A Master Control Unit for Receive Audio

Posted on Feb 7, 2014

An audio mixer to which all the receiver audio outputs go, which is described here. The audio mixer`s output is amplified and drives a common speaker. So I hear all the radios (that are turned on) through that one speaker. This is fine as far as it goes. But. Sometimes I want to turn all the sound off without having to adjust the gain of each receiver - like when the phone rings. So I need a master Audio On/Off switch.

A Master Control Unit for Receive Audio
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I also want to be able to use headphones at any of three different operating positions. I want to be able to plug the headphones in at whichever operating position I am using so the cable isn`t strewn half way across the room. And, of course, I want to be able to switch the headphone audio On and Off. I wanted to put a block diagram of the system here, but I seem to have lost it. So we will have to live with just schematic diagrams. The schematic of the Master Control Unit is shown below. I won`t go into a lot of detail describing how it works because it`s pretty straightforward. The remote gain control is a bit interesting. This unit uses a 12 volt regulator, U1, to generate a regulated reference voltage (12 volts) which is applied to a linear taper potentiometer, R6. The wiper of R6 provides a voltage that controls the gate of an FET in the audio mixer to set the overall gain of the mixer and, therefore, the system gain. Each of the Remote Units also has such a circuit. Only the unit that is in control, as selected by the Box Select switch, controls the gain of the system. Below is a picture of the enclosure I used for this unit. It started life as a Hammond 1590FF (available from Mouser). This is the part number of an unpainted model, which is the one to get. The one I used was actually painted black, but I made do with what I had on hand. I sanded it down and painted it gray, using Krylon High Gloss paint, which I...

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