Add AUX to your CD/Radio etc

To wire in an AUX to your audio equipment:

The design of some electric equipment is highly modular and straight forward to add an aux input by decoupling the the CD/Radio etc stage from the volume control/Amplifier stage and inject your AUX signal there.

Service manuals can possibly be found and downloaded for some equipment, if you find it there may be a schematic and to help you.

Always remove the power supply from its source! now take the thing apart, find an appropriate place to an AUX connection to the case, soldering wires to it, track down the audio line and at the best place desolder/disconnect it.

So how do you select between Cd/Radio source and Aux input? We'll take the brute force approach and wire in miniature Double Pole, Double Throw (DPDT) Switch. One position joins the CD/Radio etc stage to the amplifier board (like the jumpers previously did) and the other position connects the Aux input to the amplifier board.

Another way to do it is to use a 3.5mm input jack that has normally closed connections which open when the plug is inserted the Aux. (this saves a second hole in the dash and a switch that you have to flip).

With it all connected, you just run the equipment in the cd/radio etc mode, plug in an input to AUX switch the DPDT switch and ignore the display.

Notes on Input volume level setting and input impedance: you will probably notice you need to adjust the volume level on the external audio device in order to match the volume level fed from the internal line to avoid a sudden change in volume level when switching between sources. If you don't want something plugging in at max volume and making you have to turn the devices volume down, you should put an attenuator circuit in line with the Aux input. The attenuator circuit can be a simple (dual for Left/Right) resistor divider network. The same resistor values that the engineers used in the device coming out of the Cd/Radio stage (ie 2.7K Ohm for the series elements, 1.6K Ohm for the shunt elements) 1/4 W, 2% metal film resistors. This will provide approx. 4.3dB of attenuation, what resistor you use depends on the external device volume level you expect to match the CD/Radio volume level stage, thus allowing smoother volume transition when you plug the external device in.

The resistors also provide some isolation/matching between the (relatively) low impedance of your external device output and the internal amplifier stage. (The attenuator is really optional).