[Robotgroup] car audio stuff

[Andre Lamothe]

See the problem is this, the car noise is in the SAME frequency band as 
audio :) Cars run at 800 RPMs to 3-4,000 RPMs, right in the middle of the 
audio band. So you can't really dampen one without the other (the higher 
harmonics sure), but the noise is at engine RPM frequencies. So the problem 
isn't high frequency noise (which caps would help, more in the 1uF to 0.1uF 
range), but ground loops where the power and or audio is being modulated by 
noise and that's the hum. We need to get the electronics at ground with the 
rest of everything, problem 1. then see if an isolation xformer will work, 
this is usually how you have to fix this stuff.

Lastly, do me a favor, get your oscope out there if you can, run a cable 
from the house power. And see if you can see the noise better and where its 
coming from, you can probe the power, ground, and differntial signals from 
ground, +12, and the audio right and left with no signal and get some better 
idea of where you major problem area is.

But, stick with the switch, its cool! Don't let noise beat you, you will 
learn alot once you kill it.

Andre'


----- Original Message ----- 
From: <mlists at resistive.net>
To: "The Robot Group Mailing List" <robotgroup at puremagic.com>
Sent: Wednesday, March 19, 2008 10:41 PM
Subject: Re: [Robotgroup] car audio stuff


Hey Andre' -

Thanks, I'll give that a shot.  Oddly enough, the noise on the radio input
seems to be minimized when the charge line is plugged in, but the noise on
the ipod input seems to be minimized when the charge line is unplugged.  I
had a 100uF cap across the chip power and the audio output sounded pretty
muffled.  I'll have to play with it.

-Nick


> Nick,
>
> Sounds like you have a ground loop. But, tell me do you hear the noise
> WITHOUT the power lines connected to the iPod? The first step is to
> simplify
> the circuit, so you can focus on each piece. If there is still noise with
> the power disconnected to the iPod, then the things to try are
>
> 1. Put a .1, 1uF, 10uF cap in parallel across your chip first off.
> 2. Try putting an audio isolation 1:1 transformer in front of the signal
> to
> the iPod, really cheap, and will remove the ground from the iPod.
>
> Andre'
>
>
> ----- Original Message -----
> From: <mlists at resistive.net>
> To: <robotgroup at puremagic.com>
> Sent: Tuesday, March 18, 2008 11:06 PM
> Subject: [Robotgroup] car audio stuff
>
>
> Hey Robotgroup -
>
> Any audio or auto electronics folks out there?  I didn't have an aux input
> to my radio in my car, so I cut the leads where the tuner goes into the
> circuit board on my radio and put in a switch for my ipod
>
> http://www.resistive.net/schematic.jpg
>
> I also added a power line so that I could charge it too (bought a cable
> with an ipod interface) The only problem is that there's a faint noise
> when the car's running (a whirring from the spark plugs and alternator
> noise - automobiles are notoriously noisy to work in power-wise)  I get a
> little bit of a difference when the ipod charger is plugged in too, so
> that bit of capacitance across the power and ground seems to affect it as
> well.  I tried to add a cap between the power and ground but it seemed to
> make the problem worse.  Anyone have any suggestions on how I can clean up
> my signal, I've done a lot of reading on ground loops but not quite sure
> if I have some sort of loop or if the power to my audio switching chip is
> just not clean enough (or both)
>
> I'm pretty close to giving up on the chip and sticking a relay in there, I
> think I tried to get too fancy with the IC.
>
> Any suggestions would be much appreciated
>
> -Nick
>
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