This is a very simple 8 channel audiomixer but it works good enough for communication radio systems. It has 4 inputs and 4 outputs where you can individually adjust the levels. It also have a mastervolume. The amplifier is based on the OP, LM741.
The inputs can be connected to
sound out from soundcard/PC speaker.
sound out from the repeater reciever.
sound out from the linkradio reciever.
sound out from the RLM interface.
The outputs can be connected to
line in on the soundcard.
sound in (mic) on the repeater transmitter
sound in on the linkradio
sound in on the RLM interface (DTMF).
Build and adjust
Start with setting all the 8 trimpots in minimal volume (highest resistance)
and set the the master volume in a middle position.
Adjust all the in- and outputs so that the soundlevel is the same from and
to all sources. Remember not to use a fully zero ohm adjustment on a trimpot.
This will cause a impedance problem.
Use shielded wire and a good noice-free powersource. If you use the power from the RLM interface you might catch some "data-noice" that will be heard on the radio.
© 1999 SM2UMH, Peter Hansson version 990208