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Subject: Re: JukeBox in the car
From: [IDC]Dragon (idc-dragon_at_gmx.de)
Date: 2004-01-19


Chris,

> I used two (one for left and one for right)
> 25V/3300uF caps to
> couple the ~0 volt audio onto the ~10 volt audio lines.

Way too large, something like 100uF will do fine. The Archos has 220uF
internally (the older ones had less), but that's for driving headphone impedance.
It's a good idea to have external caps if the voltage you're coupling to is
so high. The internal caps are only good for 4V or so. Use pulldowns on the
Archos side, to prevent that from being raised.

> To keep
> the caps from charging to the 10 volts, I put 100Kohm pull down resistors
> on the line in side of the caps.

You don't need such high impedance. That unnecessary slows down the leveling
of the caps when you switch it on. The Archos is driving with its headphone
output, which has maybe a few Ohms impedance. It's build to drive ~32...100
Ohms, so you should be on the safe side with a 100 Ohm pulldown. If you use
higher ones, the voltage jumps up too much on power-up, the resistor won't be
able to hold it down, you're overloading the internal caps which is harming
them.
Technically, you have a voltage divider between the inner resistance of your
deck and the pulldown.

> It seems to need a minute or so after power on before I plug the JB in, or
> the audio will become rather bad.
> It had lots of distortion, static and such. I think what happens is that
> the whole cap gets driven to 10
> volts and the pull down resistors slowly pull the low side of the cap back
> to zero.

See?

Jörg

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