A bit off Ferrari topic, but a technical question for electronic types. This is about a VHF radio in a plane with a headphone and microphone jack. I currently have one comm radio with a harness to the pilot mike and headphone jacks as shown, with a standard separate push to talk (PTT) to the throttle handle. Can I add another comm radio by simply piggybacking onto the jacks as shown below? I would have a toggle between the PTT wires to select between radios to transmit. Thanks Image Unavailable, Please Login
You can't parallel the jacks without incurring unintended consequences, even if the radios are identical. Best to use a headphone mixer / combiner for the outputs and for the microphone, either a switch to choose which radio the mic is connected to or a splitter. In any case, a switch will be needed to choose ptt. Google 'headphone combiner' for hardware. The microphone device to google for is a 'microphone splitter'.
It would probably work as your diagram shows, but the speech amplifiers and audio output of one radio are paralleled to the other radio by your diagram. The microphone and headphones will then see half the impedence that it should see, even if you only activate one push-to-talk line at any given time. It would probably work, but it's not really electronically correct. And as with anything installed in an aircraft, proceed with caution. We have a helicopter that we use for our local TV news coverage, and the FAA rules are very restrictive as to what can be added, modified, etc. And don't even think about drilling a hole anywhere without authorization. It would probably violate some FAA regulation.
Here's is the more correct way. You will need a 3PDT switch (triple pole, double throw switch). The only caveat is the microphone. In simpler times, many push-to-talk (PTT) microphones simply had 3 wires: one for the mic audio, one for the PTT, and one for chassis ground. The switch inside the microphone simply grounded the PTT line. This completed the ground circuit for the relay inside the radio. A lot of modern comm gear seems to have unnecessarily complicated microphone connectors and wiring so procede with caution. This diagram illustrates the simple, old fashioned PTT switch inside the microphone. Do you have seperate RF antennas for both radios? If not, it gets more complicated because you will also need to switch 1 antenna between 2 different radios. It gets hairy and you don't want to violate an FAA regulation somewhere along the line. Image Unavailable, Please Login
thanks both Paul and Arlie!! One of the goals was to be able to listen to both at the same time. I did find this on a cursory internet search: So if I added the 10 ohm resistors to the headphone input of each radio afterwhich combining them, then used HF's schematic for the rest, would that work with the switch choosing which radio was primary and which was listen only? OBTW - each radio has a separate antenna Thanks again
ASSUMING that the info regarding the 10 ohm resistors is correct, then here is my modified schematic showing how it would be. The audio output of BOTH radios would appear in your headphones at the same time, and the switch would select which one would actually be the TRANSMIT radio. I say once again, this ASSUMES that the 10 ohm resistor trick will work. I have had to make a few mixer modules from time to time, mostly to do this exact sort of thing. Working with TV videocassette recorders, I have a module with XLR audio connectors that I use to mix 2 channels into 1, or split 1 audio into 2 outputs. I will have to dig up my very old "mixer module" info sheet that shows how to properly configure many different combinations of inputs and outputs. But most of my info is based on the broadcast studio standard of 600 ohm impedence. Most comm gear would be lower impedence. I will see what I can dig up from my locker. Image Unavailable, Please Login
The first horsefly circuit will not work since you wish to listen to both radios simultaneously, not just the one switched to. And the 10 ohm resistor trick can still lead to problems as you don't know what the headphone impedance/output impedance is. Just buy the inexpensive but proper headphone mixer and avoid potential problems. Re the antennas situation, there are antenna duplexers for that purpose depending on the radio bands. Always avoid 'probablies' when more than 6 feet above ground level. I'd suggest you find a local avionics or commercial radio or a ham operator to help you do it right. (paul - N7OCS and Motorola commercial radio specialist)
I dug out our ancient diagram showing splitter construction for multiple outputs, balanced versus unbalanced line, etc. A correct combiner for 2 unbalanced audio outputs will need 3 resistors. I have added the 3rd resistor in my 3rd diagram. All 3 resistors will have the same value, but the actual value will depend upon the impedence of the audio output of the radio(s) in question. For a pair of 600 ohm inputs and a 600 ohm output, the resistors would be 200 ohms each. I would have to do some more digging to figure out the ACTUAL values, but as I say, that depends upon the audio output impedence of the radios in question. Image Unavailable, Please Login
