[Ham-Mac] iMic Impedance

[Dick Kriss, AA5VU]

On 3/3/05 7:20 PM, "Steve Lawrence" <SMLX at earthlink.net> wrote:

> FYI - The following is from Griffin Tech support in response to my
> query of some months ago about the impedances of the iMic.
> 
> 73 - Steve WB6RSE
> _____________________
> 
> Input is 6630 ohms
> Output is 18ohms
> 

If the Kenwood TS-570 has a 4.7k ohm output and the iMic input is 6630 ohms,
I don't understand why there is a problem. It does not seem to be gross
mismatch provided we are talking about the same ports. (see note below)

I key the TS-570 via the G4's iMac/USB interface to the iMic and use the
the Speaker icon port on the iMic to key the rig via the SL-1+ interface.
This works FB!

I had/have problems using Microphone icon port on the iMac as an input to
the G4. I wonder what Griffin means by the 18 ohm output. Is this the
Speaker port (that I use for transmit) or were they referring to the output
to the Mac via the USB cable? These terms are confusing!

Steve, what do you think they were talking about?  If it is the output from
the iMac to Mac (or audio in to the sound card) this may be the problem. I
have no idea what the Mac expects.

The following is copied from my earlier message. To compound the academic
exercise Griffin's response to my early inquiry said

> 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.

To me this says the 18 ohm output is the USB interface to the Mac, not the
Speaker icon that I use to key the rig.  I am not sure how this tracks with
the 6630 ohm input Griffin provided to Steve

Yes, I am confused by something that should be easy. All I know is my G4 is
semi deaf to incoming signals from the iMic and the only fix I found was to
bypass the iMic.

Dick, AA5VU