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Schoolies’ Amp By PETER SMITH This 20W audio amplifier module sounds great and is dead easy to build! B UILDING AN AUDIO amplifier is a popular choice when it comes to the hands-on part of electronics courses. We can well imagine the classroom question “Well, does it work?” answered in a flash with “Listen to this, disbeliever!” That’s the best part of building an audio amp; you and your peers actually get to hear the final work punch out a favourite tune or two hundred! However, amplifiers that produce more than a few watts of power can be difficult to construct and expensive. This is where our new “Schoolies Amp” comes in. It features a simple board layout for easy construction, is relatively inexpensive and even includes over-temperature and shortcircuit protection. As power amplifier modules go, 16  Silicon Chip this unit may not rank at the top for raw power but you’ll be hard pressed to find a simpler circuit. The design is based on a single IC, the LM1875T 20W audio amplifier from National Semiconductor. This IC comes in a TO-220 package and, combined with a handful of other parts and a suitable power supply, delivers over 20W RMS into either a 4W or 8W loudspeaker. What’s more, the specifications are quite impressive for such a bare-bones circuit. With a signal-to-noise (S/N) ratio of 105dB and a distortion figure of less than 0.04% for 1kHz at 20W (see graphs), it could well be used as the basis for a hifi stereo amplifier. The frequency response extends from 14Hz to beyond 100kHz when measured at 1W RMS. The LM1875 includes an internal 4A current limit, preventing damage should the output be accidentally shorted to ground. It also includes “safe operating area” (SOA) protection, meaning that the current limit is dynamically reduced according to the voltage present at the output. Because so much power has to be dissipated by such as small package, the LM1875 also has in-built thermal protection. This effectively shuts the device down if there is excess heat build up in the chip itself (about 170°C). Design revamp A similar amplifier module, also based on the LM1875, was presented in the December 1993 edition of SILICON CHIP. By comparison, the Schoolies Amp has slightly better performance and is easier to build. siliconchip.com.au We achieved these improvements by dispensing with the single supply option that was part of the original design. This left plenty of room to eliminate wire links, increase pad and track size and employ some common sense noise cancelling and grounding techniques. Circuit description The circuit diagram (see Fig.1) for the amplifier module reveals just the LM1875 power amplifier (IC1) and a handful of support components. The closed loop gain of the amplifier is set to 23 by the 22kW and 1kW resistors on the inverting input (pin 2), following the standard non-inverting amplifier feedback rules (ie, voltage gain = 22k/1k + 1 = 23). The 22mF capacitor in series with the 1kW resistor sets the lower end of the amplifier’s frequency response. Another factor in the low-end response is the high-pass filter in the input signal path, formed by the 2.2mF coupling capacitor and 22kW resistor. Overall, the result is a rapid frequency response roll-off below about 10Hz (see Fig.11). Following this, a 1kW series resistor and 330pF capacitor form a low-pass filter, eliminating problems with high-frequency noise pickup on the input leads. Non-polarised electrolytic capacitors (marked “NP”) are used in these positions because the voltages present SCHOOLIES AMP (20W AMPLIFIER MODULE) Fig.1: the circuit consists of little more than the LM1875, which contains a complete low-distortion 20W amplifier with overload protection in a 5-pin TO-220 package. are too small to polarise conventional electrolytics. Keen-eyed readers will have detected that the input circuitry is not connected directly to power supply ground but instead goes via a 10W resistor. This has little effect in a single (mono) am- plifier setup but in a stereo setup, it helps to reduce currents circulating in the ground wiring which can degrade separation between channels. Finally, a 1W resistor in series with a 220nF capacitor at the output of the forms a Zobel network, designed to Shown here with its matching power supply, the Schoolies Amp (aka 20W amplifier module!) takes next to no time to build, costs very little – and will give a surprisingly good account for itself in a variety of audio projects. siliconchip.com.au December 2004  17 Fig.2: use this diagram when assembling the module. Double-check that you have the two 220mF capacitors in the right way around, as indicated by the “+” markings. neutralise the effects of the speaker’s voice coil inductance at the higher end of the frequency spectrum. Power supply The power supply for the module appears in Fig.4. An 80VA mains transformer with two 18V secondary windings or a single 36V centre-tapped winding is used. The secondary feeds a bridge rectifier and filter, formed by diodes D1-D4 and two 4700mF 35V capacitors. The output is about ±25V unloaded and is suitable for powering one or two amplifier modules. If designing your own power supply, note that the rails to the LM1875 Fig.3: full size PC board pattern for the amplifier. If you’re wondering why this looks different to the overlay pattern at left, this view is from the copper side while the overlay is “through the board” as if an x-ray. must not exceed ±30V. Voltages lower than the recommended ±25V can be used but the output power will be less than shown in the performance graphs. Refer to the LM1875 datasheet for more information (from www. national.com). The circuit also shows a ±15V preamplifier supply, based on two simple zener regulators. This supply is optional and can be left out if not required. The power supply design is virtually identical to the supply used for our SC480 amplifier, described in the January & February 2003 issues. In fact, you can use the SC480 power supply kit to power this project, if you team it The completed amplifier module is bolted to its heatsink via the LM1875. Take particular care once the amplifier is in this state – it’s quite easy to break the legs of the IC if you allow the board to flex with reference to the heatsink. 18  Silicon Chip up with the specified transformer (the higher voltage SC480 transformer can not be used). If using the SC480 power supply kit, note that the 4 x 2.2kW 1W resistors need to be replaced with 2 x 560W 1W resistors, as shown on the circuit and overlay diagrams (Figs.4 & 5). Construction Construction of the amplifier module is quite straightforward, with all parts mounting on a small PC board. Fig.2 shows the parts layout. The resistors should be installed first, followed by the capacitors. Use your meter to verify the value of resistors where necessary. Note that the two 220mF capacitors are polarised and must go in with their positive leads oriented as indicated on the overlay. The remaining two electrolytic capacitors are non-polarised and can be installed either way around. Install the fuse clips and terminal blocks next, pushing them all the way down onto the board surface before soldering. Note the retaining tabs on the fuse clips; be sure to orient these towards the outer (fuse end) side, otherwise you won’t be able to plug in the fuses later. The LM1875 is installed last of all. First, fit 10mm tapped spacers to the corner mounting positions of the board, then slip the LM1875 into position. As its leads are preformed at the factory, they shouldn’t require more than minor tweaking for a comfortable fit in the PC board holes. siliconchip.com.au Fig.4: the power supply is simple but more than adequate. The ±15V supply is for a preamplifier, if required. Make sure that the LM1875 is sitting “square” (ie, perpendicular to the board surface) and then carefully turn the assembly over and solder only the centre pin of the package. The remaining four pins should only be soldered after attachment to the heatsink, so let’s do that next. Heatsink mounting must now solder the remaining leads of the LM1875. Gingerly turn the whole assembly over, being careful not to disturb the relationship between the PC board and heatsink. Place something under the board to support its weight and keep it at right angles to the heatsink while you solder the remaining leads. It’s also a good idea to reheat and resolder the centre pin of the IC to relieve any stresses imposed during assembly. Once done, use your multimeter to confirm that the metal tab of the LM1875 is indeed electrically isolated from the heatsink. Power supply assembly Fig.5 shows how to assemble the SC480 power supply PC board to suit the Schoolies Amp. Note that the 4700mF capacitors are 35V rated but higher voltage types are fine too. Install diodes D1-D4 first, aligning the banded (cathode) ends as shown. Follow these with the two 3-way terminal blocks and then the two 4700mF capacitors. Make certain that you have Place the board and heatsink on a flat surface and bring them together, centring the LM1875 in the available heatsink width. Dependent on the particular type of heatsink, it may also be necessary to line up the hole in the tab with a gap between fins. Now gently mark around the inside of the tab hole with a sharp pencil. Centre-punch the pencilled circle and first drill a 1mm pilot hole, then step up to a 3mm (or 1/8-inch) bit for the final size. Once drilled, the edges of the hole must be deburred to obtain a perfectly smooth surface. This can be achieved by gently rotating the tip of a much larger drill in the hole opening by hand, held between the thumb and forefinger. The LM1875 can now be bolted to the heatsink using a TO-220 insulating kit (ie, a mica washer and insulating bush). Fig.12 shows the assembly details. Smear all mating surfaces with a thin film of heatsink compound before bolting the assembly together. Take care not to “skew” the LM1875 as the screw Fig.5: here’s how to assemble the power supply board. One of these can power two is tightened. modules for a stereo set-up. Note that the PC board is virtually identical to the one used in the SC480 amplifier – the full size pattern is shown at right (Fig.6). To complete your work, you siliconchip.com.au December 2004  19 Fig.7: THD versus frequency at 1W into an 8Ω load. Fig.10: THD versus power at 1kHz into an 8Ω load. AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 10.000 03 FEB 103 16:31:07 8.0000 6.0000 4.0000 2.0000 0.0 -2.000 -4.000 -6.000 -8.000 -10.00 10 Fig.8: THD versus frequency at 1W into a 4Ω load. Fig.9: THD versus power at 1kHz into a 4Ω load the positive leads of the capacitors around the right way. Leave out all the remaining components unless you specifically require the ±15V supply for a preamplifier. Wiring Use heavy-duty (7.5A) multi-strand 20 SILICON CHIP 100 1k 10k 100k 200k Fig.11: frequency response at 1W into an 8Ω load. Fig.12: this diagram shows how the LM1875 is attached to its heatsink. cable for all DC power and speaker connections. The +25V, -25V and 0V wires to the amplifier module should also be twisted together to minimise radiated noise. Now on the mains (240VAC) side, be sure to use only mains-rated (250VAC) cable and insulate all exposed con- nections. This includes the use of rubber boots or heatshrink tubing on the rear of IEC sockets, switches and fuseholders. The idea is to ensure that even with the covers off and power on, it is impossible to accidentally make contact with live mains voltages. The mains earth must be connected siliconchip.com.au Parts List – 20W Amplifier Module Fig.13: the mains earth must be securely attached to a metal chassis as shown here. Tighten the first nut very firmly before winding on the second “lock-nut”. The power supply ground (0V) must also be connected to this point. This view shows the completed power supply for the amplifier. The components at the bottom are for the optional ±15V preamp power supply and may be left out if not required. to a metal chassis using the arrangement shown in Fig.13. Return all earth wires to this point to eliminate potential earth loops. When in any doubt, refer your work to an experienced person for checkout before connecting to the 240VAC mains outlet for the first time. Never take shortcuts with mains wiring – it could be fatal! Testing Before applying power, go back over the board and carefully check that all parts are correctly located and oriented. That done, install the fuses and connect the power supply leads, taking particular care that you have the positive and negative leads around the right way! Do not connect the loudspeaker or an audio input signal at this stage. Note that you must have the heatsink fitted, as the LM1875 has to dissipate substantial power even without an audio signal present. Check the supply rail voltages first – these must be within 10% of the nominal value. Finally, check the DC voltage across the loudspeaker terminals. It should be less than ±50mV. If this checks out, the loudspeaker can be connected (switch off first) and an audio input signal applied for SC final testing. Using the cheaper DSE M-1991 transformer Dick Smith Electronics has available an alternative economy transformer, the M-1991, which is suitable for this project and offers significant savings over the specified toroidal version, priced at just $9.94! Rated at 60VA, it can be connected as 30V CT (ie, using the 0V-15V-30V taps) to give ±22.4V rails. This results in slightly reduced power output: 17W RMS into 8W and 24W RMS into 4W – not enough to be noticeable! Resistor Colour Codes o o o o o o No.   1   2   2 1 1 2 Value 1MW 22kW 1kW 10W 1W 560W siliconchip.com.au 4-Band Code (1%) brown black green brown red red orange brown brown black red brown brown black black brown brown black gold gold (5%) green blue brown gold (5%) 5-Band Code (1%) brown black black yellow brown red red black red brown brown black black brown brown brown black black gold brown n/a n/a 1 PC board coded 01112041, 80mm x 63.5mm 2 2-way 5mm/5.08mm terminal blocks 1 3-way 5mm/5.08mm terminal block 4 M205 PC-mount fuse clips 2 M205 2.5A slow-blow fuses 1 1.4°C/W heatsink (Altronics H 0580, Jaycar HH-8590 or similar) 1 TO-220 insulation kit (bush, insulating washer) & heatsink compound 4 M3 x 10mm tapped spacers 4 M3 x 6mm pan head screws 1 M3 x 10mm pan head screw 5 M3 nuts 1 M3 flat washer Semiconductors 1 LM1875T 20W audio amplifier (IC1) Capacitors 2 220mF 35V PC electrolytic 1 22mF 16V non-polarised (bipolar) PC electrolytic 1 2.2mF 16V non-polarised (bipolar) PC electrolytic 1 220nF 100V metallised polyester “greencap” 2 100nF 50V monolithic ceramic 1 330pF 50V ceramic disc Resistors (0.25W 1%) 1 1MW 2 22kW   2 1kW 1 10W 1 1W 1W 5% Power Supply 1 PC board coded 01201033, 90mm x 54.5mm 4 1N5404 3A power diodes (D1-D4) 2 4700mF 35V (or 50V) PC electrolytic capacitors 1 18V+18V 80VA torodial mains transformer (Altronics M 5118) 2 3-way 5mm/5.08mm terminal blocks 4 M3 x 10mm tapped spacers 4 M3 x 6mm pan head screws Parts for optional preamp supply section 2 15V 1W zener diodes (ZD1,ZD2) 2 100mF 16V PC electrolytic capacitors 2 560W 1W 5% resistors 1 3-way 5mm/5.08mm terminal block December 2004  21