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.
The problem with just using the sound card output level is that once
the slider gets down around 10 - where it will be if you are driving
an electret input without an attenuator - it is quite difficult to
adjust accurately. Since a mic input designed to work with a dynamic
mic will be *another 20+ dB more sensitive* it is almost impossible to
adjust between "no audio" and "overdriving the mic preamp" with the
Windows mixer.
Remember, typical audio levels are:
Sound card at maximum: 1.5 V RMS
Electet mic: 75mV RMS (26 dB below sound card)
Dynamic mic: 5mV RMS (26 dB less than electret, *50 dB* less
than maximum sound card level).
73,
... Joe, W4TV
On 10/13/2013 1:14 PM, Dave wrote:
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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