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

zerghase zerghase at in4matiker.de
Wed Nov 26 23:58:39 CET 2008

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 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


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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