connector. Note that IC1 and IC2, the two TL074 quad op amps, are powered from the ±12V rails of the computer whereas the power amplifiers, IC3, IC4 and 72  Silicon Chip IC5, are powered only from the +12V rail. The -12V rail in the computer cannot deliver lots of current but IC1 and IC2 will typically draw a total of less than 20mA. PC board assembly Fig.8: this diagram shows the details of the heatsink mounting for the TDA1519As. Be very careful when bending the legs of the IC at right angles that you do not stress the leads where they come out of the IC body. Enough of how it works, lets get into making it work. The PC board assembly is reasonably straightforward. Before mounting any components, check the board carefully for any defects such as shorted or broken copper tracks or undrilled holes. You can check the pattern against the artwork shown in Fig.10. Fig.6 shows the component layout PARTS LIST Amplifier PC board 1 PC board, code 01110961, 145 x 108mm 1 PC-mounting bracket (see Fig.9) 2 9-pin “D” females PC mounting socket 2 9-pin “D” male plugs 2 9-pin “D” backshells 1 3.5mm miniature PC mount stereo socket 1 3.5mm miniature stereo plug 2 14-pin IC sockets (optional) 1 PC mounting bracket (with holes for D-sockets, etc) 2 10kΩ multiturn potentiometers Bourns 3006P (or equivalent) Semiconductors 2 TL074 quad op amps (IC1,IC2) 3 TDA1519A dual power amplifiers (IC3,IC4,IC5) Capacitors 4 470µF 16VW electrolytic 9 100µF 16VW electrolytic 2 2.2µF 50VW non polarised (NP) electrolytic 2 2.2µF 50VW electrolytic 12 0.1µF 63VW MKT polyester 7 0.1µF 50VW monolithic ceramic 8 .01µF 63VW MKT polyester Resistors (0.25W, 1%) 2 150kΩ 2 1.5kΩ 2 22kΩ 4 100Ω 2 10kΩ 6 4.7Ω 8 4.7kΩ Fig.9: details of the PC board mounting bracket. for the PC board. Fit and solder the wire links first and don’t forget the two long jump­ers, made from insulated hookup wire, which install on the copper side of the PC board. These connect the outputs of IC5 to the respective D sockets. Leave the link marked “Power” off the board for the moment. This is connected during the testing procedure. The next step is to fit the resistors, small capacitors, trim­pots and 3.5mm jack socket. Note that the 0.1µF monolithic ceramic capacitors are used for supply filtering, so they are the ones adjacent to the 100µF electrolytics. The 0.1µF MKTs are specified in the signal parts of the circuit. The small electrolytic capacitors are next, followed by the larger ones and then the female D sockets. The two TL074s can be soldered in or if you prefer, plugged into sockets. The heatsinks for the power amplifiers are made from 3mm thick aluminium angle, 20 x 12mm. The heatsink for IC5 is 30mm long and the one for IC4 & IC5 together is 55mm long. Fig.7 shows the drill­ing details. The nine leads of each TDA1591A need to be bent at right angles before soldering into the board. With each TDA1519A facing you and the type number visible, bend the leads down at right angles, 8mm from each IC body. Note: do not put any stress on Miscellaneous 1 55mm length 20 x 12 x 3 aluminium angle 1 30mm length 20 x 12 x 3 aluminium angle 22 3mm x 20mm bolt 22 3mm nut 30 3mm flat washer 6 2.5mm x 12mm screws 6 2.5mm nuts 6 2.5mm spring washer 6 2.5mm flat washer 3 TO3 mica washers the leads where they come out of the IC body. To avoid stressing the leads, hold them with long-nose pliers when bending each one. This done, apply a smear of heatsink compound to October 1996  73 Fig.10: check your PC board against this full size etching pattern before installing any of the parts. the metal mount­ing surface of each IC and mount it on its heatsink – see Fig.8. Testing To test the amplifier board, you will need a power supply capable of delivering ±12V at several amps. We strongly sug­gest that you do not just build the amplifier board and plug it into your computer. If there is a fault on the board you could damage your computer’s power supply. Three PC stakes are provided on the PC board for supply connections. They can be seen near the edge connector, on Fig.6. Connect the power supply to the PC stakes and then use The enclosures have an internal volume of only five litres but that, combined with a spot of low down bass boost, is enough for them to put out good bass down to below 50Hz. We’ll describe their construction next month. 74  Silicon Chip your multimeter to measure voltages around the circuit. The principal voltages should be as follows: • IC1 & IC2 – pins 1, 2, 3, 5, 6, 7, 8, 9, 10, 12, 13 & 14, 0V; pin 4 +12V; pin 11 -12V. • IC3, IC4 & IC5 – pins 2 & 5, 0V; pins 1, 3, 4, 6 & 9, +6V; pin 8, floating; pin 7, +12V. For some of the pins of IC1 and IC2, designated as 0V, your multimeter may actually measure a few tens of milli­volts above or below 0V. That is normal. The current drain at this stage should be between 15 and 20mA, or thereabouts. This is the current drawn by IC1 and IC2. The current drain of the power amplifiers is negligible at this stage since we have not connected pin 8, the MUTE pin, to +12V. If all the voltage checks so far are correct, you can now install the link marked “Power” on the board. This enables the power amplifiers. When power is applied the total current drain should be around 140-160mA, with no signal applied. Well, that is as far as we can take it this month. Next month, we will describe how to build the speakers and give the parameters of the BassSC Box design.