[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:
> 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