From: Chris Muth <rockbox_at_muth.cc>
Date: Mon, 19 Jan 2004 15:33:06 -0600

On 1/19/04 at 8:21 AM [IDC]Dragon wrote:


>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.
>
The 3300's were the only two identical caps in my parts box that were large enough.
All the rest that I had two or more of were little 100,000pF (and such) ceramic caps.
The 100Kohm's were on the Archos side of the caps. They are hooked between
the (-)cap/JBR and ground. The (+) side of the cap is hooked directly to the main
board in the radio where the tape deck board would normally have hooked up.

>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.
>
I noticed that if the 10v audio line is pulled to ground, it will effectively mute
the audio that was going out to the speakers. When I had just the caps and
no pulldown resistors, plugging the JBR in would mute the audio. The JBR side
of the caps were at 10v just like the radio side. I wanted something big so that it
would not cause the audio to get muted when I turn the radio on without even
plugging the JBR in. I just had the 100Kohms sitting around from when I built
an extra 10:1 attenuator for my scope (beyond the 10:1 that the probes were
already)

>
>See?
>
Possibly.
Received on 2004-01-19