no-cap bass fix

Stefano Cavallari stefano at
Thu Mar 25 02:16:05 CET 2010

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
> ( ), 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" 
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 

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