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Studio series 20-band stereo equaliser In this second article on our 20-band stereo equaliser we give the construction details and a brief troubleshooting procedure. Three printed boards are used and the two main boards are linked together via removeable multiway cables. By LEO SIMPSON & BOB FLYNN Because this equaliser is housed in a standard 2-unit high 19-inch rack mounting case, it can therefore be mounted in an equipment rack, used in a free-standing mode, or stacked on top of other equipment. Ventilation is not a problem because it consumes very little power (less than 5 watts). While rack-mounting is a feature of the 20-band equaliser, the case design is somewhat simpler than many rack units. The chassis is a basic U-shape made of cadmium plated and passivated steel, while the wrapover lid is made from black Marviplate. The dimensions of the chassis, not including the front panel, are 427mm wide, 80mm high and 197mm from front to back. This gives plenty of room for all the circuitry. 60 SILICON CHIP There is one particular point to be noted about the construction, which was touched upon in the first article. It is very easy to run into problems with earth loops when using an equaliser with other equipment which is earthed. This applies to most big power amplifiers and mixers, especially those intended for semi-professional and professional use. To avoid the earth loop problem, the equaliser circuitry is not connected to chassis or to mains earth. The chassis itself is connected to mains earth, for safety's sake. Since there is no connection between signal earths and chassis, the input and output sockets must be isolated from chassis. Nor must there be any other path from the equaliser circuitry to the chassis. This will automatically be taken care of if you follow the wiring diagrams. Assembly procedure Most of the work in assembling the equaliser is involved with putting the three PC boards together. You can do it in any order but let's look at the slider board first. It measures 370 x 78mm and is coded SC 01107891. This board accommodates only the slider controls and does not have any other components. It connects to the main equaliser board via six multiway cables which are terminated at each end by sockets. Fig.1 shows how the parts are mounted on the board. It is simply a matter of inserting each slider and soldering its three pins to the board pattern. Make sure that each slider is square onto the board and as you solder each one in place, make sure it is parallel with its neighbour. When all 40 sliders are soldered in place, six multipin headers need to be soldered to the board to take the interconnecting cables. You will need two 4-way and four 10-way. headers. The former can be obtained by cutting one 8-way type in half. The multipin headers are ed to the copper side of the slider board, using the short pin side. The long pins take the matching socket plug. With the headers in place, the slider board is complete and can be set aside until you are ready to install it in the case. Main equaliser board The main board accommodates all the equaliser circuitry except for the power supply. It measures 262 x 150mm and is coded SC0l 107891. It has 10 LF347 quad op amp ICs and two LM833 dual low noise op amp !Cs. Fig.2 shows the parts layout for the main board. As mentioned in the first article, you must not substitute the ostensibly equivalent TL07 4s for the LF347s. In this circuit, the LF347 gives lower residual noise and better stability. We suggest that you install all the wire links and resistors first. Note that a lot of the resistors are 1 % tolerance types having five colour bands. To make it easier to select these resistors, we have included a table with the colour codes for all the resistors specified in the circuit. Even though we have given the colour codes, you should still use your multimeter to check the values, to be certain. Next, install the op amps. Note that two rows of LF347 op amps are oriented in the same direction which makes it easier to spot if you've put one in the wrong way. The two LM833s at one end of the board both point in the same direction too. Incidentally, while we used IC Fig.1 (right): this board accommodates all the slider controls. Make sure that each slider is square onto the board and parallel with its neighbour before soldering its leads. The six multipin headers are mounted on the copper side of the PCB. The power supply PCB is mounted as far away from signal circuitry as is possible and has been positioned for minimum noise. Keep all mains wiring neat and tidy and note how the earth lead is connected to a solder lug which is bolted to chassis. The connecting cables between the main board and the potentiometer board are run via multipin headers and matching plugs. The slider board is mounted behind the front panel on 10 18mm-long hollow spacers and secured with screws and nuts. If you can't get 18mm spacers, use 12mm and 6mm spacers instead. RESISTOR COLOUR CODES □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ 62 No. Value Tolerance Code 2 6 8 6 6 4 4 2 4 2 4 1MO 11 OkO 1% 1% 1% 1% 1% 1% 1% 1% 1% 1% 1% 1% 1% 1% 1% 1% 5% brown black black yellow brown brown brown black orange brown brown black black orange brown white brown black red brown grey red black red brown violet green black red brown blue grey black red brown blue red black red brown green blue black red brown green brown black red brown green blue black brown brown grey red black black brown violet green black black brown blue green black black brown blue red black black brown yellow violet black black brown orange orange red gold 2 2 12 24 2 1 100k0 91k0 82k0 75k0 68k0 62k0 56k0 51k0 5.6k0 8200 7500 6800 6200 4700 3.3k0 SILICON CHIP sockets for our prototype we don't regard them as necessary for this project. If you do decide to use IC sockets though, make sure you use good quality types (which will cost almost as much as the ICs themselves). Cheap IC sockets are not worth the trouble they can cause. You can now install the multipin headers and then the capacitors. The 8 O. lµF bypass capacitors are miniature monolithic ceramic types, blue in colour. Make sure that all electrolytic capacitors are inserted with the correct polarity. Now for the 5 % tolerance audio filter capacitors for the gyrators. We have specified Wima capacitors, kindly supplied by Adilam Electronics Pty Ltd. These are moulded . metallised polyester and polycarbonate types which are labelled in different ways, according to their RIGHT - - - - - - - - - - - 1 5 ¥ - - + 1 5 Y - - - - - OUTPUT GNO •1 .~F8,•• 2 ~- :6a 1iu ul 220pf 0.47 m~m- E3 228pFA A ~ --rnn-~· ~ GN~ lOOpF(;) (M1{3~= 1~~{3~ ~ GND• 0.47 .~~~- □ .0033LJ _ ~ 680pF I!:::!) .:{TIDe 1••· ••1 SK5a +Q e-[illJ-e .001 □ .oosa □ 1e-cmTI}e .01 □ 0.1 c:::> 0.1 eC» .015LJ .0068□ ~ .0033E!) +0100µF 1ooµFO+ ~ • : •1 .• 0.15□ .022□ 0.22□ -c=- .O~<at>.O~ ... ~ ..[illJ. .068-□ ~ 0.331!:::]i 1 +0220µF 220µFO+ ~ 0.33 □ ~ IE::!)0.22 ~ 1~FLI e-(]!0-_r..;;;;;,__ ~ ~ : ••• 0.11!::!] .0068LJ -iillD-4 ..,~ W"' --o:illJ-e ~ 0-1 «:::e 0.15 0.68 • • MLJ • : •• 0.1i;;---.i ~ • : .022□ 0.1 • 1 E:!) SK3a ..{ill[J-e SK4a 1-0~ 0.22□ <:it --mfili}e 1• e{]fil-4 .osa□ o o 2<at>o·M· --ill.ID◄ MLI 75k 0.33 • 1 8200 8 .O~s-Og : o,33c:--;t ~ o.s8D ..£illiiJ◄ w5 .068LJ ~ 0.47Ll 11 ~ .068LJ ~ O~s~O-~ 0.68□ .0068LJ e{Jfil-e .o&a □ _. 100µF ~ ~ ~i2 I 8100µF .033LJ .047LJ • 0.1 □ oc:» : ~ 1 .022E:!) : •1 1~ 0.1 0.1 This board accommodates all power supply components, including the power transformer, but not most of the bypass capacitors .osa□ .01s □ --mm◄ Power supply board : .oosa.□ 8.oosa□ ~ ..,~ ..ffiDe ~w5 ~ 0. 1 □ value. For values below .0lµF, their values are shown in picofarads but without the "pF" suffix. For example, the value .0068µF is shown as 6800. Similarly, 680pF is shown as 680. Values above .0lµF are designated in microfarads and may be with or without the " µF" suffix. All Wima capacitors are labelled with their tolerance which, in this case, is 5%. As a final step in assembly of the main board, insert the 11 PC pins for connections to the power supply and input and output shielded cables. Carefully inspect your work and then put the board aside. .0331!:::!) .0047LJ --illru-- SK2a ---m<at>J• .01E:!) -ruo-- --c:filJ◄ .0033LJ ~ • I .00¥<at>·0~ .0047LJ 1 e{ill[J-e : .047E!) .01s□ ~ <at> .0022□ ~ .. ~ --mrue-QIO-e w 5 --c:ill:}- .0022f!::!j : .022LJ .:::» 0.1 e:::,, .01s□ .0151!:::::!l e-QK}e{]fil-- .oo1s□ 1~ 0.1 .0015QI ~ SK1a 8 --oru,. w5 ~ .0047LJ 68~ <at>680pFI!::!) ... ~ 8~F .oo 100µFf!\ .0033LJ \_.} ~ 2 w47 Iii ~ 1ooµF .00471!::!) • 4 22opFA A22opF 1 100µFQ+ J: !, • 0• --OK)e l ~--mD-1 - LEFT OUTPUT GND - - - - - - - , 1 22oµFO+ "';:!; 0.33 • MLJ 47i.-=1 220µF . c:.......!! +• 75k • 0.22 7500 !,l~ ~ 1µFLI ~ 0.68 I 0.331!::j) ~ 1 ~ f 1µF □ 0 Fig.2: this is the parts layout for the main equaliser PCB. Make sure that all the resistor colour codes run in the same direction. This makes it easier to check that all values are correct. Similarly, make sure that all ICs are correctly oriented and that the electrolytic capacitors are connected the right way around. which are on the main board. The supply board is coded SC0l 103892 and measures 112 x 74mm. Its component layout is shown in Fig.3. Assembling this board is easy but correct polarity must be observed for all the components: diodes, electrolytic capacitors and the 3-terminal regulators. Note that both regulators face the same way but make SEPTEM BER 1989 63 The mains switch terminals should be shrouded with heatshrink tubing after the leads have been connected, to prevent accidental contact with the mains. Note the shield (at the end of the pen) which stops hum from being induced into the sliders. sure you don't inadvertently swap them over. The transformer is also mounted on this board, using screws, nuts and lockwashers. Terminate the primary and secondary wires and then put the board aside for the moment. You can now turn your attention to the chassis. POWER TRANSFORMER M2855 Chassis assembly We will assume you have complete metalwork for this project and so work on the chassis is basically a matter of installing the completed printed circuit boards and RCA socket panel and running th~ connecting cables between them. The screen printed front panel should not be permanently fitted until all the circuitry has been powered up and checked for correct operation. If you do put the front panei on before all the checking has been done, you are sure to have to do some troubleshooting and this increases the chances of marking or damaging the finish . The slider board is the first to be installed. It is mounted using eight 25mm-long x 3mm countersunk screws fed through 18mm-long hollow spacers and secured with nuts. The 18mm spacers are actually each a combination of a 12mm 64 SILICON CHIP levers protrude by the right amount. Incidentally, although we have not mentioned it previously, these sliders are not fitted with separate knobs - their plastic actuators are used as is. If knobs were used, the spacing between sliders would have to be increased and the whole project would not have fitted inside the specified rack case. With the slider board fitted into place, install the Tape monitor switch (S1} and the bypass switch (S2}. These are both DPDT push on/push off switches with integral mounting brackets. Both are mounted using two 12mm-long threaded pillars. Secure the two pillars to each of the switch brackets using 6mm-long x 3mm roundhead screws. This done, mount the switch bracket and pillar assemblies inside the chassis using countersunk 3mm screws, also 6mm long. Countersunk screws are necessary here, so that the front panel can mount flush against the front of the chassis. Once you have the slider board and the two switches fitted, you might like to check the fit of the front panel on the chassis. Fit it temporarily and check that all the sliders operate without fouling. Any necessary adjustments should be done now, before assembly proceeds further. Fitting the power supply ~ : '---- - A LEOl - • • • ~ ! - ~ I POWER SUPPLY BOARD + -----~ Fig.3: here's how to mount the components on the power supply board. Note that the negative supply input to the regulator uses a lO00µF filter capacitor while the positive supply uses a 2200µF capacitor. and a 6mm spacer. They give the correct spacing for the slider bodies from the rear of the front panel and ensure that the slider The power supply can now be fitted. First fit the power cord, using a cordgrip grommet to secure the cord at the rear of the chassis. The earth wire is terminated at the rear of the chassis using a solder lug secured with a screw, nut and lockwasher. The active and neutral wires are terminated to the power supply board and then figure-8 cable is run to the pushbutton mains switch at the front panel. This figure-8 cable should have 250V AC-rated insulation. Because the mains switch is very close to the slider controls in the left channel, it tends to induce hum and buzz into the lower frequency bands of that channel. To eliminate that problem a small shield is fitted between the mains switch and the G RIGHT ~ ~ LEFT TAPE RECORDER c~~ !:mi H D~ K~~ _Jl-1-~+15V 1c:::::J DV SK6a 1c::::::J SKSa P/B SK3b H SK1a SK2a ~' ~' ~' SK3a ,~ I SK4a ~' 1r::::=J SK6b 11::=J SKSb MAIN BOARD POWER SUPPLY BOARD POTENTIOMETER BOARD A ~ LED K Fig.4: this chassis wiring diagram shows how all the cables are installed. Connect SKla on the main hoard to SKlh on the potentiometer board, SK2a to SK2h and so on. Watch out when hooking up the + 15V, GND and - 15V connections between the power supply PCB and the main PCB - the order on the two hoards is different. SEPTEMBER 1989 65 jo -( ~ :;;;::;:::!::=;;;:;;;=;;:::!::=;;;;;;;:::;::!::::~=~~~ r ~---=-......:--.............- ~ . . - - - ~ -I\....J ~ ~✓ ✓✓ -( r--... " "'° ·w '-' ,n I \.... ,-A/ ;...,,, - '-o'-o r ,u,(./ -,·~ -( ~ ~ °" Fig.5: half-size reproduction of the main printed circuit board. The full size pattern measures 262 x 150mm. 28Hz left channel slider. The shield is retained by the mounting nut of the mains switch. For our prototype we made the shield from PCB copper laminate but we expect that for kits the shield will be made of light gauge steel or aluminium. When the wires are soldered to the switch, a length of heatshrink tubing should be fitted right over the switch to completely shroud it and thereby prevent accidental contact with the 240VAC mains. Alternatively, you can do as we did and fit sleeving over each of the switch terminals. You can fit the power switch temporarily at the front of the chassis so that testing can proceed. Do the same with the LED bezel. Mount the power board as shown in the photos, using four 10mm pillars, screws, nuts and lockwashers. The mounting position shown gave the minimum residual noise. Now apply power and measure the voltages at the supply outputs. They should be close to ± 15V DC. More particularly, the positive rail should be within the range from + 14.4V to + 15.6V. Similarly, the negative rail should be within - 14.4V to - 15.6V. On our prototype, the + 15V rail was + 15.12V while the -15V rail was - 15.31V. 66 SILICON CHIP If the supply rails are not correct, disconnect the power and check out the board for faults. Check also that the LED is working. Now the main board can be checked. You can do this before it is installed; that way, you don't have the hassle of removing the board to do any repairs, should they be necessary. Sit the board on an insulating surface and connect the three supply wires from the power supply board. Do not connect the five multiway cables for the slider board. Now apply power and check that the + 15V is present at pin 8 of IC1 and IC2 and at pin 4 of each LF347 quad op amp. Similarly, check that the - 15V rail is present at pin 4 of IC1 and IC2 and at pin 11 of each LF347. Where to buy the kit Complete kits for this project will be made available by Jaycar Pty Ltd for $349 (Cat. KC-5055). Note that two of the PC patterns are too large to be published full size in the magazine. For those who wish to make their own PC boards, we can supply a set of fullsize positive film transparencies for $20 .00. Alternatively, for those people who have access to enlarging photocopiers, we have published the two larger boards half-size. Next, check the offset voltage of every op amp on the board. That means checking pins 1 and 7 of the LM833 (IC1, IC2) and pins 1, 7, 8 and 14 of the LF347s. The voltage in each case should be within a few millivolts of 0V. If one of the op amp outputs is not at 0V it is likely to be at either + 15V or - 15V due to an open circuit connection on the board. If you get this sort of fault , check your soldering carefully. If all the voltage checks go as they should, you can then install the main board in the chassis. It is mounted using four pillars with screws and nuts. Next, there is the task of making six multiway cables with header plugs . Each connector pin is soldered individually to the rainbow cable and then inserted into the plug. To complete the equaliser, you need to install the RCA socket panel and run the shielded cables. The chassis wiring diagram of Fig.4 shows how to do this. Listening test When everything is complete, you can connect the equaliser into the Tape Monitor loop of your stereo control unit or amplifier with stereo headphones output. With no signal source connected, power up the equaliser and listen to the noise output via the headphones. Noise ro r. o1 0 I I I I N 0) co M 0 .,... .,... 0 (.) 0 0 Cl) 0 ~ Fig.7: full-size reproduction for the power supply PCB. Full size positive film transparencies for all PCBs are available for $20 (see panel). "'"' I i: I ;:? 0 CJ (/) L _J w 1%: <( 0 0.. "' 0 Wt0.. :::> <( 0.. I- l- o => wO I- ::> 0.. I- ::> 0 1%: w C 1%: a::a ii: 0 u w I- w 0.. 1%: t. j 0.. z <( 1- Fig.6: half-size reproduction of the potentiometer PCB. The full size pattern measures 370 x 78mm. levels should be very low and any buzz or hum should be practically non-existent. Finally, check that the noise level increases as you push sliders up and that the noise reduces when sliders are pushed down. If all that checks out, your equaliser is ready for work. ~ ::> w I- z=> _a.. ...JZ 1- 7 ::c (!J ILL ix ...J w r Fig.8: here is a full-size reproduction of the rear panel artwork. SEPTEMBER1989 67