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“A Lump In The Coax” Mini Audio Mixer We’ve published a number of audio mixers, large and small, over the years but they’ve all been “general purpose”. Not that that’s a bad idea – it’s just that when you need one for a specific purpose, you need a specific purpose mixer! I n another life, I do a lot of commentary and announcing at surf lifesaving carnivals and also do my share of MC-ing at other venues. But I have come across a problem many, many times and just as often longed for a solution to that problem. This is it! So what’s the problem? presumably well-meaning people who think they know what they are doing, invariably putting the PA system into shock (if not cardiac arrest!). You know what they say, “a little knowledge is a dangerous thing . . .”. For example, the hirers who want “more sound” and add in a couple of “real good” speakers from their home hifi. Except they don’t know that most PA systems are wired with 100V speaker lines. . . Others simply “make adjustments” which end up being maladjustments! I’m sure that anyone working in small clubs or similar venues have had this dilemma. Many clubs – as do many other community centres, halls, schools and so on – have a reasonable (and sometimes excellent) public address Lock the PA away! system installed. An increasing number of venues which I visit have their Surf lifesavers use it to warn swimmers of dangers, they use it to provide information to the beach-going public and PA system firmly locked away in a cupboard so that nofrom time to time it’s there for commentary when the club one except the person responsible for the system can get at it. And, of course, that person is never runs a carnival. The problem is that surf clubs, like Design by Nicholas Vinen around when needed. The PA often simply has an accesthe vast majority of “public” halls and Words by Ross Tester sible on/off switch while all controls sports venues, suffer badly from knoband Nicholas Vinen are pre-set to “typical” levels, so that it twiddlers and system stuff-upperers: 72  Silicon Chip siliconchip.com.au really is simple enough for any idiot to use. Idiot being the operative in many cases. Even if it isn’t locked up, getting to the business end of the system to plug anything else in – an MP3 player, for example – is often near (or totally) impossible. (I’m sure that manufacturers put all inputs and outputs on the back of amplifiers not just to tidy up the wiring but to cause the most frustration!) Such systems invariably have either an XLR socket mounted on the locked box or a coax cable emerging from it, to which is attached a wired microphone (invariably on a too-short lead) – and that’s it. Many have given up on wireless microphones, usually because the mic itself keeps on disappearing and/or they’ve suddenly found their wireless microphone is suffering from all sorts of strange interference. (See the feature earlier in this issue – wireless microphones and digital TV). So you get to a venue and find all you have to work with is a wired microphone which doesn’t even reach the balcony, so you can actually see what’s happening on the beach (I always carry a long XLRF-XLRM “extension” mic lead with me these days!). But that’s it: if you need to add music, all you can do is do it acoustically (eg, the MP3 player’s speaker to microphone) which invariably sounds awful. If you want to use a wireless microphone (for ceremonies, interviews, etc) away from the system – tough luck! And if there are two or more announcers, you’re continually swapping the fixed mic back and forward. It’s all pretty unsatisfactory – and unless you’ve been in the situation, you probably won’t appreciate just how frustrating it all is. Specifications: (22Hz-22kHz bandwidth , unweighted, 50mV RMS in/out unless otherwise stated) Signal-to-noise ratio: >65 dB Total harmonic distortio n plus noise: 0.06% (1k Hz) Common mode rejection ratio: >40dB (typically 55d B) Frequency response: 50H z-35kHz (-3dB) Gain: ~1 for microphone input, 0-0.25 for other inp uts Signal handling (microph one input): at least 1V RM S Signal handling (other inp uts): 0.2-2.5V RMS for 50m V output Supply current: 0.5mA (ty pical) Battery: CR2032 (3V Lith ium), 3.7V Li-Po or 9V alk aline/dry cell Battery life: 200+ hours with CR2032, 2000+ hou rs with Li-Po Operating voltage: minimu m 2.1V, nominal 2.7-4.2V Li-Po charger input: 5V DC <at> up to 500mA they wanted extra inputs, here would be the ideal way to do it. We mention schools here mainly because so many electronics-savvy teachers have told us that non-savvy “expert” colleagues are the bane of their lives! And we also thought of all those venues that have microphone sockets (XLRs) spread around the building wired back to the PA amp, somewhere, so that mics could be plugged in and used anywhere. With this Mini Mixer, such installations would be so much more versatile and usable. Our ideas A typical PA system uses either a dynamic or electret microphone (the latter less usual as they tend to be more easily damaged). These mics usually have an output in the region of 10-50mV and any microphone worth its name uses XLR connectors – there’s a male XLR socket built into the microphone, the coax mic lead has a female plug to match and a male plug back at the amplifier to match the female input socket. Which started us thinking . . . What if we were to come up with a mixer which simply While it’s not something every reader would need to inserted between the mic lead and the microphone and worry about (indeed, very few will ever have the problem) effectively gave a “straight through” connection for that we thought, “why not come up with a mini mixer specifimicrophone – in other words, act as if it wasn’t there? That cally designed for this purpose”. way there would be no difference in the normal operation Then we reasoned that such a mixer would be popular of the PA. with a lot of others who have PA systems – schools, for It could almost be regarded as a “lump in the coax”. example – and wanted to be able to lock But that same mixer could also accept a couple of other it away so that the twiddlers inputs – say from an MP3 or CD player for music and from couldn’t . . . twiddle. a wireless microphone receiver. The beauty of both If at any time of these is that they would be expected to be much the same output level – perhaps 1V, maybe less – which would make the mixer inputs virtually universal. We would want to make the mixer battery-operated for convenience so would need a very low power op amp if we wanted the battery to last any length of time. Speaking of batteries, a 3.7V Lithium-Ion (or even LiFePO4) would be eminently suitable, given the right design. With very low drain, even a Shown here not button cell could be used – or we too far off could use a mobile phone battery life-size, the Mini which are very flat and give a very Mixer has XLR input respectable output – that would and output sockets on theoretically last for weeks, if not the end and 6.35mm, 3.5mm months. and RCA sockets along one side. siliconchip.com.au June 2013  73 330pF 330pF 10k 100mF 100mF 100W 4.7k 10k 13 INPUT 1 CON1 1 3 2 100mF 100mF 100W 100k 100pF 100k 12 10k 10k 100pF IC2d 1mF 9.1k 14 9 MKT 10 8 IC1c 330pF Vcc 100nF 10k INPUT 2 CON3 10k 100mF 100mF 10k 6.5mm 100k 100pF VR1 10k LOG 100mF 5 6 100k IC2b 2 22k 7 4 3 IC1, IC2: OPA4348AID, AD8544ARZ OR MCP6404-E/SL 100mF 5 6 CON4 10k 11 2x 100W 100nF 3.5mm 100k Vcc 100mF 100mF 10k 100pF 7 IC1b 10k INPUT 3 1 IC1a VR2 10k LOG 100mF 3 2 100k 4 IC2a 1 22k 100nF Q3 DMP2215L 11 D S CON5 4.7k Vcc INPUT 4 4.7k 100k 100mF 100mF 100pF D3 BAT54S VR3 10k LOG 100mF 3 100k ZD1 3.6V 10 9 IC2c 8 A 22k B C Q4 BC549 E A SC 100mF K D1 1N5819 Ó2013 G 100k 2 1 100k K “LUMP-IN-COAX” LOW POWER MIXER Fig.1: the circuit is quite conventional for an audio mixer, albeit with a few clever refinements (eg low-voltage op amps) for operating at very low power. Input 4 doubles as a charging connection for Li-Po battery, if fitted. The back-to-back (series) 100µF capacitors are used because two of these are significantly cheaper than one non-polarised 50µFcapacitor. And finally, the whole thing would want to be quite small, with a minimum of controls to make it as foolproof as possible. Let’s not worry about tone controls or other “niceties”. Our design We’ve come up with a mini-mixer that fits all the criteria above (and then some!). In fact, it has some rather snazzy features and offers performance that is nothing to be sneered at! It’s small (built into a 120 x 93.5 x 35mm diecast box). It has minimal controls – just a “preset”-type gain 74  Silicon Chip control for each of the three inputs and these don’t even have knobs (again to discourage the twiddlers). We used mini pots with “screwdriver slots” on the end – they emerge just far enough from the front panel to fit a fingernail! (OK, use a small flat-bladed screwdriver if you must!). There are five sockets: an XLR female and male on the end to accept the microphone lead and the lead to the amplifier, a 6.35mm “phono” socket, a 3.5mm mini phono socket and an RCA socket. The larger phono sockets are often used on wireless microphones while the 3.5mm mini sockets are very commonly used on MP3 and other small music players, radios, etc, normally as headphone sockets. But we’ve been particularly clever with the RCA socket: feed it with audio signal, it acts as you would expect. But if you feed it with 5V DC (eg, from a USB socket or plugpack), it also serves as the charging point for the internal battery; more on this shortly. The only other control is the power switch, necessary if you use the onboard CR2032 lithium battery but almost redundant if you use a larger siliconchip.com.au 100W 330pF 4.7k 4.7k 100mF 100mF 100mF 100mF OUTPUT 13 14 IC1d 12 100W CON2 2 100k * REG1 ONLY NEEDED FOR 9V BATTERY – OTHERWISE FIT LK1 LK1 POWER OFF REG1 MCP1703-5 Vcc OUT * ON 100k Q1 DMP2215L S1 IN S G 100nF Balanced input D 1 GND 100mF 1 3 10M BATT 2 CR2032 + BATT 1 – Li-Po/ 9V 2 Q2 BD140 0.22W C E LED 100nF B SIGNAL GROUND K A 220W 1 SNS CC IC3 8 COMP BQ2057 CSN 4 TS BAT STAT Vss 6 K 5 A BC549 B CHARGE A NTC 1 2 E l C BD140 LED 1 K B LI-PO CHARGER COMPONENTS (INSIDE BLUE BORDER) SHOULD ONLY BE FITTED IF LI-PO BATTERY IS USED. MCP1703T BAT54S C GND D G OUT S phone battery, as mentioned earlier. One point to note: a lot of mobile phones, etc use headphones fitted with 2.5mm ultra-mini plugs. We haven’t allowed for a 2.5mm socket but 2.5 to 3.5mm adaptors are very common and very cheap. Finally, it’s designed to suit dynamic microphones only and then only those that use XLR plugs. No provision has been made for electret phantom power. Circuit description The balanced microphone signal from CON1 is converted to an unbalsiliconchip.com.au E OPA4348AID DMP2215L IN 3 2 1N5819 2 2.2k 1 1 A ELECTRICAL GROUND 7 2.2k 3.9k ZD1 K 3 Vcc BQ2057CSN 7 14 1 down to mono. CON5 (RCA socket) is mono only. Potentiometers VR1-VR3 are used to adjust the level of these signals respectively and in each case, the result is then buffered by an op amp and then fed to the mixing node. Why no volume control for the microphone input? The microphone volume is adjusted via the PA amplifier, so we just need three pots to set the relative level for the other inputs. Now let’s look at the circuit’s operation in a little more detail. 8 4 1 anced signal which is then mixed with the signals from the other three inputs. The result is then again converted to a balanced signal at output CON2. For all intents and purposes, the amplifier won’t even know it’s there! The gain of the balanced-unbalanced-balanced path is close to unity while the gain for the other three channels can be varied from one quarter down to zero. Inputs CON3 (6.5mm jack socket) and CON4 (3.5mm jack socket) can accept either mono or stereo plugs; if a stereo signal is applied, it is mixed The balanced microphone delivers identical but opposite polarity (out-ofphase) signals to op amp IC2d which is configured for balanced inputs but has an unbalanced (ie, single-ended output at pin 14. Both signals pass through identical RF filters comprising 100Ω series resistors and 100pF ceramic capacitors while two 100kΩ resistors provide a DC bias to 0V. Following the RF filters, both signals are AC-coupled through back-to-back 100µF capacitors to the inputs of IC2d. Note that the signal ground for IC2d (and indeed, all the op amps) has a different symbol than power supply ground and is actually at half-supply, ie, about 1.5-2V. We have used back-to-back electrolytics here because PA gear can be connected to other equipment that might have phantom power, might be faulty, etc. So all inputs and outputs tolerate ±48V DC without damage. Standard electros though are usually cheaper and smaller than nonpolarised types; two 100µF 50V capacitors connected in this manner are equivalent to a 50µF 50V nonpolarised capacitor. IC2d converts the balanced signal from the microphone to unbalanced while largely rejecting unwanted signals picked up in the cable (eg, hum and noise). The output of IC2d is the signal from pin 2 of CON1 minus the signal at pin 3. So an extraneous signals picked up equally by both lines in the microphone cable will be cancelled out or at least heavily attenuated. The two 330pF capacitors roll off the frequency response of this amplifier, forming a low-pass filter with a -3dB point at around 48kHz, rejecting signal which may be picked up that is above regular audio frequencies but low June 2013  75 which gives it the same polarity as the input signal. Here’s what our mini mixer looks like immediately before insertion into its case. Here we have used the on-board CR2032 battery option. enough to pass through the RF filters. Other inputs The circuits for unbalanced inputs 2 and 3 (CON3 & CON4) are identical. Two 10kΩ resistors down-mix the stereo to mono; if a mono jack plug is inserted, these are effectively paralleled to form a single 5kΩ resistor. A 100pF capacitor in combination with this forms the RF filter and a 100kΩ resistor provides a DC path to ground. The signal is then AC-coupled to volume control pot VR1 (or VR2). The output from its wiper is AC-coupled again to ensure that no DC flows through VR1, which would cause noise when the pot is turned. A 100kΩ resistor sets the DC bias to half-supply and the signal is then buffered by voltage follower IC2b (or IC2a) before being applied to the mixer stage. The signal path from the mono RCA connector (CON5) is the same as above but being mono, a single 4.7kΩ series resistor is used rather than a pair of 10kΩ resistors. Also, CON5 can be used to charge the onboard Li-Po battery, as we shall explain later. In this case, dual schottky diode D3 prevents current flowing into op amp IC2c as the coupling capacitors charge when DC is applied to CON5. The mixer The four signals are fed to a virtual earth mixer based around inverting amplifier IC1c which has a 4.7kΩ feedback resistor from its output (pin 8) to inverting input (pin 9). Again there is 76  Silicon Chip Virtual earth The two remaining op amp stages out of the eight (IC1a and IC1b) are used to create and buffer the half supply virtual earth. This is generated by a pair of 10kΩ resistors connected across the supply and filtered with a 100µF capacitor, so that it is effectively grounded for AC signals. Voltage followers IC1a and IC1b drive the virtual earth rail through 100Ω resistors with a 100nF capacitor to ground. The capacitor reduces the impedance of this rail at high frequencies, where the impedance of the op amp outputs could be quite high, while the 100Ω resistors isolate this capacitance from the op amps to avoid oscillation. a 330pF roll-off capacitor for further attenuation of any signals above the audio band. The output of IC2d is applied to the mixing node via a 9.1kΩ resistor and 680nF AC-coupling capacitor. This capacitor forms a high-pass filter with IC1c’s feedback resistor to remove low bass, giving a -3dB point around 50Hz. This is primarily to deal with microphone thump, etc but also attenuates any 50Hz hum which may be picked up by about 3dB. The other three inputs are applied to the mixing node via 22kΩ resistors, giving them a gain of about 0.21 (4.7kΩ÷22kΩ). The signals from these inputs will generally be at or around line level, ie, in the range of 0.5-2V RMS while the microphone signals will be much lower at around 50mV. So this attenuation gives VR1-VR3 a more useful adjustment range. Note also that the gain in this stage for the microphone input is 4.7kΩ÷9.1kΩ = 0.52. The following unbalanced-to-balanced converter has a gain of two so these cancel out. The mixed signal from the output of IC1d is applied to pin 3 of output CON2 via a 100Ω current-limiting resistor and another pair of AC-coupling capacitors with a 100kΩ DC bias resistor to ground. The mixer stage (IC1c) is inverting so its output goes to the inverted signal pin (pin3) of the balanced (XLR) connector, CON2. For the non-inverted output (pin 2 of CON2), the signal from IC1c is inverted again, without gain, by IC1d Li-Po charger There are three basic options for the power supply: an on-board CR2032 Lithium button cell, a 9V battery or 3.74.2V rechargeable Lithium Polymer (Li-Po) cell. The latter option offers the longest battery life, potentially in the thousands of hours, with the bonus that you don’t have to open up the case to change the battery if it goes flat. Instead, you simply apply 5V DC to the central pin of CON5 (the RCA connector) and an internal charging circuit brings the cell back up to full charge. Charge current starts at around 500mA and drops off as the cell approaches full charge, so for a typical 1000mAh cell, a full charge takes up to two hours. So for a two-hour charge you could get up to 2,000 hours operation! When 5V DC is applied to CON5, schottky diode D1 becomes forward biased and current flows through 3.6V zener diode ZD1 and turns on NPN transistor Q4. Q4 in turn pulls the gate of P-channel Mosfet Q3 low, allowing the power to flow through D1 and Q3 into the 100µF supply bypass capacitor for the battery charger circuit. This isolates the charger circuit from any signal applied to CON5 during normal operation, up to at least 2V RMS (2.8V peak). When Q4 is off (ie, no charging voltage is applied), Q3’s gate is pulled to its source voltage by a 100kΩ resistor, keeping it switched off. Similarly, a 100kΩ resistor ensures that a small amount of leakage current through ZD1 will not turn on Q4. IC3 (BQ2057C) is a dedicated siliconchip.com.au Lithium Ion/Lithium Polymer charging IC. There are four versions of this IC, to suit one and two cell batteries with 4.1V or 4.2V charge termination voltages, depending on the cell chemistry. Most modern Li-Po cells can be charged safely to 4.2V so that is the version we have used (see panel for details). Li-Po cells need a constant current/ constant voltage charge cycle with accurate termination to give a good life and that’s all handled by IC3. It controls PNP power transistor Q2 to regulate the current and voltage to the cell, with current sensed by the voltage drop across the 0.22Ω shunt resistor. IC3 turns LED1 on only while the cell is charging – the LED does not waste power in normal operation. IC3 has provision for an NTC thermistor which can be attached to the cell to monitor its temperature so it can stop charging if it gets too high. This is optional; if you want to fit an NTC thermistor, it should be a nominally 10kΩ type and wired across the NTC1 terminal. Otherwise, connect a 10kΩ resistor across this terminal. Note that all the charging circuitry from D1 through to Q2 may be omitted if you aren’t planning to use a Li-Po battery to power the unit. Power supply The Li-Po battery is charged via P-channel Mosfet Q1 which prevents damage in case the cell is connected backwards. With the cell in the correct orientation, Q1’s gate is pulled to ground while its source goes high (bootstrapped by its body diode) and thus it switches on, allowing power to flow from the cell to the circuit and also allowing charge current to flow into the cell from Q2. Otherwise, Q1’s gate is pulled high and being a P-channel type, it remains switched off. In this state, its body diode is also reverse-biased so no current can flow. Slide switch S1 controls power to the mixer but the unit can still be charged while off as the charging current does not flow through S1. REG1 is only needed if you want to run the circuit off a 9V battery, as IC1 and IC2 have a maximum operating voltage of 5.5V. The MCP1703-5 has a very low quiescent current so that it doesn’t spoil the mixer’s low current drain. If using a Lithium or Li-Po cell, omit REG1 and fit LK1 instead (but you siliconchip.com.au Parts list – “Lump in The Coax” Mini Mixer 1 diecast aluminium enclosure, 120 x 93.5 x 35mm (Altronics H0454, Jaycar HB5067) 1 PCB, coded 01106131, 110 x 85mm 1 PCB-mount right-angle female compact XLR socket (CON1) (Altronics P0875) 1 PCB-mount right-angle male compact XLR socket (CON2) (Altronics P0874) 1 PCB-mount switched 6.35mm stereo jack socket (CON3) (Altronics P0073, Jaycar PS0195) 1 PCB-mount switched 3.5mm stereo jack socket (CON4) (Altronics P0092, Jaycar PS0133) 1 PCB-mount switched RCA socket (CON5) (Altronics P0145A, Jaycar PS0279) 1 right-angle SPDT slide switch (Altronics S2070) 2 2-way pin headers, 2.54mm pitch (BAT1, NTC1) 4 No.4 x 9mm self-tapping screws or M2.5 machine screws 1 M3 x 6mm machine screw and nut 1 200mm length 0.7mm diameter tinned copper wire 1 110 x 85mm sheet of insulating material (eg, PET) 1 lid label Semiconductors 1 DMP2215L P-channel SMD Mosfet (Q1) 2 OPA4348AID* quad rail-to-rail micropower op amps (IC1, IC2) (element14 1706654) * AD8544ARZ and MCP6404-E/SL are also suitable but with higher minimum operating voltage. Capacitors 20 100µF 50V (Altronics R4827) (25V may be used with less margin) 1 1µF MKT/polyester (code 1U, 1.0 or 105) 5 100nF monolithic multi-layer [MMC] (code 100n, 0.1 or 104) 4 330pF disc ceramic (code 330p or 331) 5 100pF disc ceramic (code 100p or 101) Resistors (0.25W, 1% unless otherwise stated) 1 10MΩ 12 100kΩ 3 22kΩ 11 10kΩ 1 8.2kΩ 5 4.7kΩ 1 3.9kΩ 2 2.2kΩ 1 220Ω 6 100Ω 1 0.22Ω SMD 6331 (metric), 2512 (imperial) 3 10kΩ log vertical 9mm PCB-mount potentiometers (Altronics R1958) Parts needed for CR2032 button cell operation 1 PCB-mount 20mm button cell holder (Altronics S5056, Jaycar PH9238) 1 CR2032 button cell Parts needed for Li-Po cell operation 1 small 3.7V Li-Po cell with leads 2 2-way pin header plugs with crimp pins 1 BQ2057CSN Li-Ion/Li-Po charger (IC3) (element14 1652449) 1 BD140 PNP transistor (Q2) 1 DMP2215L P-channel SMD Mosfet (Q3) 1 BC549 NPN transistor (Q4) 1 1N5819 1A schottky diode (D1) 1 BAT54S dual series SMD schottky diode (D3) (Altronics Y0075, element14 1467519) 1 3.6V zener diode (ZD1) 1 3mm green LED (LED1) 1 10kΩ NTC thermistor, beta ~4000 (optional) 1 100mm length 4-way ribbon/rainbow cable 1 length double-sided, foam-cored adhesive tape 1 USB cable with type A connector at one end 1 RCA line plug Extra parts for 9V battery operation (note: battery won’t fit in specified case) 1 MCP1703-5 LDO micropower regulator, SOT-23 (REG1) (element14 1627178) 1 9V battery 1 9V battery snap with leads 1 2-way pin header plug with crimp pins 1 100mm length 4-way ribbon/rainbow cable June 2013  77 D1 CON5 Q2 3.6V 100k BUTTON CELL HOLDER 4. 7k 100k INPUT 4 ZD1 BAT2 4. 7k BC549 100k Q4 5819 10M 100pF 100nF 3 .9k 2 .2k LK1 100pF 100k 100mF BD140 100mF 4nI T CON4 100W 100W 10k 330pF 4. 7k 4. 7k 330pF LED1 4. 7k 9 .1k 1mF 22k 100k 100nF 100k 22k 100k 22k 100W + + 10k 10k INPUT 3 R S 10k 100mF 100pF 3nI 10k 100mF 100k CON3 10k + 100W 100pF + 100W + 100mF INPUT 2 100mF 100mF 100mF CON1 INPUT 1 100mF 10k LOG 100nF + 100k + + POWER 100mF xaoC nI pmuL 13160110 rexiM derewop-yrettaB S1 10k LOG 220W NTC1 2 .2k VR3 VR2 10k LOG 100mF 10k A + 100k + 100mF VR1 100nF + + 100mF 100nF + + 100pF 2nI   OUTPUT 100mF 3102 If you want to connect an iPod to the 3.5mm input socket, you can do so but you may find that it’s necessary to provide it with a lower value load resistance for it to operate correctly. This might apply to other MP3 players too although most are happy driving a 100mF 100mF 100mF 100W tupnI iPod compatibility tuptuO The performance for this mixer is pretty good considering the low voltage and power consumption. Lower-power op amps almost always have more noise and less bandwidth than their higher-power counterparts. That is because, to reduce their power consumption, the standing current in both the input pair and the voltage amplification stage (VAS) is reduced. Dynamic microphones have quite a low output signal level – typically below 50mV RMS. That, in combination with the higher input noise of low-power op amps, limits the signalto-noise ratio of the mixer. In practice though, 65dB is more than adequate for PA work. If you aren’t happy with that, there’s an easy solution – swap the OPA4348 op amps with noise of 35nV/sqrt(Hz) for a lower-noise, pin compatible part such as the TL974 which has just 4nV/ sqrt(Hz). We expect that will improve the signal-to-noise ratio by around 10dB. But it does so at the cost of much increased battery current of 16mA and somewhat reduced signal handling capability as the TL974 does not have a rail-to-rail input. If you decide to swap the op amps, you will definitely want to use a Li-Po battery. 330pF + Performance and noise 10k 10k 330pF + + 100mF 100k 10k 10k 100mF 100mF + 100k + CON2 + should fit the two bypass capacitors anyway). BAT1 + – + Fig.2: component layout for the mixer from the top (component) side. You have the option of using an on-board CR2032 button cell (as shown here), an external (rechargeable) Li-Po or even a garden-variety 9V type! high load impedance. The solution is simple: replace the 4.7kΩ series resistor at this input with a 100Ω resistor and change the 100kΩ DC bias resistor to 1kΩ. This may mean though that input can no longer be used with other signal sources, which is why we didn’t do it that way in the first place. Construction The prototype mixer was built on a single-sided PCB, coded 01106131 and measuring 110 x 85mm but production boards will be double-sided, eliminating the need for wire links (shown in green in the diagram above). Start by fitting the SMDs, beginning with op amps IC1 and IC2. Locate pin 1, which is normally indicated with a dot or stripe. If you can’t find that, check for a bevelled edge on the PCB package, also on the pin 1 side. Put a little solder on one of the IC pads and while heating that solder, slide the IC in place. When it’s lined up with its pads, double-check that the IC is orientated correctly then solder the rest of the pins. Then refresh the first one you soldered with a dab of extra solder. Remove any bridges with some solder wick. Carefully examine the solder joints with a powerful light and magnifying glass; a bad joint at this stage could cause problems later and it’s quite easy to get solder on one of these pads without it actually adhering to the component pin (something we’ve had Resistor Colour Codes o o o o o o o o o o No. 1 12 3 11 1 5 1 2 1 6 78  Silicon Chip Value 10MΩ 100kΩ 22kΩ 10kΩ 8.2kΩ 4.7kΩ 3.9kΩ 2.2kΩ 220Ω 100Ω 4-Band Code (1%) brown black blue brown brown black yellow brown red red orange brown brown black orange brown grey red red brown yellow violet red brown orange white red brown red red red brown red red brown brown brown black brown brown 5-Band Code (1%) brown black black green brown brown black black orange brown red red black red brown brown black black red brown grey red black brown brown yellow violet black brown brown orange white black brown brown red red black brown brown red red black black brown brown black black black brown siliconchip.com.au IC1 IC3 Output IC2 D3 REG1 Input 2013 In2 In3 Q1 In4 0.22W Lump In Coax 01106131 Battery-powered Mixer Q3 Fig.3: the underside (ie, normal copper side) of the PCB has seven SMD devices on it, as shown here and (partially) in the early prototype pic above right. These should be soldered in place before you start assembling the top side. While our prototype was a single-sided PCB, production boards will be double-sided. happen on more than one occasion). If necessary, add some heat and/or solder to any suspect joints. Now fit Mosfet Q1 in the same manner. It’s smaller but the pins are widely spaced. The leads should sit on the PCB surface; if they are sticking up in the air like a dead cockroach, the part is upside-down. If you will be powering the unit from a 9V battery, fit REG1 in the same manner. But we believe that most constructors will want to use the Lithium or Li-Po options; if you do use a 9V battery, you will have to fit the unit in a larger case than specified. If you are using a Li-Po and want the on-board charging facility, install the associated SMD components now, ie, D3, Q3, IC3 and the 0.22Ω shunt resistor using the same basic technique outlined above. Through-hole parts If you ordered the board from the SILICON CHIP webshop (or if it was supplied in a kit), it will already have the links as the top layer. Otherwise (eg, if you etch your own single-sided board), fit the six wire links now (shown in green in Fig.2) using tinned copper wire, plus LK1 if you aren’t using a 9V battery. Follow with the remaining resistors. Use the colour-code table as a guide but also siliconchip.com.au check their values with a DMM. Diodes D1 and ZD1 go in next but only if you are building the Li-Po version. In that case, you will also need to mount Q2 and Q4. For Q2, Bend its leads, feed them through the holes and then use the M3 screw to fasten its tab securely to the PCB before soldering the leads. The metal tab goes down, against the PCB (not as shown in the photo, which we changed). Next, fit all the ceramic capacitors (disc and monolithic multi-layer). Then solder in the 3.5mm jack socket, button cell holder (if required), slide switch S1 and the two 2-way pin headers. Follow with the single MKT capacitor and then the electrolytics; these are all the same value and in the same orientation, with the positive (longer lead) to the right side of the board. You can now mount the larger connectors, CON1-CON3 and CON5, as well as the three vertical pots. Push these down firmly into the mounting holes before soldering the two tabs and three pins. If building the Li-Po version, fit LED1 now, at full lead length, with its anode (longer lead) to the left. The PCB is now complete. Connecting the Li-Po battery We recycled a 3.7V 1500mAh “Huawei” Li-Po battery from an unloved mobile phone (the battery still charged and held charge). We found (more by good luck than good management) that this 42 x 65 x 5mm battery fitted perfectly inside the case lid, alongside the pot shafts. Even if you have to buy a new one, they’re dirt cheap on ebay – for example, one the same as we used was $3.90 including postage from Hong Kong. You can get Li-Po cells with leads attached but if recycling an old one, like ours, identify which pads correspond to positive and negative and then solder a couple of appropriately-coloured wires to these (eg, from ribbon cable). Connect a two-pin header to the other end of this cable by stripping the ends of the wire, crimping the two small pins to them (both to the uninsulated and insulated sections) and then pushing them into the plastic block. We like to solder the crimped joint too but you have to be careful not to put much solder on or the pin may not go into the block. For a 9V battery, you will need to attach a 2-pin plug to the wires from the 9V battery snap using the same method. Charging cable For the Li-Po version, you will also need to make a charging cable. You can charge from USB or a 5V DC plugpack or car adaptor. To make a USB cable, take a cable with a Type A plug on one end and chop the other end off. Strip back some of the outer insulation, solder the white and green wires together and insulate them with small diameter heatshrink tubing. Slide the rear of the RCA plug over the cable then solder the red wire to the centre pin and the black wire to the surround. Crimp the cable with the provided clamp and slide the rear cover back on. Plug it into a USB port and use a DMM to check that the centre pin is at +5V relative to the surround. If charging from a plugpack, it’s just a matter of fitting an RCA plug to the plugpack using a similar method. If fitting an NTC thermistor to moniJune 2013  79 The Lump-in-the-coax mixer installed in its diecast case, ready for the lid (with appropriate label) to be screwed on. tor the battery temperature, glue or otherwise attach it to the cell and run a couple of short leads back to another 2-pin plug as described above, to plug into the NTC1 header on the PCB. If you don’t want to fit the NTC, solder or otherwise connect a 10kΩ resistor across the NTC1 header pins. (this might take a while if it’s quite flat to start with). If you built our USB Power Monitor (December 2012) then you can use this to check that the current draw is below 500mA and slowly drops as the cell charges. Testing it A drilling template and front panel artwork can be downloaded from www.siliconchip.com.au. Use these to mark the hole positions and drill them all to the sizes shown. The holes will need to be accurately placed as the board only just fits in the case when they are in the right positions. The largest (XLR) holes will need a tapered reamer – even so, you may still need to use a round file to finish them off (many reamers only go to 20mm). Note that there won’t be much “meat” left along the rim of the case where these holes are placed, as the connectors must mount quite high for the PCB to clear the bottom of the case. You’ll also need to file flat the lip of the case lid where it would otherwise interfere with the XLR sockets. Check the unit out before fitting it into the case. Apply 3-5V DC to the BAT1 terminals via a spare 1kΩ resistor and measure the voltage across that resistor with a DMM. You should get 0.5-0.75V. If it’s much less, check the supply polarity and failing that, soldering and component placement on the PCB. If the reading doesn’t drop below 1V after a few seconds, that suggests a short circuit or other problem (eg, incorrectly orientated component) which you will need to look for. Assuming all is well, turn the pots all the way down, connect up the battery directly and attach a microphone and some sort of amplifier, as long as it has a “mic” input with either an XLR socket or you have an adaptor. Switch on and speak into the microphone; check that the output sounds OK. To test the Li-Po charger, leave the battery connected and plug in the charging cable. Unless the cell is already fully charged, LED1 will light. You can monitor the battery voltage with a DMM; it should rise to 4.2V 80  Silicon Chip Drilling the case Putting it all together Now for the tricky part, shoe-horning the board into the case. It’s a tight fit (deliberately!). First, cut a sheet of thin insulating material (eg, cut from a PET milk bottle) and place it inside the base of the diecast box, to prevent the PCB from shorting to it. If there are any particularly sharp solder joints, you can put some electrical tape over them which will stop them from puncturing this insulating layer. Next, temporarily remove the locking tab from the female (mic in) XLR socket by pressing it down and pulling it out. Now feed the RCA socket through its hole in the side of the case. It’s then a matter of rotating the PCB and pushing it down so as to get the XLR connectors into their holes. Don’t force it; it’s a very tight fit. You may even need to enlarge some of the holes in the case side before it will go in. You may also find that you have to bend the XLR connectors a little so that their lip does not prevent that end of the board from sliding into place. Don’t overdo it though as you could damage the PCB. If the board doesn’t want to go in, check that the corner cut-outs have been filed correctly and that it isn’t hitting the bottom of the case, which suggests misaligned holes that will have to be enlarged further. Once it pops in, fit the four screws to hold the XLR connectors in; this also holds the PCB in place and replace the XLR locking tab by pushing back in. The diecast box provides best shielding against hum and so on if it is connected to the circuit ground. This normally occurs through contact between the shield of the 3.5mm or RCA connector but depending on how large you’ve made the holes, they may not make reliable contact. In this case, the easiest solution is to replace one of the XLR mounting screws with an M2 x 10mm machine screw and nut and use this to attach a solder lug on the inside, under the nut. You can then run a short wire from this lug to a convenient 0V point on the PCB below (eg, a resistor lead connected to ground). This is not critical but it’s a good way to ensure that the shielding is most effective. Before putting the lid on, connect the battery. The Li-Po cell can be attached under the lid with doublesided tape, in a position where it will clear the pots and LED. It’s then just a matter of cutting out the four holes in the lid label, glueing it onto the lid, then screwing the lid on and the assembly is complete. The mixer is now ready to use. SC siliconchip.com.au