EMERGENCY TIPS: REMOTE SPEAKER

When you are in a noisy environment it is difficult to hear your radio. This may be due to the small speaker built-in to the radio or perhaps the radio is mounted so that the sound coming from the speaker is baffled or constricted or pointed away from your ears. If you turn up the volume you disturb others. You can fumble around attempting to plug a pair of headphones in the rear of the unit but that may be impossible if the radio is mounted in a cabinet or on a shelf.

So what is the answer? Well, how about a small communications type speaker mounted at ear level? Radio Shack makes a small 2 x 2 x 2 inch cube speaker that has an 1/8 inch phone plug on the end of a long connector cord. The part number is 19-318A. By itself this speaker has a lot going for it:

• It is small 2 x 2 x 2 inches
• It is lightweight
• It has a rugged case
• It has a limited frequency range that peaks in the midrange for excellent voice projection
• It will handle several watts of audio power
• It can be mounted just about anywhere - even comes with mounting hardware
• It can be customized to make it more useful

Speaking of usefulness, it would be nice to add an headphone jack to this speaker so that you could use either the speaker or a headphone.  This is easily done.  A headphone provides more privacy in a noisy environment when you need to concentrate on hearing a critical message or when you need to minimize or eliminate ambient room noise.

The diagrams below identify several methods of adding a headphone jack to the cube speaker. Note that to accomplish these modifications,  there are several things you need to be aware of:

• The small screws that hold the front of the speaker to the case are made from soft metal and the heads are easily stripped. Exercise caution when removing and replacing them. Voice of experience.
• The cable used in the cube speaker is shielded with a single center conductor. Use care when working with this flexible wire.
• Use a 1/8 inch mono or stereo jack for the headphones. There is not enough room inside the small cube to mount a 1/4 inch phone jack. Use a size adaptor (1/4 to 1/8) or stereo to mono adaptor to properly match the headphones you intend to use.
• Do not attempt to use a variable resistor volume control in place of the fixed resistor.
• Exercise caution when soldering/unsoldering the wires to the loudspeaker terminals and headphone jack.
• Use small diameter flexible wire to make connections. Do not use stiff wire as it will break the solder terminals on the loudspeaker and earphone jack.
• Use a 1/4 or 1/2 watt resistor for designs that use a resistor. The goal is to have the headphone sound as loud as the loudspeaker when plugged in.
• Make sure you position the earphone jack so that it does not obstruct the speaker from being reassembled or so that the earphone plug will short out against the loudspeaker. Mount the jack on the side of the case rather than the rear - makes it easier to get to. Cover any errors with electricians tape or black plastic resin. :^)

Simple Headphone Jack

Figure 1 shows the simplest way to connect a headphone jack. You will need the cube speaker (or any other small loudspeaker unit) plus one 1/8 inch closed circuit jack.

When the earphone is plugged in, it disconnects the loudspeaker. This is easy to wire up and works just fine - except when you plug in the headphones the sound level will be way too loud in the headphones as they are much more efficient sound reproducers than ye olde small loudspeaker. Be prepared to reach for the volume knob on the radio.

Sound Level Adjusted Headphone Jack

Figure 2 shows a better way to wire up a headphone jack. This method uses the same parts as Figure 1 but includes a resistor to drop the sound level coming out of the headphone jack.

Note that this method does not eliminate the sound coming out of the loudspeaker but it drops it to a very low level. The exact value of the resistor must be determined experimentally. A value should be chosen based on the efficiency of the earphones you will be using. Try to find a value so that the sound level heard in the headphones has the same relative loudness as the sound coming from the loudspeaker. This way when you are listening comfortably to the loudspeaker and have to plug in the headphones, you will not have your eardrums assaulted by too high a level and you won't have to reach for the volume control. Try a 180 or 220 ohm resistor to start.

The Ultimate

Figure 3 is the final design that uses both a headphone jack and a single pole, single throw switch. Yes there is room for both the switch and jack inside the small cube. You can use a simple jack rather than the more expensive closed circuit jack too!

Find a small toggle switch that mounts with a single round hole. The slide switches are difficult to work with. Mount the switch on the same side as the headphone jack or on the rear of the cube so it will be out of the way. Wire the jack and switch before attaching them to the cube as it will be much easier to solder outside the box.

Again a dropping resistor will equalize the relative levels coming from the headphones and loudspeaker. This design eliminates all sound coming from the loudspeaker. Some people don't trust the closed circuit headphone jacks - this design eliminates possible switching failures.

ENJOY

These designs are very elementary but you need to be able to solder small wires, to understand what the symbols in the diagram are, to be able to accurately drill holes and to disassemble and assemble parts in order to execute a successful project. Enlist the aid of a fellow ham if needed. You will find this external speaker system (headphone enabled) very useful for when the children are sleeping, as a sound booster for your HT, for use in mobile installations or during emergency situations. Make an extra unit and pack it in your 72 Hour Kit. You will be amazed at how useful this little speaker can be after modifications have been done.

73,
Chip -WB7TFD
Tom - KC7OZJ