Pulster fixe(s) and rework

Andy Green andy at openmoko.com
Fri Mar 6 10:40:28 CET 2009

Hash: SHA1

Somebody in the thread at some point said:

|> There are a couple of 1K resistors to 0V that will then connect directly
|> ~ to the amp outputs all the time before the new DC blocking caps you
|> will add back in.  But thanks to some recent patches by Mark Brown on
|> andy-tracking, we should  keep the amp turned off more often.
| Can these be safely removed? Are they there just to make sure the
| capacitors slowly drain, so you don't get a pop when plugging
something in?
| Or do they have other purposes as well?

The headphone action is overloaded with a digital connection to the GSM
chipset.  It's only used in factory though.  I guess the 1Ks were added
as some protection against large voltages developing on what would be
floating nets on headset insertion, etc.

|> |> Another question - is a single big capacitor enough, if it is put into
|> |> the ground line  instead of having one cap for each of the stereo
|> |> channels? Or will that wreck stereo sound? One could then use a even
|> |> bigger cap.
|> You'd need to do both channels; the one with the unchanged cap will
|> sound the same as always otherwise.
| You misunderstand. I did not propose to do only one channel.
| I planned on shorting both the small caps. Then, instead of one big cap
| on each of the stereo lines: LEave the stereo lines connected as-is.
| Break the ground line (which is common to both channels) and insert a
| single big cap there instead. Slightly less work, and perhaps a bigger
| capacitor will fit. (It'd probably have to be bigger too, as the two
| channels often enough have the same signal.)

Yes I didn't get your point.  But I don't think it's the same action as
one cap before each transducer and a direct common low-impedence ground
reference.  The two signals will mix where they join at the single
capacitor and the impedence depends on the frequency.

|> |> There is lots of easily accessible room next to the battery, above the
|> |> SIM card.
|> |
|> | I'd love to see a good answer to those questions.  Currently, it's
|> | unusable as an MP3 player and that's an important use for me (if
|> | I could use it as an MP3 player, I'd carry it with me that much more
|> | often, which would in turn increase my use of it).
|> Lifting the can and meddling with the caps is nontrivial.  Somebody did
|> give this plan a go on the list about 6 months ago and reported some

Well, point taken.

| I hope some thought goes into this for the gta03. Sound output from a
| phone obviously has low power, but it should be as hi-fi as the sound
| chip allows. Ideally, a balanced output that don't use (or need) caps.
| One can play uncompressed wav files, and use a high-quality headset or
| connect to a regular stereo system.  Openmoko could have sound
| enthusiast customers, as well as linux enthusiasts.

We have 47uF series caps on this circuit right now on 3D7K (new name for
old GTA03).

- -Andy
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