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High performance 10-Octave STEREO GRAPHIC Part II EQUALISER By JOHN CLARKE F Last month we described the circuit and performance of our new 10-octave Stereo Graphic Equaliser. Now we conclude with assembly details of the PCB and the acrylic case with its very smart-looking front panel, which is actually a black screen-printed PCB – with slots for the sliders but no tracks! or such a large circuit, the double-sided PCB for this project is surprisingly compact at only 198 x 76mm. It is coded 01105171. It is very compact because we have mounted the 10 ganged slider pots on one side and all the active circuitry, with 12 (or 13) LM833 low-noise dual op amps, on the other side. Virtually all of the resistors are surface-mount types but fortunately you can read their printed values (with a magnifying glass). All of the capacitors, apart from 12 (or 13) 100nF surface-mount ceramic bypass caps, are through-hole types, 82  Silicon Chip so while some components are quite small, they are also quite straightforward to solder in place. And the benefit of soldering the surface-mount components is that you don’t have to clip off their pigtails after soldering. The PCB front panel shown in the prototype above is a little smaller than the acrylic case but we have since modified it so that the outside dimensions of the front panel and case are the same – it looks neater. Mind you, the acrylic case is not needed if the equaliser is to be mounted into existing equipment or into a half rack width 2U case – but you will still need some form of front panel. Choice of supplies We have provided two component overlays for the PCB, one for the ACpowered version and the other for the DC-powered version. The main difference is that the DC version omits the components for the low voltage AC power supply but adds the circuit components associated with IC13, as depicted on page 24 of last month’s issue (June 2017). To assemble the PCB, you will need a fine-tipped soldering iron bit, siliconchip.com.au 0.71mm-diameter solder, a good light and a magnifying glass or spectacles to be able to solder the surface-mount components in place. Begin by mounting the surfacemount ICs. As already noted, IC13 is only installed if you intend to use a DC supply. Each IC is firstly oriented correctly and note that the chamfered side is the 1k 100pF 100k 2.7k L LK2 R 1k 100k 2.7k CON2 = 2 F 470 RIGHT 680pF C 2017 01105171 17150110 10 OCTAVE GRAPHIC EQUALISER 100nF 91k 680 100nF 100k 680 100nF 100nF 1 F 110k 680 33nF 220nF 680 91k 100nF 68nF 390nF IC2 820nF 680 100k 220nF 820nF R 0V ~ CON5 ~ ~ 47k – ~ IN 470 F + OUT 10 F REG2 7915 REG1 7815 (78XX) W04 BR1 10 F 680nF + V– siliconchip.com.au SLIDER SHIELD 220nF L 680 110k 100nF 680nF =1.5 F IC1 =1.5 F 31.25Hz SINGLE SUPPLY LINK 62.5Hz 100nF 68nF 100nF IC3 125Hz 1 F 680 82k 15nF 82k 680 IC4 250Hz 390nF 33nF 10nF 47nF 100k 680 IC5 500Hz 680 100k 220nF 100nF 680 82k 15nF 22nF 4.7nF 82k 680 IC6 1kHz 100nF 10nF 100nF 680 91k 47nF 6.8nF 10nF 2.2nF 91k 680 IC7 2kHz 4.7nF 1nF 100nF 680 110k 22nF 110k 680 IC8 4kHz 2.2nF 100nF 680 82k 10nF 3.3nF 82k 680 IC9 8kHz 1nF 680pF 100nF 680 62k 6.8nF LEFT 62k 680 IC10 16kHz 3.3nF 1nF CON1 L IN 2.7k 1 F L OUT 100pF 10 100nF 1 F IC11 100pF 10 + +/– SUPPLY LINKS LK1 470 100pF 1M = 2 F 2.7k 1nF CON3 R IN 100pF 1 F 470nF L1 R OUT CON4 (and for IC13 if used). Then the surface-mount resistors can be soldered in place including that for LED1 and those resistors used for the DC version, if that is the version being built. We said that the surface-mount resistors have the values printed on them but some “interpretation” is required. A 3 or 4-digit code is used, with the last digit being the number of zeros. 10 100nF 1 F IC12 100pF 10 1M 470nF L2 IC1-13 LM833 pin 1-4 side of the IC. Place the IC in position over the PCB pads and solder one corner pin. Check its alignment and remelt the solder if the IC needs adjustment. When the IC is aligned correctly, solder the remaining seven pins. Make sure that there no solder dags bridging any of the adjacent pins. Then align and solder the 100nF supply bypass capacitors for IC1-IC12 Fig.7 (left): use this component overlay (and the matching photo at right) if you want to use an AC supply. It contains the bridge rectifier, smoothing capacitors and most importantly the positive and negative 15V regulators. Note also the supply links (top left) – both are in place. In the photo these are shown as header sets but as these would normally be set once and forgotten, wire links (from component lead offcuts) would be the way to go. July 2017  83 16kHz 8kHz VR8 1kHz 250Hz 500Hz 2kHz 01105171 VR7 VR6 VR5 125Hz VR4 62.5Hz VR3 31.25Hz VR2 LED1 VR1 84  Silicon Chip When mounting the RCA sockets, the white ones are for the left channels and the red are for the right channels. The 3-way screw terminal CON5 is mounted with the opening to the edge of the PCB. Take care when mounting the bridge rectifier, making sure that its pin labelling matches the screen printing on the PCB. REG1 (and REG2 if used) can be installed next, seated as far down onto 4kHz VR10 Then install the MKT polyester capacitors. Note that the 820nF and 680nF capacitors for the 32Hz gyrator are connected in parallel to make up a value of 1.5µF. Alternatively, you could use 1µF and 470nF capacitors instead, if the 680nF and 820nF values prove difficult to obtain. The electrolytic capacitors are mounted next, taking care to orient each one with the correct polarity. VR9 So the 680Ω resistors will be labelled 6800, ie, 680 with no extra zeros. The 100kΩ resistors will be 100, with three zeroes, ie, it is labelled as 1003. Once all the surface-mount components have been installed, the throughhole components can be mounted. Start with the resistors and then fit the two ferrite beads, using a resistor lead offcut to feed through each bead before soldering them in place. Fig.8: the top side of the PCB contains only the 10 slider pots and the power LED; everything else is on the underside. Again, the matching (same size) photo at right will assist you in PCB assembly. The square hold in the board is to accommodate the power switch, itself attached to the front panel. siliconchip.com.au Fig.9: use this alternative PCB overlay if you are using a DC supply. Only the two end sections of the PCB are shown – the centre of the PCB is identical. Note the absence of links for LK1 and LK2 but the link over three pads at the bottom (this would be easiest achieved on the underside of the board). the PCB as they will go. For the DC supply version, you can use a 15V regulator (7815) if the DC source is between 18V and 25V (maximum). If the supply is less than 18V, a 12V regulator (7812) can be used provided the DC input is 15V or more. Below this 15V, you can dispense with the regulator and connect a wire link between the IN and OUT terminals; the two outer pads for the component). Naturally, this will mean the supply is unregulated. Headers LK1 & LK2 or LK3 can be installed next. LK1 & LK2 are for the AC version and LK3 for the DC version. Install the jumper links on LK1 & LK2 for the AC powered version and a jumper link on LK3 for the DC version. That should complete all the components installation, apart from the 10 sliders and LED1, which are mounted on the other side. So it is most important that you carefully check that you have installed and soldered all the parts correctly before moving the to the next stage (with the sliders). In particular, double check parts placement for the capacitors that mount directly opposite the sliders. Once the sliders are installed, you will not have access to the soldered connections for any of these capacitors. Before mounting the sliders on the front of the PCB, make sure that all siliconchip.com.au of the capacitor leads that were soldered on this side of the PCB have been trimmed back. This must be done so that the sliders can be fully seated onto the PCB. Note that the sliders only fit with one orientation. So if they don’t seem to fit, try the alternative 180° orientation. LED1 also needs to mount with the correct orientation (longer lead is the anode) and with the top of the lens 12mm above the PCB. Initial testing Power can now be applied to the equaliser circuit to test for voltage at the op amps. For the single 16VAC supply, connect the supply leads between an AC input (one of the outer terminals of CON5) and the centre 0V terminal. If your supply is from an existing piece of equipment with a 30V centre tapped transformer, connect the two AC voltages to each of the outer terminals of CON5 and the centre tap to the centre 0V terminal. The transformer must be capable of supplying the extra current drawn by the equaliser circuit (55mA typical, so allow for, say, 100mA). Power up the circuit and the LED should light. Now measure the DC voltage between pin 4 and pin 8 of one of the op amps. This should be close to 30V if you are using the AC supply and 15V (or less depending on whether you have a 12V regulator or if it is bridged out). For the DC supply version, check that voltage between pin 4 of any IC to pins 3 and pins 5 shows half the supply voltage. In other words, this voltage should be +7.5V or thereabouts if you have a 15V supply between pin 4 and pin 8. The low cost and ease of assembly of our new Graphic Equaliser is due in no small part to the laser-cut “case”, shown here with the power switch and DC supply socket fitted. July 2017  85 M3 x 25mm tapped spacer M3 x 6.3mm tapped spacer * * Equaliser PCB Slider Pot Front panel (PCB) * Laser-cut black acrylic case pieces (ends not shown) M3 x 15mm screw The PCB is in position, with the slider-pot shafts poking through the front panel and the board held in place with threaded spacers. The diagram at right (Fig.10) shows how the PCB and case components fit together Case installation Fig.10 shows the assembly of the Acrylic case. Note that we show the mains transformer in the circuit for the centretapped 30V supply but a transformer will not fit in the acrylic case. In addition, the power switch used in the case is not intended for switching mains voltages which could otherwise induce hum into the graphic equaliser circuitry. The power switch is only intended for low voltage switching. For the DC supply, the polarity needs to be correct and this depends on the wiring to the plug that connects to the socket. There will be no power supplied to the circuit if polarity is incorrect. You need to have the positive connected to the outer terminal of CON5, so swap the two leads to the DC socket if the voltage is reversed. The wiring to the switch and socket are covered in heatshrink tubing. The case is assembled as shown with the front panel PCB attached to the front of the case using M3 x 15mm screws secured with tapped M3 spacers 6.3mm long. These are placed at the four corner mounting positions on the PCB. A washer is placed under each spacer first to increase clearance. The two mounting holes in the middle of the PCB, top and bottom are se86  Silicon Chip cured to the front of the case with M3 x 10mm screws and M3 nuts. The main equaliser PCB then is placed over the screws protruding through the 6.3mm long spacers and with the slider adjustment shafts protruding through slots in the front panel and front PCB. The PCB is secured using the M3 x 25mm spacers. The rear panel of the case is secured to these spacers using M3 washer * * M3 x 10mm screw * M3 x 10mm screws after the top and side pieces of the case are attached in place. The holes in the rear of the case for the RCA sockets are made with large enough clearance, so that RCA plugs can pass through hole and onto the sockets. So connect up your new equaliser for a new listening experience. SC Enjoy! And finally, the case components are slotted together ready for the PCB/front panel assembly to be slipped into place and screws fitted to the four threaded spacers to complete assembly. siliconchip.com.au