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Studio series 32-band 1/3-octave equaliser In this second article on our 32-band 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 & JOHN CLARKE Because this equaliser is intended for use by bands and in recording applications, it is housed in a standard two-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). Incidentally, a number of readers got wind of this project before the first article was published and contacted us for details. They particularly wanted to know about provision for balanced input and output with XLR sockets. We have not provided for balanced lines in this design but will do so in a later article, with a small add-on board. While rack-mounting is a feature 46 SILICON CHIP of the 32-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. 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. Slider controls If you look at the parts list for the equaliser (published last month) you will note that we have used the same type of slider for the main level control as for the 32 equaliser bands. We have done this for a number of reasons. First, and the most practical, it means that the whole design can be standardised on the one type of slider. Second, even though the slider has a special resistance taper, with an S-shaped inflection around the detent as described in the first arti- cle, it does not cause audible problems when used as a slider control the signal level increases smoothly as the slider is moved upwards, with no apparent discontinuities. The power supply PCB is mounted vertically on the rear panel to minimise residual noise. Keep all mains wiring neat and tidy and note how the earth lead is soldered to a solder lug which is bolted to chassis. Centre detent It is also possible to make good use of the centre detent available on the slider control, although the circuit needs to be changed slightly, as compared with that presented in the first article. By making this small change, it is possible to set the overall gain of the equaliser to unity, with the input level control set to the centre detent position; ie, lined up with the "flat" setting for all the other sliders. To achieve this, the lkn resistor between pins 6 and 7 of IC la should be changed to 1.5kn. This change makes the overall gain of the equaliser equal to 2.25 with the input level control at maximum setting. However, when the gain control is at the centre detent position, the overall gain of the circuit will be exactly unity because of the loading effect of the 1ookn input resistor (connected to pin 5 of ICla) on the 50kn slider pot. This is making a virtue of the centre detent. In practice, we think that most users will normally set the input level control to the centre position, for unity gain, and rarely move it from that setting. However, they then have the option of increasing the gain by up to + 7dB (2.25 times) by moving the slider to the maximum setting. The mains switch should be shrouded with heatshrink tubing after the leads have been connected to prevent accidental contact with the mains. Some users may not like the idea of using the centre detent of the input slider control in the way we have just described. They may prefer to have the unity gain setting with the slider control all the way up. In that case, Rl (the 1.2kn resistor connected to pin 6 of ICla) should be omitted. This converts ICla to a unity gain "voltage follower". Note that the component overlay diagram for the equaliser board incorporates the change mentioned above (ie, lkO changed to 1.5kO), so there is no need to worry about it just follow the diagram. 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 APRIL 1989 47 0 POWER TRANSFORMER M2855 Fig.l(a): here's how to mount the parts on the power supply PCB. You can use PC stakes to terminate the + 15V, - 15V, GND and LED connections but be sure to . solder the mains leads and transformer leads directly to the board. The completed power supply should be checked out as detailed in the text before connecting it to the remaining circuitry. IC2-IC! *OMIT FOR UNITY GAIN POTENTIOMETER BOARD • • • • • • • • • nnnnnn· • 1• • • • • • • • S b • • 16Hz • •• • 20Hz • • • 25Hz 32Hz • • 40Hz • • 50Hz • • 63Hz • • 80Hz • • 100Hz • • 125Hz • • 160Hz • • 200Hz • • 250Hz 1••··· •• • SK2b • • 320Hz • • 400Hz • 1• • • • SKJb • 500Hz • • 630Hz • SILICON CHIP • 800Hz Fig.l(c): this board accommodates all the slider controls. Make sure that each slider control is square onto the board and parallel with its neighbour before soldering its leads. The five multipin headers are mounted on the copper side of the PCB, along with 6 PC stakes which terminate the shielded cable connections. 48 • • • 1kHz I : LF347 ONLY Fig.l(b): this is the parts layout for the main equaliser PCB. Push all the parts down onto the PCB as far as they will go before soldering the leads and take care to ensure that the ICs are all correctly oriented. Connections to the slider board are made via multi-way plugs and sockets. • nn· • • • • • • • 1• • • • • • • • SK4b • • • • 1.25kHz 1.6kHz • • 2kHz • • 2.5kHz • 3.2kHz • • 4kHz • • 5kHz • • 6.3kHz • • 8kHz • • • • • • 10kHz 12.5kHz 16kHz • • 20kHz measures 373 x 75mm and is coded SC 01103893. This board accommodates only the slider controls and does riot have any other components. It connects to the main equaliser board via five multiway cables which are terminated at either 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 33 sliders are soldered in place, five multipin headers need to be soldered to the board to take the interconnecting cables. The 8-way type we used are stocked by Jaycar (Cat. HM-3210). You will need one 4-way and four 8-way headers. The latter can be obtained by cutting an 8-way type in half. Don't throw the unused half away - you'll need it for the main equaliser board. The multipin headers are soldered 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 280 x 146mm and is coded SC 01103891. It has eight LF347 quad op amp ICs and one LM833 dual low noise op amp IC. As mentioned in the first article, you must not substitute the ostensibly equivalent TL074 for the LF347 as the TL074 is liable to go into oscillation momentarily after the power is switched off, giving rise to an audible "chirp". 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. It is quite easy to confuse the values of 5-band resistors if you are not used to them so it is a good idea to check each value with a digital multimeter (switched to "Ohms") before it is installed. APRIL 1989 49 The connecting cables between the main board and the potentiometer board are run via multipin headers and matching plugs. Forget the insulated wire links on the back of the potentiometer board - we goofed on the prototype. The PCB pattern has been corrected. To make it easier, we have tabulated the specified 1 % values below. Note that the tolerance band is brown. 1Mn - brown, black, black, yellow, brown. 220kQ - red, red, black, orange, brown. lOOkn - brown, black, black, orange, brown. 10kn - brown, black, black, red, brown. 1.5kQ - brown, green, black, brown, brown. 1.2kQ - brown, red, black, brown, brown. 1. lkQ - brown, brown, black, brown, brown. lkn - brown, black, black, brown, brown. 9100 - white, brown; black, black, brown. 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 the middle row of op amps is oriented differently from the outer two rows and that the LM833 (ICl) is oriented differently to IC6 and IC7, in the same row. Check the 50 SILICON CHIP component overlay diagram before you insert each IC and double check it before you solder it in place. Incidentally, we don't recommend IC sockets for this project. You can now install the multipin headers and then the capacitors. Make sure that all electrolytic capacitors are inserted with the correct polarity. Lastly, insert the 7 PC pins for connections to the power supply and input and output shielded cables. Carefully inspect your work and then put the board aside. Power supply board This board accommodates all power supply components, including the power transformer, but not some of the bypass capacitors which are on the main board. The supply board is coded SC 01103892 and measures 112 x 74mm. 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 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. 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 running the connecting ea bles 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 panel 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 ten 25mm-long x 3mm countersunk screws fed through 18mm-long MAIN BOARD POTENTIOMETER BOARD SK3•D 0 TPUT~ oo=~ ""~ "'4 m~\, '\2 · ·~ ~ + ,, . SK3bo ""~ ,~~ >O ., z > ., I + ~ ~ POWER SUPPLY BOARD TO TRANSFORMER ). PRIMARY~ r-r- -~~\: \ \ 8 7 6 N (BLU) S A (BRN) / CORO CLAMP GROMMET (~ ~~(GRN/YEL) ~ ~ EARTH LUG Fig.2: this chassis wiring diagram shows how all the cables are installed. Connect SKla on the main board to SKlb on the potentiometer board, SK2a to SK2b and so on. Watch out when hooking up the + 15V, GND and - 15V connections between the power supply PCB an the main PCB - the order on the two boards is different. APRIL 1989 51 -- ~ ~ Use cable ties to separately bind the shielded audio cables and the power supply wiring to keep everything neat and tidy. Note that the two RCA sockets on the rear panel must be insulated from chassis to prevent earth loops. to hollow spacers and secured with nuts. The 18mm spacers give the correct spacing for the slider bodies from the rear of the front panel and ensure that the slider levers protrude by the right amount. 52 SILICON CHIP cTT 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 may not have fitted inside the specified rack case. With the slider board fitted into place, install the bypass switch. This is a DPDT push on/push off switch with integral mounting bracket. It is mounted using two tt::::::::::.--i rf~\ 0 .ii: The potentiometer 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 15mm spacers instead. 12mm-long threaded pillars. Secure the two pillars to the bracket using 6mm-long x 3mm roundhead screws. This done, mount the bracket and pillar assembly 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 bypass switch 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 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 four insulated wires are run to the DPDT toggle switch at the front panel. These wires should have 250V AC-rated insula- tion and then preferably be sleeved to hold them neatly together. 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 240V AC mains. 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 board vertically at the rear of the chassis using four pillars, with screws, nuts and lockwashers. Incidentally, while there is plenty of space to mount the power supply on the bottom of the chassis, we mounted it vertically because that position 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 although generally they will be fairly close to 15V. On our prototype, the + 15V rail was + 14.78V while the - 15V rail was - 15.24V. If the supply rails are not correct, disconnect the power and 0 I· 0 ~ - 0 Fig.4: half-size reproduction of the potentiometer PCB. The full size pattern measures 373 x 75mm. 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 APRIL 1989 53 This vi~w shows the completed equaliser before the installation of the front panel. Note that all the slider potentiometers must be lined up accurately, so that their actuators pass through the front panel slots. 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 + 15V is present at pin 8 of ICl and at pin 4 of each LF347'quad op amp. Similarly, check that the - 15V rail is present at pin 4 of ICl and pin 11 of each LF347. Next, check the offset voltage of every op amp on the board. That means checking pins 1 and 7 of the LM833 (ICl ) and pins 1, 7, 8 and 14 of the LF347s. The voltage in each case should be within a few millivolts of OV. If one of the op amp outputs is not at OV 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 five pillars with screws and nuts. Alternatively, you could use plastic PC mounts. Next, there is the task of making five multiway cables with header plugs (Jaycar Cat. HM-3220). Ea ch lo 0 N C') C0 (') 0 .,.... .,.... 0 (.) (,/) 0 of I I I I ~ Fig.5: full-size reproduction for the power supply PCB. Full size positive film transparencies for all PCBs are available for $20 (see panel). 54 SILICON CH IP connector pin is soldered individually to the rainbow cable and then inserted into the plug. To complete the equaliser, you need to install the two insulated chassis-mount RCA sockets and run the shielded cables. Fig.2 shows how to do this. Testing When everything is complete, you can connect the equaliser to a mixer or control unit with headphone outputs. With no signal source connected, power up the equaliser and listen to the noise output via the mixer and headphones. Noise 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 forwo~. ~ Where to buy the kit Complete kits for this project will be made available by Jaycar Pty Ltd for $229.00. 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 full-size positive film transparencies for $20.00 . Alter· natively, for those people who have access to enlarging photo· copiers, we have published the two larger boards half-size .