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A Master Control Unit for Receive Audio


I have a number of receivers and I like to be able to use any number of them at the same time without the complexity - and inflexibility - of a big switching matrix. So I made 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.

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.

Additional goals include:

So, I decided to make three Receive Audio Control boxes. One is designated as the Master and the other two are Remotes.

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.

audio control unit schematic

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 found at ACE Hardware.  The part number of this paint is Krylon 1606 Pewter Gray Gloss.  It’s actually pretty close to the color of the trim rings on Collins S line gear. But that’s not why I picked it.  I picked it because I wanted a nice shade of gray that was light enough to provide good contrast for black lettering but not so light that it appeared to be a dirty shade of white.  This color looks very electronic – like a lot of military and industrial gear.


I used Visio to lay out the front, back, and top panels. First I made a perimeter outline that is about the size of the panel. Then I made circles that approximated the profile of the perimeter of each component that mounts on the panel, including space for connections.  This allows me to space all the controls and connectors properly.  Visio makes it easy to move these around until I get them all where I want them. Here’s the layout.  The labels that will ultimately appear on the panels are not yet in their final form here.


I print this out on paper and cut out each panel separately. I then tape the appropriate layout to the appropriate panel with masking tape, positioning it carefully using the outer perimeter as the main guide. When I’m happy with it, I use it as a drilling template to drill and punch all the holes.

Once that is done, I paint the unit. I spray on four or five coats of paint, wet sanding in between coats with 1000 grit wet-or-dry sandpaper. I just keep doing it until the finish passes my perfection filter - the width of which varies with time and mood. One of the tricks to getting a really good finish is to only spray horizontal surfaces. This requires spraying one side, letting it dry very well, rotating the unit so a different side is up, spraying the “up” side, and repeating until done. It doesn’t take much actual labor time, but it takes a lot of elapsed time – like a week. For me, it’s hard to be that patient. But it’s worth it, and I guess I must be getting older and more mellow.

The next step is to make a version of the Visio layout that only has the perimeter outline and the text labeling. This is easily done by making a copy of the file and then erasing the parts that are not needed, such as most of the component outlines.  But it is important to leave the drill points because we need them to locate the label correctly - I line up the center drill points in the center of their appropriate holes.

Then I print this on one 8 ½” by 11” clear label. I get these labels from Desktop Publishing Supplies (www.desktopsupplies.com). They sell them in packages of ten for about $8. The laser print version is Part # 60000-10. The Inkjet version is Part # 61000-10. The labels for the front, top, and back all fit on one 8 ½” by 11” label. Then I cut out around each perimeter outline with scissors. Cut on the inside edge of this line so that no line is left on the label unless, of course, you want to keep the outline - such as I did on the label that goes on the top of the unit. Then I position the label carefully on the panel by centering the drill points in their respective holes. I concentrate on the lower left hole and the upper right hole. When I think it’s centered pretty well, I stick it on and press out any bubbles.

This is the best equipment labeling technique I have found so far. It sounds much more complicated than it really is.  Basically you print, cut, and stick it on. I haven’t been doing this long enough to have a reading on how these labels age.

Here are some pictures of the unit before wiring it up.  I just glued the LEDs in their holes using RTV and let it sit overnight.

Notice that there are few rough edges involving the labeling.  I cut the label material on the front panel a bit big, so the edges try to wrap around the radiused edges of the enclosure.  This is not good, the edge of the label tends to want to peel up.  So I learned to cut the label a bit smaller and will do so next time.  On the back panel, I did the same thing.  I failed to cut away all of the perimeter outline, so one can see remnants of the black perimeter outline.  Live and learn.

For wiring, I use the “ugly” method of construction.   Here's a shot of the board just as I began.  I have just glued some of the relays to the PCB.  I used superglue, mostly because I am impatient and wanted to get on with it, not to wait overnight again for some glue to dry.

The main PC board is double-sided un-etched PC board material. I tin the whole board on both sides with Liquid Tin (from Mouser). This keeps the copper from corroding and makes it easy to solder to - both now and in the future.

I then cut some strips of this unetched but tinned PC board about ¼” wide. Then I cut this strip into roughly square “chips”, about ¼” by ¼”.  These form the node connection points in the “ugly” construction scheme.  I solder each one in place to the PC board using a 225 watt soldering gun.  The top side of each one is NOT connected to the bottom side - watch out for solder bridges. 

Here’s a picture of the unit all wired up. It’s not pretty, but I chose to keep lead length as short as possible rather than dress the wires nicely. Every wire that goes into or out of the unit has a ferrite bead on it as an RF choke.  Not much to it, just some switching using relays and a little diode logic.

Conclusions:  The unit works well and has made my operating much more pleasant. It has shown no propensity to pick up RFI (at least so far).  I'm satisfied, even gratified, by the labeling.  The only thing I plan to change is to reduce the current through the LEDs. I’m using superbright LEDs so they won’t fade in daylight. However, they are REALLY bright, distractingly so. I want to tone them down a bit.  I also haven’t implemented the remote gain control in the mixer yet, so there is some chance I will wind up changing something in that portion of the circuit.

Other than that, it’s a keeper.

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