Audio project help ? For a newbie...

[RobertE]

Hi. I am hoping some expert here can help me.

I have built a radio/ipod/aux audio controller for my car.

I have dedicated audio ground lines and I use an IC to switch between the inputs - controlled by an arduino micro controller.

The problem I have is that for the removable sources (iPod and aux)...

If I select the source and nothing is connected, I get lots of noise. As soon as I connect the device it is clear - even when not playing. I would like it to be quiet when nothing is connected.

How do I fix this? Do I need to connect the audio source lines to the audio ground via a resistor or capacitor ???

[RobertE]

To clarify, when the device is not attached, there are "dangling" audio and ground lines feeding the amp - causing the noise I assume, as neither would be referenced to the other. ???

[Mooly]

Try connecting a resistor as you say across each input and ground. Wiring in cars is full of noise and spikes. You can go really low in value too, see what works. I would start with around 10k and go down to even as low as 100 ohms if needed. Any portable device that drive h/phones will cope with that just fine. You can also try a small cap as well in parallel to the resistor. Guessing around 470pf for a 10k.

[RobertE]

It seemed to help, but the lower I went the lower the volume seemed to be, which brings up one more issue - the controller basically switches line-level sources, but it appears the amp wants something a bit stronger as the maximum volume is quite low. Could this also ge an impedance mismatch?? Do I need to add a small amp to the circuitry? Any other options or things to try ? Thanks for your help !

[Mooly]

It's hard to say without knowing all the details.

Your using a controller of some sort you say. Is there any way that you could use it to turn on a muting device (FET, analogue switch etc) to short the inputs that aren't in use.

If that's no good, then you have to look at why the floating inputs are picking up noise. If the value of resistor needed to shunt the inputs is so low that it attenuates the signal, then you have to look at fixing the issue at it's source... which means shielding the input area of the switching unit.

Adding an amp would give a low impedance source to the inputs it works into, but you would still have to shield the input to the amp if it's location picked up noise. A simple opamp gain stage is all that is needed.