Its a tangled web we weave
BlackRV7 said:
My totally inept electrical mind said, "Dana, that braided shield on the RCA cables has to go somewhere". Apparently not. I was only going to use a very short, 1", portion of the existing RCA black cable to splice in aviation wire to go to the intercom harness. Thanks for the thoughts.
Yes an audio speaker needs a positive and a ground. That ground can come from different places. We're talking about connecting two amp, the audio source amp to the intercom amp.
YOUR PS Engineering does have an audio ground and you may want to use it. You can try the one wire approach but I would use the audio ground provided, which I am pretty sure PS Eng has.
You are using a Car audio device CD/DVD or an audio source; some of these amps have separate "grounds" for each channel. This is a balanced audio output and really is two HOTs and a ground null, but for conversation we will call it discrete grounds. This is typical of high powered hi-end car audio. Make sure you know what you have. A little portable device and high powered radio are two different things.
Others amps typical of little portables or less expensive car audio have the same ground for both channels L&R (un-balanced).
Some have NO ground and its all in common ground through the chassis which the radio and speaker are attached. This "no ground" approach was done in (old) cars for years. They stopped because its terrible, but in those installations there was ONE wire to the speaker.
Know what kind of intercom and audio source you have. I can guess but you have to confirm.
Besides the source, you need to know the kind of audio amp the intercom has. THAT IS WHERE THE MISMATCH OCCURS. Most aviation intercoms are the weak link and sometimes can be a poor match for the audio source. The PS engineering is one of the best so you should have an audio ground. There are solutions I'll go over.
Remember we are talking AUDIO not power distribution, so things get a little more complicated in that very small voltage difference can cause noise. The ground is a great place for noise to get in, e.g., common-mode current, sometimes called ground-loop. Also audio is more like AC current than DC. So think AC not DC with audio signals. That is why audio is susceptible to AC power 60 Hz humms. DC is good for audio but mixing DC power and AC audio can cause noise as well, especially with a AC based alternator. The AC ripple in the power system can be picked up by the audio easily if grounds are not isolated. That's why good intercoms have discreat grounds for the output amp and input (headset phone and mic jacks are isolated).
Apparently from BlackRV7's post, his intercom does not have ANY provision for audio ground. That's fine when dealing with a source of the same kind. Bad news is many audio sources are isolated and balanced. There is a way around it.
SO WHERE DOES THE AUDIO GROUND COME FROM?
Mixing power ground and audio ground is typical of most cheap amps, but it's far from ideal for Hi-Fi. It does not mean its bad, just not super Hi-Fi and is more susceptible to noise. You do what you have to do to make it work.
Here are the three choices to deal with this, I see:
My first advice is do nothing with audio ground as I suggested as see what happens. You can't hurt any thing, and if you can hear the "tunes", leave the audio grounds disconnected. Keep the shield on the center audio wire up to the intercom. Keep the jack and shield grounded on the audio source side.
If the above does not work, ground the audio device chassis to the airframe, if possible. With portable devices, typically battery powered or powered thru isolated power supply (adapters), there is no way to ground it to the airframe. That's fine but may put you in a pickle if your intercom has no audio ground input. Which leads us to the last choice.
Last choice, ground the audio ground'(s) to the airframe. This will be a good place (a must) for a noise filter (isolation filter) as I mentioned, resistor or capacitor or both. You have to play with the values. Too much resistance Ohms, volumn is shot. Check the output impedance (Ohm) and use that resistor value to start. (iPods I think are 32 ohms) Too much "attenuation" go with less resistance. For the capacitor, a small non-polarized 0.1 or 0.22 uF non-polarized type. Try to ground the audio grounds exactly at the same location the intercom pwr is grounded (the ground bus). EE types please add $0.02, you may be laughing. Feel free to correct me if I'm off base.
I do know balanced outputs are not a good match for airplane intercoms, and NOT to ground either BAL lead to ground (see comments below).
What to do with ONE or NO audio ground for both channels (L/R)?
It gets a little sticky because some aviation intercoms have either NO audio ground or one audio ground for both L & R channel. So every case is different.
Those little amps are not that temperamental and that is normal and fine since that is all they usually output. Portable audio devices with little phone jacks have ONE ground for both L & R channels. If you tie the hots together for mono you can damage the amp.
Balance amps are typically hi-powered, hi-end, hi-fi stereos, some not. This may apply to you.
If it's a balanced amp, like a high powered car stereo, do not ground the either lead to ground. You will blow it up. You must either isolate them or possibly try the one wire trick? How do you isolate? Carefully. It may be involve more than capacitor & resistors (larger ones). You may also audio isolation transformer or small amp, but this is at the limits of my ignorance. I think they sell a "Bal to Un-Bal converter" commercially (
radio shack isolator, model:270-054?).
I highly recommend NOT using high powered balanced car stereos w/ little aviation intercoms. Its a bad match. If you must, be careful.
Stereo to mono?
NEVER tie stereo L & R hot or positive audios together, stereo to mono. People do that with little portables and get a way with it, but its ugly. Again filters / isolation with capacitor and resistors will do the trick just like the audio ground. Again DO not do this with high powered amps, only low powered portable devices.
If you have your heart set on a car stereo get one with aux low gain output or low power. El-cheap stereo is fine because you are running it thru a milli-watt intercom and aviation headsets. No need for a $2000 high power stereo to listen thru a cracker-jack box.
Most of the so called stereo aviation intercoms are really not stereo inside or made for Hi-Fi inputs. They are for low powered portable devices. This is the quandary, but we can force it with enough capacitors, resistor, transistors and isolation transformers.
Here is an article that goes into detail:
http://www.equitech.com/support/wiring.html
(PS you can just cut the RCA patch cable and splice but some are poor quality. Good quality patch cables are better but hate to ruin it just for an RCA plug. My choice is buy RCA plugs and connect with aviation shielded wire from the get go, it's cleaner.)
