Combine two audio signals

I'm currently building an in-car alert system using a Raspberry Pi. My car has an AUX port (3.5mm jack) which I currently use with my phone - to play music.

I wish to be able to play music, and play alerts from the Raspberry Pi through the AUX channel.

As it stands, the phone is connected to the AUX jack via a 3.5mm jack cable. If I was to wire the Pi's audio output into the back of this jack would the two audio signals (phone & pi) be played together or would there be distortion or any other issues?

Appreciate any help here!

• I'm not following where the two signals are coming from and exactly how/where they are being combined. The only signal source you mention is the Raspberry Pi. Feb 13, 2014 at 14:43
• He is probably referring to the left and right stereo outputs. Feb 13, 2014 at 15:06
• I think the two signals are i) the phone output plugged into the jack ; ii) the pi audio wired into the back of the jack. Wiring as a current summer T with resistors would probably work. Will add a diagram as a proper answer if I get some time. Feb 13, 2014 at 15:17
• Not sure why this is downvoted, the two sources are as akellyirl notes: the phone output connected to the jack, the pi audio output wired to the back of the jack. Will update question to clarify. @akellyirl, would appreciate it if you can add a diagram! Feb 13, 2014 at 15:38

A T current summer should work.

A circuit like this adds the currents from the Jack and the R-Pi and the resistor R3 converts the added currents into a Voltage that gets applied to the radio's amp. The 600Ohm value is set for 0dB Line-Level.

The capacitors block any DC from the sources if it's present.

• Thanks for this. When you say this assumes 0V DC level inputs, what does that actually mean & how can I find this out? Sorry - electronics rookie here! Feb 14, 2014 at 11:22
• Edited to show caps. They would work no matter how the source is DC biased. Feb 14, 2014 at 11:50
• Are you saying the capacitors are required for use with the Pi? I'm not sure I understand the need for them? Feb 14, 2014 at 12:08
• Just in case there are DC levels on the inputs. They will block any DC that may be present there. The Amplifier only want to Amplify AC signals. It will have its own DC bias level. Feb 14, 2014 at 12:11
• Understood, thanks. Additionally, as I understand it, a jack cable has 2 positives & then a common ground - is that correct? Your diagram only shows 1 wire from each input, so a little confused there? Feb 14, 2014 at 12:52

While akellygirl's answer of a summer may work for you, if you find you're getting excessive noise when the pi is connected, you may wish to include an isolation transformer, which will prevent ground loops. They're commonly available in car stereo shops.

This is a photo of a cables to go 4000, available from Amazon and many other sources etc.