[TenTec] Head-Phone volume in Orion2

[Dr. Gerald N. Johnson]

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