Without wanting to start another discussion like that on impedance verses
radiation resistance recently, I did say "and I would always advise..."
The isolation avoids earth loops, the variable resitor avoids overdriving,
if you are taking the speaker output straight to the mic input then the PC
speaker audio level can be used, but like Joe, I'd always prefer a system
with a variable resistor external to the PC, but that's a personal
preference. You can still reduce the audio from the soundcard program and,
infact, this is what another bit of kit called the "PSK Meter" effectively
does. http://www.ssiserver.com/info/pskmeter/
Dave (GDJA)
----- Original Message -----
Sent: Sunday, October 13, 2013 12:01 PM
Subject: Re: [Elecraft] Interface and Ham Radio Deluxe
An isolation transformer does not solve the excessive level problem.
For that you need an attenuator or voltage divider. Typically one
would connect a 4.7K or 10K resistor in series between the sound card
output and the mic input and a 470 or 1000 Ohm resistor from the mic
input to the mic ground. That 20dB is about right for the sound card
line/speaker output to an electret mic input. Substitute 47K or 100K
for dynamic mic inputs or use a 500/1000 Ohm pot with the wiper to
the mic input instead of the fixed shunt resistor.
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