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View Full Version : Patch Cords

09-18-2018, 06:11 PM
Anyone have a good schematic for a Low-Z (Military Impedance) patch cord design? Preferably with a rheostat for adjusting the output volume. Thanks! I know we can just wire the headphone output straight into the comm cord, but it usually sounds like crap. I'd like to build something with a balanced circuit and something that won't draw too much current from the audio source.


UAL T38 Phlyer
09-18-2018, 07:02 PM
You need a resistor bridge. I made one years ago, with a little help from the electrics shop. :)

Your typical output from a phone, ipod, etc, requires an 8 Ohm load. Been a long time, but I think the input to the aircraft intercom is about 100 Ohms.

Heres a fixed system (assuming I have the intercom impedance correct...ask the avionics guys):

UAL T38 Phlyer
09-18-2018, 07:04 PM
If you wanted to put a volume control in the patch cord, Id use a 50 Ohm fixed resistor, and a 50 Ohm variable resistor:

09-20-2018, 02:01 PM
Thanks! My first just job as a reservist is building some patch cords, this is perfect.

09-20-2018, 03:04 PM
Thanks! My first just job as a reservist is building some patch cords, this is perfect.

So for reference, is the correct configuration always going to be a parallel load on the inputs equal to what that system expects and then a series load on the output for the impedence of that system?

UAL T38 Phlyer
09-25-2018, 10:47 AM
The way I was taught/remember, in this example: if you are an 8-Ohm device, “looking” at your terminal connections on the patchbridge, you will see 8-Ohms as your closed loop.

If you are the 100-Ohm intercom, looking back at the output terminals, you see 100-Ohms in THAT loop.

Easy in the fixed-volume bridge.

For the variable, this “looking back” impedance could vary from 50-100. It would mean at zero resistance in the variable resistor (max volume), you’d have only 50-Ohms total in the bridge...might lead to some distortion, due to impedance mismatch.