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Build the CHAMP: a handy audio amplifier based on a single IC What’s the same size as a 9V battery, more useful than a deck of cards and uses only a half a dozen components? The CHAMP – a Cheap & Handy AMPlifier that will deliver 0.5W into eight ohms from a 9V supply for those little audio projects. By DARREN YATES Well, this is about as small as you can get with standard sized components – a single channel audio power amplifier than will produce 0.5W into eight ohms with a 9V battery and with variable internal gain from 20 to 200. It can also drive a 4Ω loudspeaker at lower power levels and with increased distortion (see Figs.2 & 3). We don’t claim this to be an original design but it is tiny! You’ll be surprised by the number of projects that use an audio amplifier of some kind. Most 46  Silicon Chip of the time, they are only low power modules hanging off the end of some noise-maker but it seems a pity to have to re-engineer the wheel every time an amplifier is needed. This module uses the well-known LM386 audio amplifi­er IC. It’s small and most importantly, cheap. It will fit into the tightest of spaces – you could even glue it to the back of a 9V battery if you wanted to! Mind you, running it from a small 9V battery would not be an economical prop- osition – better to run it from a 6V lantern battery, four 1.5V AA cells or a 9V DC plugpack. Circuit diagram The circuit diagram for the CHAMP is shown in Fig.1. As you can see, there’s not much to it. The power supply can be anything from 4 to 12VDC and connects straight to pins 6 and 4. The input signal is fed to a 10kΩ trimpot and then straight to pin 3 of the IC. The 10µF capacitor connected to pin 7 provides supply decoupling and reduces the effect of hum on the power supply if it comes from a 9V DC plugpack. The gain of the amplifier can be changed from 20 to 200 by changing the value of the 1kΩ resistor at pin 1. Reducing the resistor increases the gain. As presented, the circuit gain is 41 or 32dB. By replacing the resistor with a wire link, the gain becomes 200. A 220µF capacitor couples the out- Fig.1: the circuit is based on IC1, an LM386 audio amplifier IC. The gain of the amplifier is controlled by the 1kΩ resistor on pin 1. +4-12V 10 16VW INPUT 6 VOLUME VR1 10k LOG 2 1k 8 3 1 IC1 LM386 4 100 16VW 7 220 16VW 5 0.1 8W 10 16VW 10  PARTS LIST 1 PC board, code 01102941, 46 x 26mm 6 PC pins 1 10kΩ trimpot Semiconductors 1 LM386 low-power audio amplifier IC (IC1) Capacitors 1 220µF 16VW electrolytic 2 10µF 16VW electrolytics 1 0.1µF 63VW MKT polyester THE "CHAMP" Resistors (0.25W, 1%) 1 1kΩ 1 10Ω Miscellaneous Tinned copper wire, speaker cable, solder. Fig.2: device dissipation vs output power for a 4Ω load. The three curves shown are for 6V, 9V & 12V supply rails. Note that the maximum power output into a 4Ω load is about 0.3mW at 3% THD. Fig.3: device dissipation vs output power for an 8Ω load. In this case, four curves are shown, corresponding to supply voltages of 6V, 9V, 12V & 16V. The maximum power output is about 0.7W at 3% THD. put signal from pin 5 to an 8Ω speaker. You can also use a 4Ω ohm speaker if the supply voltage is 6V or less. A Zobel network consisting of a 0.1µF capacitor and 10Ω 0.25W resistor prevent high-frequency oscilla­tion from occurring due to long speaker leads. Finally, a 100µF 16VW capacitor provides supply decoupling and aids in the operation from a battery supply. Power output will vary depending on the supply voltage and whether a 4Ω or 8Ω loudspeaker is used. The graphs of Fig.2 and Fig.3 show what can be expected with 4Ω and 8Ω speakers at vari­ous supply voltages. 1k VR1 1 SPEAKER + 10uF 10  100uF 220uF 0.1 GND We designed a teensy weensy little PC board for this pro­ject but although it’s small, it’s a snack to put together. The board measures just 46 x 26mm and is coded 01102941. Apart from the LM386 IC, it has two resistors, a trimpot and five capacitors on the board. +4-12V 10uF IC1 LM386 INPUT Construction SPEAKER GND Fig.4 (left): take care to ensure that the IC & electrolytic capacitors are all installed the right way around during the PC board assembly. The power supply to the board can be anywhere in the range from 4-12V DC. Fig.5 at right is a full-size etching pattern for the PC board. Check the board carefully for any defects in the copper pattern such as shorted or broken tracks. If there are any, fix them before proceeding further. Begin the assembly by installing six PC pins at the external wiring points, followed by the two resistors, then the capacitors, trimpot VR1 and the IC. Be sure to install all polarised parts the right way around – ie, the IC and electrolytic capacitors. Testing Connect a 9V power supply to the amplifier module, with your multi­ meter (switch­ed to the 200mA range) in series with one of the leads. Do not connect a loudspeaker at this stage. With no input signal, you should get a quiescent current of about 8-10mA. Any more than this and you should switch off immediately and check the PC board against the overlay diagram to see if you have made any errors. Once everything appears to be OK, connect a loudspeaker and do a “blurt” test. You do this by winding the trimpot anti-clockwise and then putting your finger on the input. This injects a hum/hash signal into the amplifier which is heard as a “blurt” from the speaker. If it blurts, it’s working. Finally, the installation of the CHAMP is basically left up to you. Make sure you keep it away from mains transformers or anywhere large SC amounts of hum are present. February 1994  47