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Versatile 4- . . . with tone controls and This low-cost 4-input mixer features low-noise input preamps, each of which can be configured to suit a wide range of signal sources: microphones, guitar pick-ups, tape decks, synthesisers or CD players. Other features include a built-in equaliser with bass, midrange and treble controls along with a monitoring amplifier which can drive stereo headphones. By JIM ROWE 58  Silicon Chip siliconchip.com.au -Input Mixer a built-in headphone amp! Specifications Input Sensitivity (for 2.0 V RMS output, each ma in preamp configuration Dynamic mic, low impeda ): nce: ................................... ......................... 2.6mV RM Electric guitar: ................. S ........................................ ............................. 28mV Tape deck: ......................... RMS ........................................ ......................... 145mV RM CD player: ......................... S ........................................ ......................... 463mV RM S Frequency response: ..... ................ -3dB at 23Hz and 40kHz, -1dB at 40Hz and                    22kHz (with tone controls flat; see Fig.4) Maximum output: ............ ...................... 3.2V RMS (9V p-p) before clipping; see Fig.6 Output noise level (with respect to 2V RMS output , maximum gain & volum e, tone inated with 1kW, unweig hted 22Hz-22kHz bandw CD player input, ............... idth): ............... controls flat, inputs term ............ -92dB unweighte Tape deck input ............... d; -96dB A-weighted ............................ -92dB unweighted; -96dB A-weig Guitar input .................... hted ............................. -85dB unweighted; -89dB A-weig Low-Z mic input ................. hted ......................... -67dB unw eighted; -70dB A-weighte d Total harmonic distortio n (THD):.................. Less than 0.01% up to 3.2V RM S output Graphic equaliser: Bass: ......................... +13 dB & -12.5dB at 100Hz, ±18dB at 40Hz, ±0.5dB at Mid Range:...................... 1kHz ............ ±11dB at 1kHz, ±1d B at 100Hz, ±2.5dB at 10k Treble:............................. Hz ....... ±10.5dB at 12kHz, ±1d B at 1kHz, ±11.5dB at 15k Hz Headphone amplifier: Output voltage before clip ping: ............................5 90mV RMS into 2 x 33W THD for 500mV RMS into loads 2 x 33W loads: ................. .....................................0 .8% Supply voltage: ............... ........................................ ......... 12V DC (nominal) Maximum current drain: – see text ........................................ ........................................ ..... 45mA siliconchip.com.au June 2007  59 B ACK IN JANUARY 1992, we published the design for a low-cost four-input guitar mixer module for small bands and groups. It turned out to be very popular and the kit people tell us that kits for it were still selling steadily until quite recently. However, in its original form, it apparently wasn’t quite as flexible as many users wanted, particularly in terms of the ability to configure the input preamps for signal sources other than guitar pick-ups – eg, for dynamic mics, tape decks, CD players and synthesisers. It also didn’t include a built-in headphone amplifier for monitoring. These shortcomings have been eliminated in this new design. It retains all the features of the original January 1992 unit but there’s now more flexibility in configuring the input preamps, together with a built-in headphone amplifier. Block diagram Fig.1 shows the block diagram of our new Versatile 4-Input Mixer. As shown, it still provides four inputs, each with its own preamp stage and gain control. However, unlike the earlier design, each of the four input preamps can now be configured by the user, to provide the appropriate gain and input impedance values to suit a wide range of signal sources – from the millivolt or two of a low-impedance dynamic mic to the 1-2V signals of a CD/MP3 player or keyboard synthesiser. This makes the new unit much more versatile. Following the input gain controls, there’s a standard mixer stage, to allow the signals to be combined in whatever proportions you wish. The resulting composite audio signal is then fed to a 3-channel “mini equaliser” stage, where three tone controls (bass, mid-range and treble) allow you to adjust the tonal balance. This equaliser stage is basically an expanded version of a standard “Baxandall” feedback tone control, with three controls instead of two. From there, the output of the equaliser stage is passed to the master volume control and finally to the output jack via an output buffer amplifier operating with a gain of 2.2. This section is similar to the 1992 design but the headINPUT 1 phone amplifier (shown above the output buffer) is a new addition. It simply allows the output audio signal to be monitored via a pair of standard stereo headphones. The new design also differs from its predecessor in another way, not evident from Fig.1. The original unit needed a regulated power supply of ±15V DC but we’ve designed the new unit to operate from a single 12V DC supply. This can be provided either by a mains plugpack or a 12V battery, making the unit suitable for portable and mobile use. The current drain is less than 50mA. These additional features have been provided without sacrificing any of the key features of the original mixer. All components are still mounted on a single PC board for ease of assembly and although the board is a little larger than before, we’ve made it just the right size to fit snugly into a 225 x 165 x 40mm low-profile plastic instrument case. Circuit details Fig.3 (overleaf) shows all circuit details of the new mixer. It’s quite easy to relate each circuit section to its corresponding block in Fig.1. At the far lefthand side are the four signal input jacks CON1-CON4, each connected to its own preamp stage and gain control. These preamps each use one section of an LM833 low-noise dual op amp IC – ie, two ICs are used (IC1 & IC2). Although the four preamps shown in Fig.3 all have exactly the same circuit configuration, some of the components in each stage do not have specific values. Instead they have symbolic values like Rm, Rin, Rza, Rzb, Rf and Cf, to indicate their basic function rather than their value. This is because their values need to be chosen when each preamp is configured to suit a particular signal source. Specifically, Rm, Rin, Rza and Rzb are given values to provide the appropriate input impedance for the source, while Rf and Cf are given values to provide the appropriate gain and/or signal handling capability. The table in the circuit diagram gives the values for each of the various input sources. As the mixer is a mono device and there is a good chance that stereo devices may be connected to it (eg, an MP3 or PREAMP 1 GAIN (EACH CHANNEL) INPUT 2 TREBLE HEADPHONE AMPLIFIER MID RANGE PREAMP 2 MONITOR PHONES BASS INPUT 3 VOLUME PREAMP 3 OUTPUT TONE CONTROL (EQUALISER) MIXER/AMPLIFIER INPUT 4 OUTPUT BUFFER PREAMP 4 STEREO TO MONO MIXERS 60  Silicon Chip Fig.1: the block diagram of our new Versatile Mixer. The four inputs are amplified, mixed and then fed to the tone control/equaliser stage before passing to an output buffer, to be fed into an external power amplifier and/or a low-power headphone amplifier for monitoring. siliconchip.com.au Look mum, no wiring! This inside-thecase pic shows how everything is mounted on one PC board. It’s an early prototype so there are a few minor differences to the final design. CD player) all four channels have the capability of being “summed” to mono via Rma and Rmb – again, the values are shown in the table. Some devices, such as microphones, are generally mono, so Rma and Rmb may be substituted with links and/or omitted completely. Yes, we know there are stereo microphones out there but these are the exception, not the rule. For example, to configure a preamp for an electric guitar input, Rin, Rza and Rzb are 1MW (giving an input impedance of 330kW), while Rf is 22kW (to give a gain of 19 times, or about 25dB). Finally, Cf is given a value of 100pF to ensure stability. Similarly, to configure a preamp for the much higher stereo output from a CD player or synthesiser keyboard, Rza and Rzb are given values of 100kW while Rin is changed to 2.2kW. Rma and Rmb are given values of 47kW. These values give an input impedance of close to 50kW. Resistor Rf is made 27kW, lowering the preamp gain to unity so that it can handle the much larger input signals without overloading. Note that resistors Rza and Rzb must always have the same value. That’s because they also form the bias voltage divider for the preamp concerned. No provision has been made for powering electret microphones but in a permanent installation, this could be easily achieved through the use of a suitable bias resistor (10kW is commonly used) from the nominal 12V line to the “hot” input of the electret. The outputs from the preamp stages are fed via 2.2mF capacitors to gain control potentiometers VR1-VR4. The signals at the wipers are then fed via 47kW mixing resistors and a 2.2mF capacitor to the pin 2 input of mixer/amplifier stage IC3a. IC3a operates as a standard inverting amplifier with a gain of -2 (100kW/47kW) for each of the four inputs. It also provides a low “virtual earth” input impedance, to ensure that there is no interaction between the four gain controls (VR1-VR4). siliconchip.com.au A half-supply rail bias (+6V) for IC3a is provided by op amp IC3b. This is connected as a voltage follower with its pin 5 input set at +6V by a voltage divider consisting of two 47kW resistors across the supply rail. The resulting +6V bias voltage from pin 7 of IC3b is applied to pin 3 of IC3a via a 100kW resistor. It’s also used to bias op amps IC4a (pin 3) & IC4b (pin 5). Tone control stage IC4a forms the heart of the tone control/equaliser stage. As mentioned previously, this is an extended version of the standard Baxandall feedback tone control configuration – ie, it has three controls instead of the usual two. The operation is exactly the same though, with each pot (VR5, VR6 & VR7) acting as a gain control for signals within a set frequency range. Fig.2: this shows the operation of the bass tone control stage. June 2007  61 +12V Rm1a INPUT 1 Rm1b CON1 47 F Rza1 2.2 F Rin1 1k Rzb1 5 6 8 7 IC1b 1.2k 8 5 2.2 F GAIN 1 VR1 10k LOG Rf1 100nF 47k 47k 47 F 47k 7 IC3b 6 +6V SUPPLY RAIL SPLITTER Cf1 PREAMP 1 22 F 100k Rm2a INPUT 2 Rm2b CON2 Rza2 2.2 F Rin2 1k Rzb2 2.2 F 3 2 4 1 IC1a 1.2k 4 47k 2.2 F 100k Cf2 22pF PREAMP 2 22 F MIXER/AMPLIFIER STAGE (A = -2) +12V Rm3a INPUT 3 Rm3b CON3 47 F Rza3 2.2 F Rin3 1k Rzb3 2.2 F 1 IC3a 2 GAIN 2 VR2 10k LOG Rf2 3 2.2 F 5 6 8 7 IC2b GAIN 3 VR3 10k LOG Rf3 1.2k 2.2 F 47k IC1– IC4: LM833 IC5: LM358 4 8 1 Cf3 PREAMP 3 22 F K LED Rm4a INPUT 4 Rm4b CON4 Rin4 1k Rzb4 1.2k 22 F SC  2007 3 2 4 1 IC2a A 2.2 F GAIN 4 VR4 10k LOG Cf4 PREAMP 4 VERSATILE FOUR INPUT MIXER In operation, the pots vary the effective negative feedback ratios for their respective frequency bands. Fig.2 shows a simplified scheme for the bass control. When the pot is in its centre position, IC4a has equal input and feedback impedances for the frequencies in its control 62  Silicon Chip E B C Rza4 2.2 F Rf4 CON1-4 ALL STEREO SWITCHED TYPE BC328, BC338 A 47k D1, D2: 1N4148 K ZD1 + PREAMP COMPONENT VALUES FOR VARIOUS INPUTS Rma Rmb Rin Rza,Rzb Rf Cf ELECTRIC GUITAR (50mV) (OMIT) LINK 1M 1M 22k 100pF DYNAMIC MIC (Mono, Lo-Z) (OMIT) LINK 680 10k 220k 12pF DYNAMIC MIC (Mono, Hi-Z) (OMIT) LINK 1M 100k 120k 18pF TAPE DECK (Stereo, 300mV) 47k 47k 2.2k 100k 82k 22pF CD PLAYER/SYNTH (St, 2V) 47k 47k 2.2k 100k 27k 82pF INPUT range, thus giving it unity gain for those frequencies. However, when the pot is turned to the “maximum boost” (fully clockwise) position, the ratio of the feedback and input impedances increases to 11:1 (110kW/10kW), so the stage gain for those frequencies increases to 11 times or +21dB. siliconchip.com.au 47 +12V 4.7k 2200 F 25V 1000 F 16V A +6V CON7 A K 100k LIN 33 10 F 10k VR5 BASS 10k ZD1 16V 1W +12V K  LED1 22nF 10k 10 100nF 2.2nF 10k 10k 100k LIN 6.8k 100k LIN 1.5nF TONE CONTROL (EQUALISER) STAGE 5 2.2 F VR6 MIDRANGE 10nF 6 6.8k 8 470 VOLUME VR8 10k LOG VR7 TREBLE 39pF 7 IC4b 2.2 F 100 OUTPUT CON5 68pF 4.7 F 10k 22k 100 OUTPUT BUFFER (A = -2.2) 2 +6V 1 IC4a 3 4 100nF +12V 2200 F 16V 10k 4.7pF B 330k PHONES VOLUME VR9 50k LOG 100k A 47k 270nF 2 6 10 F 8 IC5b 7 100k +6V 33 470 F K 1 4 330k K 68 PHONES CON6 A 33 D2 270nF SUPPLY RAIL SPLITTER IC5a Q1 BC338 E D1 3 5 C B E C 10k 68 Q2 BC328 HEADPHONE AMPLIFIER Fig.3: don’t be daunted by the size of the circuit diagram – it really is quite an easy project to understand (especially when you compare it to the block diagram overleaf). And the good news is it’s even easier to put together because all components mount on a single PC board. No wiring should mean no mistakes. Conversely, when the pot is turned to the “maximum cut” (fully anticlockwise) position, the ratio of feedback and input impedances reduces to 1:11 (10kW/110kW). As a result, the stage no longer amplifies those frequencies but attenuates them instead – ie, by about 11 times, or -21dB. siliconchip.com.au Going back to Fig.3, all three tone controls act in this same way but each covers its own range of frequencies, as determined by the values of the various capacitors in the feedback networks. IC4a’s output appears at pin 1 and is AC-coupled to June 2007  63 AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 25.000 24 APR 2007 17:43:54 20.000 u 15.000 10.000 x w 5.0000 v 0.0 -5.000 y -10.00 -15.00 -20.00 -25.00 10 100 1k 10k 100k Fig.4: this complex frequency plot is the result of five frequency sweeps with different tone control settings. The green trace u is taken with maximum bass boost, midrange flat (centred) and maximum treble cut. The yellow trace v is taken with all tone controls flat (centred). The red trace w is taken with maximum bass cut, midrange flat (centred) and maximum treble boost. The purple trace x is taken with bass and treble controls flat and maximum midrange boost while the pink trace y is taken with bass and treble controls flat and maximum midrange cut. Note that the tone controls do interact with each other. VR8, which is the master volume control. This controls the signal level fed to output buffer stage IC4b which is configured as a standard inverting amplifier with a gain of 2.2 (22kW/10kW). Its output is in turn fed to output jack CON5 via a 2.2mF DC blocking capacitor. Headphone amplifier The output signal at CON5 is also used to feed the headphone amplifier (IC5a), via a 100W isolating resistor and potentiometer VR9 (the headphone volume control). The headphone amplifier itself is based on IC5a, which is half of an LM358 low-power dual op amp. IC5b is wired in a similar manner to IC3b (ie, as a voltage follower) and is used to bias pin 3 of IC5a to +6V. Transistors Q1 and Q2 are used to boost the output current capability of IC5a, to provide sufficient drive for both sides of a standard low-impedance stereo headphones/ear buds (33W per earpiece). These transistors are configured as complementary emitter followers, with diodes D1 and D2 setting their quiescent bias levels. Negative feedback for the stage is taken from the junction of the two 33W emitter resistors and applied to pin 2 of IC5a via a 330kW resistor, ie, transistors Q1 & Q2 are inside the feedback loop. This reduces the distortion level of the headphone amplifier and also flattens its frequency response. The 4.7pF capacitor across the 330kW resistor rolls off the response above 100kHz to ensure stability. Power supply To make it as versatile as possible, power for the mixer is derived from either an external 12V DC regulated plugpack supply or from a 12V battery. This is applied via connector CON7 and powers all the mixer circuitry. Reverse polarity protection is not provided by a series 64  Silicon Chip diode but instead by a 10W series resistor and zener diode ZD1, which also protects the circuit from over-voltage damage. If you connect a plugpack with the wrong polarity (ie, centre negative instead of the more usual centre positive) the 10W resistor should burn out, cutting power from the circuit. A single 3mm “power on” high-brightness LED connects across the 12V supply via a 4.7kW current-limiting resistor. The 2200mF capacitor across ZD1 decouples and filters the supply rail, while the rail to the headphone amplifier is further decoupled using a separate 33W resistor and 2200mF capacitor. This is done to prevent unwanted interaction between the headphone amplifier and the rest of the circuit due to supply rail fluctuations. Additional supply decoupling for the +12V rail to the LM833 op amps is provided by a 47W resistor and 1000mF capacitor. This eliminates any possibility of low frequency “motor-boating” when high gain is used on all the input channels, together with maximum bass boost. It also makes it possible to use an unregulated 9V DC plugpack in a pinch; hum will be higher but at least it might get you out of trouble if the specified regulated 12V DC plugpack is unavailable. Self-contained battery power? We know it’s going to be asked, so we will answer the question already: can you make the mixer portable and run it from internal batteries – say a couple of 9V alkalines? The answer, with a couple of reservations, is yes, it is possible – because the op amps set up the half-supply rails. The two batteries could occupy the vacant real estate in the middle of the PC board. (You’d obviously need to fix these in position to the PC board but that shouldn’t be difficult, given the amount of earth track in this area). A couple of riders, though: the mixer draws about 20mA without the headphone amplifier being used, so even new alkaline 9V batteries are only likely to give you a few hours operation at best. If you use the headphone amp, expect even less. But that period might be long enough for your application. And to use an 18V supply, you would need to change the 16V zener to a 22V type. You would also probably want to fit a small power switch. Construction Another of the major features of this new design, one that we haven’t mentioned earlier, is the fact there is no wiring to be done! Everything – including the input/output sockets and control pots – is mounted on the single PC board. This makes building this mixer very easy. This PC board is coded 01106071, measures 198 x 156mm and fits neatly inside a standard low-profile ABS instrument case measuring 225 x 165 x 40mm (available from Jaycar and Altronics). As can be seen from the photos, all but one of the control pots are mounted along the front of the board, the exception being the headphone volume control pot (VR9). There simply wasn’t enough room for it on the front, so it was mounted adjacent to headphone jack (CON6) on the rear panel. siliconchip.com.au AUDIO PRECISION SCTHD-HZ THD+N(%) vs FREQ(Hz) 5 26 APR 2007 10:16:22 Parts List – Versatile 4-Channel Mixer 1 1 PC board, code 01106071, 198 x 156mm 6 6.35mm stereo jack sockets, PC board mounting (CON1-6) [eg, Jaycar PS-0190, Altronics P-0073] 1 2.5mm concentric DC socket, PC-mount (CON7) 9 16mm diameter aluminium knobs 5 8-pin DIL sockets (for IC1-IC5) 1 200mm length of 0.25mm tinned copper wire 1 Low profile ABS instrument case, 225 x 165 x 40mm (eg Jaycar HB-5972, Altronics H0474) 0.1 0.010 0.001 20 100 1k 10k 20k Fig.5: this graph shows total harmonic distortion versus frequency at an output of 2V RMS. The measurement bandwidth is 22Hz to 80kHz. AUDIO PRECISION SCTHD-W THD+N(%) vs measured 5 LEVEL(V) 26 APR 2007 10:11:38 1 0.1 0.010 0.001 10m 0.1 1 5 Fig.6 shows total harmonic distortion versus output level at a frequency of 1kHz. The measurement bandwidth is 22Hz to 22kHz. The rising value at lower signal levels is solely due to the residual noise at around –92dB with respect to 2V. Since the residual noise is fixed, it results in higher THD values as the signal level is reduced. In reality, the harmonic distortion is less than .003% at 1kHz, for all signal levels up to 2V RMS. Note that the board has been designed to suit standard low-cost 6.35mm jacks for CON1-CON6 (like the Jaycar PS-0190/Altronics P-0073) but the board will also accept the unswitched stereo type. The reason we use switched stereo sockets is so that unused inputs are shorted to earth, thus minimising noise. Fig.7 shows the parts layout on the PC board. Begin by carefully inspecting the PC board for etching defects, then start the assembly by fitting the six wire links. Follow these with the resistors. You will have to decide how you wish to configure each input and then choose resistors Rma, Rmb, Rin, Rza, Rzb and Rf from the table on the circuit diagram accordingly. We’ve shown the resistor colour codes (and capacitor codes) but you should also check the resistor values using a digital multimeter, as some colours can be difficult to decipher. siliconchip.com.au Semiconductors 4 LM833 dual low noise op amp (IC1-IC4) 1 LM358 dual op amp (IC5) 1 BC338 NPN transistor (Q1) 1 BC328 PNP transistor (Q2) 1 16V 1W zener diode (ZD1) 1 3mm high-brightness LED (LED1) 2 1N4148 diodes (D1,D2) Capacitors 2 2200mF 25V RB electrolytic 1 1000mF 25V RB electrolytic 1 470mF 25V RB electrolytic 4 47mF 16V RB electrolytic 4 22mF 16V RB electrolytic 2 10mF 16V RB electrolytic 1 4.7mF 16V RB electrolytic 13 2.2mF 16V RB electrolytic 2 270nF MKT metallised polyester 3 100nF multilayer monolithic 1 22nF metallised polyester 1 10nF metallised polyester 1 2.2nF metallised polyester 1 1.5nF metallised polyester 1 68pF disc ceramic, NPO 1 39pF disc ceramic, NPO 1 22pF disc ceramic, NPO 1 4.7pF disc ceramic, NPO 4 ceramic caps, selected values (Cf1-Cf4) Resistors (1%, 0.25W) 2 330kW 4 100kW 2 6.8kW 1 4.7kW 1 100W 2 68W 7 47kW 4 1.2kW 3 33W 1 22kW 4 1kW 1 47W 8 10kW 1 470W 1 10W Up to 8 47kW input mixer resistors, (Rm1-4 and Rmb1-4) [omit for mono sources and use some links instead] 4 input resistors, selected values (Rin1-Rin4) 8 bias divider resistors, selected values (Rza1-Rza4 & Rzb1-Rzb4) 4 feedback resistors, selected values (Rf1-Rf4) 4 ceramic capacitors, selected values (Cf1-Cf4) Potentiometers 5 PC-mount 16mm 10kW log pots (VR1-VR4,VR8) 3 PC-mount 16mm 100kW linear pot (VR5-VR7) 1 PC-mount 16mm 50kW log pot (VR9) June 2007  65 Fig.7: here’s how it all goes together. Don’t worry about all that PC board real estate with not much on it – the size is basically dictated by the pot spacing and the availability of suitable cases! The MKT and non-polarised capacitors can go in next. Again, the feedback capacitors (Cf1-Cf4) will have to be selected from the circuit diagram table. The polarised electrolytics can then be fitted, taking care to ensure they go in with the correct polarity. Next fit the sockets for the five ICs, making sure you orientate them with their “notched” ends as shown in Fig.7 (above). Follow these with diodes D1 & D2, zener diode ZD1 and transistors Q1 & Q2, again making sure they have 66  Silicon Chip the correct orientation. Potentiometers VR1-VR9 can now be fitted. Before doing so, though, cut each pot’s spindle to a length of 10mm using a small hacksaw and then use a small file to remove any burrs. This step will not be necessary if you use “metric” pots with 10mm-long splined shafts and matching splined knobs. Note that the three 100kW linear units (usually marked “B100K”) must be fitted in the VR5, VR6 & VR7 positions siliconchip.com.au And here’s the matching completed PC board photo, shown very close to full size (again, this early prototype has some minor component placement differences). This is ready to “drop into” the ABS case. along the front of the board. The five 10kW log pots (marked “A10K”) go in positions VR1-VR4 and VR8, while the remaining 50kW log pot (marked “A50K”) is fitted as VR9 at the rear. It’s just a matter of pushing each pot as far down onto the board as it will go and soldering its pins. Once they’re all in, scrape or file away some of the plating at the top of each of the VR1-VR8 pot bodies and solder them together using a 170mm length of tinned copper siliconchip.com.au wire. A second length of tinned copper wire is then used to connect VR3’s body to an adjacent earth point on the PC board – see Fig.7. This step earths the pot bodies to prevent hand capacitance effects as the controls are adjusted. The seven 6.35mm jack sockets CON1-CON7 are fitted along the rear in much the same way, except there is no earth wire to be soldered on. Once the sockets have all been fitted, the next step June 2007  67 www.siliconchip.com.au INPUT 1 SILICON CHIP VERSATILE MIXER – SILICON CHIP VERSATILE MIXER 12V DC IN PHONES VOL PHONES OUTPUT – INPUT 4 + – INPUT 3 + INPUT 2 + VOLUME TREBLE MIDRANGE BASS INPUT 4 INPUT 3 INPUT 2 INPUT 1 Front (left) and rear panels for the Versatile Mixer. The white panels underneath each input pot are used for writing on the input source (using a fine felt-tipped pen) – especially if your mixer is not permanently installed (and even if it is). Instantly knowing which input is which can save a lot of embarrassment when you need to adjust levels! 68  Silicon Chip is to attach the rear panel to them (and to VR9). This simply involves passing the threaded ferrules through their matching panel holes and then fitting the washers and nuts. Don’t tighten the nuts up fully yet though – just leave them “finger tight” for the time being. The front panel is fitted in exactly the same way, this time over the threaded ferrules of VR1-VR8. Again leave the pot nuts finger tight – they’re not fully tightened until the assembly is fitted into the case. Once this has been done, you’re now ready to slide the completed board/panel assembly down into the lower half of the case, with the panel ends mating with the front and rear case slots. That done, the PC board can be fastened to the integral standoffs on the base using nine of the small self-tapping screws provided. The connector and pot mounting nuts can now all be carefully tightened with a small shifting spanner. Don’t tighten them too forcefully though, otherwise you’ll strip the threads. Just nip them up tight enough to ensure they don’t loosen with use. That done, you can fit the control knobs to the pot spindles. The “power on” LED mounts so its front is flush with the front panel – a tiny dob of super glue is enough to hold it in place. The LED leads will probably not be long enough to reach down to their respective holes on the PC board so use some resistor lead cut-offs to lengthen them. If there is any danger of shorting the LED leads to the potentiomenter earthing wire, you can slip some short lengths of insulation over the leads. There’s now just one more step to complete the construction and that’s to plug the five ICs into their sockets. Be sure to fit the LM358 into the IC5 position and take care to ensure that they are correctly orientated (IC1 & IC2 face in one direction, while IC3, IC4 & IC5 face in the opposite direction). Checking it out There are no circuit adjustments to be made but you should give it a quick visual check-out to make sure everything is in the right place and you haven’t, for example, put any of the ICs, other semiconductors or electrolytic capacitors in backto-front. If it all checks out, you should make a simple current check before pronouncing it ready for use. This is easy to do – you actually do it by measuring voltage! First, turn control pots VR1-VR4, VR8 and VR9 fully anticlockwise and set VR5-VR7 to the centre of their ranges (ie, at the top). That done, connect a 12V DC power supply to the mixer’s power socket. Make sure the power supply plug’s centre pin is positive, otherwise the 10W resistor will let its smoke out and the mixer will definitely not work. Now turn on the power supply and make sure the front panel LED comes on. That’s a pretty good clue that everything is working properly. But it’s not foolproof! Connect your multimeter, on its lowest voltage range, across the 10W resistor at the DC input socket on the PC board. It should read somewhere between 200 and 300mV (200mV across the 10W resistor means that the mixer is drawing 20mA). If so, you can be reasonably confident your mixer is working properly. However, if the reading is higher than 300mV, switch off immediately because this indicates that there’s some kind of error. At least you can be assured that it isn’t a wiring error because there is no wiring! So what is wrong? There are quite a few possibilities – you may have connected the siliconchip.com.au DC power lead with reversed polarity, fitted one of the ICs, transistors, diodes or electrolytic capacitors the wrong way around, or accidentally shorted adjacent tracks or pads on the PC board with solder. (Kit suppliers tell us that 99% of problems are due to poor soldering.) In that case, it’s a matter of going over your work and carefully checking everything until you find the problem. As we mentioned earlier, if you have reversed the power supply polarity, the odds are that the 10W resistor at the input (ie, between the power input socket and the zener diode) will have said “too much” and given up the ghost. Assuming that the voltage across the 10W resistor is correct (at 200-300mV), switch the multimeter to a suitable DC voltage range (eg, 0-20V) and use it to check the voltage at various key points in the circuit. The easiest way to do this is to connect the meter’s negative lead to the wire earthing the pot bodies and then use the positive lead to probe the key voltages. Remember that you have many identical stages to compare voltages. First, check the voltage at the rear centre pin of CON7 - it should read 12V, or whatever your battery or power supply is delivering. That done, check that pin 8 of either IC4 or IC3 is about 1V lower. You should also find this voltage at pin 8 of IC1 and IC2 as well. Now check the voltage at pin 8 of IC5. This will be slightly lower again – something like 11.8V or so, if you’re using a 12V source. If everything seems OK so far, check the voltages at pin 7 of IC5 and at pin 7 of IC3. In both cases, you should get a reading of about 5.5V, because these pins are the outputs of the “half supply rail” splitters. If these voltages are correct as well, your mixer is almost certainly working correctly. It’s just about finished! The last check is to wind down the headphone volume pot to minimum, connect a set of headphones and then slowly increase the level to maximum. Depending on the headphone sensitivity, at maxiNo. Value o 2 330kW Capacitor Codes o 4 100kW o 7 47kW Value mF code EIA Code IEC Code o 8 10kW 270nF 0.27mF 274 270n o 2 6.8kW 100nF 0.1mF 104 100n o 1 4.7kW 22nF .022mF 223   22n o 4 1.2kW 10nF .01mF 103   10n o 4 1kW 2.2nF .0022mF 222   2n2 o 2 470W 1.5nF .0015mF 152   1n5 o 1 100W 68pF NA   68   68p o 2 68W 39pF NA   39   39p o 3 33W 22pF NA   22   22p o 1 10W 4.7pF NA   4.7   4p7 siliconchip.com.au This inside view from the back shows the input and output sockets, ’phones volume control, DC input plus the internals of the front panel. mum you will probably hear some hiss or noise but not much else. Plug in a suitable signal source (taking into account what components you have selected for the various inputs) and make sure that the input level pot for that source varies the level from zero to maximum. Check all four inputs in a similar way with other audio sources and also make sure that there is output at the output socket by connecting it to an amplifier. All that remains is to fit the top half of the case and fasten everything together using the four countersink head machine screws supplied. Your mixer is now complete and SC ready for use. Resistor Colour Codes 4-Band Code (1%) orange orange yellow brown brown black yellow brown yellow violet orange brown brown black orange brown blue grey red brown yellow violet red brown brown red red brown brown black red brown yellow violet brown brown brown black brown brown blue grey black brown orange orange brown brown brown black black brown 5-Band Code (1%) orange orange black orange brown brown black black orange brown yellow violet black red brown brown black black red brown blue grey black brown brown yellow violet black brown brown brown red black brown brown brown black black brown brown yellow violet black black brown brown black black black brown blue grey black gold brown orange orange black gold brown brown black black gold brown June 2007  69