On Sun, 2009-08-30 at 10:48 -0600, Dr. Gerald N. Johnson wrote:
> On Sat, 2009-08-29 at 20:08 -0700, pfizenmayer wrote:
> > The headphone output IC in the OII is a stereo IC LM4811. I measured about
> > 2.75 ohms output impedance at the headphone jack before I installed the RF
> > filter . The RF filter in later OII's and the board that TenTec supplies
> > has
> > surface mount ferrite loaded inductors and the total series resistance in
> > each lead is about 24 ohms.So output Z is about 27 ohms each side.
>
> 27 ohms Z with 32 ohm phones cuts the available power by a factor of 4.
> 6 dB. When the headphones are weak already, that's a serious drop. There
> might be a need for old fashioned, non tiny inductors to get the
> resistance down to a few ohms. Clearly driving 32 ohm headset through an
> impedance step up transformer is not a viable option.
>
> So there's one other option, adding an outboard headset power amp.
Like a pair of RS 277-1008 or single 55021916 or single 33-118 or others
from other sources. The first ones you'd have to make a custom cable,
the rest just plug in. A sound card external speaker power amp would
probably do just fine.
>
<SNIP>
73, Jerry, K0CQ
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