Hello, I am installing a single kx-155 for nav/com operation driving a cdi for vor navigation and as primary com. I have been reading that I need to wire in resistors in series to nav audio hi out and hedphones/comm audio hi out to the headphones ( im using a portable intercom in a 2 seater) so i can avoid using an audio panel. I just want to be able to control the nav audio (ID tones) by turning it on and off on the radio. Any advice as to how to go about it? I have read that by just combining the two wires together i may damage the built in amp inside the radio.
single nav/com installation K155
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big.sky.wagon
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I'm trying to troubleshoot my kx-155 and have the same question. What the heck does the comm audio hi/lo and the nav audio hi/lo go to? I know that this thread is a few years old, but did you ever make any headway on what to do with these four wires?
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I’ll take a stab…
The “hi” is the signal. Like the + on the back of an old stereo speaker output.
The “lo” is basically ground.
So… wiring the two outputs together effectively halves the resistance, which drives the amperage up to keep E=IR. So the amplifier works too hard, overheats and dies. Think back to basic electricity and look at two resistors in parallel.
So… wiring an appropriate resistor in series (a resistor equal to whatever the “characteristic resistance” of that output) brings everything back into what the amplifier wants to see.
No such thing as a free lunch, your effective volume will be halved, usually not a big deal.
Ok real electrical engineers, am I close? Ha!
Can put a switch in so the headsets are only connected to one at a time, so overall impedance remains the same.
The “hi” is the signal. Like the + on the back of an old stereo speaker output.
The “lo” is basically ground.
So… wiring the two outputs together effectively halves the resistance, which drives the amperage up to keep E=IR. So the amplifier works too hard, overheats and dies. Think back to basic electricity and look at two resistors in parallel.
So… wiring an appropriate resistor in series (a resistor equal to whatever the “characteristic resistance” of that output) brings everything back into what the amplifier wants to see.
No such thing as a free lunch, your effective volume will be halved, usually not a big deal.
Ok real electrical engineers, am I close? Ha!
Can put a switch in so the headsets are only connected to one at a time, so overall impedance remains the same.
Ok then, connect a 220 ohm 1/4 watt resistor in series from pin 9 of the kx155 (com audio high) to the aircraft headphone jack TIP connector and a second 220 ohm resistor from pin 10 of the kx155 (nav audio high) to the same aircraft headphone jack TIP connector. See this document for wiring up the rest of the connections on the jacks and the SPA-400: https://www.sigtronics.com/_pdf/air_pdf/air_intercom_pdf/SPA-400_King_KX-155-165_Inst.pdf