[sound]: capacitor that act like a high-pass filter and so removes the bass on the headphones jack

[Al Johnson]

On Wednesday 26 November 2008, zerghase wrote:
> Thank you for your advice.
> An adapter with an additional power source is out of the question for me. I
> do have a USB-headset, but that's out of the question, too, because I want
> to be able to play music while charging the Freerunner.
> So, I disassembled my Freerunner, and shorted the two caps you were talking
> about in the other thread[1][2]. I used conductive silver, because I don't
> own the equip that is needed for SMD soldering, and the danger of
> destroying something without heat is minor. Then I built an adapter for my
> headphones, and tested it with different capacitors. With 1µF, the bass was
> cut off just as with the internal 1µF caps. Then I tried 10µF, 100µF and
> even without any capacitor, and I had always a good sound with full bass,
> and there was absolutely no difference between the three solutions.
> I have no clue why the capacitor is needed (or why it's needed at all), but
> for me it works great without it. Maybe it is handled already inside the
> IC? Is this possible?

It's good to hear someone's actually tried a mod :-)

Caps in that position are usually to block dc on the output, especially with a 
single sided power supply. You may want to check for dc on the output before 
running without caps for an extended period if you value your headphones. I 
haven't checked the datasheet for this amp so it's possible it's handled by 
the chip already, but I would be surprised. Bigger caps will give a lower 
cutoff frequency, but if 10uF sounds good you may as well stick with it as 
they'll be physically smaller.

> It would be great if there were no objections. Then everybody could do the
> HW fix for the sound issue, even without soldering skills and without
> risking damage.
>
> [1] http://lists.openmoko.org/pipermail/hardware/2008-September/000558.html
> [2] http://www.die-informatiker.eu/zerghase/capacitors.jpg
>
> Sincerely,
> zerghase
>
> Al Johnson wrote:
> > On Wednesday 26 November 2008, zerghase wrote:
> >> Can someone please explain, how such an adapter had to look like (maybe
> >> a circuit diagram), because I'd like to build one.
> >
> > It'll be an active circuit so will need power. That means batteries, or
> > getting power from USB. Most likely it'll use an opamp or headphone amp
> > chip
> > in an active filter arrangement. It's almost a textbook case of a bass
> > boost
> > circuit - just pick the frequency so the boost starts at the same
> > frequency
> > as the bass rolloff starts on the Freerunner. This should give you some
> > ideas:
> > http://headwize.com/projects/showfile.php?file=equal_prj.htm
> >
> > Having said that it'll be quicker, cheaper and easier to buy a usb
> > headset.
> > You can always cut off its headphones and solder on a 3.5mm socket
> > instead.
> >
> >> I think in most cases, people will use an adapter
> >> from the 2.5mm to a 3.5mm jack anyway.
> >> Will the audio quality be equal with an adapter to the hw fix?
> >
> > It'll be close if you do it well.
> >
> >> If not, is  there a description
> >> of a hw fix for it (Joerg, you say there are different ways, so i assume
> >> you have already
> >> done or at least designed one)?
> >
> > This thread is the closest so far AFAIK. I haven't actually tried it yet.
> > http://lists.openmoko.org/pipermail/hardware/2008-September/000558.html
> >
> >> Sincerely,
> >> zerghase
> >>
> >> Joerg Reisenweber wrote:
> >> > Yes, sounds like a good idea (input impedance should be (much) > [&GT]
> >> > 1000Ohm
> >> > though, not <1k)
> >> > This adapter also could implement some hw-equalization for the
> >> > highpass-filter
> >> > created by the 1uF*(1/(1/1k + 1/(33R + adapter-impedance)). (where, in
> >> > sequence, values are from: C4111:1u, R4117:1k, R4407:33R. Right
> >> > channel analogue)
> >> >
> >> > Requests for assistance welcome.
> >> > cheers
> >> > jOERG
> >> >
> >> > Am So  16. November 2008 schrieb Scott Carlson:
> >> >> Would also be possible to create a small inline adapter for head set
> >> >> that have <1k input impedance? It may be convenient to make a batch
> >>
> >> and
> >>
> >> >> sell them
> >> >> cheap? (As a non-intrusive) hw fix.?
> >> >>
> >> >> SCarlson
> >> >>
> >> >> > Sorry the audio is definitively broken due to the capacitor issue
> >> >>
> >> >> you're
> >> >>
> >> >> > mentioning. There is no sw-fix either.
> >> >> > You may get (semi)decent audio by:
> >> >> > o- using a home-stereo line in, which has >1k input impedance
> >> >> > o- using high impedance headphone (>600 Ohm)
> >> >> > o- do a rework on the capacitors (there are different ways to do
> >>
> >> that,
> >>
> >> >> all
> >> >>
> >> >> > need excellent soldering skills)
> >> >> > o- use a bt-headset
> >> >> >
> >> >> > very sorry I didn't achieve to fix this for MP yet
> >> >> > cheers
> >> >> > jOERG
> >
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