I would like to feed the pilotAware(raspberry 2b classic) audio output into my KMA 24.
The input impedance to the KMA is 500 ohms, whereas the PA output impedance is much lower I believe, so the audio is very quiet, at least I think this is the problem from my school boy electronics knowledge!
Can I use a transformer(would be nice as power not req’d) or amplifier to solve this problem?
Recommendations please!
Thanks
Pete
What do the PA audio output connections look like? Is it just a single wire, plus a ground, or is it two wires intended to directly drive a loudspeaker? Is there any documentation you can post a screenshot from?
The impedance matching is not the issue. A low impedance source will drive a high impedance load just fine. It is only for maximum power (in watts) transfer that the two impedances need to match but that is not relevant here; it is relevant for e.g. drawing a few hundred amps from a battery into a starter motor. In your case you want the max voltage, only.
It’s works loudly.
You may experience some EMI even with shielded wire.
Try different device positions first, with all electrical load On and then you may select the correct position.
Enjoy.
PA- pilotAware, 3.5mm jack, just needs to be louder. I think an audio transformer would help?
No, you may connect the pilot aware straight to kma 24. ( at the proper pin – free pin not used )
3.5mm jack,
Mono or stereo jack? Can you post a photo of it, or the suggested wiring?
I’ve tried feeding a tablet audio output into a KMA26 audio panel — essentially the same thing that PeteD is proposing. It works but the sound level is very low. From whatever specs I could find that was not unexpected. You would need an amplifier — or possibly an impedance matching transformer would do.
The reason I asked about the detail is in case the output is balanced, or some such eg stereo and the two channels are shorted, driving the amp into current limit. Without this information, one is just guessing.
Otherwise, a little audio transformer from ebay can be used to boost the signal, assuming the source impedance is low enough.
So the audio output from the pilotaware comes from a raspberry pi 2B and is a 3.5 mm stereo jack, designed to drive headphones like the ones used for mobile phones etc and is of low output impedance.
The 3.5mm female jack on my panel is connected to pin J and audio ground(wired for mono i.e. L&R shorted)
https://www.scribd.com/document/87127092/KMA-24-Install
page 2-19, which is “HF/TEL 500 ohm input”, the nav/com inputs are also 500 ohm
When I connect my handheld radio aircraft headphone output to it with patch lead all works well, as expected.
When the pilotaware is connected it is very quite, as expected.
So I hoped an audio transformer would work(given the low impedance output into high impedance), just not sure of the spec I need?
A google for “raspberry pi 2B schematic” digs out this which shows
and a google on the NC7WZ16 chip digs out this
which, powered apparently from +2.5V and pulse width modulated to generated a crude form of audio, and going through the lowpass filter made up of the two 100R resistors and the 100nF capacitors, produces a stereo output with a peak amplitude of 1.25V (under no load conditions).
The first Q is whether the sound appears on both channels or just one, so you have to pick off the correct channel (or just one channel if they are both the same). You cannot connect to both channels! Well, you can, using two resistors to mix the two signals, but this should not be necessary.
The source impedance will be 50 ohms, which is absolutely fine for a 600 ohm load. You lose only about 10% which is negligible.
And 1.25V peak is loads of signal. A standard “line level” signal, in the standard 600 ohm system, is 0.77V rms, which is about the same.
So I would make sure the PA box is actually generating the right volume. There may be a software setting somewhere. It is easy to verify this with an oscilloscope.
Can you post a photo of the 3.5mm jack plug you are using, and if possible a pic of the socket on the PCB?
I found this for the socket
which looks like a standard stereo socket, so the plug should look like this
