[Ham-Mac] Griffin iMic

[Dick Kriss, AA5VU]

Jack,

Thanks for the tip on the Radio Shack 1000 : 8 ohm transformer with the 8
ohm side to the radio.  Maybe I could add the transformer inside the
SignaLink SL-1?  FYI, the following is a quote form Griffin when I
questioned the interface of to the iMic.
------------
> Snip ............... An impedance mismatch with the line amplifier
> (inside the iMic) might cause the signal coming out of the iMic to be lower
> than normal but we rarely see this problem anywhere.  The iMics signal level
> is enough to drive more headphones which are typically of around 30ohm
> impedance.
> 
> The output impedance of the iMic is AC coupled 18ohms at 150uF.
------------
I never obtained a good understanding of the expected iMic input and each
time I ask the question they talked about the output of the iMic. My guess
is the iMic input is 300 ohms, like many sound systems. Jack's analogy of it
being like having a 200:1 SWR on an antenna is right on!

Dick, AA5VU


On 3/1/05 3:42 PM, "Jack Brindle" <jackbrindle at earthlink.net> wrote:

> 
> 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. 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.
> 
> Good luck and 73!
> 
> -Jack Brindle, W6FB
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