Mono signal won't work with a stereo aux in - no sound at all ?

[alesserfate]

Hello folks,

Looking for some basic help, I have a feeling it's something simple.

I use an xlr microphone, with a preamp powered by a 12v power source in a vehicle, to connect to auxiliary inputs on multiple tour vehicles in our fleet. By auxiliary inputs I mean a standard automotive audio deck with a female minijack stereo input.

It works in 2/3 of the vehicles, in the third one, it does not work.
- I have tested the microphone and amplifier setup in other vehicles as well as using computer speakers and it works.
- I have tested the minijack aux input in the 3rd vehicle using an ipod and it works, tested it using a cellphone mp3 player as well and it works.

The details about the output from the microphone amplifier: it sends a mono signal across a stereo cable using a mono-minijack male to stereo-minijack male cable. In other words, it sends the same signal on both the L and R positive but using a common ground.

Why does this not work in the third vehicle ? I am using the same audio cable. The 3rd vehicle has a factory auxiliary input. What can I do differently to make it work ?

All the best, any input greatly appreciated.

 

[KJ6EAD]

Can you check for opens, shorts and miswires in your cables and connectors? It could be a simple signal to ground type fault.

 

[KrisBlueNZ]

Try a different cable between the mic preamp and the automotive audio system - either mono-to-mono or stereo-to-stereo. If that works, the automotive system that it doesn't work with has a mono input and the ring contact is grounded; that would short out the signal when you plug in a stereo plug with the tip and ring connected together. You could either modify the input, or carry two different cables or a cable and an adapter.

I would post some images, but google.co.nz is down! Aaaargh! How can I find anything now? End of the world! LOL

Edit: google.co.nz is back up. Here's a picture of the plug with tip, ring and sleeve marked.

 

[alesserfate]

Thanks, I just tried it with a bunch of other cables. Still not working. Works with all said cables with other audio systems, and the aux-in jack works with everything else including ipod, mp3 player, cellphone. Kind of stumped.

 

[alesserfate]

Any ideas anyone?

 

[KrisBlueNZ]

Have you checked that the microphone preamp is receiving power in that vehicle?

If that's not the problem, I'm out of suggestions :-(

 

[alesserfate]

Yes it does work, powers on, clip light flashes, and works if i use portable battery operated speaker with it.

Starting to lean towards the aux-in having an issue, although it does work with other devices, it's nowhere near as loud or crisp as the radio or bluetooth streaming it can do. There is probably an issue with the harness between it and the factory sound system, I have a feeling potentially a short between the ground of one channel and the signal of the other, hence why there's nothing heard when they are linked in mono. I'm going to take the van back to the dealer and have them look at it under warranty.

 

[KrisBlueNZ]

The aux input on the vehicle's amplifier system may have a low input impedance, so it needs to be driven by a something that's designed to drive headphones directly. The MP3 player you connected to it would have provided a strong enough signal, but the microphone preamp probably doesn't. That could explain the problem.

Most likely that could be fixed by increasing the value of a resistor in the amp, or adding a buffer circuit externally. The amplifier may be working according to specification and the dealer may not consider it a warranty issue, or even a problem.

If you need more help, post the brand and model number of the amp, and be prepared to open it up and make a minor modification to it.

 

[alesserfate]

Tested with other trucks of the same make at the dealer, works in some, doesn't work in others. Narrowed it down to being a hardware revision of the specific audio headunit. Mine being an earlier one. Also narrowed down that the problem lies with the left channel in the vehicle having a partial short to ground, so when you use a true stereo input it still works as you hear the right channel, and when you use a mono in such as a PA where the input is bound, the short in the left channel also screws up the right. New head unit is on the way and hopefully this one will work, if not it's a harness problem.