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.
Though I agree that a radio the price of the Orion II ought to have
matched headset and speaker levels from the factory, every version of
the firmware.
>
> 14060 MHz 50 uv in to RX Antenna.(essentially S9) Spot Tone 400 Hz ,
> Sidetone
> set at 100 (max) . Received signal output level set to comfortable headphone
> level.(Approximately half of the audio bar level readout with my phones)
>
> ver 2.037j firmware . I have not measured the latest beta.
>
> OPEN CIRCUIT LEFT MAIN RIGHT MAIN
>
> set for comfortable 55.1 mv 55.5 mv
> level (approx 1/2 bar)
>
> SPOT 101.8 mv 102 mv
>
> CW Sidetone 50.7 mv 51.0 mv
>
> NO signal 3.7 mv 3.7 mv
> residual noise
>
> So from the above with about 27 ohms source impedance , you can deduce what
> the level will be with various phones.
>
> I use Sony MDR V6 phones which are about 70 ohm drivers and not very
> sensitive (I cannot remember the spec right now) and the cw spot tone level
> to me is marginal.
>
> With input signal as above S9 , Maximum output level open circuit wide open
> 1.2 volts RMS , about 3.5 V pk-pk and no visible clipping of clipping. All
> measurements on a Fluke 179 true RMS meter.
>
> I know of several guys who have done what was suggested and they just use
> the speaker output
For the phones?
> with appropriate attenuation. You of course lose the
> ability to listen to the sub rx in one ear and the main rx in the other ear
> but you solve a whole lot of other issues.
>
> 73 de Hank K7HP
>
73, Jerry, K0CQ
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