The simplest form of radio set up, uses the speaker in the radio. This can be improved for emergency use by adding headphones. But what happens when you need to add a person as a logger. You can use a Y cable for the headphones, but what if the two people have different levels of hearing. It would be nice to have separate volume controls for each person.
Now along comes someone who needs to hear a message that is coming in on the radio. You can hand over one set of headphones or pull out the headphone cable from the radio so everyone can hear, but wouldn't it be nice to have an amplified speaker with a volume control, so you can just turn up the volume for this person and then turn it down again.
What if you want to record the radio audio or add a speaker in another room. Adding an audio distribution amp to the radio output, will provide multiple independent audio outputs. Now add a couple simple amplifiers and you have a headphone amp for each user, an amplified speaker and if the distribution amp has 600 ohm outputs, you can extend this over the internal phone cables and add a speaker at the other end using a 600 ohm to 8 ohm transformer with the speaker, or for more volume, add an amplified speaker.
These are simple circuits to build and there are lots of examples on the internet or in the ARRL handbook. The idea for this project is to bring them all together into one compact unit.
There is an extension to this project idea, which would allow more than one microphone to connect to the radio. This way the logger/backup operator can talk when they need to, no handing off of the microphone, changing headsets for the boom mic. There are some issues with feeding a portion of the transmit audio from the mic into the headsets, so both operators can hear. Now you can split the operators up a bit if that makes it easier to work.
There are many low-cost headsets for cell phones and cordless phones available on the market. The one on the right is an example.
In theory, these headsets can be wired for use on amateur radios, by ensuring the microphone's bias voltage is correct and changing the connectors.
EMRG is interested in finding anyone who has already tried this. If you are looking for something to do, here is an interesting project!