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Last month we introduced our new Hifi Headphone Amplifier which features very low distortion and noise. It can even drive efficient 8-ohm speakers. This month, we show you how to build and test it. By NICHOLAS VINEN Hifi Stereo Headphone Amplifier, Pt.2 T HE ASSEMBLY of the Hifi Stereo Headphone Amplifier is straightforward, with all the parts mounted on a single PCB coded 01309111 and measuring 198 x 98mm. Apart from the PCB, there is no other wiring. Fig.9 shows the parts layout on the board. Before starting assembly, it’s a good idea to test-fit the larger components (eg, the jack socket, heatsinks, RCA sockets and so on) to check that their mounting holes are large enough. That done, begin by installing the 10 wire links using 0.7mm-diameter tinned copper wire or component pig86  Silicon Chip tail off-cuts (don’t forget the one near CON3). Once those are in, install the resistors, noting that two (both 100Ω just below the RCA sockets) have ferrite beads on their leads. Check each resistor with a DMM set to Ohms mode before soldering it into place. Follow with the 14 1N4004 diodes, taking care to ensure they are all correctly orientated. In each case, the stripe faces to the left or the bottom of the board. The four BAT42/BAT85 small-signal Schottky diodes (D15D18) near IC1 (upper-left) can then go in. Their orientations vary so take care. If you are using sockets for IC1-IC3, install them now with the notches to the right as shown. Alternatively, you can solder the ICs direct to the board with the same orientation. The MKT and ceramic capacitors are next on the list, followed by the 20 small-signal transistors. There are four different types so be sure to install the correct type at each location. Use a small pair of needle-nose pliers to crank the transistors leads so that they mate with the board holes and take care to ensure that each transistor is correctly orientated. siliconchip.com.au 680 + TINNED COPPER WIRE SOLDERED TO POT BODY 22 F 220 F + 4004 220 F D15 + D9 4004 220 F 1k reifilpmA enohpda e H ifi H 22 F + + The two air-core inductors (L3 & L4) siliconchip.com.au + + + 4004 4004 Q24 TIP32 4004 10nF 10nF 22 220 68 BC549 Q4 Q2 1.8k 1.8k Q9 2.2k Q8 BC549 + 2.2k + D7 BC328 Q26 TIP31 1.2 1.2 1.2 1.2 Q12 TIP32 4004 BC328 Q25 100nF D3 4004 150nF 150nF L4 4.7 H 10 POWER LED A K TINPLATE SHIELD C D6 + 220 F BC559 Q5 Q7 Q3 Q1 BC559 1.1k 100 910 100nF Q21 220 F L3 4.7 H 10 4004 220 F 100nF + 4004 12V AC INPUT Q11 CON4 HEADPHONE OUTPUT 7912 7812 TP1 BD139 30k D4 CON3 4004 47 F Q10 22 D8 D5 4004 TP2 CS D2 4004 1102 D1 * 2200 F BC338 10k TP3 TIP31 1.2 1.2 1.2 1.2 BC559 220pF 680pF VR2 500 22 + + 220 F 43 47 10 10k 10k 68 1.8k 1.8k 2.2k 47 F Q22 VR1 2x10k LIN 680 22k + 43 Q14 68 100 TP4 + Winding the inductors 2.2k Q23 Installing the LED POWER SWITCH F1 CON3b Before installing the 3mm blue LED, you need to bend its leads down by 90° exactly 4mm from the rear of the lens. Be sure to bend the leads in the right direction though, so that the longer anode lead goes through the hole marked “A” on the layout diagram. Once the leads have been bent down, solder the LED in place with the horizontal section of its leads 6.5mm above the PCB surface (a cardboard spacer can be used to set the height). This ensures that it will later line up with its front panel hole and will be in line with the centre shaft of the adjacent switch (S1). BC559 BD139 The two 500Ω trimpots and the fuseholder clips can now go in. Check that the small fuse clip retention tabs are towards the outside before soldering the fuse clips, otherwise you won’t be able to fit the fuse later on. That done, fit PCB pins at test points TP1-TP4 plus another two to support the tinplate shield between inductors L3 & L4. Once they’re in, fit the electrolytic capacitors but leave the two 2200µF filter capacitors out for the time being. 2.2k BC549 BC338 10k 220 F Q6 BC559 BC559 220pF VR3 500 2.2k 22 + * 2200 F 220 BC559 Q17 Q20 BC549 680pF 47 F 220 F D11 2.2k + 910 LEFT IN Q19 Q13 BC559 1.1k Q15 68 100 2.2k 100nF SC Note: Use 105° capacitors BC549 220 F BC559 2.2k BC559 L1 Hifi Headphone Amplifier © 2011 Q18 2.2k 47 10 10k 10k FERRITE BEADS 100 CON1 01309111 220 F D13 100k BC549 Q16 100pF 10k D12 47 F 4004 L2 D14 100nF IC3 LM833 100pF + 470nF 10k 100pF 4004 RIGHT IN 100 CON2 10 K A 100nF IC2 LM833 D10 D17 BAT42 100k 100 100k 100nF 22k D16 IC1 LM833 10 100pF BAT42 D18 100k BAT42 470nF BAT42 S1 01309111 * 1A FAST BLOW 10nF 22k * WHEN DRIVING LOUDSPEAKERS, INCREASE RATING OF F1 TO 2A & REPLACE THESE CAPACITORS WITH 4700 F 25V Fig.9: follow this PCB overlay to assemble the headphone amplifier. Note that the specified case has no ventilation, so we recommend the use of 105° electrolytic capacitors for long-term reliability. are wound on small plastic bobbins. It is much easier to wind them if you make a winding jig, as shown in the adjacent panel. To wind the first coil, first secure the bobbin to the jig with one of its slots aligned with the hole in the end cheek. That done, feed about 20mm of a 1m-length of 0.8mm-diameter enamelled copper wire through the October 2011  87 A Winding Jig For The Inductors ➊ ➋ The winding jig consists of an M5 x 70mm bolt, two M5 nuts, an M5 flat washer, a piece of scrap PC board material (40 x 50mm approx.) and a scrap piece of timber (140 x 45 x 20mm approx.) for the handle. The flat washer goes against the head of the bolt, after which a collar is fitted over the bolt to take the bobbin. This collar should have a width that’s slightly less than the width of the bobbin and can be wound on using insulation tape. Wind on sufficient tape so that the bobbin fits snugly over this collar. Next, drill a 5mm hole through the centre of the scrap PC board material, followed by a 1.5mm exit hole about 8mm away that will align with one of the slots in the bobbin. The bobbin is then slipped over the collar, after hole, then carefully wind on 20.5 turns before bending the end down so that it passes through the opposite slot in the bobbin. Trim the “finish” end of the wire to 20mm (to match the start end), then secure the winding with a layer of insulation tape and remove the bobbin from the winding jig. A 10mm-length of 25mm-diameter heatshrink tubing is used to finally secure the winding. Slip it over the outside and gently heat it to shrink it down (ie, be careful to not melt the bobbin). The second coil is wound in exactly the same manner. Once it’s finished, scrape the enamel off the leads on both inductors and tin them before fitting them to the PCB. Completing the PCB assembly The PCB can now be completed by fitting the remaining large items, starting with the SPDT power switch. Make sure it sits flat against the PCB and is at right-angles to it before soldering 88  Silicon Chip ➌ which the PC board “end cheek” is slipped over the bolt. Align the bobbin so that one of its slots lines up with the exit hole in the end cheek, then install the first nut. The handle is then fitted by drilling a 5mm hole through one end, then slipping it over the bolt and installing the second nut. its pins. The power socket can then go in, followed by the RCA sockets (CON1 & CON2). Use a red RCA socket for CON1 (right) and a white RCA socket for CON2 (left). Be sure to push the sockets all the way down onto the board, so that their plastic locating tabs go into the corresponding holes, before soldering their pins. The tinplate shield between the two inductors can now be installed. This shield measures 35 x 15mm and can be cut from the lid of a large Milo tin using tin snips. File the edges smooth after cutting, then temporarily position it between the two PC pins and mark their locations. That done, hold the shield in an alligator clip stand and melt some solder onto either side at the marked locations. It may take 10 seconds or more to heat it enough for the solder to adhere. Finally, melt some solder onto the tops of the two PC pins before fitting the shield in position and remelting the solder to secure it. Preparing the potentiometer The 16mm dual-gang potentiometer (VR1) may need to be modified before installing it on the board. Take a look at the pot – the flat section of the shaft must extend all the way back to the threaded mounting bush. If not, this flat section must be extended. To do this, lightly clamp the tip of the shaft in a vice with the flat section facing upwards and use a file to extend this section back to the threaded bush. Once that’s done, cut the shaft to a length of 7mm and file off any burrs. It’s also necessary to remove a small area of the metal passivation layer on the top of the pot body (use a file), after which the pot can be soldered to the PCB. The metal body of the pot must be earthed. This is done by first soldering an 80mm-length of tinned copper wire to an adjacent pad immediately below the pot (ie, between it and the adjacent 220µF capacitor). This wire is then looped across the top of the pot, pulled down and soldered to the topright pad on the PCB and to the pot’s body (ie, where you exposed the bare metal earlier). Mounting the heatsinks The two regulators and six power transistors are mounted on six large flag heatsinks. These have two posts which pass down through the PCB for support. Start by loosely fitting the 7812 and 7912 regulators to their heatsinks as shown in Fig.10(A). Note that, in each case, the regulator’s metal tab must be isolated from its heatsink using an insulating bush and silicone washer. That done, fit the 7812 regulator assembly through the lower set of holes just above CON3 and D3 (see Fig.9). If the heatsink has “solderable” pins, flip the board over and solder one, then double-check that it is sitting perfectly flush with the board before soldering the other. Since you have to heat up quite a bit of metal, it could take 15 seconds or more before the solder adheres to the post. Alternatively, if the heatsink doesn’t have “solderable” pins, use pliers to bend the tabs outwards far enough so that it is secured to the board. Having secured the heatsink, check that the insulating washer is properly siliconchip.com.au The PCB assembly is a neat fit inside the recommended Altronics case. Note how the body of the volume control pot (top, left) is earthed using a length of tinned copper wire. This wire is looped across the top of the pot’s body and is terminated in solder pads on either side (see Fig.9). aligned with the regulator and tighten the mounting screw. The regulator’s leads can then be soldered. Repeat this procedure for the 7912 regulator. The two TIP32 power transistors (Q12 & Q24) are mounted in identical fashion to the regulators. By contrast, the heatsinks for the two TIP31 power transistors (Q11 & Q23) have the BD139 VBE multiplier transistors mounted on the other side. Fig.10(B) shows the mounting arrangement. Be sure to insulate all the transistors from the heatsinks using silicone washers and insulating bushes as necessary. You can now fit the 6.35mm jack socket. The type we used does not sit right down on the board due to the shape of its pins but rather sits above the board by about 4mm. If your jack socket does not have “necked” pins, you will either need to extend them or its front panel hole will have to be lowered by 4mm when you drill it later. Finally, fit the two 2200µF capacitors. As mentioned in Pt.1, if you use 4700µF 25V capacitors (ie, for more output power), they must be no taller than 30mm and no more than 16mm in diameter, otherwise the assembly will not fit into the specified case. Test & adjustment The assembled board can now be tested. First, turn both trimpots and the volume control potentiometer fully anti-clockwise, then clip a multimeter siliconchip.com.au set it to its highest AC amps mode across the fuseholder (without the fuse in place). The easiest method is to use alligator clip leads. Next, connect the 12VAC plugpack and apply power. You should get a reading of 120mA ±20mA (no op amps installed) or 160mA ±20mA (op amps installed). If the current does not fall inside this range after about a second, switch off the plugpack at the wall and check the board for faults such as solder bridges between pads and tracks. Assuming it’s OK, switch off, install the op amps if they aren’t already on the board and check the current consumption again (ie, it should be 160mA ±20mA). Now turn the power off, install the 6021 TYPE TO-220 HEATSINK M3 x 10mm SCREW M3 NUT FLAT WASHER SILICONE INSULATING WASHER (A) 7812 & 7912 REGULATORS, ALSO TIP32 TRANSISTORS INSULATING BUSH PC BOARD fuse and connect a multimeter set to volts/millivolts mode between TP1 & TP2. That done, switch on and check the reading – it should be very close to 0mV. Now slowly adjust VR2 clockwise. At first nothing will happen but eventually the reading should start to rise. Adjust it for a reading of 28.5mV. This sets the quiescent current in the left channel to 47.5mA. Note that this reading may slowly rise as the transistors warm up so leave it on for a few minutes and then re-adjust it. Once that’s done, switch off and connect the multimeter between TP3 & TP4. VR3 can now be adjusted for a reading of 28.5mV, to set the quiescent current in the right channel. 6021 TYPE TO-220 HEATSINK M3 NUT SILICONE INSULATING WASHERS BD139 INSULATING BUSH M3 x 10mm SCREW FLAT WASHER TIP31 (B) PC BOARD Fig.10: follow this diagram to install the regulators and output transistors on their heatsinks. Make sure that the metal tabs of all devices are isolated from the heatsinks using insulating washers and bushes as required. Note that the heatsinks should be either soldered or clamped to the PCB before soldering the device leads, to avoid stress fractures. October 2011  89 A CL 208 78.5 Drilling the case 23 5.25 D FRONT PANEL B 40 40 REAR PANEL 10 C 4.5 A 20 A ALL DIMENSIONS ARE IN MILLIMETRES 53.5 45.5 CL 77.5 11 8 HOLES A: 8.0mm DIAMETER HOLE B: 6.0mm DIAMETER HOLE C: 4.0mm DIAMETER HOLE D: 11.00mm DIAMETER CL CL The final test is to connect a signal source and headphones and slowly turn the volume up. If you hear clear, undistorted sound from both channels then the board is working properly. Provided the quiescent current is set correctly for both channels, the idle current will be about 340mA (AC RMS), giving a power consumption of about 4W. With headphones, this does not usually increase but it may be higher when driving loudspeakers, depending on the volume level and speaker efficiency. Fig.11: the drilling templates for the front & rear panels. Start each hole with a small pilot drill and then enlarge it using larger drills and a tapered reamer as necessary. The rectangular cut-out is made by drilling and reaming a 6mm hole in the centre and then shaping it using a flat needle file. Be sure to correctly position the templates before drilling the holes. 90  Silicon Chip A half-size 1-unit steel case (Altronics H4995) is used to house the PCB assembly. Other cases are also suitable provided the PCB fits, although you will probably have to chassis-mount the RCA input sockets and power connector. If chassis-mounting the RCA connectors, it will be necessary to use shielded cable to connect them to the PCB. The drilling templates for the Al­ tronics case are shown in Fig.11. Disassemble the case entirely first, by removing all the screws. It separates into three pieces: the aluminium base (and rear panel), the front panel and the steel lid. Remove the feet as well and place them and the screws into the provided snap-lock plastic bag for safe-keeping. Next, download and print out the drilling templates and attach them to the front and rear panels. Use a punch to mark the centre of each hole. Alternatively, you can start the holes with a small bit (say 1mm) and a handdrill. Either way, drill pilot holes (eg, 1.5mm) in each location before enlarging them to size using larger drills and a tapered reamer. The hole which must be the most accurately placed is that for the power switch. The LED leads can be bent to compensate for any inaccuracy in its mounting hole position and those for the output socket and volume control can just be made slightly oversize. Note that the hole for the power LED is drilled to 4mm to suit a plastic LED clip. The rectangular cut-out for the power socket is made by first drilling and reaming a 6mm hole in the centre before carefully enlarging it to a rectangular shape with a flat needle file. Once the drilling has been completed, download the front and rearsiliconchip.com.au TABLE 1 Sound Maximum Recommended Pressure Level Exposure (per 24 hours) 88dBA(SPL) 4 hours 91dBA(SPL) 2 hours 94dBA(SPL) 1 hour 97dBA(SPL) 30 minutes 100dBA(SPL) 15 minutes 103dBA(SPL) 7 minutes 106dBA(SPL) 3 minutes 109dBA(SPL) 1 minute 112dBA(SPL) 30 seconds 115dBA(SPL) 15 seconds panel labels (in PDF format) from the SILICON CHIP website and print them out. These labels can then be trimmed and laminated before affixing them to the panels using double-sided adhesive tape. The holes are cut out using a sharp hobby knife. Mounting the board The PCB assembly is mounted on the same machine screws that secure the rubber feet to the case. Remove the supplied short machine screws from the feet and insert M3 x 15mm screws instead, then re-attach them to the base. Once they’re all in place, slip three M3 flat washers over each screw thread, then fit a Nylon nut/ washer combination over the top, with the larger “washer” section at the top (note: if you can’t get these, use separate Nylon nuts and washers instead, siliconchip.com.au Another view inside the completed unit. Make sure that the screws used to secure the lid clear the underside of the PCB – see text. Note that the PCB shown here is a prototype and differs slightly from the final version shown in Fig.9. with the washers on top). Next, undo the two rear two screws until only a tiny bit of thread is sticking out above the Nylon washers (say 1mm), then introduce the board by pushing the RCA sockets and DC input connector through their respective holes. It’s then just a matter of dropping the front of the board down onto the screw threads, after which you can re-tighten the rear mounting screws. The lid is held in place by two screws on each side and these should just clear the underside of the PCB. Temporarily fit these screws (ie, without the lid) to check this. If any of screws do foul the PCB you will need to remove it and add more M3 flat washers under the Nylon nuts. Once it’s correct, fit M3 nuts to all four screws to secure the PCB in place, then remove the nuts and washers from the jack socket and volume control pot. The front panel can then be attached by slipping it into place and installing the two screws at the bottom. Once it’s secured, push the plastic LED clip into place and push the LED into the clip from the back. The assembly can now be completed by reinstalling the washers and nuts for the jack socket and volume control, attaching the knob and fitting the lid. Using it Finally, here are a couple of tips for using the headphone amplifier. First, always turn the volume knob right down before donning the headphones and then turn it up to a comfortable level. If you don’t do that, you risk hearing damage. This particularly applies if somebody has left the volume control turned fully up or if the signal source is much louder than it was the last time you used the headphone amplifier. Similarly, do not listen at high volume levels for long periods. This is especially critical with a headphone amplifier as it’s easy to expose yourself to damaging sound pressure levels without too much apparent discomfort (and without anyone else noticing). Table 1 shows the recommended maximum exposure periods for various sound pressure levels (SPLs) ranging from 88-115dBA. In short, don’t make a habit of listening to loud music SC via headphones. October 2011  91