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Main Features • • • • • • • • Balanced input for microphone Balanced and unbalanced output Level control 3-band equaliser Runs from battery, plugpack or phantom power Battery indicator Ground lift Rugged diecast housing W Balanced Microphone Preamp This Balanced Microphone Preamp comes with a 3-band equaliser and is suitable for Karaoke, public address or many other applications. It can run from a plugpack, its own internal 9V battery or phantom power. by JOHN CLARKE 42  Silicon Chip HETHER IT IS FOR karaoke, public address or for a band, a microphone connection to an amplifier is a basic requirement. This Balanced Microphone Preamplifier includes a 3-band equaliser and can be used to drive a guitar amplifier, any stereo amplifier or provide an additional channel for a public address amplifier. Balanced microphones are desirable since they prevent the injection of hum and noise into the sound system. A balanced microphone has a 3-wire cable usually connected via XLR plugs and sockets. XLR pin 1 is the return or ground and the other two terminals (pins 2 & 3) are for the signals. The signals are in anti-phase; in other words when one line goes positive, the other line swings negative by the same amount. Any hum that is picked up along the lead is effectively cancelled because the same level of hum will be present in both signal lines. The 3-band equaliser (bass, mid and treble controls) is handy for enhancing a musical instrument so that it sounds natural when played through the microphone or to remove sibilance (the whistle sound from a voice particularly when pronouncing the letter “s”) by reducing the treble level and boosting the mid range. Or the bass control can be reduced to suppress popping noises which occur when speakers hold the microphone too close. A level control is included to prevent overload and a “ground lift” siliconchip.com.au siliconchip.com.au August 2004  43 Fig.1: the circuit is based on two low-cost dual op amps: IC1a & IC2. IC1a functions as a balanced to unbalanced preamplifier, while IC1b functions as a noninverting amplifier with a gain of 46. IC2a, VR2, VR3 & VR4 make up the equaliser stage, while IC2b provides an out-of-phase signal for pin 3 of CON3. Parts List 1 PC board, code 01108041, 102 x 89mm. 1 metal diecast box, 119 x 94 x 57mm (Jaycar HB5064) 1 front panel label, 112 x 88mm 2 SPST ultra-mini rocker switches (S1-S2) 1 momentary-contact pushbutton switch (S3) 1 PC-mount 9V battery holder 1 mono 6.35mm panel-mount jack socket 1 3-pin male XLR panel-mount connector 1 3-pin female XLR panel-mount connector 1 2.5mm PC-mount DC socket 1 PC-mount 10kΩ 16mm log potentiometer (VR1) 3 PC-mount 100kΩ 16mm linear potentiometers (VR2-VR4) 4 knobs to suit potentiometers 4 stick-on rubber feet 4 M3 tapped x 6mm Nylon spacers 12 M3 x 6mm screws 1 M3 x 10mm screws 1 M3 nut 3 M2.5 x 6mm screws 1 3mm eyelet crimp connector 12 PC stakes 1 200mm length green hookup wire 1 200mm length pink hookup wire 1 200mm length orange hookup wire 1 200mm length blue hookup wire switch can reduce hum in some situations. Circuit details Let’s now have a look the circuit in Fig.1. It uses two low-cost op amp ICs, four potentiometers, an XLR socket and plug, a 6.35mm jack socket, several switches and a few other lowcost parts. Op amp IC1a functions as a balanced to unbalanced preamplifier with a modest gain. The balanced microphone signal is fed to pins 5 & 6 of IC1a via 22µF capacitors and 1kΩ resistors. Gain for the inverting input is set at -3.3 by the 3.3kΩ feedback resistor from pin 7 to pin 6. Frequencies 44  Silicon Chip 1 200mm length red hookup wire 1 200mm length purple hookup wire 1 7812T regulator (REG1) Semiconductors 2 TL072 dual op amps (IC1, IC2) 1 1N5819 Schottky diode (D1) 3 1N4004 diodes (D2-D4) 1 12V 1W zener diode (ZD1) 1 5.6V 1W zener diode (ZD2) 1 5mm red LED (LED1) Capacitors 3 100µF 16V PC electrolytic 1 100µF 16V PC electrolytic (optional) 2 22µF 16V PC electrolytic 3 10µF 16V PC electrolytic 2 10µF 16V non-polarised (NP or BP) electrolytic 2 2.2µF 16V PC electrolytic 1 470nF MKT polyester 1 220nF MKT polyester 1 15nF MKT polyester 1 12nF MKT polyester 1 2.7nF MKT polyester 1 1.5nF MKT polyester 2 1nF MKT polyester 1 330pF ceramic 1 220pF ceramic 1 100pF ceramic 1 22pF ceramic Resistors (0.25W 1%) 2 100kΩ 2 3.3kΩ 2 18kΩ 1 2.2kΩ 2 12kΩ 7 1kΩ 4 10kΩ 1 220Ω 3 10kΩ (optional) 3 150Ω above 48kHz are rolled off by the 1nF capacitor across the 3.3kΩ feedback resistor. For the non-inverting input (pin 5), the input signal is attenuated by a factor of 0.77 due to the 3.3kΩ resistor connecting to Vcc/2. Overall gain for this signal path is therefore 0.77 x 4.3 or +3.3. Thus, the signal gain for both signal paths is the same. The 330pF capacitor between pin 2 and pin 3 of the XLR socket shunts high frequencies so that the Preamplifier does not detect radio frequencies. The output of IC1a is fed to the level potentiometer, VR1, via a 2.2µF capacitor and then to pin 3 of op amp IC1b. This provides a gain of 46 by virtue of the 100kΩ feedback resistor between pins 1 & 2 and the 2.2kΩ resistor to the half supply rail (Vcc/2). IC1b drives the following 3-band equaliser stage via a 2.2µF capacitor. EQ controls The equaliser stage is based on op amp IC2a and potentiometers VR2, VR3 and VR4. These potentiometers and their associated resistors and capacitors are in the feedback path between pins 6 & 7. This circuit is identical to the 3-band equaliser used in the DI Box for Musicians described in August 2001. Each of the Bass (VR2), Midrange (VR3) and Treble (VR4) feedback networks are effectively in parallel and act more or less independently (ie, with modest interaction). When the tone pots are all centred, the gain over their respective frequency ranges is unity (-1) and therefore the overall frequency response is flat. Let’s now look at the Bass control in more detail. When we wind the wiper of VR2 fully clockwise toward the output of IC1b, the input resistance for IC2a now decreases to 18kΩ while the feedback resistance increases to 118kΩ. At the same time, the 15nF capacitor is completely in the feedback circuit across the 118kΩ resistance. Without this capacitance the gain would be -118kΩ/18kΩ or -6.5 (ie, +16dB boost). The addition of the capacitor forces the circuit to give this gain below 100Hz and this reduces towards -1 as the frequency increases. Conversely, when the pot’s wiper is wound towards IC2a (anti-clockwise), the gain without the capacitor is 18kΩ/118kΩ or -0.15 (ie, -16dB cut). The 15nF capacitor is now on the input side so the gain rapidly increases to -1 at frequencies above 100Hz. Maximum bass cut is below 100Hz. The midrange section with VR3 works in a similar manner except that there is now a 12nF capacitor in series with the input. This combines with the 2.7nF capacitor across VR3 to give a bandpass filter. Finally, the treble control (VR4) operates with only a 1.5nF capacitor in series with the wiper. As a result, this control produces a high frequency boost or cut at 10kHz. Response curves for the tone controls are shown in Fig.2. The 220pF capacitor across IC2a’s siliconchip.com.au feedback path provides high frequency rolloff to prevent instability. Similarly, the 1kΩ resistor at the inverting input acts as a stopper for RF signals to prevent radio pickup. IC2a’s output at pin 7 drives the unbalanced output at CON2 via a 10µF capacitor and 150Ω resistor. IC2a’s output also drives pin 2 of the XLR output socket CON3, again via a 10µF capacitor and 150Ω resistor. As well, IC2a’s output drives inverting amplifier IC2b. This has a gain of –1 to derive the out-of-phase signal for pin 3 of CON3. The remaining pin on the XLR plug is the ground pin (pin 1). This is either directly connected to ground via switch S2 or AC-coupled to ground via a 470nF capacitor. Opening the ground lift switch (S2) prevents a hum loop if the input is separately earthed. This is not likely to occur with a microphone but there may be separate grounds connected when the unit is used to convert a balanced line to an unbalanced output. Power supply Power for the circuit can come from a DC plugpack, internal 9V battery or via phantom power. Diode D4 provides reverse polarity protection for external DC power sources such as a plugpack. The DC supply rail is then filtered and applied to 3-terminal regulator REG1 to provide the +12V rail which is then fed to IC1 and IC2 via diode D2. The internal battery supply is fed to the op amps via Schottky diode D1. A Schottky diode has a lower voltage drop than a standard diode and this extends the battery life. Note that the negative return of the battery goes via the DC power socket. Hence, the battery is disconnected whenever a plug is inserted into the DC power socket. Phantom power is delivered via pins 2 & 3 of the XLR plug and applied via two 1kΩ resistors to diode D3. Zener diode ZD1 regulates the voltage to 12V before it is applied to the rest of the circuit. This phantom power is usually produced from a source of either 48V with a 3.4kΩ impedance or from 24V with a 600Ω impedance. We can draw up to 7.5mA from each supply or 15mA in total at 12V. Diodes D1, D2 & D3 isolate each supply so that only one source can deliver power to the circuit. Essentially, where more than one supply is connected, siliconchip.com.au Fig.2: this graph shows the responses generated by the bass, mid-range and treble controls. The maximum bass boost is 12dB at 100Hz, while maximum mid-range boost is about 9dB at 850Hz. The treble boost is limited to about 7dB at 11kHz. it is the highest voltage source that powers the unit. The half-supply rail (Vcc/2) is obtained using two 10kΩ resistors connected in series across the power supply. The half supply point is decoupled using a 100µF capacitor to filter out any supply ripple. Switch S3, LED1, ZD2 and the series 220Ω resistor form a simple battery indicator. If the voltage is 9V, the voltage across the 220Ω resistor will be 9V - 5.1V - 1.8V (the LED voltage drop) or 2.1V. As a result, a current of 9.5mA will flow through LED1 when S3 is closed. This will cause the LED to glow brightly. As the battery voltage goes down, the current through the LED drops accordingly and so its brightness also decreases. For example, a battery voltage of 7.5V will only leave about 0.6V across the 220Ω resistor and so just 2.7mA will flow through the LED which will then be quite dim. Building it Most of the parts for the Balanced Microphone Preamplifier are mounted on a PC board coded 01108041 meas- Specifications Sensitivity ................................................................. 6.8mV input for 1V output Signal Handling ................................................ 2.3V RMS with equaliser set to flat response and 12V supply; 1.8V RMS at 9V supply Input Impedance ���������������������������������������������������������������������������������������� 1kΩ Frequency Response ................................................. -3dB at 30Hz and 19kHz Equaliser Response ...................................... +11db and –11db boost or cut at 100Hz; +9.6 and –10dB boost or cut at 1kHz; +7.4 and –8.4dB at 10kHz Signal-To-Noise Ratio ................................... -80dB with respect to 1V out and 20Hz to 20kHz bandwidth; -85dB A-weighted Phase Difference at Balanced Outputs ������������� 180° at 1kHz; 160° at 20kHz Battery Current .............................................................................. 8.8mA at 9V August 2004  45 multimeter, as the colours can be hard to recognise. The diodes can be installed next, making sure that D1 is the 1N5819. Be careful not to mix up the two zener diodes. ZD2 is the 5.1V zener and may be marked 1N4732 or C5V1. ZD1 is the 12V device and will be labelled 1N4742 or C12V. Next, install the two ICs and the capacitors. Non-polarised capacitors can be installed either way around but standard electrolytics with negative lead markings must be placed in the PC board with the correct polarity. The DC socket and REG1 can now be installed, followed by the PC stakes. The four pots can then be mounted on the PC board. LED1 should be installed about 20mm above the PC board. It is later bent over to mount in a hole in the side of the case. Finally complete the PC board by installing the 9V-battery holder using three M2.5 screws. Make sure the leads are soldered to the PC board. Drilling the box Fig.3: install the parts on the PC board as shown here. The components marked with an asterisk are optional and are installed only if you are using a phantom powered microphone or an externally powered microphone. uring 102 x 89mm. This is housed in a metal diecast box measuring 119 x 94 x 57mm. The diecast case serves to provide shielding for the audio circuitry and makes the unit extremely rugged – a necessary requirement for stage work. Fig.3 shows the PC board assembly details. Begin by checking the PC board for any shorts or breaks in the copper tracks. Check also that the PC board fits neatly into the case. If it doesn’t, file the corners and edges of Kit versions will probably be supplied with the case holes already drilled. If you’re starting from scratch, the first job is to drill out the four corner mounting holes in bottom of the case to 3mm. That done, attach the four 6mm tapped spacers to the underside of the PC board using M3 x 6mm screws. Note that the 6mm spacers must be nylon or insulated types to prevent the tracks on the PC board from shorting to the case. Next, mark out the positions for the pot shafts. The shaft centres are about 22mm above the outside base of the box. Drill the holes for the pot shafts, then use a rat-tail file to elongate the the board so that it fits when seated on 6mm standoffs. These can be temporarily attached for testing the PC board fit. Position the assembled PC board within the box and mark out the four corner mounting holes. Install the two wire links first, then the resistors. Note that the resistors marked with an asterisk are only used if the microphone needs an external supply. Table 1 shows the resistor colour codes used in the circuit. It is wise to check each value with a digital Table 1: Resistor Colour Codes o o o o o o o o o o o No.   2   2   2   4   3 (optional)   2   1   7   1   3 46  Silicon Chip Value 100kΩ 18kΩ 12kΩ 10kΩ 10kΩ 3.3kΩ 2.2kΩ 1kΩ 220Ω 150Ω 4-Band Code (1%) brown black yellow brown brown grey orange brown brown red orange brown brown black orange brown brown black orange brown orange orange red brown red red red brown brown black red brown red red brown brown brown green brown brown 5-Band Code (1%) brown black black orange brown brown grey black red brown brown red black red brown brown black black red brown brown black black red brown orange orange black brown brown red red black brown brown brown black black brown brown red red black black brown brown green black black brown siliconchip.com.au Table 2: Capacitor Codes Value 470nF 220nF 15nF 12nF 2.7nF 1.5nF 1nF 330pF 220pF 100pF 22pF μF Code EIA Code 0.47µF 474 0.22µF 224 .015µF 153 .012µF 123 .0027µF 272 .0015µF 152 .001µF 102   – 331   – 221   – 101   – 22 IEC Code 470n 220n   15n   12n   2n7   1n5    1n 330p 220p 100p   22p holes vertically. This will make it easier to insert the pots through the holes when the final assembly is inserted into the box. Now mark out and drill the mounting holes for the 6.35mm jack socket, the XLR connectors, the switches and the LED and DC socket. Use the front panel artwork as a guide to positioning these holes. The switch cutout and XLR holes can be made by first drilling a series of holes around the outside perimeter, then knocking out the centrepiece and carefully filing to shape. The switches must be a snug fit so that they will be held correctly in position with the integral plastic retaining lugs. The XLR connectors are secured with M3 x 6mm screws that are tapped directly into the case. We used an M3 tap to The PC board is secured to the bottom of the case using machine screws, nuts and spacers. All external wiring to the board is terminated using PC stakes. Note the earth wire between the case and pin 1 and shield terminals of CON3. make the thread and first drilled the hole out to 3/32” (2.4mm). If you use nuts instead of tapping the hole you will find it difficult to attach the lower nut unless it is glued in position first. Finally, drill a 3mm hole for the case earthing connection. Now fit the PC board and secure it with M3 x 6mm screws. That done, mount the remaining hardware and complete the wiring as shown. The wiring to the the XLR connectors and switches is easier to install if they are not attached to the box but remember Above: this view shows the location of the battery test switch (S3), the power socket (CON4) and the battery test indicator LED on the rear panel. Note that S3 should be a pushbutton switch, not a rocker type as shown here. Right: this end of the case carries (from left to right) the 3pin male XLR socket (CON3), a 6.5mm jack socket (CON2), the Ground Lift switch (S2) and the Power switch (S1). The 3-pin female XLR socket mounts on the other end of the case. siliconchip.com.au August 2004  47 Fig.4: follow this wiring diagram to connect the external switches and sockets to the stakes on the PC board. Note that CON1 (balanced input) is a 3-pin female XLR socket, while CON3 (balanced output) is a 3-pin male XLR socket. The jack socket (CON2) provides the unbalanced signal output. to pass the leads through the holes in the case before soldering to the terminals. The connectors and switches can then be mounted in place after the wiring is completed. The LED is inserted into its hole in the side of the box by bending its leads over and pushing it into position. Fit the panel label to the lid and install the knobs to complete the final assembly. Testing Fig.5: this is the full-size etching pattern for the PC board. Both the board pattern and a full-size front panel artwork can be downloaded from the SILICON CHIP website at www.siliconchip.com.au. Check your board carefully for etching defects before installing any of the parts. 48  Silicon Chip Apply power using a 9V battery and check that the battery test LED lights when the test switch is closed. Note that this LED will not operate if you are using a plugpack or phantom power. Test for 9V (when a fresh battery is powering the unit) or 12V when a plugpack is supplying power between pins 4 & 8 of IC1 & IC2. Further testing can be done with a microphone and amplifier. Check the operation of the level control and the equaliser controls. The ground lift should only be used when there is a SC hum present in the signal. siliconchip.com.au