car radio aux mod

[chris2001net]

I modified my car radio to switch between FM radio or a stereo aux in (using the stereo radio lines to a sound fader chip)

The problem I have is the AUX is not as loud as the radio and when i turn my mp3 player up it distorts badly.

I use a 3.5 jack connector which switchs between radio / line in.

The jack wires connect to input pins on the sound chip TEA6320
http://www.datasheetcatalog.org/datasheet/philips/TEA6320_2.pdf

I've read I need to put 4.7k resistors in the AUX L & R, but testing with 1.5k and 10k proved it didn't work.

Any ideas how I can get the aux to match the radio volume without distortion?

Thanks in advance.

 

[crutschow]

Is the volume from the aux loud enough if you turn up the volume on the fader chip? If so you can match the volume levels by attenuating the level from the FM radio.

A resistor in series with a 10kΩ resistor to ground should work. You will need to experimentally determine the value of the series resistor.

Alternately you could use a 10KΩ pot in each of the two stereo lines for attenuation. Connect one end of the pot element to ground and the other end to the FM signal. Connect the wiper outputs to the fader inputs.

 

[chris2001net]

so i would be reducing the Radio volume? Thats possible.

I still don't understand how my aux is quieter than radio, but when i turn the mp3 up to FM's volume level the AUX distorts. I will try a voltmeter across the radio and aux and see what the difference is.

 

[MikeMl]

Duplicate, please ignore!

 

[MikeMl]

so i would be reducing the Radio volume? Thats possible.

I still don't understand how my aux is quieter than radio, but when i turn the mp3 up to FM's volume level the AUX distorts. I will try a voltmeter across the radio and aux and see what the difference is.

The mp3 player is designed to drive low-impedance earbuds/headset. Since it runs on <3V, it can only produce less than 2Vp-p audio. I'll bet that the internal level between your in-dash radio's detector and its volume control is much higher than that.

You need a stereo preamp with a voltage gain of ~3 between the mp3's line out and your newly hacked input. A more expensive in-dash stereo which has a "line-in jack" has the amplifier built-in.

I have had to do this in interfacing personal audio equipment to aircraft entertainment systems. Either a gain of three amplifier, or an 500Ω to 8Ω audio transformer hooked up backwards (voltage step up) will do it.

 

[audioguru]

The sound fader chip has a voltage gain of 20dB which is 10 times. Then its output is 2V when the MP3 player's output is only 200mV.
Isn't that enough to feed the car radio's amplifier?