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
----- Original Message -----
From: "Dr. Gerald N. Johnson" <geraldj@storm.weather.net>
To: <tentec@contesting.com>
Sent: Tuesday, September 04, 2007 9:03 PM
Subject: Re: [TenTec] O II and headphones?
On Tue, 2007-09-04 at 21:36 -0600, Richard Williams wrote:
Tom,
Yep, I have the same problem. I didn't notice the problem with my
older
Heil headset with the switchable elements; but it didn't have the noise
canceling feature.
Maybe it is something Ten-Tec needs to look into!
Dick K8ZTT
OK its solvable. What it means is that the headphone output has a
resistor to limit the power applied to the headphones and the headphones
are presumed to need a great deal less power than the speaker or the
noise canceling headphones. You can use an outboard amplifier with the
noise canceling headphones or identify that resistor and shunt it with
something having lower resistance to give the headphones more audio
power. It could be that resistor is wired on the headphone jack, but its
more likely on the audio output board. I don't think its a matter of a
coefficient in the DSP because most solid state audio amplifiers have a
very low output impedance so different loads shouldn't vary the audio
voltage much and the low impedance loads will naturally take more
power.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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