You get many solutions because there ARE many workable solutions.
Including a simple few hundred ohm series resistor while connected to
the speaker circuit to cut the headphone level.
I would say that MOST Scout users use the Scout mobile, and don't drive
with headphones.
The matching transformer works when you have acquired headphones with
the wrong impedance to match the radio. Headphones with impedance higher
than 600 ohms work all the way out to 10,000 ohm headphones, I'm sure.
Headphones need a few milliwatts of audio and the radio can supply a few
watts, so the only thing critical is to get the level comfortable
without excess noise or excess overdrive of the headphones. A simple
series resistor or rheostat suffices. Might lose 25 or 30 dB, that's
alright, you WANT to lose that much.
Is there something WRONG with surplus headphones? All my life I've used
surplus headphones, something like 52 years since my first crystal set.
They have quite redeeming features, with narrow bandwidth being prime.
That is very appropriate to the communications bandwidth of CW and SSB
radios, and so ignore some of the circuit hash and rumble. Then they
generally have low cost. Last year at FD I used some Icom radio with
super wideband headphones. All the time I was regaled with the rumble as
if I was sitting along side the UP main line. Had I been using surplus
headphones I would not have noticed that VLF rumble and I'd have copied
exactly the same number of stations. The 4 ohm stereo headphone is not
the BEST headphone for amateur radio, its about the least suitable.
It could well be that the Scout was designed with 500 ohm output
impedance to be a HINT to NOT use 4 ohm hifi stereo headphones. The book
does say that 600 ohm (or higher) impedance phones are most appropriate.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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