On Wednesday 24 March 2010 16:22:15 Joerg Reisenweber wrote: > This is *STRONGLY DISCOURAGED* and will break quite a couple of things. > Details see inline below > > [Stefano Cavallari Di 23. März 2010]: > > Yesterday night I was going to fix the poor audio response of the > > Freerunner. > > > Just before starting to solder (having opened the phone and the metallic > > plate) I discovered the caps I got were the wrong ones. > > So I looked at the scheme for an alternative solution, and I decided to > > try > > to > > > replace the audio caps with 0R, thus losing DC blocking. > > You're not only losing the DC-decoupling, you're also losing the negative > half of the sine wave, when amp is basically shorting output to GND to > create the negative current by discharging the coupling capacitor. See > operation principles of analog bridge amp outputs in some good book about > electronics. This is NOT a class-A amp > (http://en.wikipedia.org/wiki/Electronic_amplifier#Class_A ), means it > never opens both the pullup and pulldown transistor concurrently. The > LM4853 is a class-B push-pull bridge. I should have read the data sheet. Audio does not seem distorted though, except at high gains. There is a some white noise in the background, not noticeable when music is running though. Anyway I was ready to use external capacitors in the adapter cable. > > The plan was to measure the DC component and if low enough, leave it as > > it. If not, putting the DC filter in the minijack adapter. > > This is basically feasible, but will most surely break JACK_INSERT logic, > by applying a voltage >0V to the GPIO detecting if a jack is inserted or > not. Each time you enable the amp to output some sound the headphones, it > will latch up and not detect jack removal. And it's quite unlikely jack > insertion is correctly detected each time as well, there also might be > both false positives and false negatives. Jack sensing still works and seems reliable. Anyway if it stop working or it proves to be not reliable I can just disable it. I'll never use an headset, just headphones for playing music. I can force the output to the jack when starting the player. > > So I did that, and it seem to work. I tried first with a multimeter. It > > reads > > > 0.2 V DC, but I have to confirm it with an oscilloscope. > > This reading probably is with headset amp disabled. Correct reading should > be Vmid, i.e. ~1.6V > > > I tried the audio with very cheap headphones first (I was afraid of > > burning them), > > Chances are you actually will end up with broken headset speakers, just > because of this > > > then with decent ones. It seems to work way better! > > Now I just need a better adapter cable (mine need to be inserted middle > > way, it's not the right one), and then I have usable audio :) > > Don't you think, if this was a viable workaround for the problem, we at OM > (particularly me in this case) came up with this suggestion some 1.5 .. 2 > years ago?
It depends on what you mean for "viable". It is not something correct, and I was aware of that. Building a custom cable with non-SMD capacitors is easier than finding "right" capacitors. Risking some cheap headphones that will sound way better than good headphones on a unfixed phone is even easier. I just wanted to share my experience, maybe someone finds this compromise useful. -- () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments
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