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msg · June 30th 07, 09:31 PM posted to rec.radio.amateur.homebrew

wrote:

snip
During the last field day some members of our club ask if it is
possible to connect to the main station two headsets and a speaker at
the same time. Each headset should have his own control of volume. The
volume of the speaker will be control by the vol of the radio.

Pads are commonly used to control volume at the jack in audio distribution
arrangements; I also use headphones which contain a built-in pad for
volume (a slider mounted in one of the earpieces). In the U.S., Radio
Shack used to be a source however I cannot find any at their website.

Use a pad that maintains a nearly constant input impedance to audio across
its adjustment range; the bridged-T pad has both constant input and output
impedance. Make sure the total parallel impedance of the connected pads
matches your audio distribution impedance, (say 600 ohms); use a transformer
to match the audio output impedance of your radios' speaker jack (say 8 ohms).

Consider controlling the radio's speaker with a pad as well so that the line
level remains unaffected by local volume settings.

A site describing pad construction and theory is at:

http://www.uneeda-audio.com/pads/

Regards,

Michael