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Building the “Tiny Tim” Stereo Amplifier Part 2 – By Leo Simpson & Nicholas Vinen Having introduced this compact stereo hifi amplifier in our October issue, let’s start building it. Here we describe how to assemble the amplifier board and power supply and also how to prepare the case. A s explained earlier, this amplifier is a beef-up version of the Hifi Stereo Headphone Amplifier and is suitable for driving a wide range of speakers – especially the “Tiny Tim” speakers featured in our October issue. Note that to complete the amplifier, some additional parts will be required beyond those specified in Part 1 – see the parts list in this article. Start by assembling the main amplifier/preamplifer PCB. It’s coded 01309111 and measures 198 x 98mm. First, though, there is a little surgery required on the PCB tracks which will allow it to operate with higher power. On the PCB overlay there are eight links shown. However, the PCBs purchased from the SILICON CHIP online shop – www.siliconchip.com.au/shop 80  Silicon Chip – (and, presumably, any supplied in kits) will be double-sided so these links will already be in place, courtesy of PCB tracks. However, these boards may also have top-side tracks connecting the points labelled “A” to “D” (near the heatsinks at lower left). If so, you will need to cut these (use a sharp knife to cut sections out of the tracks and then check, with a multimeter, that there is no continuity between the points). If you have made your own board or yours is supplied as a single-sided type, you will obviously need to install the links. Use 0.7mm diameter tinned copper wire. And regardless of whether yours is a single or double-sided board, you will need to cut the two tracks on the underside shown of the board, marked with red “x”s on the PCB overlay diagram. As before, use a sharp knife then check that there is no continuity between A&B and C&D with your multimeter. Now you can start fitting the components, starting with resistors, noting that two (both 680Ω) have ferrite beads slipped over their leads before they are soldered in place. Check each resistor against the colour-code table and also with a DMM set to Ohms to ensure you have the right value. Note that you may want to leave the resistor pads labelled A and B unsoldered at this point, with the resistor leads left long; this will make it easier to join wires to them later. With the resistors in place, follow with the 14 1N4004 diodes, taking care to ensure they are all correctly siliconchip.com.au The main PCB for the Tiny Tim amplifier, containing both preamplifier and power amplifier. The board is the same as that used in the HiFi Headphone Amplifier project from September/ October 2011 but requires slight modification and of course an upgrade of components. With the mods described here it will achieve 10W music power into 4 or 8Ω speakers and 8W RMS into 4Ω. orientated. In each case, the stripe faces to the left or the bottom of the board. The four BAT42/BAT85 smallsignal Schottky diodes (D15-D18) near IC1 (upper-left) can then go in. Their orientations vary so take care. If you are using sockets for IC1-IC3, solder them in 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 16 small-signal transistors. There are three 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 check that each transistor is correctly orientated. The two 500Ω trimpots can now go in. That done, fit PCB pins at test points TP1-TP4 plus another two to support the tinplate shield between inductors L3 & L4. Then, mount the electrolytic capacitors but leave the two 4700µF filter capacitors out for the time being. Note that four of the capacitors are labelled as 50V types (a higher rating such as 63V is fine). As with the resistors, the capacitor leads labelled C and D are best left unsoldered until later. The four BD139/140 transistors which are not mounted on heatsinks siliconchip.com.au can go in next. You will need to bend their leads to fit the triangular pad pattern originally intended for a TO92 transistor, as shown on the overlay diagram and photos. The metal mounting faces of each pair face towards each other. Note that some BD139/140 transistors may lack a metal face; in this case you will need to look at which side has the transistor type number printed on it (which is opposite the mounting face) and ensure that these sides face away from either other. Winding the inductors The two air-core inductors (L3 & L4) are wound on small plastic bobbins. It is much easier to wind them if you make a winding jig, as shown in the panel on P84. 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 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. Further modifications The tracks cut earlier allow us to reconfigure the power supply so that the output stages run off the unregulated ±20V rails but to do that, we also need to run four insulated wires on the underside. It is simply a matter of connecting the pads labelled A-A, B-B, C-C and D-D. To join A-A and B-B you can use light duty wire as these only need to be able to carry enough current to power the preamplifier; even Kynar (wire wrapping wire) or bell wire is suitable. The two shorter runs, C-C and D-D, can carry in excess of 1A so medium- or heavy-duty hook-up wire is more suitable. Completing the PCB assembly The tinplate shield between the two inductors can now be installed. This shield measures 35 x 15mm and can December 2013  81 Fig.6: complete overlay and wiring diagram for the main PCB. Note the two tracks to be cut and the insulated wire links to be installed to make it suitable for higher power operation. be cut from the lid of a large tin (or similar) 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 82  Silicon Chip 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, flow some solder onto the tops of the two PC pins before fitting the shield in position and remelting the solder to secure it. 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 siliconchip.com.au MAINS CORD TO MAINS POWER SWITCH Blue N © 2013 A T1 SW Blk 15V+15V 20 OR 30VA F1 1A Slow Blow DANGER Here’s the underside of the power supply board, completely covered with a sheet of fibre insulation. Live 230VAC 10k 10k + 4700F 25V Orange BR1 Yellow Black Red ~ ~ ~ W04M +20V CON3 Fig.7: same-size PCB component overlay with matching photo below. This PCB can also be used as a general-purpose supply with appropriate transformer. + 4700F 25V + "Tiny Tim" Power Supply 18110131 – ~ + - -20V TO AMPLIFIER PCB –20V 0V +20V 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. Be sure to insulate all the transistors from the heatsinks using silicone washers and insulating bushes as necessary. The power connector, power switch and LED, input and output sockets and volume control potentiometer are not fitted to the board; instead, most of them are chassis-mounted and connected with flying leads We’ll get to that later. First, let’s assemble the power supply. Power supply support. Two of the heatsinks have two transistors mounted on them, one each side (see overlay and photos). Start by loosely fitting the 7812 and 7912 regulators to their heatsinks. 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. If the heatsink has “solderable” pins, flip the board over and solder one, then siliconchip.com.au 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 aligned with the regulator and tighten the mounting screw. The regulator’s Before fitting any components, use the power supply PCB as a template to cut a sheet of fibre insulation (often sold as Presspahn or elephantide) to 100 x 75mm and drill through the four mounting holes to make corresponding holes in the Presspahn sheet. Also make a hole corresponding with the transformer mounting hole and enlarge this to 5.5mm diameter. Now begin assembly, follow the overlay diagram of Fig.7. Fit the two resistors, then the bridge rectifier – make sure its + symbol lines up with that shown on the PCB overlay. Follow with the terminal block (wire entry holes towards board edge) and then the fuse holder. We need to install the two pin headers next but there’s a bit of a trick here. In the October issue, we showed the power switch connected between the Neutral pin of the mains power plug and the transformer primary/fuse. While this will work, it means that the transformer and fuse are live even when the power switch is off. December 2013  83 SWITCH OUTPUT + OUTPUT – A K INPUT + INPUT – LED Fig.8: finally, the only other PCB which requires assembly, the “MiniReg” universal power supply (used here to power the DAC) which we published in the December 2011 issue. 4004 R2 D1 REG1 LM317 110 4004 D3 2.2k 10F CON1 CON2 CON3 CON4 VR1 1000F 100F 1111ERCJ Of course, when opening up the unit for any reason (eg, to replace the fuse) it is always a good idea to ensure that it is unplugged but in case somebody fails to do this, it is safer to have the switch between the mains plug Active pin and the rest of the circuit. Note that it’s possible for mains Active and Neutral to be swapped in house wiring so this doesn’t guarantee safety (hence the advice to always unplug a device before servicing it) but this is a safer arrangement most of the time, ie, when the house wiring is correct. Now since we’re recycling the mains cord from a set-top box (or whatever other device you decide to rat), we don’t know how it’s wired. We checked two set-top boxes – both from the same manufacturer – and found that the mains cords were wired opposite to each other. So you will need to set your DMM on continuity mode and work out which pin of the header plug is wired to Active (normally indicated with an “A” or “L” moulded into the plastic mains plug housing). Once you’ve determined that, you can install the two pins headers with an orientation such that the Active wire will go to the terminal marked “A” on the board (ie, the one directly adjacent to the switch header). This is easier than trying to swap the pins to the polarised plug. With the two headers in place, connect the mains cord to the lefthand header (leave the other end unplugged!) then double-check that the Active pin on the plug is electrically connected to the left-hand pin of the switch header. If not, you will have to remove the left-most header and re-install it the other way around. Once you have verified that, fit the two electrolytic capacitors. Now before mounting the transformer, feed a cable tie through one of the two large holes at upper-right and then back up through the other, so that it passes under the board in the space between them. Make sure it’s the D2 4004 right way around to do the tie up later, then place the sheet of Presspahn you prepared earlier under the board and feed the transformer mounting bolt up through this and the hole on the PCB. Check that the corner screw-holes more or less line up and then slide the transformer’s rubber pad over the bolt, place the transformer on top (with wires exiting on the top side) and use the rest of the mounting hardware supplied with the transformer to loosely hold it in place. Typically this consists of another rubber pad, a metal dish, a spring washer, a flat washer and a nut. Rotate the transformer so that the wires line up with the wire pads on the right-hand side and then tighten the nut (but not too tight!). How you proceed depends on which transformer you are using. Jaycar MT2086: The primary and secondary leads will need to cross over to reach the appropriate pads. Luckily, the primary leads are double-sheathed and so pro- A Winding Jig For The Inductors 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. 84  Silicon Chip 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 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. siliconchip.com.au The power supply PCB is in the left rear corner, with a protective shield alongside. The DAC is in the opposite corner with its “MiniReg” power supply in front. Pretty much the whole of the rest of the case is taken up by the main PCB. vide sufficient insulation to remain safe in this configuration. Trim both the primary and secondary leads to length so that they reach their respective pads, leaving a little bit of slack and allowing for the fact that we are going to tie the primary leads down to the PCB before soldering them to the two pads. You can check this by pushing them down onto the PCB with a finger, between the two tie holes, then arching them over to reach the solder pads. The secondary wires are colour-coded and go to the appropriate labelled pads at the lower-right of the PCB. You will probably need to trim these to slightly different lengths so they will all reach their respective pads. Altronics M4915A: This has the opposite wiring arrangement to the Jaycar transformer so the primary and secondary leads to not need to cross over. Note that the colour coding is different though; the white lead goes to the pad labelled “yellow” while the others match up with their respective colours. As with the Jaycar transformer, you will need to allow a bit of extra length for the primary (blue and brown) leads to be siliconchip.com.au tied to the board before being soldered to the pads labelled “Blue” and “Bl.” (it doesn’t matter which goes to which). Finishing the power supply With the transformer leads trimmed and stripped, run the two primary leads through the cable tie you inserted earlier and do it up tight, then trim off the excess length. Solder all six leads to the appropriate pads, as explained above. Use two or three more cable ties to lace the secondary leads together well so that should one break loose, it won’t go floating around (and also to contain the magnetic field as much as possible). You can now fit the four tapped spacers with the PCB and fibre insulation panel sandwiched inbetween. Use a Nylon M3 screw in the upper-right corner, near the mains tracks, to ensure that a metal screw in the other end of the Nylon spacer can’t possibly make a connection through to the top of the board, where a stray wire could make the chassis live. Insert a 1A slow-blow fuse into the holder and clip the top cover on. We’ll test the power supply board later once it’s in the case. DAC power supply We’re using the MiniReg, described in the December 2011 issue of SILICON CHIP, to power the DAC, which runs off 6V DC at about 50mA. The MiniReg is fed from the 12V rail from the amplifier via the 2-pin plug soldered earlier. Follow the instructions in the December 2011 issue (or instructions accompanying the MiniReg kit) to assemble it. Don’t worry about adjusting the output voltage, we can do that later. You will need to make up a short (~50mm) 2-wire cable with a polarised header plug on one end and a 2.5mm inner diameter DC jack plug at the other end, to suit the DAC. This should be wired so that the inner conductor of the DC plug is positive. Refer to the MiniReg instructions to see which pin is the positive output and which is the negative. You will also need to short out the switch terminal (eg, with a jumper shunt). The amplifier power indicator LED can also be run from the MiniReg and again this will require a 2-core cable with a polarised header plug at one December 2013  85 end. Make this one a bit longer – say 100mm. Cut the LED leads short and solder the other end of the wires to these, with the cathode (flat side of LED lens) going to the terminal marked “K” on the MiniReg PCB. Put these cable assemblies aside, for now. Wiring Cables for power, signal input and output leads must be soldered to the amplifier board along with shielded cable to connect to the volume pot. While you could solder these wires directly to the board, doing so with everything already in the case is awkward. Hence, we fitted PC pins to most of these pads and soldered the wires to these later. There are a total of 17 required – two for each input, three for the outputs, six for the potentiometer connections, three for the power supply wires and one for the speaker ground returns. However, upon reflection, we recommend soldering the power supply wires directly to the underside of the board, leaving 13 PC pins to fit. Solder the pins in now, to the pads shown on the overlay diagram. Note that most of these holes are much larger than required for PC pins and some will let the whole pin pass through. So you will need some sort of a clamp (eg, self-closing tweezers) to hold the pins in while you solder them. For the power supply, solder 100mm lengths of heavy-duty wire to the 4700µF capacitor terminals. We have left fitting these capacitors until now so that you can wind the wire around the leads before soldering. Colour code the wires as shown. Two more black heavy-duty wires then need to be soldered to the large ground plane area above these capacitors, for the speaker outputs. If you have a commercially-made board, you will need to scrape away some of the solder mask to allow this. If you like, you can drill a hole through the board and feed the wires in from the top and you can even fit a PC pin or two so the wires can be later soldered to the top of the board, if you want to. You will also need to connect wires to run the DAC from the regulated +12V rail on the amplifier board. Take light duty figure-8 cable about 50mm long (or two strands from a ribbon cable) and crimp/solder them into a 2-way polarised header plug. The other ends go to the pads shown in 86  Silicon Chip Parts List (in addition to parts listed in Part 1) 17 PCB pins 2 chassis-mount RCA sockets, one red & one white (or black) 1 panel-mount DPDT miniature slide switch (Jaycar SS0821, Altronics S2010) 1 sheet fibre insulation (eg Presspahn or elephantide), at least 100 x 115mm 1 100mm length 8mm diameter black heatshrink tubing 1 200mm length 5mm diameter black heatshrink tubing 4 M3 Nylon tapped spacers & various M3 Nylon nuts (to suit DAC installation) 6 M3 x 10mm Nylon machine screws 2 M3 x 6mm Nylon machine screws 4 M3 x 5mm machine screws 3 M3 Nylon nuts 2 M2 x 10mm machine screws and nuts 1 jumper shunt 3 2-pin polarised header plugs with crimp pins 20 small cable ties 3 small adhesive wire saddles/clamps 1 100mm length 8mm diameter red heatshrink tubing 1 panel, 2mm plastic or 1mm aluminium, large enough to cover rear panel of case 1 5mm LED bezel clip (optional) Fig.7, with 12V to pin 1 of the plug. A pin 1 indicator is normally moulded into the plastic plug housing. Chassis preparation A number of holes must now be drilled in the front, rear and base of the case, to attach the various connectors and mount all the modules. Start with the rear panel which needs holes or cut-outs for the four speaker terminals, analog RCA input sockets, analog/digital selector switch and DAC inputs. If you are using a case which originally housed a commercial piece of equipment (in our case, a set-top box), there will be many holes in the rear panel, most of which are not in the right location to re-use. The simplest way to solve this is to attach a new rear panel on top of the existing one, covering these up, which you can then drill and cut new holes in. This panel can be metal or plastic, providing it is strong enough. We used a 2mm thick plastic front panel from an instrument case that we had spare. Don’t use thinner plastic as it isn’t strong enough. A sheet of aluminium or tinplate is also suitable. Cut the panel to the same size as the rear panel of your case, or at least large enough to cover up all the holes except that for the mains cable. Place this over the rear of the case and drill at least two 3mm holes through both. We put one at the end near the mains cable and another in the middle. Feed through short machine screws and tighten these onto nuts to hold the panel in place. If one of the holes is near where the mains power supply will go, use a Nylon screw and nut there. You can now mark out the positions for the four binding posts which should go near the middle of the rear panel, but not too close to the power supply mounting location – leave at least 10mm separation. We spaced them apart by about 20mm with 5mm extra between the two pairs; if you put them much closer together than this, this makes connecting wires awkward. Now mark positions for the TOSLINK and RCA socket inputs of the DAC board in the right-hand rear corner, as well as a rectangular cut-out for its selector switch to fit through. Since this switch body sticks out further than the TOSLINK connector, a slot will need to be cut to fit the whole thing through. We elected to place the stereo RCA analog input sockets and analog/digital selector slide switch underneath the DAC inputs as there wasn’t enough room in our case to place them side-byside. You may want to do the same. In this case, make sure the holes for the DAC inputs and switch are towards the top of the case. With the positions for all these connectors marked out, start by drilling pilot holes right through both the original rear panel and the new panel on top. Enlarge the holes for the binding posts and RCA sockets until the connectors fit through. Ideally, the siliconchip.com.au binding posts and RCA connectors should be a snug fit. The TOSLINK input, DAC selector switch and analog/digital selector switch require rectangular cut-outs and these are too small to easily nibble so you will probably have to drill a row of holes in each case and then slowly file it into a rectangle using needle files. You may find it easier to temporarily remove the new rear panel and file holes in the two panels separately before re-fitting it. Note that it’s more important that the holes are neat in the outer panel than the inner one. Test-fit the DAC board and make sure that it can be butted right up against the rear panel. For the analog/ digital selector slide switch you will need to file a slot for its actuator as well as two small mounting holes for screws. Once you have it in place and do the screws up, check that the slot is large enough for it to smoothly slide to the end stops in both directions. Make sure to de-burr all the holes on both sides before fitting the connectors. Front panel You will also need to drill some holes in the front panel, or if possible, enlarge existing holes. Make sure you don’t compromise the insulation for the existing mains switch when doing so as you will want to re-use it. As you can see from the photos, we drilled a hole at the left end for a 6.35mm headphone socket and enlarged existing holes at right, near the power switch, for the 16mm volume control pot and 5mm power LED. You may also have to cut away some of the internal structure of the front panel in order to get these to fit. We used a plastic bezel to make the power LED a snug fit in the hole, then glued it in place using hot melt glue; you could also use silicone sealant. If there are any remaining holes in the front panel near the mains switch, file a piece of plastic to the shape of each hole and glue it in place. We used black plastic, to match the existing front panel, and glued them with cyanoacrylate (“super glue”). As you can see, the resulting seams are quite subtle. First place the amplifier board near the front-left corner and mark out its four corner hole positions in the base. Then drop the power supply PCB in at left rear, close to but not right up against the rear panel, and mark out its four mounting hole positions. Mark out two more holes, just to the right of the power supply board, one roughly in line with the rear mounting holes and the other about 50mm closer to the front of the case. These will be used to hold a small Presspahn shield in place, for extra safety. With the DAC in position, mark the locations directly below its four mounting holes (eg, using a sharp drill bit) although note that you may not be able to fit pillars to suit all four if you are putting the RCA sockets and switch underneath it; also consider where the wiring for these will go. Two or three mounting holes are sufficient. Finally, choose a location for the MiniReg near the amplifier and DAC boards and mark out positions for its mounting holes too. You can then remove all the modules from the case, drill all the holes to 3mm and de-burr them. There should be about 17 mounting holes in total. Testing the power supply You can now temporarily install the power supply PCB in the case, with the mains connectors towards the rear and plug in the mains cord and switch. Make sure that the mains cord goes into the right socket, ie, that closest to the transformer. Ensure the fuse is in place and the cover clipped on. It’s a good idea to connect a DMM (or two) to the low voltage outputs with short lengths of wire (that can’t short together!) and clip leads so that you can check the output without having to hold probes in place. But you can use regular probes as long as you are careful not to go anywhere near the mains side of thing while the unit is plugged in. Check that there is no continuity between either mains plug pin and the case and that there are no loose conductors near the power supply board and switch the unit’s mains switch to on. Then stand back, plug in the mains cord and switch on the power point. Check the voltages at the output screw terminal of the power supply. You should get pretty close to 20V between the middle terminal and those on either side, with the positive output being to the left. Ours measured around +21.5V and -21.5V. If that tests OK, switch off and unplug the unit. If you didn’t get anything, there could be an open circuit connection somewhere on the board while if the fuse blows, that suggests there is a short circuit somewhere. In either case, you will have to remove the power supply board and inspect it carefully. Wiring it up With the modules built and all the holes in the case drilled or cut and de-burred, all that’s left is to fit the modules and wire them up. We’ll go through these remaining steps in Part 3 next month and also present some performance data for the complete amplifier. SC Module mounting holes The next step is to drill a series of 3mm holes in the bottom of the case for mounting the various modules. siliconchip.com.au Here’s a view from the back to the front, showing how we made the bits fit into what was originally a set-top-box case. Once the lid goes on you’d never know! December 2013  87