The OII has an entirely separate headphone amplifier IC from the speaker
amplifier IC. The master AF gain affects both speaker and headphones - but
the basic levels are settable in firmware . Both IC's are basically 3 or so
ohms output impedance but the headphone path is stereo - the speaker is
mono. A later mod of the headphone output from TT added a RF filter on the
headphone output which adds about 24 ohms in each side of the headphone
output.
The speaker IC is a TDA1013B - the headphone output amp is a LM4811 - the
LM4811 is driven from a PT2258 and out5 and out6 of the PT2258 are tied in
parallel to feed the input of the TDA1013B - theTDA1013 has a a 100K
resistor to ground fed from two 560 ohm isolation resistors from out5 and
out6 of PT2258. That 100K could be lowered to lower speaker path gain- or
perhaps raising both 560 will also but that is more involved.
All this is on the Orion II schematics on the web site . A15 566 Audio.
FWIW -
73 Hank K7HP
----- 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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