On Tue, 2007-09-04 at 22:14 -0700, Ron Castro wrote:
> I'm afraid it's not that simple. Looking at the schematic, identified as
> "566AUDIO Model (1).pdf", you can see one of the more significant
> differences between the Orion and the Orion II. The O2 runs the audio
> through a PT2258 switcher, which I assume controls what audio sources is fed
> to the left and right phones. The outputs 1 and 2 are combined and fed to
> input 1 of an LM4811 stereo headphone amplifier
> (http://www.national.com/ds/LM/LM4811.pdf) and outputs 3 and 4 are combined
> and fed to input 2. The output of the LM4811 is fed directly through a 100
> uf 25 V cap to the headphones, unless you have the factory RFI mod board
> which simply adds an LC filter. No resistor in the circuit to bypass.
> Also, note that the specs of the 4811 show it has 16 digital steps of 3 dB
> each, and an output of 70 mW into 32 Ohms and 105 mW into 16 Ohms.
>
> Meanwhile, the speaker amp is fed from the PT2258 Out 5 and 6, which are
> combined and fed to U3, a TDA1013B 4-Watt amplifier
> (http://www.ortodoxism.ro/datasheets/philips/TDA1013B_CNV_2.pdf) which in
> turn feeds the speaker.
>
> The volume on the LM4811 is digitally controlled, so the solution the the
> problem is best handled in the firmware, although since the speaker
> amplifier is analog controlled, an appropriate resistor from pin 8 to ground
> would reduce the speaker level to match the headphones without burning up
> power in the amp. I have no idea what that value resistor would be, and it
> would no doubt be a major PITA to mount it on the board!
>
> Ron N6IE
> www.N6IE.com
>
So, what could have been done simply as it has been done in radios for
65 years has been done in a complex way that prevents the user from
adjusting the relative gains. Or is there a hidden menu option to set
the headphone gain? Shouldn't there be?
Or is it that the Heil noise canceling headset simply demands too much
audio power to work with a typical headphone level circuit?
Is there some other reason for the complexity, audio products that can
be fed to the headphones but not to the speaker, such as transmit audio
monitor?
Would it be practical for the Heil noise canceling headset to add the
closed circuit headphone jack to the speaker with a suitably selected
series resistor as used for many decades? A little box to be connected
between the radio and the speaker with a headphone jack and a headphone
level pot? Without breaking the speaker ground it might take a more
complex jack than a simple closed circuit jack, but such jacks are
available.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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