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

[zerghase]

I think I understand. I built one 10µF capacitor into the adapter (at the
mass), so the problem with the headphones should be fixed. I also measured a
1,59V current at the capacitor when plugged in and silent. Sound is good as
before.

Unfortunately, the internal speaker sounds really bad now (distorted). So I
think i'll remove the silver again and find somebody who can help me with
the soldering...

Al Johnson wrote:
> 
> 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
>> >
>> > _______________________________________________
>> > hardware mailing list
>> > hardware at lists.openmoko.org
>> > http://lists.openmoko.org/mailman/listinfo/hardware
> 
> 
> 
> _______________________________________________
> hardware mailing list
> hardware at lists.openmoko.org
> http://lists.openmoko.org/mailman/listinfo/hardware
> 
> 

-- 
View this message in context: http://n2.nabble.com/-sound-%3A-capacitor-that-act-like-a-high-pass-filter-and-so-removes--the-bass-on-the-headphones-jack-tp1502610p1583669.html
Sent from the Openmoko Hardware mailing list archive at Nabble.com.