Production intercom channel splitter

This is a simple device that splits a single ClearCom or similar intercom line into two separate audio channels, sharing a single power supply. It creates a new audio termination point for the second "universe" to allow independent conversations, and maintains functionality of the CALL light.

It implements the circuit shown near the end of this document that explores how such systems work, and the particulars of the termination circuit are corroborated by ClearCom's own documentation (PDF).

I've been meaning to build one of these for years, and never got around to it until faced with the possibility of needing two channels on an event where only a single-channel base station was going to be supplied.

[Click on small pictures for larger copies.]

A collection of parts I had the appropriate parts in stock -- the sexy black Neutrik solder-cup connectors, and a suitable little project box. Intercom lines traditionally follow which way the pins point similar to sound connections, but even if it doesn't really matter for the shared-bus architecture I'd do it right with one female and two male connectors.

Drilling doesn't work Hardest part was making the holes for the connectors. I scribed then onto the box lid around a suitably sized washer [shown later] and without more sophisticated hole-drilling tools at hand figured I'd have to just nibble or dremel them out to size. But that needed a large enough starter hole. Problem is, drilling plastic like this is almost impossible as a drill bit tries to bite its way through all at once and just winds up grabbing and spinning the whole mess. Clamping it would probably just crack everything.

Solution: punch it The right answer was to load up my largest punch and die, and rough out most of the area in the form of many overlapping holes.

Roughed-out holes Next problem was to continue out to my nice round scribed boundaries; the template for drawing same is also shown here.

Reasonably nice holes A combination of nibbling and dremeling and hand-whittling yielded fairly decent holes; the irregularities would be hidden under the connector flanges anyways. At this point I dug around to find a uniform set of screws and nuts.

Front panel ready to assemble I figured the panel needed a label so someone randomly picking the device up would have some idea what it is and how to connect it; but real estate shared between the connectors, my lettering, and the tape to hold it down was at a premium.

Splitter circuit wired in The termination circuit went in with totally old-school wiring technique. Pins 1 and 2 straight through; pin 3 is where the magic happens.

All done! Done! The label wound up a little wonky, but it's good enough for the purpose.

Join/split switch A couple of years later, after the specific set of intercom gear I was most familiar with had all fallen under my care anyway, I added a channel-join switch to this widget.  Again, a simple enough thing -- for the third connector's pin 3, either connect to the terminator or the pin 3 bus of the other two.

An additional label was added to the front documenting this, and advising "switch briskly!" as moving the switch slowly would leave the new channel's bus completely unterminated for just a moment.  An open mic anywhere downstream of that could easily cause brain-destroying feedback until the internal rocker of the switch could contact one or the other side.  I didn't have any make-before-break switches in stock, as they're a somewhat specialty item which would nonetheless be the correct answer for this.

Bigger and better splitter box And a couple of months after that fixup, I ordered more connectors and put together two of the dee-luxe-o version of the same idea: a *three* way splitter, with the option of creating a new channel on the third output.  The "Bud" boxes were already sitting around in stock and happened to be a perfect size for this, which helped create the idea.  As the 4.7K side of the terminator actually does emit a little heat especially in a 30V system, here I made it a pair of quarter-watt resistors in series to total the right value, protected in a fold of gaff tape with the capacitor sitting on top.  The rest of the wiring is again obvious.

_H*   120123, rev 160225