Fullscreen HTML5Med Res (??MB)HTML4

High-performance microphone preamplifier Some recording devices, especially computer sound cards, have poor sound quality or insufficient gain when used with certain microphones. This tiny module provides a line level output from an unbalanced or balanced microphone and has very low noise and distortion. It runs off 5-20V DC, consuming just 6mA. By NICHOLAS VINEN T HE REASON THAT a microphone preamplifier is necessary is that most microphones, especially unpowered types, have a low output signal level. A typical microphone will deliver 10-200mV RMS at maximum volume. Audio “line level” is around 775mV RMS (0dBu) or higher but a great deal of audio equipment can actually handle 1V RMS or more. Higher signal levels usually mean more dynamic range. So to interface a microphone to a mixer, computer sound card, amplifier etc, we need to insert a preamplifier in-between to boost the signal level. Otherwise it may be impossible to get enough volume. Some such devices contain internal amplifiers but they don’t always perform well. Their internal microphone preamplifiers can be noisy and may not 42  Silicon Chip provide enough gain for some microphones (ie, those with very low output levels). Many, if not most, computer sound cards do not use high-quality analog components. Adding a microphone preamplifier does not guarantee good results, as the line level circuitry can still introduce noise and distortion but it certainly improves your chances of getting acceptable sound quality. On the other hand, a preamplifier is a necessity for connecting a microphone to any gear which only has line level inputs. Performance As can be seen from the specifications and graphs, this preamplifier has very good performance despite its low supply requirements. Signals below 50mV RMS will result in worse performance while higher level signals will provide better performance. For a 25mV RMS input, the signal-to-noise ratio will be reduced by 6dB, for 12.5mV by 12dB and so on. With a 100mV RMS input, the S/N ratio goes up to 94dB and THD+N improves to below 0.002%. The performance doesn’t vary with signal frequency. The frequency response is very flat with -3dB points at around 1Hz and 1MHz (see Fig.1). The total harmonic distortion plus noise (THD+N) level is the same across the audible band (see Fig.2) and at typical microphone levels consists mostly of noise. Under our test conditions with 50mV RMS input and 775mV RMS output, harmonic distortion accounts for just 12% of the total distortion measurement and is primarily second harmonic. siliconchip.com.au +0.1 Frequency Response: 50mV in, 1V out, 6V supply 07/20/10 15:47:03 0.1 THD+N vs Frequency: 20x gain, 10Hz-80kHz BW 07/20/10 15:42:15 +.08 Total Harmonic Distortion + Noise (%) 0.05 Amplitude Variation (dBr) +.06 +.04 +.02 +0 -.02 -.04 -.06 0.02 0.01 .005 .002 -.08 -0.1 20 50 100 200 500 1k 2k 5k 10k 20k .001 20 50 100 Frequency (Hz) CMRR The Common Mode Rejection Ratio (CMRR) is a measure of how well a 500 1k 2k 5k 10k 20k Frequency (Hz) Fig.1: this graph plots the frequency response of the Mini Microphone Preamplifier. Note that the vertical scale is greatly magnified, as the frequency variation is within just ±0.01dB. This figure is at the limit of our Audio Precision System One test gear’s resolution – the response is about as flat as it gets. Such a wide frequency response is not necessary but is the result of making this project as small and simple as possible. There is no low-pass filter except for the internal compensation of the op amps. We are assuming that most devices which accept line level signals will have their own bandpass filters to remove frequencies outside the audio spectrum. 200 Fig.2: this graph shows the total harmonic distortion (THD) with respect to frequency. Distortion levels are higher than quoted because this is measured over a wider bandwidth (10Hz-80kHz), so more noise is registered. The slight drop at high frequencies is due to the 80kHz cut-off. Again this is essentially a flat measurement. device with a balanced or differential input is able to reject a signal that is common to both inputs. In other words, if the same amount of 50/100Hz hum is coupled into both signal conductors in the cable, this is the amount by which that hum is attenuated. For our first prototype, we used standard 1% resistors throughout and measured a CMRR of -55dB. Our second prototype used more expensive 0.1% resistors in the differential amplifier which improved the CMRR to -88dB. In practice, -55dB is perfectly adequate unless you have a very long microphone cable run. To get the best performance, either the power supply ground or signal ground should to be connected to earth. This reduces the possibility of mains 50/100Hz hum entering the circuit. However, you should avoid earthing both so that an earth loop cannot be created. In other words, earth the supply ground but only if neither the input nor output signal grounds are already earthed. The diecast aluminium box Main Features • Unbalanced or balanced mono input (3.5mm mono/stereo socket) • Unbalanced mono output (3.5mm mono socket) • • • • • • Very low distortion and noise Small and easy to build Runs off a 5-20V DC plugpack or battery Adjustable gain over a wide range Line level output to at least 1.5V RMS Provision for electret microphone bias (approx. 390µA) siliconchip.com.au September 2010  43 REG1 LM2931 +5-20V DC IN GND 10k CON1 +5V OUT 100 F LOW ESR 100nF A  LED1 10k 4 12 100nF 14 IC1d 100 F 13 10k K +2.5V MIC BIAS LK1 100k IC1: AD8648ARZ 100k 5 6 10k 10k 4.7 F NP MIC INPUT 4.7 F NP CON2 3.5mm STEREO 7 10k* 10k* * USE 0.1% RESISTORS FOR IMPROVED CMRR 10k* GAIN VR1 10k LIN 9 10 180 100k IC1b IC1c 100 8 100k 100k 3 IC1a 1 10k* MINI MICROPHONE PREAMP CON3 3.5mm STEREO 10k* LED SC LINE OUT 11 10k* 2 2010 10 F LM2931 GND K A IN AD8648ARZ 14 OUT 7 1 Fig.3: the circuit is based on quad op amp IC1, with IC1a & IC1b forming a balanced amplifier stage. This provides the gain and drives differential amplifier stage IC1c which converts from a balanced to an unbalanced output signal. Regulator REG1 provides a +5V supply rail to power the circuit, while IC1d and the 10kΩ divider resistors on its pin 12 input provide a +2.5V half-supply rail to bias IC1a-IC1c. is connected to ground (and therefore earth) to improve its magnetic shielding properties. It is also very important to use shielded cables. Most of the distortion we encountered while testing the preamplifier’s unbalanced performance was in the form of hum entering via the input lead. This changed depending on how the lead was routed. We tested both the AD8648ARZ and AD8694ARZ quad op amps and found the overall performance to be the same. Use whichever one is easiest or cheapest to obtain. Balanced input While this circuit was designed with cheap, unbalanced microphones in mind, it is able to handle balanced signals too. These have the advantage of good noise cancellation, eliminating hum, especially with long cable runs. However, because the unit is so small we cannot fit the standard XLR type connectors. Instead, we are using the 44  Silicon Chip tip and ring of a 3.5mm stereo connector for the positive and negative balanced signals respectively. If you intend using unbalanced microphones, we assume that they will be fitted with a mono 3.5mm jack plug. Inserting this into the 3.5mm stereo socket will ground one side of the balanced input, to give unbalanced operation. To test its balanced capabilities, we used an XLR to 6.5mm Tip-RingSleeve (TRS) cable with a 6.5mm-to3.5mm stereo adaptor on the end. Both XLR and 6.5mm TRS connectors are used for balanced audio connections on professional gear, so getting such cables is easy enough. Unfortunately, professional gear does not come cheap. The cable probably cost more than the preamplifier! Using a balanced input cable isn’t strictly necessary but our tests showed that it is by far the best way to eliminate mains hum from the equation. With an unbalanced input cable, we could only eliminate the hum by watching the signal on the oscilloscope and moving the cable around until the 50Hz component disappeared. In practice, hum will always be a problem when using unbalanced microphones. Substituting a balanced cable (and signal) completely eliminates it, regardless of the cable routing. Note that the metal enclosure is less critical if you are using a balanced microphone. This is because the low-level signals on the PC board are all differential – by the time the signal is converted to unbalanced, it has already been amplified so mains interference is less of a problem. With a balanced signal, even if the PC board is mounted in a plastic enclosure, performance should be good. Tests at our office show no loss in performance running the bare board with a balanced input signal. Op amps The AD8646/7/8 and AD8691/2/4 siliconchip.com.au op amps we have used in this project provide excellent performance from a low supply voltage. They both feature a low input noise of 8nV/√Hz – the same as an OPA2132/4. This is not quite as good as an NE5532, NE5534 or LM4562 but it is impressive nonetheless, especially as they operate from such low supply voltages. The AD8648 has a gain bandwidth (GBW) of 24MHz while the AD8694 has a GBW of 10MHz. The AD8694 features a THD+N figure of 0.0006% and a low input offset voltage of 400µV (with low drift), while the AD8648 can deliver 120mA from its outputs and handles 600Ω loads gracefully. Both have very low input bias current (<1pA) and low quiescent supply current (<2mA per amplifier). Both op amp series are only available in surface-mount packages – Small Outline Integrated Circuit (SOIC) or the finer-pitched Thin Shrink Small Outline Package (TSSOP) or Mini Small Outline Package (MSOP). That is the trend these days and many modern, high-performance ICs are no longer available in through-hole packages. Having said that, these SMD packages are reasonably easy to solder. These op amps are ideal for highquality audio processing in batteryoperated equipment. The AD8646/7/8 (single/dual/quad version) can even do a decent job of driving a headphone load. In this application, we have chosen them primarily for their low noise and distortion, as well as their reasonable price. Circuit description Refer now to Fig.3 for the circuit details. As shown, power is supplied via PC-mount DC socket CON1, with green LED1 indicating operation. The 10kΩ current-limiting resistor is a much higher value than usual and as a result, the LED glows dimly. This is done to conserve power if the preamplifier is being run from a battery. If you don’t plan to use a battery or don’t mind a few milliamps of extra current drain, then you can change the 10kΩ resistor to 1kΩ so that the LED is brighter. Since the LED runs off the unregulated supply, its brightness will depend on the supply voltage. This means that it can also be used as a crude battery level meter. Regulator REG1 is an LM2931 lowdropout (LDO) type, so its output voltage is stable with an input as low as siliconchip.com.au Specifications Supply voltage: 5-20V DC (operates at 2.8-5V with reduced performance) Supply current: typically below 6mA Voltage gain: 3-111 Input sensitivity (line level output): 14mV RMS Input sensitivity (1V RMS output): 18mV RMS Input impedance: 50kΩ (8.3kΩ with bias enabled) THD+N ratio: 0.0035% THD+N ratio (10mV RMS in): 0.014% Signal-to-noise ratio: -90dB (-93dB A-weighted) CMRR* (1% resistors): -55dB CMRR* (0.1% resistors): -88dB Frequency response: 20Hz-20kHz ±0.01dB Signal handling: >1.5V RMS output Signal handling (3.0V supply): >1.0V RMS output Note 1: CMRR = Common Mode Rejection Ratio Note 2: all specifications relative to 50mV RMS input, 775mV RMS output, 20Hz-22kHz bandwidth and a 6V supply, unless otherwise stated. 5.1V. Its quiescent current is typically below 1mA, again contributing to good battery life. Below 5.1V, REG1 ceases regulating but the circuit can still run, as long as the input supply is at least 2.8V. However, the maximum output signal level is lower with a supply below 5.1V. With a 2.8V supply, the maximum output level is 950mV RMS, which is still above line level. There is no supply polarity protection diode as REG1 can withstand negative voltages and its input filter capacitor is a non-polarised type. The 100µF capacitor at its output filters the regulated voltage and is necessary for stability. Because REG1 is an LDO type, the output capacitor must be at least 100µF and its Equivalent Series Resistance (ESR) has to be between 0.03Ω and 1Ω. That is why we have specified a low-ESR type (listed as 0.22Ω). In reality, many other 100µF capacitors are probably suitable but they would need to be tested using an ESR meter before installation to ensure that they are within the acceptable range. The two 10kΩ resistors between REG1’s output and ground form a voltage divider, the junction of which is at half the supply voltage (normally +2.5V). This is necessary because the op amps use ground as their negative rail. Their input and output AC signals must be biased to this virtual ground potential so that the signals always stay between the two supply rails (5V and 0V). The second 100µF capacitor filters this virtual ground. This is important as otherwise supply noise could couple into it and noise on the virtual ground will couple directly into the signal path. The half-supply voltage is fed into op amp IC1d which is configured as a voltage follower. Its output is the same voltage as its input but has a much lower impedance, so any current fed into the virtual ground has no effect on its level. The two input signals at 3.5mm socket CON2 (one of which is grounded with an unbalanced microphone) are DC-biased with 100kΩ resistors in case the signal source is floating. If a jumper link is placed across LK1, these signal lines are pulled up via 10kΩ resistors to provide a 390µA bias current for an electret microphone – see panel. Regardless of the DC biasing, the signals pass through the two 4.7µF non-polar AC-coupling capacitors. Next, the signals are biased to a DC level of 2.5V by two 100kΩ resistors and then enter the differential amplifier. Op amps IC1a, IC1b and IC1c are configured in the classic instrumentation amplifier layout. IC1a and IC1b have a high impedance input and September 2010  45 R IC1 AD8648 100nF 10k 10k T S T 10k 100k 10k 100 F 10k 100k R S + NP 10k* 10k* 10k* 100k 5V BIAS LK1 1 + + T 4.7 F 180 S NP (UNDER) R CON2 + + IC1 VR1 4.7 F 100 F L/ESR 100nF S 100k 100k 100 T 10 F R CON3 REG1 10k* 10k* 10k* + LED1 CON1 A * USE 0.1% RESISTORS FOR BEST CMRR TOP OF BOARD (COMPONENT SIDE) UNDERSIDE OF BOARD (COPPER SIDE) Fig.4: here’s how to install the parts on the PC board. IC1 should be installed first – it goes on the copper side of the board and must be orientated with pin 1 at bottom right (see photo). The jumper is installed for LK1 only if you intend using an electret microphone – see panel. Note that prototype board shown in the photo differs slightly from the final version. provide the gain which is varied by potentiometer VR1. VR1 and its series 180Ω resistor form a voltage divider, along with the 10kΩ resistors to the outputs of IC1a (pin 1) and IC1b (pin 7). As a result, when VR1 is turned clockwise and its resistance decreases, the gain of both IC1a and IC1b increases. Note that, in each case, the “bottom end” of the divider network is not connected to ground but rather to the output of the opposite op amp. This provides much better common-mode rejection. That’s because the gain of each op amp can vary due to resistor tolerances but since the gain is differential, it does not matter. The buffered and amplified signals are now passed to IC1c which is connected as a differential amplifier. It converts the balanced signals from IC1a & IC1b to an unbalanced signal. The resulting waveform is then AC-coupled via a 10µF electrolytic capacitor to CON3, the 3.5mm stereo output socket. Note, however, CON3’s ring termi- nal is grounded which means you must use a mono jack plug. The associated 100kΩ resistor references the output signal to ground while the 100Ω series resistor isolates the output from capacitive loads to ensure stability. Construction All the parts are mounted on a PC board coded 01109101 and measuring 56 x 49.5mm. This board has corner cutouts to clear the corner pillars in the specified diecast metal case. Fig.4 shows the parts layout. Begin by checking the copper side of the board for any defects (cracks, short circuits, etc). Check also that it is the correct shape to fit in the box. If necessary, make the corner cut-outs using a small hacksaw and file. Make sure that the board goes all the way down into the box. The inside of the box tapers slightly and it may be necessary to file the edges of the board so that it fits. The next step is to solder the surface-mount IC (IC1) into place. This is a 14-pin SOIC package and is fairly easy to solder provided due care is taken. You will need a soldering iron with a fine tip and a good light (preferably a magnifying lamp). First, orientate the PC board copper side up and with the SMD pads posi- This view shows the completed PC board mounted inside its diecast metal case. This case makes for a rugged assembly and provides the necessary shielding. Table 1: Resistor Colour Codes o o o o o No.   5 11   1   1 46  Silicon Chip Value 100kΩ 10kΩ 180Ω 100Ω 4-Band Code (1%) brown black yellow brown brown black orange brown brown grey brown brown brown black brown brown 5-Band Code (1%) brown black black orange brown brown black black red brown brown grey black black brown brown black black black brown siliconchip.com.au Biasing Electret Microphones Parts List Electret microphones have an internal Field Effect Transistor (FET) which amplifies the very low level signal they generate. This FET requires a source of current to operate. Some such microphones contain an internal battery, in which case they can be treated like any other microphone. However others require power to be sent along the input cable, in a similar manner to “phantom power”. If you have an unbalanced electret microphone which requires external power, this unit can deliver it. A jumper shunt placed on the 2-pin header (LK1) enables the bias current. Assuming the microphone’s bias voltage is 1V, it will receive 390µA. Some electrets require more current – up to 800µA – but others can be damaged if more than 400µA is supplied. If your microphone needs more current then you can change the two 10kΩ resistors near the non-polarised capacitors to 5.1kΩ. In this case the bias current will increase to 775µA with a bias voltage of 1V. This unit will also provide power for balanced condenser microphones, using the same 2-pin header. 1 PC board, 56 x 49.5mm, coded 01109101 1 die-cast sealed aluminium box, 64 x 58 x 35mm (Jaycar HB5030) 1 2.5mm PC-mount DC power socket (Jaycar PS0520, Altronics P0621A) 2 3.5mm PC-mount stereo switched socket (Jaycar PS0133, Altronics P0092) 1 2-pin header (2.54mm pitch) 1 10kΩ linear 9mm vertical PCmount potentiometer (Altronics R1946) 1 jumper/shorting block 1 50mm length of tinned copper wire or 0Ω resistor tioned as shown in Fig.4. That done, apply a small amount of solder to one of the pads – eg, the upper-right pad if you are righthanded or the upper-left pad if you are lefthanded. Now place the IC alongside the pads with the bevelled edge on the righthand side and the pin 1 dot at the bottom-right. Check that it is correctly orientated, then melt the solder on the pad (taking care not to spread it to adjacent pads) and gently slide the IC into place. Do not apply heat for more than a few seconds. Next, press down gently on the IC and re-heat the pad, allowing the solder to melt. This ensures that the IC is sitting flat on the board. Now check that the pins are all aligned with the pads. If not, re-heat the soldered pad and slide the IC until all the pins are lined up, then apply solder to the diagonally opposite pin. It’s now simply a matter of flowing a small amount of solder onto the remaining pads. Ensure that it adheres to both the pad and the pin in each case. Generally, this is achieved by ensuring that the soldering iron remains in contact with the pad for about one second after the solder is applied. If you accidentally join any of the adjacent pads or pins together, remove the excess solder using solder wick. Finally, apply a small amount of additional solder to the first two pads you soldered to hold the IC in place, to ensure the solder has flowed correctly. Once it’s finished, use a magnifying glass to check that all the pins have been correctly soldered and that there are no bridges. siliconchip.com.au With the IC secured, the parts can now be installed on the top of the board. Begin by fitting the single wire link using 0.71mm tinned copper wire or a 0Ω resistor, then install the resistors. Table 1 shows the resistor colour codes but you should also check each one using a DMM before installing it. If you are using 0.1% 10kΩ resistors to get the improved CMRR figure then be sure to install them in the locations marked with asterisks on Fig.4. The remaining 10kΩ resistors can be 1% types without affecting the performance. Now mount the two 3.5mm stereo sockets. First, remove the nuts from both and discard them then press the sockets down so that they sit flat on the PC board. Check that they are aligned with the edge of the board before soldering all the pins. Follow these with the two multilayer ceramic capacitors. They are the same value and can go in either way. The LM2931 regulator in the plastic TO-92 package can then go in. Use small pliers to bend its legs out by 45° and then back down parallel again so that they will fit through the holes in the PC board. Make sure its flat face is orientated as shown on the overlay. The 2-pin header is next on the list, followed by the polarised electrolytic capacitors. Check that the 100µF low ESR type goes in next to the regulator and check that they are all orientated correctly. Don’t get the 10µF and 100µF capacitors mixed up. The two 4.7µF non-polar electrolytics can be fitted either way around. Install these now, then fit the DC power Semiconductors 1 AD8648ARZ or AD8694ARZ quad low noise rail-to-rail op amp (IC1) 1 LM2931Z-5.0 or LM2931AZ-5.0 low dropout 5V regulator (REG1) 1 5mm green LED (LED1) Capacitors 1 100µF 16V low-ESR electrolytic (Jaycar RE6310) 1 100µF 16V electrolytic 1 10µF electrolytic 2 4.7µF non-polar electrolytic 2 100nF multilayer ceramic (code 100n or 104) Resistors (0.25W, 1%) 5 100kΩ 1 180Ω 11 10kΩ 1 100Ω Optional: use 6 x 10kΩ 0.1% for improved CMRR – see Fig.4 socket. The latter should sit flush with the board and its pins soldered using generous amounts of solder. Finally, install the 5mm green LED. This goes in with the bottom of its plastic body 19mm above the PC board and its flat edge towards CON1 – see Fig.4. A strip of cardboard cut to 19mm can be inserted between its pins when soldering it in to set the correct height. Testing It is a good idea to test the board before installing it in the box. Once it’s installed, it can be difficult to remove. The first step is to install the shorting jumper on the 2-pin header if your September 2010  47 5-20V DC HOLES A: 7.0mm DIA. HOLE B: 5.0mm DIA. HOLE C: 6.5mm DIA. HOLE D: 8.0mm DIA. Power D CL Output 1.5 (ALL DIMENSIONS IN MILLIMETRES) (TOP) UPPER LONG SIDE CL CL (TOP) A 9.5 7 C CL A 9.5 1.5 LEFT END TOP (LID) Fig.5: these are the drilling templates for the case. Use a small pilot drill to drill the centre of each hole initially, then carefully enlarge them to full size using a tapered reamer. microphone requires a bias current (ie, if it is an electret – see panel). That done, apply power (a plugpack is the easiest) and check that the LED lights. If it doesn’t, then either the supply polarity is reversed or you have a short circuit between two tracks. Once it has power, turn the gain all the way down and connect a signal source to the input socket. You can use a microphone or some other mono or balanced signal source. A stereo signal will not work very well however, as the two channels will be subtracted from each other by the differential amplifier. Next, connect the output socket to an amplifier or use some other method to monitor the output signal (eg, a scope). Now slowly increase the gain control on the preamplifier and check that the input signal is being correctly fed through to the amplifier. If you are using a microphone for this test, be careful to avoid feedback between the monitoring speakers and the microphone. Final assembly Assuming all is well, use the drilling templates shown in Fig.5 (also available on the SILICON CHIP website) to 48  Silicon Chip Gain SILICON CHIP Input Mini Mic Preamplifier B 18 + make the five holes in the diecast box. It’s best to initially drill the centre of each hole with a small-diameter bit (eg, 3mm) and then carefully enlarge them to full size using a tapered reamer (this will ensure that they are accurately placed). Deburr each hole using an oversize drill. Once the holes have been made, insert the board with the two 3.5mm sockets angled downwards. Push these into the appropriate holes and then lever the board down. The DC socket should clear the edge of the box, allowing you to lay the board flat on the internal “shelves”. If it won’t go in, you may need to either file the board edges where it is catching on the box or slightly enlarge the holes for the 3.5mm input and output sockets. Once the board is in, secure it in position with the two screws provided with the box. Now, using a multimeter set on continuity mode, check that the board ground is electrically connected to the box. You can use the exposed metal tab on the side of the DC socket as a ground test point. It is also a good idea to ensure that there is no short circuit between the Fig.6: this front panel artwork can either be photocopied or you can download it in PDF format from the SILICON CHIP website. exposed wire link on the top side of the PC board and the box, or between the exposed tab on the rear of the DC socket and the box. If there is, your power source will be shorted out when it is plugged in. If you do get a short, remove the PC board and check for any leads or other metal pieces sticking out the underside which may be contacting the box. Assuming there are no short circuits, plug your power source back in and check that the preamplifier still works. If not, there may be a short circuit from one of the signal paths to the box for the same reason stated above. The board has been designed so that the component pads clear the box edges and shelves (except for the ground track) but there may be some circumstances under which they can make contact. All that remains now is to fit the front panel and attach the lid. You can either photocopy the artwork shown in Fig.6 or you can download it in PDF format from the SILICON CHIP website and print it out. It can be protected using wide strips of clear tape (or laminated) and attached using a smear of silicone sealant. The gain control shaft and LED should project through the holes in the lid by a few millimetres – just enough to allow the gain control to be adjusted with your finger tips while making it difficult to accidentally alter it if it is bumped. If you wish, you can press the provided neoprene seal into the recess underneath the lid just prior to fitting it. However, the holes drilled earlier mean that the box is no longer dustSC proof or waterproof. siliconchip.com.au