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This simple adaptor allows commonly available electret lapel and headset microphones to be used with public address systems. It features a balanced output and is built into a compact case that can be clipped to a belt or slipped into a pocket. By JOHN CLARKE Lapel microphone adaptor for PA systems W HILE STANDARD HANDHELD microphones are generally used for most public address (PA) applications, there are times when a lapel microphone is much more convenient. A lapel microphone not only frees up a user’s hands but also allows the wearer to roam about easily. They are ideal when giving talks and lectures, and for certain types of theatre work. Another advantage of lapel microphones is that they provide a reason54  Silicon Chip ably consistent output, even when the person speaking turns their head. That’s because a lapel microphone is usually clipped to the user’s clothing around the chest area and so remains at a similar distance from the mouth regardless of head movement. By contrast, hand-held microphones must always be held close to the mouth, otherwise the signal level will vary drastically. Lapel microphones are generally available in two forms. By far the most common form for PA use at the present time is the radio microphone. This consists of the lapel microphone itself plus a small radio transmitter which is worn by the user – eg, inside a shirt pocket or by attaching it to a belt. The signals from the transmitter are picked up by a corresponding receiver which then feeds the signal to the PA system. The big advantage of the radio www.siliconchip.com.au Fig.1: the circuit uses op amps IC1a & IC1b to provide a balanced output signal, while relays RLY1 & RLY2 shunt the signal to ground when activated, to provide muting. microphone is that it allows the user to roam freely over several tens of metres without being tethered to a lead. However, this freedom comes at a high cost, with complete radio microphone systems typically costing around $600. Despite its advantages, this high cost cannot always justified, especially when full use of the radio transmitting feature is not exploited. This particularly applies to applications where the user doesn’t need to roam too far. In those situations, a much cheaper solution is to dispense with the radio system and instead use a tethered lapel microphone – ie, one that’s tethered to the PA amplifier via a lead. www.siliconchip.com.au However, obtaining such a wired lapel system is quite another matter. Music shops are keen to sell the wireless microphones but are usually at a loss when asked to supply a wired type. The older-style dynamic lapel microphones simply no longer appear to be available, while the smaller electret microphones require a power source. So why can’t you simply use an electret microphone and power it from the phantom supply that’s sometimes available in PA mixers? Unfortunately, it’s not as simple as that, for a couple of reasons. First, many mixers do not have phantom power and if they do, the Main Features • • • • • • Uses standard electret lapel microphone Adaptor attached to belt or in pocket Battery powered (9V) Balanced output Muting facility Battery indicator current available is well in excess of that required for an electret microphone. Electrets require only 0.5mA or less for correct operation, whereas January 2004  55 Parts List 1 PC board, code 01101041 (86 x 64mm) for Jaycar and DSE cases; or code 01101042 (81 x 61mm) for Altronics case 1 case measuring 135 x 70 x 24mm with battery compartment (DSE Cat. H 2949 (grey), Jaycar Cat. HB 6510 (black), Altronics H 0342 (grey)) 2 panel labels, 59 x 16mm and 114 x 50mm 1 belt/pocket clip (Farnell 353 6294 (grey) or 353 6282 (black)) 1 lapel microphone (Jaycar AM4092 or Altronics C 8907 or C 8913) 2 5V reed relays (RLY1, RLY2) (Jaycar SY 4036) 1 double-pole 3-position (DP3W) slide switch (S1) with 2 x M2.6 mounting screws (Altronics S 2030) 1 3.5mm PC board socket (Jaycar PS 0133) or 3-pin chassis male miniature XLR connector (Altronics P 0891) – see text 1 right angle stereo 6.35mm jack plug to 3-pin XLR line plug lead (Altronics P 0902 XLR line plug and P 0047 jack) 5 metres of dual-screened microphone cable (Altronics W 3028) 1 stereo 6.35mm metal line socket (Altronics P 0080A, Jaycar PS 0194)) 1 9V battery clip lead 1 9V battery 3 M3 x 6mm screws 1 M3 x 10mm countersunk screw 1 M3 x 20mm countersunk screw 1 M3 x 10mm tapped spacer 1 50mm cable tie 13 PC stakes the phantom power from a PA mixer is usually between 14mA and 60mA – enough to destroy an electret unless precautions are taken. Second, an electret microphone provides only a single “unbalanced” output. This means that there are just two output connections – ie, the shield and the signal wire. However, any leads that are several metres long or more in a PA system can readily pick up 50Hz mains frequency hum which is then amplified and fed through to the loudspeakers as an annoying buzz. In this case, both signal leads still pick up mains frequency hum but because the lines are balanced, the hum signal can be rejected to just leave the wanted microphone signal. This is done in the PA mixer – it receives the balanced signal and subtracts the non-inverted microphone signal from the inverted microphone signal. This removes the mains hum signal, since the same signal will be present in both Balanced output The way around this problem is to use what’s known as a “balanced” output. This type of output has two signal outputs plus a shield lead, with one output inverted with respect to the other. 56  Silicon Chip Semiconductors 1 TL072 dual op amp (IC1) 1 BC328 PNP transistor (Q1) 1 4.7V 1W zener diode (ZD1) 1 1N5819 Schottky diode (D1) 2 1N4148, 1N914 diodes (D2,D3) 1 3mm green LED (LED1) Capacitors 1 470µF 16V PC electrolytic 4 100µF 16V PC electrolytic 1 22µF 16V PC electrolytic 2 10µF 16V PC electrolytic 1 100nF MKT polyester 1 1nF MKT polyester Resistors (1% 0.25W) 1 100kΩ 1 680Ω 1 22kΩ 2 560Ω 6 10kΩ 1 220Ω 2 6.8kΩ 2 100Ω 1 1kΩ 1 22Ω Specifications Frequency response: 16Hz to 16kHz (actual response depends on the microphone used) Output level: typically 100mV Current consumption: 4mA when on, 11mA on mute, 0.1µA when off leads. By contrast, the microphone signal is doubled, since subtracting an inverted signal from the non-inverted signal gives twice the signal level. Lapel microphone adapter That’s where the Lapel Microphone adapter comes in – it not only provides power to a standard electret microphone but also includes all the necessary circuitry to provide balanced output signals. In addition, it also includes a muting facility which shorts the signal output to ground, so that sound is no longer heard through the PA system. This muting function is completely silent in operation – ie, there are no clicks and pops in the sound when the muting is switched in or out. As shown in the photos, the unit is housed in a small case which contains a separate battery compartment. The lapel microphone plugs into a socket at the top of the case, while the output lead plugs into a 6.35mm stereo socket on one side. A single 3-position slide switch is used to switch the power on/off and to select the muting. An adjacent green indicator LED flashes when the power is switched on and this can also be used to indicate the battery condition. A bright flash indicates a good battery, with the LED becoming increasingly dim as the battery goes flat. In addition, the LED serves as an indicator by glowing faintly when the switch is in the Mute position. It also flashes brightly and decays when the unit is switched off, to acknowledge the switch selection. Circuit details Fig.1 shows the full circuit details of the Lapel Microphone Adaptor. It includes a dual op amp package (IC1) to do the audio signal processing, plus two relays to shunt the signal on each balanced line to ground during muting. Power for the circuit is derived from a 9V battery and is applied via reverse polarity protection diode D1 and power switch S1. The electret microphone is plugged into a mini XLR male socket or a 3.5mm jack socket, depending on the type of electret used. It is powered from the 9V battery via 1kΩ & 22kΩ resistors and a 100µF filter capacitor. This decoupling is necessary to keep supply noise and ripple from degrading the microphone signal. www.siliconchip.com.au This is the view inside the completed prototype. The 6.35mm jack socket has its outer cover removed and is secured to the PC board using a cable tie. The socket is then further secured by its threaded boss when the lid is fastened down. The output signal from the microphone is fed to the pin 5 (noninverting) input of op amp IC1a via a 100nF capacitor. This capacitor and its associated 100kΩ resistor roll off the low-frequency response below 16Hz Note that IC1a’s pin 5 input is biased at half-supply (ie, Vcc/2) via the 100kΩ resistor which is connected to a voltage divider consisting of two 10kΩ resistors across the 9V rail. This allows the op amp’s output to swing symmetrically above and below Vcc/2. IC1a is wired as a non-inverting buffer stage and provides an output which is in phase with the microphone signal. By contrast, IC1b is connected as an inverting amplifier. It operates with a gain of -1, as set by the two 10kΩ input and feedback resistors. IC1b is fed from IC1a’s output (pin 7) and provides a complementary out of phase signal at its pin 1 output. The 1nF capacitor across the feedback resistor rolls the signal off above about 16kHz to ensure stability. As a result, IC1a’s output provides the in-phase signal while IC1b’s output provides the out-of-phase (or inverted) signal. The op amp outputs are then AC-coupled to the output socket via series 10µF capacitors and 560Ω resistors. The 560Ω resistors provide a www.siliconchip.com.au nominal 600Ω output impedance and prevent the op amps from oscillating (due to the extra capacitance) when the balanced microphone cable is connected. The 10µF capacitors are necessary to remove the DC levels that are present at the outputs of IC1a and IC1b. Muting As previously mentioned, the outputs can be muted and this is achieved using relays RLY1 and RLY2 which short the outputs to ground when powered. In addition, the outputs are muted at ELAN Audio The Leading Australian Manufacturer of Professional Broadcast Audio Equipment switch-on. This is necessary because when power is initially applied to op amps IC1a & IC1b (via switch S1b), their outputs quickly rise to half supply (Vcc/2). Without muting, this voltage would be coupled into the PA system and cause large switch-on thumps. To circumvent this, relays RLY1 & RLY2 are switched on at power up to short the signal outputs to ground until the voltages settle. The relays are switched via switch S1a and its associated circuitry based on transistor Q1. This works as follows. Switch S1 is a double-pole 3-position switch and when S1 is in position 1, no power is applied to the circuit. In position 2, S1b’s contacts feed power 2 Steel Court South Guildford Western Australia 6055 Phone 08 9277 3500 Fax 08 9478 2266 email poulkirk<at>elan.com.au www.elan.com.au RMA-02 Studio Quality High Power Stereo Monitor Amplifier Designed for Professional Audio Monitoring during Recording and Mastering Sessions The Perfect Power Amplifier for the 'Ultimate' Home Stereo System For Details and Price of the RMA-02 and other Products, Please contact Elan Audio January 2004  57 This close-up view shows the wiring details to the double-pole 3-position slide switch. The three switch terminals at the top connect to their corresponding PC stakes via short lengths of tinned copper wire. Fig.2: here are the parts layouts for the two different PC board versions (Altronics top, Jaycar & DSE bottom). Make sure that all polarised parts are correctly oriented and that the correct component is installed at each location. Note that the Altronics version uses an XLR connector for the microphone (ie, there’s no provision for a 3.5mm socket). to op amp IC1, while the corresponding contacts in S1a connect transistor Q1’s 10kΩ base resistor to ground via a 100Ω resistor. As a result, Q1 turns on and applies power to the relays. As shown on Fig.1, the relay coils are connected in series, with one side going to ground via a 470µF capacitor and 680Ω resistor connected in parallel. Initially, the 470µF capacitor is discharged and so the full 9V is applied across the series-connected relay coils – ie, 4.5V for each relay. This is quite sufficient to activate the 5V relay coils and close the contacts. As the 470µF capacitor charges, the voltage across the relay coils de58  Silicon Chip creases. However, the relays remain closed because their dropout voltage is much lower than the voltage required to activate them. The 680Ω resistor sets the minimum voltage across the relay coils to around 2.7V per relay. This resistor is included to reduce the current drawn from the battery while the relays are closed. The resistor and capacitor also cause LED1 to momentarily flash when the power is switched on. Initially, when power is applied and the 470µF capacitor is discharged, LED1 is fed via a 4.7V zener diode (ZD1) and the series 220Ω resistor. The LED will glow brightly with a fresh battery but as the battery voltage falls to around 7.2V, there will be insufficient current to light it at full brightness. It works like this: since there is 4.7V across ZD1 and a nominal 2V across the LED, this leaves only 0.5V across the 220Ω resistor when the battery is at 7.2V. As a result, the LED current is only about 2.3mA and so the LED will only glow dimly. By contrast, if the battery is at 9V, the resistor will have 2.3V across it and so the LED current will be around 10mA. As a result, LED1 will glow brightly. However, the LED does not light for long, as the 470µF capacitor quickly charges via the relay coils and turns LED1 off again. When S1 is placed in position 3, IC1 is still powered but Q1’s 10kΩ base resistor is disconnected from ground. As a result, the 22µF capacitor is now left to supply Q1’s base current for a short time as it charges towards the 9V supply rail via the two series 10kΩ resistors. After about 1s, Q1 switches off and the relays also turn off, thereby releasing the shorts across the output lines from IC1a and IC1b. Diode D3 quenches the back-EMF voltage that’s generated when the relay coils are switched off. This back-EMF voltage is further damped by the 100µF capacitor at D1’s cathode. Note that the muting can be reactivated at any time by switching S1 back to position 2, so that the relays are switched on again. In addition, when the power is fully switched off www.siliconchip.com.au (S1 switched to position 1), the relays remain on for one second while the 22µF capacitor charges. This ensures that IC1 is fully powered down before the relays are switched off, to prevent loud switching thumps in the PA system. As a further precaution, the 100µF capacitor that’s used to decouple IC1’s supply rail is quickly discharged via a 100Ω resistor and position 1 of S1a. Diode D2 is included to ensure that the 470µF capacitor also discharges, so that the relays turn on if power is quickly applied again. The 22Ω resistor in series with pin 8 of IC1 limits the surge current through the switch when power is applied. Similarly, the 100Ω resistor at position 2 of S1a limits the discharge current from the associated 22µF capacitor when S1a switches this contact to ground. A separate battery compartment accommodates the 9V battery that’s used to power the circuit. The screw in the back of the case (just above the 6.35mm jack socket) is used to secure the 10mm tapped spacer to the PC board (see Fig.4). Construction The assembly is straightforward since all the parts are mounted on a single PC board. There are two board versions: one coded 01101041 (86 x 64mm) to suit a Jaycar or Dick Smith Electronics (DSE) case; and one coded 01101042 (81 x 61mm) to suit an Altronics case. Note that the Altronics version assumes the use of a mini XLR socket for the microphone. There’s no provision for a 3.5mm socket on this board. Regardless of its origin, the specified case measures 135 x 70 x 24mm and includes a separate battery compartment. A small panel label measuring 59 x 16mm is affixed to the top panel of the case. Begin by checking the PC board for any possible shorts between tracks or you can use a PC-mount 3.5mm socket instead. In that case, you won’t need to make the cutout. You should also check that the two front corners of the PC board have been cut out to the shape shown. These cutouts are necessary so that the board clears the internal pillars in the case. breaks in the copper pattern. Check also that the hole sizes are correct. Note that a cutout will need to be made in the board to provide space for a mini XLR panel-mount socket if you are using a lapel microphone fitted with a mini XLR (female) plug. The XLR cutout is shown as an outline on the PC board. You also need to file the edge of the PC board slightly where shown, to allow room for the XLR securing nut to encroach into the PC board space. Alternatively, if you are using a microphone with a 3.5mm jack plug, Table 2: Capacitor Codes Value µF code IEC Code EIA Code 100nF 0.1µF 100n 104   1nF .001µF 1n0 102 Table 1: Resistor Colour Codes o o o o o o o o o o o No. 1 1 6 2 1 1 2 1 2 1 www.siliconchip.com.au Value 100kΩ 22kΩ 10kΩ 6.8kΩ 1kΩ 680Ω 560Ω 220Ω 100Ω 22Ω 4-Band Code (1%) brown black yellow brown red red orange brown brown black orange brown blue grey red brown brown black red brown blue grey brown brown green blue brown brown red red brown brown brown black brown brown red red black brown 5-Band Code (1%) brown black black orange brown red red black red brown brown black black red brown blue grey black brown brown brown black black brown brown blue grey black black brown green blue black black brown red red black black brown brown black black black brown red red black gold brown January 2004  59 Fig.3: here are the full-size etching patterns for the two versions of the PC board (Jaycar & DSE left; Altronics right). screws and by fitting the securing nut to the 3.5mm jack socket. That done, the LED’s leads can be bent at right angles about 4mm from its body and the LED slipped into position so that it protrudes through the front panel. Adjust its leads as necessary and make sure that it is oriented correctly before finally soldering it into position. In particular, note that anode lead (A) is the longer of the two. This lead goes towards the bottom edge of the PC board as shown on Fig.2. 6.35mm jack socket A right-angle stereo 6.35mm jack plug to 3-pin XLR line plug lead is used to connect the balanced output signal from the Lapel Microphone Adaptor to the PA amplifier. Fig.2 shows the assembly details for the two versions. Start by installing all the PC stakes at the wiring and switch terminal points, then install the resistors, diodes D1-D3, zener diode ZD1 and the IC. Make sure you place each component in its correct position and with the correct orientation. Table 1 shows the resistor colour codes but it’s also a good idea to check the values using a digital multimeter as some of the colours can be difficult to distinguish. The relays and transistor Q1 can go in next, followed by the capacitors. Be sure to install the electrolytic capacitors with the polarity shown. The 3.5mm socket can also now be 60  Silicon Chip installed if it is being fitted. The 3-position switch (S1) is mounted on its side, with its top face aligned with the edge of the PC board. Five of its bottom terminals are soldered directly to the previously installed PC stakes as shown on Fig.2, while three of the top terminals connect to their PC stakes via short lengths of tinned copper wire. Drilling the front panel The front panel can now be drilled to accept the switch, LED and microphone input socket. That done, attach the front panel label, then attach the front panel to the PC board assembly by installing the supplied switch A hole is needed in the side of the box for the 6.35mm jack socket which is used without its outer cover. Mark the hole location with the case clipped together, noting that the socket sits directly on the PC board and against the battery compartment. The mounting hole must be drilled and reamed out to 10mm diameter, which will not be large enough for the threaded section of the socket. That done, place the PC board in the case and secure it in position using three M3 screws (two at the top and one at bottom right). Next, position the socket in its mounting hole and tighten down the case lid with the four self-tapping screws supplied. Now heat the socket using your soldering iron until the plastic case begins to melt, at the same time pressing the case together so that it forms a tight fit around the socket and closes correctly. Finally, remove the iron and wait www.siliconchip.com.au Fig.4: this diagram shows how the M3 x 10mm tapped spacer is secured to the PC board. This helps secure the 6.35mm socket when the lid is screwed down. Fig.5: this artwork can be used as a drilling template for the front panel. for the heated case to cool. The case will now have formed a moulding around the threaded section of the 6.35mm jack socket. It should then be prised open again and the socket secured in position using a cable tie which passes through a hole in the PC board and then around the edge of the board. To further secure the socket, a 10mm M3 spacer is installed on the PC board adjacent to it so that the lid can be firmly screwed down at this point. To do this, the mounting post in the base of the case adjacent to the socket is drilled out to 3mm and this hole goes right through the case. In addition, you have to drill out the post in the case lid directly above this point. That done, countersink the holes and cut off the post in the lid using a sharp utility knife. The 10mm M3 spacer can then be fitted in position and secured using an M3 x 20mm screw installed from the bottom of the case as shown in Fig.4. All that remains now is to complete the wiring to the stereo socket and connect the battery clip lead. Note that the leads from the battery clip will have to be fed through from the battery compartment before soldering them to the supply terminals on the PC board. Testing To test the unit, apply power and check that the relays close and that the LED flashes. If not, check that transistor Q1 has been installed correctly and check its associated components. If the www.siliconchip.com.au Silicon Chip Binders REAL VALUE AT $12.95 PLUS P &P Fig.6: this is the full-size artwork for the case label. relays do close but the LED doesn’t flash, check that the LED has been installed with the correct polarity and check the orientation of ZD1. Finally, check that pins 1 & 7 of IC1 are at about 4.5V (ie, Vcc/2). This voltage should also be present on pins 3 & 5 (ie, the non-inverting inputs). If everything checks out, then it is likely that the unit is working correctly and is can be tested by connecting it to a PA system and plugging in a SC microphone. H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A12.95 plus $A5.50 p&p. Available only in Australia. Just fill in the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. January 2004  61