Many moons ago, at a large Field Day site, I realized that a single headphone jack just wasn’t sufficient. This set me off on a never ending pursuit of an ideal audio distribution box for amateur radio.
Initially this was a simple three 1/4″ mono phone jack box utilizing adapters to accommodate 1/8″ and stereo plugs. This was neat but the adapters kept getting misplaced and headphones and a speaker, to let the ‘guests’ hear’ just didn’t work at the same volume.
A potentiometer was added for each ‘channel’ which by trial and error ended up at 100 ohm / 2 watts. 100 ohms is the lowest practical value available and 2 watts held up well to ta radio’s audio output.
Finally the multi-jack / stereo / mono switch arrangement shown here was found to be ideal. I know, it is not real stereo but it does allow stereo plugs, such as on most ear buds etc. putting audio into both ears. The ‘mono’ position keeps a mono plug from shorting out the unused connection.
Amplifiers: I tried various amplifiers one / box and one / channel without much success. RF caused too much distortion in the audio and I was never able to get better results with anything ‘active’. sometimes KISS is the way to go.
The circuit shows one ‘channel’. 3 channels, in parallel, has been found to be most useful. This allows for two headsets and a speaker. This all fits in a standard electric box, plastic or metal (metal has been found to help with RF, and a standard blank switch plate. Of course you can put this into any box… my go-box unit is in a home-brew aluminum box to match the rest of the components.
To use the audio distribution box, plug it into the radio’s speaker jack, the headphone jack not have enough volume. Switch to stereo or mono, depending on the plug you are using and insert the plug in the correct jack. This was designed to have one output, speaker, headphone etc. per channel so each channel has its own volume level. There is no reason why you could not plug a 1/4″ and 1/8″ plug into one channel so long as they are both stereo or mono.
Audio Distribution Box Parts List and Cost:
To make 1 – 3 channel audio distribution box.
(Thanks to Rick, AJ4FW for putting this list together)
|Item||Description||Vendor / Part #||$/each||#||$/box|
|J1||¼” Stereo Phone||Parts Express P/N 090-320||$1.10||3||$3.30|
|J2||1/8” (3.5 mm) Stereo Phone||Mouser P/N 161-3402-E||$ .93||3||$2.79|
|S1||SPST Switch||Parts Express P/N 060-510||$ .78||3||$2.34|
|R1||100 Ohm 2W variable Resistor||Parts Express P/N 023-604||$3.78||3||$11.34|
|KN1||Knob, R1||Parts Express P/N 249-208||$ .63||3||$1.89|
|P1||Plug, Mono (Radio Audio)||Parts Express Part # 090-327||$ .98||1||$ .98|
|C1||Cap .1 uf||Mouser P/N 21RZ310-RC||$ .08||1||$ .08|
|B1||Box, Electrical Plastic(1)||Lowes P/N 70998||$ .38||1||$ .38|
|CV1||Cover, Blank||Lowes P/N 72440||$ .50||1||$ .50|
|Misc||Wire, Solder, and labor||Junk Box||$ .00||1||$ .00|
|Total Per Box ~||$23.60|
(1) a metal box may be substituted if needed for RF