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Build The TwinTen . . . a gutsy little stereo amplifier with surprisingly good performance by JIM ROWE & PETER SMITH Want a compact amplifier to team with your Discman, MP3 player, games machine or whatever? This tiny stereo amplifier puts out a surprising amount of power, considering that it runs from a 16VAC 1.5A plugpack. N OT ONLY IS THIS a gutsy little amplifier, it is an ideal learnwhile-you-build-it project for high school and tech college students. It all fits onto a single largish PC board with no 240VAC mains voltages present – that’s taken care of by the 16VAC plugpack. The TwinTen has inputs for three stereo program sources such as CD player, MP3 player and FM/AM tuner and has front-panel LEDs to indicate which source has been selected. The other controls are Bass, Treble, Volume and Balance. At the rear, there are RCA line inputs, binding posts for the speaker connections, a finned heatsink and a socket for the plugpack supply jack. As part of the final checking of this 32  Silicon Chip amplifier, one of our staff members took it home for a blast on his large stereo system. His daughter was so impressed with the performance from such a tiny package that she wanted it straight away. We thought that was a pretty good endorsement. The reason why it packs such a punch is that it can deliver around 17 watts music power per channel into 4-ohm loudspeakers. In a typical room with reasonably efficient speakers, that’s enough to blast you out of your seat. And did we say it’s tiny? It measures just 225mm wide, 40mm high and 165mm deep, including the knobs and rear heatsink. It’s also pretty quiet and its other performance parameters are pretty good as well, considering that we have kept a very tight rein on the overall cost of the design. One reason it performs so well is that it is based on the same National Semiconductor LM1875T 20W audio amplifier IC used in the Schoolies’ Amplifier, published in the December 2004 issue. This IC has inbuilt thermal protection so that even if you abuse it or short out its output, it won’t be damaged. The power amplifier circuits are very close to the Schoolies’ Amplifier circuit but inevitably there are component differences to provide different gain and so on. How it works As you can see from the circuit diagram, the amplifier is quite straightforward. The left and right channels are identical, with an input preamp stage followed by a tone control stage and finally a power amplifier stage. We’ll follow the signal path through the left (upper) channel in detail but the right (lower) channel is exactly the same. Source selector switch S1a allows the user to select input signals from any one of three sources. These are labelled CD, MP3 and TUNER but any siliconchip.com.au The new amplifier is built into a low-profile ABS instrument case measuring just 225 x 165 x 40mm. Construction is straightforward, since all the parts except for the loudspeaker terminals are mounted on a single PC board. Power comes from an external 16VAC 1.25A plugpack supply. of these inputs can be used for line level audio signals from sources such as a DVD player, a MiniDisc player, games console or a VCR. From the switch, the selected signal passes through a simple low-pass filter formed by a 2.7kW resistor and 680pF capacitor. This filters out any RF (radio frequency) signals or noise that may be present with the incoming audio signals, to prevent them from causing trouble. After this, the signals are applied directly to volume control potentiometer VR1a. From the wiper of VR1a, the signals pass through a 220nF coupling capacitor to the input of IC1a, which is one half of a TL072 dual FET-input op amp IC. IC1a is used as an input preamplifier stage, with its voltage gain set to 3, using negative feedback provided by the 5.6kW and 2.7kW resistors. The amplified signals from IC1a’s pin 1 output are then fed to this channel’s tone control stage, which is based on IC2a – half of a second TL072 dual op amp. IC2a is connected in what is known as a Baxandall negative feedback tone control circuit, where the op amp’s negative input is connected siliconchip.com.au to the wipers of both the bass and treble control pots (VR2a and VR3a). As shown, the incoming signals from IC1a are fed to one end of each pot’s filter circuit, while the feedback signals from IC2a’s output (pin 1) are connected to the other end of each filter circuit. As a result, when both VR2a and VR3a are set to the middle of their ranges, the tone control stage provides the same gain (ie, unity or 1) for signals of all audio frequencies. This gives the amplifier a flat frequency response. However, if VR2a is turned clockwise (towards input preamp IC1a), the stage provides more gain for frequencies lower than about 500Hz, thus giving SPECIFI CATION S Power Output Both channels driven into 8W ........................................................... 6W RMS Both channels driven into 4W ........................................................... 6W RMS One channel driven into 4W ............................................................ 10W RMS IHF Burst Peak Output (Music Power) Both channels driven into 8W ..............................................................13.5W Both channels driven into 4W...............................................................17.1W General Distortion at 1W continuous into 8W..............................................0.04% THD Frequency response (-3dB points).............................................. 10Hz - 72kHz Signal-to-noise ratio <at> 10W RMS into 4W ..............................................-80dB Input impedance, all channels................................................... 12kW//600pF Treble tone control range at 20kHz............................................ +11dB/-12dB Bass tone control range at 50Hz............................................... +12dB/-13dB February 2005  33 Par t s Lis t – TwinTen Amplifier 1 PC board, code 01102051, 215 x 156mm 1 low-profile ABS instrument case (225 x 165 x 40mm) with prepunched front and rear panels (see text) 1 16VAC 1.25A plugpack 1 3-pole 3/4-position rotary switch 2 M205 PC-mount fuse clips 1 M205 2A slow-blow fuse 1 finned heatsink, 84 x 24mm (Altronics H 0668) 3 PC-mount dual RCA sockets 1 PC-mount 2.5mm concentric power socket 4 binding posts, 2 red & 2 black 5 20mm skirted instrument knobs 1 knob actuator, 40 x 30 x 2.5mm (see text) 4 6G x 6mm self-tapping screws 3 6G x 9mm self-tapping screws 2 M3 x 16mm screws, nuts & flat washers 2 TO-220 insulating washers and nylon bush sets 1 small tube of heatsink compound 1 300mm length of 0.7mm tinned copper wire 1 200mm length figure-8 speaker cable Semiconductors 2 TL072 dual op amps (IC1,IC2) 2 LM1875T audio amplifiers (IC3, IC4) 2 12V 1W zener diodes (ZD1,ZD2) bass boost. On the other hand, if VR2a is turned anticlockwise (towards the output of IC2a), the stage provides lower gain for frequencies below 500Hz, thereby giving bass cut. In the same way, VR3a can be used to give a variable amount of boost or cut to frequencies above 2kHz; ie, treble boost or cut. And as both of these tone controls are part of a negative feedback circuit connected around IC2a, the boost or cut action is achieved without causing any significant distortion or increase in signal noise level. Balance control The output signals from IC2a are then fed via a 4.7kW series resistor and 1mF non-polarised (NP) capacitor to the positive input of IC3, which 34  Silicon Chip 1 3mm green LED (LED1) 1 3mm red LED (LED2) 1 3mm orange LED (LED3) 4 1N5404 3A diodes (D1-D4) Capacitors 4 2200mF 25V electrolytic 2 470mF 16V electrolytic 4 220mF 25V electrolytic 2 4.7mF 16V NP electrolytic 2 1mF 16V NP electrolytic 4 220nF 50V MKT polyester 8 100nF 50V MKT polyester 2 10nF 50V MKT polyester 4 1.5nF 50V MKT polyester 2 680pF 50V disc ceramic 2 330pF 50V disc ceramic 2 33pF 50V disc ceramic Resistors (0.25W 1%) 2 270kW 4 4.7kW 4 82kW 4 2.7kW 2 56kW 3 1kW 6 27kW 2 470W 4 10kW 2 100W 2 5.6kW 2 10W 2 2.7W Potentiometers 1 PC-mount 16mm dual 10kW log. pot (VR1) 1 PC-mount 16mm dual 100kW linear pot (VR2) 1 PC-mount 16mm dual 50kW linear pot (VR3) 1 PC-mount 16mm 50kW linear pot (VR4) is the power amplifier IC for the left channel. We’ll look at this in a moment but first note that one end of pot VR4 (the Balance control) is connected to the junction of the 4.7kW resistor and 1mF NP capacitor. Because the moving wiper of VR4 is connected to ground, this means that the 4.7kW resistor and the associated “half” of VR4 form a variable voltage divider, while the other “half” of VR4 forms a similar voltage divider with the 4.7kW resistor in the right channel. In addition, because the two dividers share VR4, their division ratios vary in opposite fashion; when the wiper is turned one way from the centre position, the gain is reduced in the left channel but increased in the right channel, and vice-versa. Fig.1 (right): the circuit consists of identical input preamp stages (IC1a & IC1b), followed by a tone control stage (IC2a & IC2b) and finally power amplifier stages IC3 & IC4. Switch S1 selects the input signal, with LEDs1-3 indicating the selected signal source. So VR4 provides a “see-saw” or differential adjustment of the gain in the two channels, allowing them to be matched or balanced (to compensate for any difference in the incoming signal levels). Power amplifier IC IC3 is an LM1875 power amplifier IC, which comes in a TO-220 package with five connection leads and a metal tab for connection to a heatsink. It’s connected as a basic amplifier stage, with a voltage gain of about 18 times, as set by the 82kW and 4.7kW negative feedback resistors (ie, 1 + 82k/4.7k). The 4.7mF capacitor in series with the 4.7kW resistor is used to give the amplifier full DC negative feedback, for maximum thermal stability. In addition, the series 220nF capacitor and 2.7W resistor connected between IC3’s output and ground form a Zobel filter network, to ensure that the amplifier is stable at high frequencies with varying loudspeaker loads. Power supply Both IC3 and IC4 operate from dual supply rails of ±22V DC. This gives a total DC supply of 44V – necessary for the power amplifier stages to develop the power we want into 8W loudspeakers, with low distortion. We generate the +22V and -22V supply rails from the external plugpack’s supply voltage of 16VAC using four half-wave rectifier circuits, one each for the positive and negative rails for IC3 and IC4. Each rectifier circuit uses a single 1N5404 diode and a 2200mF reservoir capacitor and they’re all kept separate to minimise crosstalk between the two channels. The input preamp and tone control stages around IC1 and IC2 don’t need to operate from the same high voltage rails, because the signals they handle are at a much lower level. IC1 and IC2 are therefore operated from +12V and -12V rails, derived from one of each pair of high voltage rails using 470W series resistors and 12V zener diodes ZD1 and ZD2. There’s also a 470mF siliconchip.com.au siliconchip.com.au February 2005  35 Fig.2: the frequency response curves for the left and right channels of the amplifier. The response is 3dB down at 10Hz and 72kHz. capacitor across each zener diode for added filtering and decoupling, while the supply lines to IC1 are also given a small amount of extra decoupling via the 100W resistors and 100nF capacitors. Finally, note that the selector switch has three poles. Two of these (S1a & S1b) select the input signals for the left and right channels as described previously, while the third (S1c) pole is used to switch about 10mA DC to one of three LEDs, to show which input source has been selected. The LEDs have different colours, so they’re easily identified from a distance. Construction The new amplifier is very easy to build, with all components apart from the four loudspeaker terminals mounting directly on a single PC board. This board is coded 01102051 and measures 215 x 156mm. It’s designed to fit inside a low-profile plastic instrument case measuring 225 x 165 x 40mm. The box is fitted with a rear panel cut from 2mm thick sheet aluminium which acts as the heatsink for the two output amplifier ICs (IC3 and IC4). However, by itself, the heatsinking provided by the rear panel is insufficient. Therefore, an external finned heatsink is bolted to the rear panel, to help keep the power amplifier ICs cool. As shown in the photos, the front panel on the prototype was also made from aluminium but a standard plastic panel could also be used. To reduce off-board wiring to a minimum, we have mounted input selector 36  Silicon Chip Fig.3: this plot shows how the noise and distortion (THD) varies with frequency for a power output of 1W into 8W. It’s better than .05% from 20Hz to 20kHz. switch S1 directly on the PC board with its control spindle vertical. The only complication arising from this is that you need to convert a standard 20mm knob into a custom side actuator lever knob, to allow the switch to be operated via a lever which emerges through a matching slot in the front panel. It’s not hard to make this special knob, as we shall see later. Board assembly Before installing any of the parts, it’s a good idea to carefully inspect the copper side of the PC board for manufacturing defects. In particular, make sure that there are no bridges between pads or tracks or breaks in the tracks and make sure that all the holes have been drilled. If everything checks out OK, you’re ready to start the assembly. Fig.6 shows where all the parts go. Begin by fitting the six wire links to the board, followed by the resistors and the MKT and ceramic capacitors. Table 2 shows the resistor colour codes, although it’s a good idea to also check each value using a digital multimeter before it is installed on the board. That’s because some of the colours can be difficult to read. Next, install the four rectifier diodes (D1-D4), zener diodes ZD1 & ZD2 and the two TL072 ICs (IC1 & IC2). These parts are all polarised so make sure they go in the right way around. By the way, don’t fit the rectifier diodes with their bodies right down against the board – instead, mount them so that each diode body is about 4mm above the board. This provides better ventilation and also reduces any interference coupling to the signal track which passes underneath them. That done, fit the electrolytic capacitors, again taking care to ensure that they go in with the correct polarity. The only exceptions here are the 1mF and 4.7mF electrolytic capacitors which are non-polarised. It’s now simply a matter of completing the board assembly by fitting the large hardware items. These parts include the DC power socket, the three dual RCA connectors and the control pots. Before fitting the control pots, you will probably need to shorten their spindles, so that the knobs sit close to the front panel when the amplifier is finally assembled. This job can be done using a small hacksaw – cut each spindle at a point 9mm from the threaded bush and remove any burrs with a small file. Each pot mounts directly on the PC board. Be sure to fit each dual pot (VR1, VR2 & VR3) in its correct position, as they all have different values. Each pot should be pushed down onto the board as far as it will go before soldering its pins. Once all four pots have been mounted on the board, cut a 160mm length of 0.7mm tinned copper wire and pass one end down through the small hole in the left front corner of the board just to the left of VR4. Solder it to the copper underneath, then run the free end across the top of all four pots (see Fig.6 and photo). It’s then soldered to the metal body of each pot, to make sure siliconchip.com.au Fig.4: THD + noise vs. output power into 4W. The right channel (red) has higher distortion at the lower power levels due to noise from the power supply circuitry. that all four (and also the metal front panel, when it’s fitted) are connected to the amplifier’s signal earth. Note that you will need to scrape away a small area of the anodising on each pot body using sandpaper, to ensure that the solder properly bonds to the metal. The rotary switch is fitted next but this also needs its spindle cut short. In this case, cut the spindle at a point just 6mm from the threaded bush and again remove any burrs using a small file. There’s another operation to be performed on the switch before it’s mounted. This is to make sure that it Fig.5: THD + noise vs. output power into 8W loads. Once again, the right channel is worse due to power supply noise but it’s still generally substantially less than 0.1%. is set for only three positions. This is done by first unscrewing the mounting nut and removing the star lockwasher underneath. That done, prise up the switch’s stop washer and turn the switch spindle anticlockwise with your fingers until it won’t turn any further. After this, refit the stop washer with its cranked locating pin passing down into the second hole from the anticlockwise end of the series moulded into the switch body (the hole between the moulded “3” and “4”). Finally, refit the star lockwasher and nut, threading the nut down against the star lockwasher just hard enough to fasten everything in place. If you now try turning the switch spindle by hand, you should find that it has only three positions. The switch can now be fitted to the PC board. It must be orientated so that the round spigot moulded into the top/front of its body is directly behind the spindle, as indicated in the wiring diagram (Fig.6). The pins on the rear of the switch can then be pushed down through the corresponding holes in the board, until the moulded black lugs that hold the switch body together are resting on the board. Finally, the pins can all be soldered to the copper pads. The rear panel carries three pairs of RCA input sockets (Tuner, MP3 & CD), the heatsink, and the left and right channel speaker terminals. There’s also an access hole for the power socket. siliconchip.com.au February 2005  37 Fig.6: install the parts on the PC board as shown here but refer to the text before mounting the audio amplifier ICs (IC3 & IC4) and switch S1. The three source indicator LEDs can now be installed. These mount vertically at the very front of the board, immediately in front of rotary switch S1. All three are orientated so the flat 38  Silicon Chip on the side of the body is towards the right. First, solder their leads to the pads underneath so the underside of each LED’s body is just 12mm above the board. After that, it’s just a matter of bending each pair of leads forward by 90° at a point 3mm below the LED body, so that the LEDs will later protrude through the matching holes in siliconchip.com.au This is the view inside the completed unit. Take care to ensure that all polarised parts are correctly oriented – ie, the ICs, LEDs, diodes, zener diodes and the electrolytic capacitors. the front panel of the amplifier. At this stage, the PC board assembly should be complete except for the two LM1875 audio output amplifiers (IC3 and IC4). Don’t install these yet – that step comes later, after they have been bolted to the rear panel. Side actuator knob With the board assembly now complete, you can make the special side actuator knob for the selector switch. This is made by first modifying a standard knob by cutting off its top and bottom. This is then fitted with an actuator lever cut from a small piece of 3mm Perspex or acrylic sheet (the accompanying diagram gives you a siliconchip.com.au template). The two are then cemented together using epoxy adhesive. First, cut out the actuator paddle from a small piece of 3mm-thick Perspex using a hacksaw and then smooth it all around using a small file. Lightly sand one side as well using fine garnet paper, so it has a matt surface. Next, cut off the top and bottom of a 20mm skirted knob at the positions marked in Fig.7. This should leave only the knob section immediately around the brass insert. It’s a good idea to smooth both cut surfaces (top and bottom) with fine garnet paper, so they’re flat and free from burrs. Finally, mix up a small amount of Araldite (or similar) epoxy adhesive and cement the lower surface of the knob to the matt surface of the Perspex actuator paddle. This should be done with the knob positioned as close as possible to the centre of the round hole in the paddle and with the knob orientated so its grub-screw hole is pointing at “12 o’clock” when the paddle arm is positioned at “5 o’clock” – see Fig.7. Make sure that you have a layer of epoxy adhesive between the two surfaces, as well as a fillet all around the lower circumference of the knob. The assembly should then be left untouched for 24 hours so the adhesive can cure. Once it has cured, attach the side actuator knob to the rotary switch by pushing it down onto the spindle as far as it will go. Check that the grubscrew is in the centre of the spindle’s February 2005  39 a photocopy of the panel artworks as a template. The small rectangular slot in the front panel for the input switch paddle is best cut by drilling a row of 2.5mm holes fairly close together and then opening them into the slot using jeweller’s needle files. The larger round holes for the input sockets and the control pot spindles are best made by first drilling a small hole in the centre of each position. It’s then simply a matter of opening each hole out to the correct diameter using a hand-held tapered reamer. Finally, remove any burrs from the holes using a countersink bit. Once both panels have been prepared, you can prepare dress labels by photocopying the artworks onto adhesive-backed A4 label paper. The stickers can then be given a protective coating using wide clear cellulose packaging tape, before cutting them to size. After that, you just peel of the backing, carefully affix each one to its panel and cut out the various holes using a sharp hobby knife. How To Make The Switch Actuator Fig.7: follow this diagram to make the side actuator knob. The actuator paddle is made from 3mm-thick Perspex and must be glued to the modified knob exactly as shown in the diagram at right – ie, in the 5 o’clock position. flat, then tighten the grub-screw using an Allen key and move the actuator lever to the centre position. Preparing the case The next step is to remove 12 of the Table 1: Capacitor Codes Value 220nF 100nF 10nF 1.5nF 680pF 330pF 33pF μF Code 0.22µF 0.1µF 0.01µF .0015µF   NA   NA   NA EIA Code    224   104   103   152   680   330    33 IEC Code       220n   100n   10n   1n5    680p    330p    33p integral mounting pillars in the bottom of the case, so that they don’t foul the PC board assembly. Leave only the four pillars that line up with the corner mounting holes in the PC board itself. You can use an oversize drill to remove the unwanted pillars. Front & rear panel assembly Now for the rear panel assembly. Begin by installing the four binding posts used for the loudspeaker terminals (red to the top, black to the bottom), then slide the rear panel into position in the bottom half of the case. That done, slip the front panel over the pot shafts, LEDs and switch actuator and slide this assembly into the case. The PC board can now be temporarily secured by installing four 6G x 6mm screws at the corner positions. Follow this by fastening the three dual Drilling the panels If you buy a complete kit of parts, you won’t have to worry about this next step – the panels will come precut, drilled and screen printed. Alternatively, if you’re building the amplifier from scratch, you’ll have to first cut the front and rear panels to size (219 x 34.5mm) from 2mm sheet aluminium. That done, you can drill the various holes in each panel, using Table 2: Resistor Colour Codes o o o o o o o o o o o o o o No.   2   4   2   6   4   2   4   4   3   2   2   2   2 40  Silicon Chip Value 270kW 82kW 56kW 27kW 10kW 5.6kW 4.7kW 2.7kW 1kW 470W 100W 10W 2.7W 4-Band Code (1%) red violet yellow brown grey red orange brown green blue orange brown red violet orange brown brown black orange brown green blue red brown yellow violet red brown red violet red brown brown black red brown yellow violet brown brown brown black brown brown brown black black brown red violet gold brown 5-Band Code (1%) red violet black orange brown grey red black red brown green blue black red brown red violet black red brown brown black black red brown green blue black brown brown yellow violet black brown brown red violet black brown brown brown black black brown brown yellow violet black black brown brown black black black brown brown black black gold brown red violet black silver brown siliconchip.com.au Above and right: these two close-up views clearly show the construction of the side actuator knob. Note that it’s vital to glue the actuator paddle to the knob in the correct position; ie, with the arm at 5 o’clock to the grub screw – see text and Fig.7. RCA connectors to the rear panel using three 6G x 9mm screws, then secure the pots to the front panel using the supplied nuts and washers. You are now ready to bolt the two LM1875 audio output devices (IC3 & IC4) to the rear panel, along with the finned heatsink. Fig.9 shows the mounting details. First, check that the area around the mounting holes is smooth and free of any burrs. That done, apply thermal grease to the various mating surfaces (see Fig.9), then slip the two LM1875 devices into position on the PC board. Finally, bolt them to the rear panel along with the heatsink using two M3 x 16mm screws. Don’t leave out the insulating bushes that go through the device tabs and make sure that the mica washers are properly aligned before tightening the mounting screws. Tighten both mounting screws so that the tab of each IC is clamped flat against its mica washer and the rear panel. Once that’s been done, set your multimeter to a low ohms range and use it to confirm that the device tabs siliconchip.com.au Fig.8: these full-size artworks can be photocopied and used as drilling templates for the front and rear panels, if necessary. They can also be copied onto adhesive-back paper and used to make the final dress panels – see text. February 2005  41 Rear Panel & Heatsink Assembly Fig.9: follow this diagram to mount the heatsink and audio amplifier ICs (IC3 & IC4) on the rear panel. Note that the amplifier tabs must be isolated from the panel metalwork using mica washers and insulating bushes. are indeed electrically isolated from the rear panel. If either device tab shows a short to the rear panel, the assembly must be dismantled and the fault located. Assuming everything is OK, remove the PC board and panel assembly from the case and solder the pins of each LM1875 device. The internal wiring can now be completed by running figure-8 speaker cable between the loudspeaker terminals and the PC board. Finally, refit the assembly to the case and fit the knobs to the pot spindles. You are now ready for the smoke test. Checkout time OK – connect a 16VAC plugpack to the input socket and apply power. If all is well, the centre red LED on the front panel should light (assuming the Source switch is in that position). The other two LEDs should light when the switch actuator is moved to their respective positions. If the LEDs do light correctly, the supply voltages are probably OK. However, if you’d like to make sure, you can check the DC supply voltages using a multimeter. The simplest way to do this is to connect one lead of the multimeter to the tinned copper wire that connects the bodies of the control pots and then use the other lead to touch various supply voltage points. As a guide, several convenient voltage test points are marked on the PC board overlay diagram (Fig.6). For example, near the ends of the two 470W resistors near the centre of the board, you should be able to measure -22V, +22V, -12V, and +12V as shown. Similarly, you should be able to measure -22V on the anode of diode D3 and +22V on the cathode of diode D1. If all of these voltages measure correctly (ie, within ±0.5V), your amplifier is probably working correctly. To confirm this, switch off, connect suitable loudspeakers and connect a suitable audio source (eg, a CD player or AM/FM tuner) to a pair of input sockets. Finally, apply power and check that the unit is working properly by varying all the front-panel controls. All that remains now is to finish the assembly by fitting the top half of the case and fastening it all together using the four 25mm-long M3 machine screws provided. Your new TwinTen Stereo Amplifier is now ready for use. Happy SC listening! The finished amplifier is quite compact and is ideal for use in a rumpus room or study. 42  Silicon Chip siliconchip.com.au