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e t i M y t h g i M Powered Loudspeaker Build the Revitalise the sound card in your computer with the Mighty-Mite Powered Loudspeaker. It uses a miniature surfacemount IC amplifier which only requires a 5V supply to deliver a 1W output. By JOHN CLARKE Sound cards for computers and multimedia are all the go at present. But without suitable loudspeakers, much of the impact of the sound can be lost. By building the Mighty-Mite Powered Loud­speaker, you can obtain sound quality that’s far superior to that available from low-cost multimedia loudspeaker systems. The system to be described is based on an LM4860M integrat­ ed circuit (IC) audio amplifier and this drives a 100mm dual-cone loudspeaker. Both the amplifier circuitry and the loudspeaker are housed in a compact sealed plastic case. The only front panel control is for volume while at the rear are the signal input and DC supply sockets. The amplifier circuit is powered from a 5V rail and this can come from 60  Silicon Chip VDD CS 0.1 Rf 10k AUDIO INPUT C1 1 Ri 10k VDD 13 -IN GAIN-OUT Vo1 10 14 +IN 40k AMP1 5 BYPASS Vo2 15 VDD/2 AMP2 Av = -1 50k 6 HP-IN1 7 HP-IN2 RL 8 40k 50k LM4860 CB 0.1 PARTS LIST BIAS 3 HP-SENSE 2 SHUTDOWN GND 1,4,8,9,16 Fig.1: the internal arrangement of the LM4860M audio amplifier IC. Amp1 is the main amplifier & is connected in inverting mode. Its output appears at pin 10 & also drives inverting amplifier stage Amp2 to derive an out-of-phase output at pin 15. L1 FX115 or sim. 47 16VW +5V 6.8pF Semiconductors 1 LM4860M surface mount 1W audio amplifier (IC1) 100k INPUT VOLUME VR1 10k LOG 1 16VW 22k 13 14 12 11 IC1 LM4860 5 2 3 1,4,6, 7,8,9, 16 10 15 4 OR 8 10 16VW MIGHTY-MITE Fig.2: this diagram shows the complete circuit details for the Mighty-Mite Powered Loudspeaker. It operates with a gain of nine & this provides an input sensitivity of about 320mV for 1W output into 8-ohms. the computer itself – either from the games port or from the sound card input/output port (see Fig.4). Alternatively, the circuit may be powered from a 9-12VAC plugpack via an option­al 5V regulator circuit which is also described here. Of course, the Mighty-Mite is not just suitable for multi­media applications. It can be used anywhere a powered loudspeaker system is required; eg, as part of a low-cost audio system or in a workshop. If you do use them with a computer system though, be sure to heed the accompanying warn- 1 PC board, code 01305951, 33 x 25mm 1 sealed ABS box, 171 x 121 x 55mm (Jaycar HB-6128 or equival­ent) 1 dual-cone 100mm loudspeaker, 4Ω or 8Ω (DSE Cat. A9651 or equiv­alent) 1 10kΩ 16mm log pot (VR1) 1 FX115 ferrite bead or equivalent (L1) 3 self-adhesive labels 1 16mm dia. knob 1 DC panel socket 1 chassis mount RCA panel socket 2 25mm long x 3mm dia. screws & nuts 2 9mm tapped spacers 2 6mm long x 3mm screws 4 black countersunk 4mm diameter screws & nuts (to attach loud­speaker) 9 PC stakes 1 25mm length of 0.8mm tinned copper wire ing panel. Unlike most multimedia loudspeakers, these units are not magnetically shield­ed, so don’t place them too close to the monitor. The audio amplifier IC One of our first tasks in designing this system was to choose a suitable audio amplifier IC. There are many such units available, most capable of providing excellent results. We final­ly settled on the LM4860M because of its excellent specifications (considering its small surface-mount package), its 1W (RMS) power output and its ability Capacitors 1 47µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 1 6.8pF ceramic Resistors (0.25W, 1%) 1 100kΩ 1 22kΩ Optional 5V Regulator 1 PC board, code 01305952, 59 x 35mm 1 heatsink, 26 x 29 x 13mm 1 7805T 3-terminal regulator (REG1) 1 B104 bridge rectifier (BR1) 1 470µF 25VW PC electrolytic capacitor 1 10µF 16VW PC electrolytic capacitor 4 PC stakes 1 6mm long x 3mm dia. screw & nut to operate from a 5V supply. At first glance, it might seem impossible to obtain a 1W output into 8-ohms from such a low supply rail. August 1995  61 SPEAKER DC INPUT 6.8pF L1 100k 22k IC1 INPUT 47uF 1 IuF VR1 10uF Fig.3: install the parts on the PC board & run the external wiring as shown in this diagram. Note that IC1 is a surface mount device & is mounted on the copper side of the board (see text). This is because, to obtain 1W, the amplifier would have to deliver 2.83V RMS or 4V peak into the load. In other words, it would have to deliver 8V peak-to-peak, which is greater than the supply voltage. However, the LM3860M is a bridge amplifier which drives both terminals of the loudspeaker. Thus, when one terminal of the loudspeaker is driven high, the other terminal is driven low with the opposite phase. As a result, the effective power output from a bridge amplifier is four times that available from a standard amplifier (P = V2/R). Fig.1 shows the internal arrangement of the LM4860M and the typical external connections. Amp1 is the main amplifier and this is connected in inverting mode. Its gain is set by the ratio of the feedback resistor (Rf) to 62  Silicon Chip the input resistor (Ri), and in this case is set to -1. The non-inverting input is set to half-supply by two internal 50kΩ voltage divider resistors and is decoupled using capacitor CB. The output of Amp1 appears at pin Fig.4: a 5V rail to power the Mighty-Mite can be derived from a games or sound card port of a PC. This diagram shows the supply connections. You will need to make up a suitable power cable which is fitted at one end with a matching DB15 connector. 10 of the IC and also drives the inverting input of a second internal amplifier. Desig­nated Amp2, this amplifier is also connected in inverting mode, with its gain set to -1 by two internal 40kΩ resistors. Its signal output appears at pin 15 and is 180° out of phase with the signal at pin 10. This arrangement forms the bridge amplifier configuration. Compared to a single-ended amplifier, it effectively doubles the output voltage swing applied to the loudspeaker and thus quadru­ples the power. Note that the overall gain of the amplifier is 2Rf/Ri, due to the bridge configuration. With no signal applied, the outputs of Amp1 and Amp2 will be at the same voltage because both amplifiers are biased at half supply. Consequently, there is no need for an output coupling capacitor to prevent DC from flowing in the voice coil. This not only reduces the component count but also improves the low fre­ quency response. As well as the internal amplifiers, the IC also contains a shutdown feature which can be used to reduce the power consump­tion when the amplifier is not in use. It is activated by con­necting pin 2 to the positive supply rail (or to some other point above 3V). This reduces the no-signal supply current from a nominal 7mA to 500µA. Alternatively, the shutdown feature can be activated via an internal OR gate which has its pin 3 output connected to pin 2. The amplifier is then shut down by feeding control signals to the pin 6 and pin 7 OR gate inputs. When either or both of these inputs are at a logic high, the amplifier is disabled. These control inputs are typically used to shut down the amplifier in situations where a set of headphones is plugged into a preceding stage. In this case, the control input is derived by switching in a suitable voltage via an internal switch in the headphone socket. Circuit details Refer now to Fig.2 for the final circuit details of the Mighty-Mite Powered Loudspeaker. In this circuit, the gain has been set to nine by the 100kΩ feedback and 22kΩ input resistors. This provides an input sensitivity of about 320mV for 1W output into eight ohms. In addition, a 6.8pF capacitor has been connected across the feedback path and this rolls off the high-frequency response above 230kHz to prevent instability. The incoming audio signal is applied to IC1 via volume control VR1 and a 1µF coupling capacitor. This coupling capacitor is necessary to prevent DC current from flowing through the 22kΩ input resistor and VR1. It rolls off the response below 7Hz. The 10µF capacitor decouples the half-supply rail at pins 5 & 14 to improve supply rejection and reduce the distortion below 100Hz. Note that pins 6 & 7 (the OR gate inputs) are tied low, while the OR gate output at pin 3 is tied to the shutdown input at pin 2. Because the OR gate output is always low in this design, the amplifier is permanently enabled. Finally, the power supply to IC1 is isolated using a ferr­ite bead and decoupled by a 47µF capacitor. This measure helps to reduce noise injection into the amplifier if it is powered from a 5V computer supply (a computer supply rail usually has a fair degree of hash and high frequency noise). This view shows the completed amplifier module. Note that the volume control potentiometer (VR1) is mounted by soldering its terminals to three PC stakes at one end of the board. Construction The Mighty-Mite is built onto a PC board coded 01305951 and measuring 33 x 25mm. Fig.3 shows the wiring details. Begin construction by installing PC stakes at the external wiring points; ie, at the loudspeaker outputs, the +5V and 0V supply inputs, and at the signal inputs. In addition, install PC stakes at the three wiring points for VR1. Once the PC stakes are in, IC1 can be installed. Because this is a surface-mount component, it is mounted on the copper side of the board. Before soldering IC1, the copper lands should be pretinned using a fine-tipped soldering iron. This done, place the IC on the board with the notch in its plastic body towards the 1µF capacitor position, then carefully tack solder a couple of pins to the pretinned lands by heating them gently with the iron. The pins can then all be carefully soldered. Be sure to use only small amounts of solder during this job, to prevent unwanted shorts between adjacent pins of the IC. In fact, it is a good idea to carefully inspect the completed job under a magnifying glass to ensure that all is correct. This close-up view shows the mounting details for the amplifier board. It sits 27mm above the floor of the case on two 9mm-long spacers which are screwed onto 25mm long x 3mm dia. mounting screws. Performance of Prototype Output power ����������������������������� 1.3W into 4Ω at onset of clipping; 1W into 8Ω at onset of clipping Distortion ����������������������������������� <1% see graphs Signal-to-noise ratio ������������������ 76dB with respect to 1W with 1kΩ input resistor & 20Hz to 20kHz bandwidth; 91dB A weighted Frequency response ������������������ -2dB at 10Hz & 100kHz Sensitivity for 1W out ���������������� 320mV RMS Supply voltage ��������������������������� 2.7-5.5V Quiescent current ���������������������� <15mA; typically 7mA Output offset voltage ����������������� <50mV August 1995  63 Building The Optional 5V Regulator Board BR1 B104 9-12VAC INPUT IN 470 25VW REG1 7805 GND OUT +5V 10 16VW 0V +5V REGULATOR I GO Fig.5: you will need this simple regulator circuit if you intend powering the unit from an AC or DC plugpack supply. BR1 The regulator board only takes a few minutes to assemble. Make sure that all parts are correctly oriented & use PC stakes at external wiring points. 470uF REG1 7805 9-12V AC INPUT 10uF HEATSINK GND 5V OUTPUT Fig.6(a): here’s how to install the parts on the regulator board. Note that REG1 is bolted to a small U-shaped heatsink. If you wish to power the unit from a 9-12VAC (or 9-12V DC) plugpack, then you will need to add the 5V regulator circuit shown in Fig.5. As shown, the output from the plugpack is fed to a bridge rectifier (BR1) and this in turn drives 3-terminal regulator REG1 to derive a regulated 5V rail. The 470µF and 10µF electrolytic capacitors provide filtering for the IN and OUT terminals of the regulator The LM4860 IC (IC1) is mounted on the copper side of the PC board as shown here. Use a fine-tipped soldering iron for this job & make sure that the device is correctly oriented. 64  Silicon Chip Fig.6(b): this is the full-size etching pattern for the regulator PC board. Check the board carefully before installing any parts. respectively. A PC board (code 01305952) has been designed to accommodate the regulator components – see Fig.6(a). Install the parts on this board exactly as shown and note that REG1 is bolted to a small finned heatsink to ensure adequate heat dissipation. Apply a thin smear of heatsink compound to the metal tab of the regulator before bolting it down. The remaining components mount on the top of the PC board. Take care with the electrolytic capacitors - they must be orient­ed with the correct polarity, as shown on Fig.3. The resistors mount end on, while L1 simply consists a short length of tinned copper wire fed through the ferrite bead. Finally, the board assembly can be completed by soldering VR1’s terminals to the top of the PC stakes. The completed amplifier, along with the loudspeaker, is housed in a sealed ABS box measuring 171 x 121 x 55mm. In no circumstances should you power the amplifier from a voltage greater than 5.5V. The audio amplifier chip could fail if you do. The 5V regulator board can either be mounted inside the case of the Mighty-Mite, or mounted in a separate case and used externally. Take care to ensure that the supply connec­tions to the amplifier board are correct. This box is fitted with three adhesive labels – two on the front panel and one on the rear. Fit these labels to the locations shown in the photographs, then drill mounting holes in the rear panel for the DC power socket and the RCA input socket. Similarly, on the front panel, drill a hole for the volume control shaft. Note that it’s best to start with a small pilot hole and then carefully enlarge the hole to the correct size using a tapered reamer. Once this has been done, mark out and drill the loudspeaker YOU CAN AFFORD AN INTERNATIONAL SATELLITE TV SYSTEM SATELLITE ENTHUSIASTS STARTER KIT Fig.7: this graph shows the distortion as a function of output power into an 8-ohm load. Note that the distortion is less than 1% for output powers up to 1W & rises steeply beyond this level of output. YOUR OWN INTERNATIONAL SYSTEM FROM ONLY: FREE RECEPTION FROM Asiasat II, Gorizont, Palapa, Panamsat, Intelsat HERE'S WHAT YOU GET: ● ● ● ● ● ● 400 channel dual input receiver preprogrammed for all viewable satellites 1.8m solid ground mount dish 20°K LNBF 25m coaxial cable easy set up instructions regular customer newsletters BEWARE OF IMITATORS Direct Importer: AV-COMM PTY. LTD. PO BOX 225, Balgowlah NSW 2093 Tel: (02) 9949 7417 / 9948 2667 Fax: (02) 9949 7095 VISIT OUR INTERNET SITE http://www.avcomm.com.au Fig.8: distortion vs. output power into a 4-ohm load. The distortion is less than for 8-ohm loads, while slightly more output power (1.3W) can also be obtained. mounting holes plus a circular pattern of holes in front of the cone position to let the sound escape. Next, carefully measure out, mark and drill the mounting holes for the PC board in the base of the case. The PC board assembly is then installed in the case as shown in one of the photos. To do this, first fit two 25mm long x 3mm dia. screws to the mounting holes and secure them with nuts. A 9mm spacer is then fitted to each screw. Screw these spacers down until their top surfaces are 27mm above the base of the case, then fit the PC board and secure it to the spacers with 6mm long screws. YES GARRY, please send me more information on international band satellite systems. Name: __________________________________ Address: ________________________________ ____________________P'code: __________ Phone: (_______) ________________________ ACN 002 174 478 August 1995  65 Below: the input & DC power sockets are mounted on the rear panel. At left is the view inside the prototype. All that remains now is to run the small amount of internal wiring – see Fig.3. This consists of: (1) connecting the supply leads from the DC socket to the PC board; (2) running a short length of shielded cable from the RCA socket to the input termi­ nals; and (3) running a length of figure-8 cable from the board to the loudspeaker terminals.   Warning Unlike most multimedia loudspeakers, the MightyMite design does not include magnetic shielding. As a result, the strong magnetic field around the loudspeaker can cause colour distortion if placed too close to a monitor screen, due to magnetisation of the shadow mask. Usually, this problem will be cured by the internal de­gaussing circuitry of the monitor each time it is switched on. Severe cases, however, will require the use of a degaussing wand, which means a trip a professional service organisation. To avoid this problem, do not place the Mighty-Mite Powered Loudspeaker any closer than about 300mm from a monitor or TV set. Testing To test the unit, first connect a 5V DC supply to the DC socket, taking care to ensure that the polarity is correct. This done, switch on and check that the wire link through the ferrite bead is at +5V with respect to ground. If this is correct, check that the accessible lead of the 22kΩ resistor is at 2.5V (ie, half supply). The two loudspeaker terminals should also each be at 2.5V, give or take 50mV. If all checks out, then you are ready to try the Mighty-Mite out. This simply involves completing the case assem- VOLUME Fig.9: here are full-size artworks for the three labels plus a full-size etching pattern for the Mighty-Mite amplifier board. 66  Silicon Chip + MIN MAX Power supply As mentioned previously, the Mighty-Mite can be powered directly from a games or sound card port. Fig.4 shows the +5V and GND (0V) connections for these ports. You will need to make up a suitable power cable which is fitted at one end with a matching DB15 connector. Note that the +5V rail can be derived from pin 1, pin 8 or pin 9. Alternatively, you can assemble the optional 5V regulator board & power the unit from an AC or DC plugpack SC supply. 5VDC MAX. + MIGHTY-MITE bly, fitting the knob and feeding in a suitable signal from your computer’s sound card, or from some other suitable audio source (eg, a tuner or tape deck). + SIGNAL IN - +