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Looking for an easy-to-build audio power amplifier with more power than the 25W module in the December 1993 issue? This single-chip power module will provide 50W RMS continuous into 8 ohms with extremely low distortion. It’s a sign of the times and how far electronics has come when you can buy a 50W audio power amplifier on a single chip which has better specifications than many of the discrete modules currently available. This 50W amplifier module is based around the newly-released LM3876T from National Semiconductor. Not only can it deliver 50W RMS continuous into 8Ω loads but it has on-board protection and an input mute function. See the data article on this device elsewhere in this issue for the full details. This amplifier module is quite robust and requires no setting up – all you do is build it then use it. It will also run on a lower supply voltage, with no changes to the circuit required. Build this 50W audio amplifier module By DARREN YATES 22  Silicon Chip A glance at the specification panel in this article will show that this amplifier module has very respectable performance, better in fact than the Twin 50W power amplifier module pub­lished in the February 1992 issue of SILICON CHIP. In particular, note the very low distortion, excellent signal-to-noise ratio and very high damping factor. Circuit details Looking at the circuit diagram in Fig.1, you could be forgiven for thinking that the LM3876T is just a big power op amp – and that’s really all it is, although it has a lot of enhance­ments in the way of internal protection circuitry. A handful of passive components and a power supply complete the circuit. The input signal is connected to the non-inverting input at pin 10 via an RC network consisting of a series 1kΩ resistor and a 220pF shunt capacitor. This network is an RF attenuator to prevent pick-up of radio interference. The voltage gain of the module is set to 19 by a negative feedback network consisting of an 18kΩ and 1kΩ resistive divider and a 22µF capacitor. The 1kΩ resistor and 22µF capacitor togeth­er set the low frequency -3dB point to about 7Hz. Also connected to the output at pin 3 is a fairly savage Zobel network comprising a 2.7Ω resistor and 0.1µF capacitor. This RC network and the associated RL network consisting of a 10Ω resistor in parallel with a 0.7µH inductor ensure that variations in the load impedance at supersonic frequencies do not cause instability. F1 2A 220 63VW 1k 220pF INPUT 22k 10 1 9 IC1 LM3876 Construction All of the components for the 50W module except the heat­sink are installed on a small PC board measuring 83 x 58mm and coded 01103941. Before you begin any soldering, check the board thoroughly for any shorts or 2. 7  1W 22 16VW 8W 0.1 MUTE S2 27k F2 2A 220 63VW 22 63VW -35V 0.1 .01 250VAC T1 ALTRONICS M-3030 S1 A BR1 PW04 25V +35V 240VAC 25V 2200 63VW N GND E 2200 63VW -35V L1 : 10T, 0.4mm DIA ENCU WOUND ON 10  1W RESISTOR 1 11 50W AUDIO AMPLIFIER MODULE Fig.1: the module is based on IC1, an LM3876T audio amplifier IC with comprehensive internal protection circuitry. No setting-up adjustments are necessary. IC1 LM3876 1 22uF F2 220pF 220uF 1k 1k 0.1 TO S2 -35V GND +35V 18k 220uF 0.1 The power supply uses a 50V centre-tapped transformer feeding a bridge rectifier and two 2200µF 63VW electrolytic capacitors. This results in balanced supply rails of around ±35V, although the exact voltage will depend on the mains voltage and transformer regulation. To obtain the quoted power output of 50 watts, you will need a transformer rated at 80VA or more. We suggest the 80VA toroidal type sold by Altronics (Cat. M-3030). A cheaper alterna­tive would be the 44V centre-tapped 66VA transformer sold by Jaycar Electronics (Cat. MM-2010). This would reduce the module’s maximum power output to about 40 watts. 10  1W 4 1k 27k Power supply L1 0.7uH 18k Muting An optional feature of this module is the mute function at pin 8. We’ve shown pin 8 connected via switch S2 and a 27kΩ resistor to the negative supply rail. With the switch closed, the amplifier operates normally but with the switch open the audio signal is attenuated by 110dB (typical) which is near enough to completely off. The 22µF capacitor also connected to pin 8 provides a slow turnon feature. If you don’t want to use this feature, you can replace switch S2 with a wire link. The proto­type board, shown in the photo, was wired this way. 0.1 3 7 8 +35V 22uF 10 / L1 2. 7 0.1 F1 O/P 22k GND I/P GND Fig.2: the parts layout on the PC board. Make sure that all polarised components are correctly oriented. breaks in the copper tracks. These should be repaired with a small artwork knife or a touch of the soldering iron where appropriate. When you’re sure that everything is correct, you can install the wire links, followed by the resistors and capacitors. Make sure that you install the electrolytic capaci­tors correctly. March 1994  23 Fig.3 (above): the LM3876 IC is insulated from the heatsink using a mica washer & insulating bush (note: the pins on the IC are cranked differently to those shown here). Smear all mating surfaces with heatsink compound before bolting the assembly together. Fig.4 at right shows the PC artwork. L1 consists of 10 turns of 0.4mm enamelled copper wire wound onto a 10Ω 1W resistor and soldered at both ends. To wind it, scrape the enamel off the start of the copper wire and solder it to one end of the resistor. This done, neatly wind 10 turns onto the resistor body, scrape the enamel off the end of the wire, and solder it to the other end of the resistor. You then install the resistor-cum-inductor as you would a normal resistor. Following that, you can continue by installing the seven PC stakes and the PC mounting 2AG fuse clips. Note that these clips have little lugs on one end which stop the fuse from moving. If you install the clips the wrong way around you cannot fit the fuses. Finally, you can install the LM­ 3876T IC. Make sure that the tab of the device is lined up with the back edge of the PC board so that it can be properly mounted onto the heatsink. Once installed, you can add the four 15mm spacers and then line up the heatsink against the IC so that you can drill the hole for the mounting screw. After drilling, use a standard TO-3P mounting kit to mount the device to Performance measurements Output power .......................... 50W into 8 ohms, 55W into 4 ohms Frequency response ............... 15Hz - 110kHz ±1dB Input sensitivity ....................... 1V RMS (for clip point onto 8 ohms) Harmonic distortion ................ < .06% from 20Hz to 20kHz; typically <.002% Signal-to-noise ratio ............... 106dB unweighted (20Hz-20kHz); -114dB A-weighted Protection ............................... 2A fuses plus SPiKe (TM) Damping factor ....................... >150 (for 8-ohm loads) Stability ................................... unconditional the heatsink (see Fig.3) and make sure that the heatsink is electrically isolated from the device (use your multimeter switched to a high “Ohms” range). The heatsink used needs to be sub­stantial and should be rated at about 1.5°C/W or less. A suitable model is Altronics Cat. H-0580. If you use a smaller heatsink, the IC will run hotter and its internal protection circuitry will reduce the maximum avail­able power output accordingly. As presented in this article, the heatsink is attached to the PC board via the leads of the power IC. In practice, both the heatsink and the PC board should be attached to a suitable chas­ sis, together with the power supply. Testing To test the unit, first connect up the power supply and apply power. The supply rails should be around ±37V (no load condition). Now check the quiescent current. This can be done in one of two ways. The first is to remove one fuse (while the power is off) and connect your multimeter, switched to an “Amps” range) across the fuse clips. With no input signal and no load, the quiescent current should typically be around 30mA but may range up to RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 1 1 1 1 1 24  Silicon Chip Value 27kΩ 22kΩ 18kΩ 1kΩ 10Ω 2.7Ω 4-Band Code (1%) red violet orange brown red red orange brown brown grey orange brown brown black red brown brown black black brown red violet gold brown 5-Band Code (1%) red violet black red brown red red black red brown brown grey black red brown brown black black brown brown brown black black gold brown red violet black silver brown PARTS LIST Capacitors 2 220µF 63VW electrolytic 1 22µF 16VW electrolytic 1 22µF 63VW electrolytic 3 0.1µF 63VW MKT polyester Resistors (0.25W, 1%) 1 27kΩ 1 1kΩ 1 22kΩ 1 10Ω 1W 1 18kΩ 1 2.7Ω 1W Power supply 1 25V + 25V 80VA mains transformer (Altronics Cat. M-3030 or equivalent) 1 100V 6A bridge rectifier 2 2200µF 50VW or 63VW electrolytic capacitors 70mA. Alternatively, you can connect a 100Ω 1W resistor across the fuse clips and measure the voltage across it. For a quiescent current of 30mA, the voltage across the 100Ω resistor should be 3V DC. The DC voltage at the output should be within ±15mV of 0V DC. Next, connect suitably rated speaker and check that you get an output. If you touch the input PC pin on the PC board you should get an “audible” blurt from the loudspeaker. If you don’t, check that the mute circuit is disabled. To disable the mute facility, switch S2 must be closed or replaced with a wire link. If the circuit isn’t working, check all audio paths from the input through to the output for continuity. You should also make sure that the PC stakes are well soldered into position. Some brands don’t take solder easily and SC can cause dry joints. SILICON CHIP FLOPPY INDEX WITH FILE VIEWER Now available: the complete index to all SILICON CHIP articles since the first issue in November 1987. Now you can search through all the articles ever published for the one you want. Whether it is a feature article, a project, a circuit notebook item, or a major product review, it doesn’t matter; they are all there for you to browse through. The index comes as an ASCII file on a 3.5-inch or 5.25-inch floppy disc to suit PC-compatible computers and you can use a word processor or our special file viewer to search for keywords. Now with handy file viewer: the Silicon Chip Floppy Index now comes with a file viewer which makes searching for that article or project so much easier. You can look at the index line by line or page by page for quick browsing, or you can make use of the search function. Simply enter in a keyword(s) and the index will quickly find all the relevant entries. All commands are listed on the screen, so you’ll always know what to do next. Note: requires CGA, EGA or VGA graphics card, IBM-compatible PC, MSDOS 3.3 and above. Disc size:   ❏ 3.5-inch disc   ❏ 5.25-inch disc ❏ ❏ ❏ ❏ ❏ ❏ ❏ Floppy Index (incl. file viewer): $A7 + p&p Notes & Errata (incl. file viewer): $A7 + p&p Bytefree.bas /obj / exe (Computer Bits, May 1994): $A7 + p&p Alphanumeric LCD Demo Board Software (May 1993): $A7 + p&p Stepper Motor Controller Software (January 1994): $A7 + p&p Printer Status Indicator Software (January 1994): $A7 + p&p Switchers Made Simple – Design Software (March 1994): $A12 + p&p Note: Aust, NZ & PNG please add $A3 (elsewhere $A5) for p&p with your order Enclosed is my cheque/money order for $­__________ or please debit my ❏ Bankcard   ❏ Visa Card   ❏ Master Card Card No. Signature­­­­­­­­­­­­_________________________ Card expiry date______/______ Name _____________________________________________________ PLEASE PRINT Street _____________________________________________________ Suburb/town __________________________ Postcode______________ Send your order to: SILICON CHIP, PO Box 139, Collaroy, NSW 2097; or fax your order to (02) 979 6503; or ring (02) 979 5644 and quote your credit card number (Bankcard, Visacard or Mastercard). ✂ 1 PC board, code 01103941, 84 x 58mm 4 10mm x 3mm machined screws 4 15mm x 3mm tapped spacers 1 125 x 75mm heatsink 1.5°C/W (Altronics Cat H-0580 or equivalent) 1 LM3876T 40W audio amplifier (IC1) 4 M205 PC-mounting fuse clips 2 2A M205 fuses 7 PC pins 1 30cm length of 0.4mm-dia. enamelled copper wire March 1994  25