Simple Multiuse Amplified Speaker

J. Frank Brumbaugh KB4ZGC R (X Box 30 - c/o Defendini Salinas PR 00751-0030

So you've built that nice kit transceiver, and you want to use it in your shack. You've got two problems right away First, it was probably designed to run off batteries, and second. it probabh doesn't have enough audio power to drive a speaker.

Many kits are designed that way to save mone\ and to make them portable. The power supply and audio amplified circuits add weight raise costs and create current drain that makes the units difficult to use in the field.

For ever\da\ use, main hams add a regulated power supply and use their field units at home* The power supply doesn't solve the audio problem, however If an audio amplifier and speaker is added to an existing receiver or transceiver it is not available for any other use, it cannot also be used on a different receiver, nor as a test speaker when checking designs of audio circuitry.

What is needed is a compact, lightweight, self-contained amplifier and speaker with volume control, and a headphone jack that can mule the speaker. It should be capable of producing normal room volume while requiring minimum current with no or minimal distortion. It should he capable of being powered h\ either an internal 9-Volt batten or an external 12-Volt battery or 13,8-Volt power supply. It should use few and inexpensive parts, and be easily constructed and used by any hanv The unit described here meets all these requirements.

The Circuit

Fig. 1 is the schematic diagram of this amplified speaker. S) is a SPOT toggle switch with a center-off position (on-none-on). In one position it connects the internal 9-Volt battery to power the amplifier. In the opposite position it connects the amplifier to J3, to which is connected an external storage batter\ or regulated power supply. When the unit is not in use. the switch is centered,

U1 is an LM386 audio amplifier chip in an 8-pin DIP package, wired to produce maximum gain at minimum current. In operation, low level audio is fed through .11 (audio in) through isolation capacitor C I to the uip of the volume control Rl, a 10k audio-taper potentiometer. Audio taken from the wiper of Ri passes through isolation capacitor C2 and is applied to one input of amplifier Ul, This input is lightly bypassed by C3 to prevent UI from picking up stray RF, including that from an\ local AM broadcast station, and serves to stabilize the amplifier Amplified audio is taken from pin 5 of Ul through coupling capacitor C7 to closed circuit jack J2 < PHONES f and then to the speaker, LSI. Mute the speaker by plugging headphones into J2,

Construction

Ul and associated components should be mounted on a small piece of perf board or one of the general purpose printed circuit boards available at Radio Shack™. C3 should be mounted with short leads between pins 2 and 3 of Ul. Mount C4 at the end of UL where pins 1 and 8 are located. Mount C6 with the shortest lead possible to pin 6 of Ul. You may uish to use an 8-pin DIP socket for Ul, but it is not necessary.

This unit should be constructed in the smallest case that will hold your components. The speaker should he small, yet have a large magnet and be rated for 1/2 watt or more. Recommended is a 1-Watt, 8-Ohm speaker intended for cellular phone use. It is L8 inches in diameter. Magnet diameter is 1.35 inches. It is available from All Electronics, P. O, Box 567, Van Nuys CA 91408-0567. The catalog number is SK-175, However, you can use any 4 or 8-Ohtrl speaker you have in the junk box.

A small aluminum box will make the hesi enclosure, but you can use a plastic box, or make one from picces of printed circuit board material. Yon can drill a series of small holes in the circular area of the panel hehind which the speaker will be mounted. You could also cut a circular hole for the speaker and use a piece of screen wire or perforated aluminum as a grille over the hole to protect the speaker cone.

Mount LSI, Jl, J2„ SI andRl on the panel. Mount J3 on the rear of the enclosure, Put rubber feet on the bottom to prevent scratches.

Battery BTI can be mounted internally in a dtp, or you can use self-adhesive strips of hook-and-loop.

Operation

Connect a source of audio, such as the output from the headphone jack of your receiver, to Jl, Switch SI to INT

"Self-contained, lightweight—it uses almost no power, yet you get wonderful sound! At home or in the field, you'll be proud of this project."

to use the internal 9-Volt battery, or to EXT, if an external power source is connected to J3. Adjust R1 for a com-fonahle volume level from LSI. If there is loo high a level of audio at Jl, it should be reduced at its source. If you plug headphones into J2 the speaker LSI will be muted, and you will have to reduce the volume level with Rl for a comfortable listening level. Because this unit is essentially noise-free, any noise you hear will be coming from the source plugged into Jl, and will be amplified along with the desired audio.

When this unit is working, the LED D1 will be lit- When it's not being used, place SI in the center (OFF) position; the LED will go out.

Using the internal battery, or with an external 9-Volts applied to J3t and no audio input, the amplifier draws K mA. With Rl fully clockwise (maximum gain} you will hear no, or almost no, white noise from ihe speaker or in the headphones using the internal battery for power. Any noise heard with an external power supply will originate in the power supply itself under these conditions.

With the audio ieveJ set aI comfortable listening level, current drawn on CW or voice peaks can reach 50-75

mA. However, since this occurs only on peaks, the average current drain will be between one-third and one-half the peak current drawn on peaks while a signal is present. ^

Parts List

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