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Combined radio/cans box


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Wasn't sure where to put this, but I guess here is as good a place as any.


At my venue we have a set of 4 Stonewood (Granite) wired intercoms, with a set of 4 pin headsets. These are primarily used for calling the show and other communications during a performance.


Due to the intercoms being wireless, at the interval it is common practice to switch to the Motorola CP040 radios, which are also the emergency form of communication. For the radios I have a covert acoustic tube earpiece.


My idea is to create an all-in-one box if you will that will allow me to connect the radios and intercom into one headset. From what I understand, it's bad practice to combine audio signals, as the waveforms will interfere with each other causing destructive interference? However due to only one of the devices sending a headphone signal at a time this should not be an issue? I would like the input to the box to be 4 pin XLR, which would allow me to use the intercom headsets, and by replacing the end of the radio headset with a 4 pin XLR would allow me to bypass the box if I wanted to use the radio headset with the intercom only. The removed Motorola plug could then be a direct output of the box.


The other issue is that the Motorola will transmit when the microphone circuit is closed, but the intercom headset does not have a PTT switch like the radio headset. Therefore would a simple toggle switch between intercom and radio be acceptable, with a PTT button built into the unit? The PTT on the box would only need to be active when used with the intercom headset.


I've found suitable XLR sockets and connectors on the CPC website, as well as an enclosure, but I'm looking for advice on the cable and switches.



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Do you actually need to combine the audio? It sounds like you're saying you never expect to have audio from both at the same time - comms during the show, radio in the interval.


Let's just confirm though, you're looking to just use the comms headset all the time, and direct the radio audio through it during the interval? A comms headset is a bulky thing to be wearing for 3+ hours. I know my ears breathe a sigh of relief when I take mine off after an hour long half. I think a different earpiece would be a breath of fresh air!


But, assuming you want to carry on, why not have a simple changeover switch? Double pole, double throw. Centre teminals to the earpiece, then each side to either the radio or the comms. You could maybe common the grounds of both, but I'm not 100% sure what effect that would have on the otherwise "floating" radio - being connected to the proper ground of the comms system.

Be careful with the mic connection - are the comms pack and radio providing phantom or electret power? If the answer is yes then connecting the 2 could be interesting. The comms might use a dynamic type, but I bet the radio is expecting a small lav type electret. A changeover switch on this may be the answer, but only if the mic on your comms headset will work with both.

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The bespoke Motorola radio headset connector is a 3.5mm mono connector for the earpiece, and a 2.5mm mono connector for the mic, so I would assume the mic is a dynamic? Ideally, I'd like to be able to use both with the comms, hence adding the 4 pin XLR to the radio headset, but I could add the radio in the mix by using the box if required. I like the idea of a dpdt switch, but there have been occasions where both the comms and radios are used at the same time, ie. calling the show and searching for a missing cast member, but a dpdt switch on the mic would be a great idea as I don't want comms people getting radio messages!
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The bespoke Motorola radio headset connector is a 3.5mm mono connector for the earpiece, and a 2.5mm mono connector for the mic, so I would assume the mic is a dynamic?



Motorola rarely, if ever, use dynamic microphones for their radios. It is a safe bet that the radio is supplying a bias voltage of around 3 to 5 volts and expecting to see an electret microphone. It is also unlikely that an unamplified dynamic microphone will give the required level. The transmitter keying (PTT) is also done on the mic connector , measuring the change in current as the microphone is is connected by pressing the PTT button.


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  • 2 weeks later...

To anyone thinking along these lines, we found this to be the most frustrating piece of kit we have ever used. Trial it first.


It's been one of the most useful pieces of kit we've ever used - once all the crew have got used to using it!

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