[Ham-Mac] Griffin iMic

[Chris Smolinski]

>On Mar 1, 2005, at 1:11 PM, Dick Kriss, AA5VU wrote:
>
>>Heads up to Kenwood TS-570 users on the iMic. I use the Griffin iMic for
>>sound out of the G4 only.  It was totally useless for sound into the G4 due
>>to a gross impedance mismatch with the audio output from my Kenwood TS-570.
>
>This is like having a 200:1 SWR on an antenna. You wouldn't expect 
>the antenna/rig combination to work very well at all. The iMic has 
>an input impedance in the 1K ohm range. Driving it from the 4.7 ohms 
>out of your radio gives an impedance mismatch that results in an 
>approximate 200:1 SWR on the audio line.

Since we're not dealing with power transfer here, we don't care about 
impedance mismatches. The iMic is acting purely like a voltage 
sensor. We're not trying to transfer any significant amount of audio 
power into the iMic. This is different, say, than driving a speaker, 
where we are trying to transfer as much power as possible.

>This really needs an impedance transformation, something that is 
>commonly done with audio transformers. Try an inexpensive Radio 
>Shack 1000 : 8 ohm transformer, with the 8 ohm side to the radio. 
>Not only should this work better, but it will also provide ground 
>isolation in case you are getting any ground loop problems between 
>the computer and radio. These usually show up in the transmit audio.

Use of isolation transforms on all audio cables in the shack is 
certainly a good idea, I use them myself.
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Chris Smolinski
Black Cat Systems
http://www.blackcatsystems.com