[CQ-Contest] SO2R Sound mixer

k6ll at juno.com k6ll at juno.com
Tue Jul 25 17:47:40 EDT 2000

On Tue, 25 Jul 2000 12:28:06 -0800 "Chris Hurlbut" <wl7ky at gci.net>

> Does anyone have plans for a good SO2R audio mixer?  I built one 
> that =
> has 3 positions:  Radio 1 in both ears, Radio 2 in both ears, and 
> both =
> radio in both ears.  Both radios in both ears is really hard to get 
> =
> figured out in my head, so I am trying to get left radio in left 
> ear, =
> right radio in right ear.  However with the switch etc that I have, 
> I =
> can't figure out how to do it.  Does anyone have plans for a switch 
> box =
> that will do that?

The following diagram will do that, if you leave out the 180 ohm
resistor. It rather difficult to read, but it's the best I could
do. Make sure you view the diagram with Courier Font.

On the 3 position switch,
position 1 is radio 1 both ears
position 3 is radio 2 both ears
position 2 is a mix of both radios in both ears, but R1 is louder
   in the left ear, and R2 is louder in the right ear. If you leave
   out the 180 ohm resistor, you will hear only R1 in the left ear,
   and only R2 in the right ear. 

Get a little aluminum box from Radio Shack and mount
the following Radio Shack parts on it:

2 rca jacks
1 2p3t rotary switch
1 stereo phono jack
2 33 ohm resistors (to prevent headphone hiss from the spkr outputs)
1 "mixing" resistor ~180 ohms
Wire it up like so: (view this with Courier Font)
R2 and R1 are the external speaker outputs from radios 1 and 2.

R2----/\/\33OHMS--a----0 <------------RIGHT HEADPHONE

                     --0 <------------LEFT HEADPHONE

Connect a jumper from d to b (too hard to show these above)
connect a jumper from a to c
connect an experimental resistor, about 180 ohms, across
the left and right headphone leads. Adjust this resistor
for your preferred mix. You could also use 47 ohms in series
with a pot instead of hand-selecting a fixed resistor.

Dave Hachadorian, K6LL
Big Bear, CA
K6LL at juno.com

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