[CQ-Contest] Mixer

[Tom W8JI]

Thank goodness, a non-volatile post.    :-)

> Does anyone have some design to home brew a multiple headphone ummm
> Mixer? Ya know plug into a headphone jack and be able to have multiple
> headphones plugged in.

I have a distribution system in my rooms. I use the speaker outputs of 
radios into a backwards connected 1-2 watt transistor output transformer, 
like one of these from Mouser:

 42TU120-RC 42TU048-RC


Almost any transistors-to-voice coil transformer will work.

The low-Z secondary goes to radios, and the ~40 ohm primary goes to the line 
that headsets bridge across. I load the line with 100 ohms or so, to swamp 
out load changes.

Unless someone has a really low impedance headset, like 16 ohms or less, or 
the headset has really non-standard sensitivity, it works great.

http://www.w8ji.com/images/New%20Contest%20Room/NEW%20shack%20web/run-position-best.jpg

http://www.w8ji.com/images/New%20Contest%20Room/NEW%20shack%20web/all-ops.jpg

The little metal boxes have two SP12T switches for selecting any audio line 
for each ear independently. None of the audio lines have a ground path, 
except of course on the radio side of the transformers. This is important!!!

While rated at less than 1 watt, these transformers do not saturate or clip 
well over 1 watt for almost all load impedances, so there is lots of volume.

To drive a speaker, I use a MFJ-616 bridged into a pair of switches than can 
sum up to two channels, with a fader control between those channels.

This lets any operator listen to any radio in any ear he wants. I had 
provisions for variable L pads, but found it unnecessary.

73 Tom