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Looking for a lowcost audio power module that’s easy to assemble? This compact module will deliver 25W RMS into an 8-ohm load & can be powered from single or dual supply rails. By DARREN YATES A Low-Cost 25W Amplifier Module A S POWER amplifier modules go, this unit may not rank at the top for raw power but you’ll be hardpressed to find a sim­ pler or more versatile circuit. It’s 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 25W RMS into 8 ohms and 20W RMS into 4 ohms. What’s more, the specifications are quite impressive for such a barebones circuit. With a signal-to-noise (S/N) ratio of 110dB and a distortion figure of just 0.025% for 1kHz at 20W, it could well be used as the basis for a hifi stereo amplifier. The frequency response extends from 14Hz to 32  Silicon Chip beyond 100kHz when meas­ured at 1W RMS. The module is also easy to construct and no setting-up adjustments are necessary. And, as mentioned in the introduction, it can be powered from either single or dual supply rails (you can build either version on the same board). The supply voltage can range from 20V to 50V for a single supply rail, or from ±10V to ±25V for dual supply rails. Depending on the supply voltage, the output power ranges from 4W into 8 ohms (20V supply) to 25W into 8 ohms (50V supply). To guard against device failure, the LM1875 includes inter­nal short circuit protection. This protects the device if the output is shorted to ground via either an AC or a DC path. It also has current limiting to 4A to prevent damage when driving reactive loads, which makes it a highly robust module that can handle more than its share of knocks. Because so much power has to be dissipated by such as small package, the LM1875 also has in-built thermal protection. This shuts the device down if there is excess heat build-up in the chip itself (in excess of about 175°C). Other specifications of the device include a supply rejec­tion figure of -94dB, an open loop gain of typically 90dB and a power bandwidth of 70kHz. If you’d like more information on the LM1875 audio amplifier, refer to the data article elsewhere in this issue. Because there are two possible power supply arrangements, we’re presenting two circuit diagrams – see Figs.1 & 2. Both circuits have low component counts and differ only in a few minor details. 10 35VW Dual supply version The dual supply version (see Fig.2) uses the same feedback and Zobel network components as the single supply version. Apart from the power supply itself, the main difference between the two circuits is the input DC biasing arrangement. Pin 1 of IC1 is connected to the 0V rail via a single 22kΩ resistor. In addition, the 2200µF output coupling capacitor is omitted, since pin 4 of IC1 normally sits within ±50mV of 0V, with no signal present. The power supply uses a mains transformer with a centre-tapped 35V secondary. The resulting outputs from bridge rectifier BR1 are filtered using two 2200µF capacitors to give nominal ±25V supply rails and these go to pins 5 and 3 of IC1 via 2A fuses. 1 1k INPUT 1 1M 2 2200 63VW 5 4 IC1 LM1875 +25V 3 1 180k 0.22 4- 8  10k 22 63VW S1 A F1 1A BR1 PW04 .01 250VAC 17.5V +50V 240VAC 2200 63VW 17.5V N E CASE 25W AUDIO POWER MODULE SINGLE SUPPLY Fig.1: the single supply version of the 25W Amplifier Module. IC1 drives the loudspeaker via a 2200µF capacitor. +25V 220 35VW 0.1 1 1k INPUT 1M 1 22k 2 5 4 IC1 LM1875 1 3 180k 4- 8  0.22 10k -25V 22 63VW S1 A .01 250VAC 0.1 220 35VW F1 1A F3 2A F2 2A BR1 PW04 17.5V +25V 240VAC Construction Construction of the amplifier is quite straightforward – see Figs.3 & 4. All you have to do is follow the diagram for the version you require. In either case, start by checking the PC board carefully for any defects by comparing it with the published pattern. This done, begin the board assembly by soldering in the wire links and by installing PC stakes at the external wiring points. The resistors and capacitors can now be installed, 220 63VW 0.1 22k +50V 22k Single supply version Fig.1 shows the circuit for the single supply version. As shown, the input signal is coupled via a 1kΩ stopper resistor and a 1µF capacitor to the non-inverting input of IC1 at pin 1. This input is biased to ½Vcc (ie, half the supply rail) via the three 22kΩ resistors and the associated 10µF capacitor. The closed loop gain of the amplifier is set to 19 by the 180kΩ and 10kΩ feedback resistors on pin 2 and follows the stan­dard non-inverting amplifier feedback rules (ie, G = 180/10 + 1 = 19). The 2.2µF capacitor and the 10kΩ resistor set the lower 3dB frequency point to 7Hz. The output from the amplifier appears at pin 4 of IC1 and drives the loudspeaker via a 2200µF coupling capacitor (to prev­ent DC from flowing in the speaker coil). Also connected to the output is a series 1Ω resistor and a 0.22µF capacitor. These components form a Zobel network and this provides high-frequency stability when driving capacitive loads. Power for this circuit is derived from a mains transformer with a 35V secondary winding (either a single 35V winding or two 17.5V windings connected in series). The resulting AC voltage drives bridge rectifier BR1 (PW04), the output of which is then filtered with a 2200µF capacitor to give a nominal +50V DC rail. Further on-board supply decoupling is provided by a 220µF 63VW capacitor, while a 2A fuse protects against any external shorts to ground. Finally, a .01µF capacitor is connected across the power on/off switch (S1) to minimise the switch-off “thump”. F2 2A 22k +25V 17.5V 2200 35VW N E CASE 2200 35VW -25V 25W AUDIO POWER MODULE DUAL SUPPLY Fig.2: the dual supply version of the 25W Amplifier Module. December 1993  33 IC1 LM1875 22k 180k 1uF 10k 180k 2200uF 1uF 22uF 10uF 1M 1 1M 10k 22k 0.22 0.22 GND 220uF IN 1k F2 0.1 1k 220uF F2 0.1 IN 0.1 GND 1 22k 22k 10uF IC1 LM1875 220uF F3 GND V+ POWER SUPPLY GND V- SPEAKER Fig.3: parts layout for the single supply version. GND POWER SUPPLY V+ GND SPEAKER Fig.4: parts layout for the dual supply version. Fig.5 (above): this diagram shows how the LM1875 audio amplifier IC is insulated from the heatsink using a mica washer & insulating bush. Smear all mating surfaces with heatsink compound before bolting the assembly together, then use your DMM to confirm that the device is correctly isolated. Fig.6 at right shows the full-size PC artwork. This is the dual supply version of the 25W Amplifier Module. Check the supply rail voltages, the quiescent current & the DC offset voltage across the output terminals before connecting a loudspeaker – see text. 34  Silicon Chip followed by the fuse clips. Make sure that the electrolytic capacitors are correctly oriented, otherwise they may be destroyed when power is applied. The fuse clips must also be correctly oriented, with the retaining tabs towards the outside. Once these parts are in, install the LM1875 and then fit 15mm spacers to the corner mounting positions of the board. The board and the heatsink can then be placed on a flat surface and the mounting hole marked out for the IC. Drill this hole to 3mm and carefully remove any metal swarf using an oversize drill to ensure a perfectly smooth surface. The IC is now bolted to the heatsink using a TO-220 insu­ lating kit (ie, a mica washer and insulating bush). Fig.5 shows the assembly details. Smear all mating surfaces with heatsink compound before bolting the assembly together, then use your multimeter to confirm that the metal tab of the IC is indeed electrically isolated from the heatsink. Note that no provision has been made for the power supply components on the PC board. This has been done deliberately to avoid potential hum problems due to circulating earth currents when using two modules in a stereo amplifier. If you wish to run two modules, you should use a common power supply and ideally the transformer should have a rating of about 80VA, although for most applications a 60VA unit will suffice. The transformer used to test the prototype was a 60VA unit with two 17.5V secondary windings from Dick Smith Electronics (Cat. M-6676). A 30V 60VA power transformer could also be used, although this will result in reduced power output. Do not use a 30V 1A transformer as its rating will be insufficient. Testing Before applying power, check that all parts are correctly located and oriented. This done, install the fuse(s) and connect the power supply leads with your multimeter (switched to Amps) in series with the positive rail. Do not connect the loudspeaker or an audio input signal at this stage. Now switch on and check that the current settles down to 50-70mA following a brief surge to charge the main filter capaci­tor(s). Note that you must have the heatsink fitted, otherwise the thermal overload protection circuit may cut in and switch the device off. Check the supply rail voltages – they should be within 10% of the values shown on the circuit. If the quiescent current is correct, check the DC offset 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 SC signal applied for final testing. PARTS LIST Single Supply Version 1 PC board, code 01112931, 87 x 64mm 6 PC stakes 4 15mm x 3mm tapped spacers 1 TO-220 heatsink mounting kit (ie, mica washer & insulating bush) 4 15mm x 3mm machine screws 2 2AG fuse clips 1 1 amp 2AG fuse 1 heatsink (Altronics Cat.H-0580 or equivalent). Semiconductors 1 LM1875T 20W audio amplifier (IC1) Capacitors 1 2200µF 35VW electrolytic 1 220µF 63VW electrolytic 1 22µF 63VW electrolytic 1 10µF 35VW electrolytic 1 1µF 63VW electrolytic 1 0.22µF 63VW MKT polyester 1 0.1µF 63VW MKT polyester Resistors (0.25W, 1%) 1 1MΩ 1 10kΩ 1 180kΩ 1 1kΩ 3 22kΩ 1 1Ω PARTS LIST Dual Supply Version 1 PC board, code 01112931, 87 x 64mm 7 PC stakes 4 15mm x 3mm tapped spacers 1 TO-220 heatsink mounting kit (ie, mica washer & insulating bush) 4 15mm x 3mm machine screws 4 2AG fuse clips 2 1 amp 2AG fuse 1 heatsink (Altronics Cat.H-0580 or equivalent). Semiconductors 1 LM1875T 20W audio amplifier (IC1) Capacitors 2 220µF 63VW electrolytic 1 22µF 63VW electrolytic 1 1µF 63VW bipolar electrolytic 1 0.22µF 63VW MKT polyester 2 0.1µF 63VW MKT polyester Resistors (0.25W, 1%) 1 1MΩ 1 10kΩ 1 180kΩ 1 1kΩ 1 22kΩ 1 1Ω Power supply parts 1 35V 60VA power transformer (DSE Cat. M6676 or equivalent - see text) 1 PW04 bridge rectifier (BR1) 1 mains switch (S1) 1 2200µF 63VW electrolytic capacitor 1 .01µF 250VAC polyester capacitor Power supply parts 1 35V centre-tapped 60VA power transformer (DSE Cat. M6676 or equivalent - see text) 1 PW04 bridge rectifier (BR1) 1 mains switch (S1) 2 2200µF 63VW electrolytic capacitors 1 .01µF 250VAC polyester capacitor Miscellaneous Tinned copper wire, solder, screws, nuts & washers. Miscellaneous Tinned copper wire, solder, screws, nuts & washers. RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 1 3 1 1 1 Value 1MΩ 180kΩ 22kΩ 10kΩ 1kΩ 1Ω 4-Band Code (1%) brown black green brown brown grey yellow brown red red orange brown brown black orange brown brown black red brown brown black gold gold 5-Band Code (1%) brown black black yellow brown brown grey black orange brown red red black red brown brown black black red brown brown black black brown brown brown black black silver brown December 1993  35