[Ham-Mac] iMic Impedance

[Chris Smolinski]

>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!

No confusion, this is the audio (speaker) output impedance. You are 
not concerned with the USB cable.

>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

It says that there is 150 uF capacitance in series with the audio 
output circuit, which has an 18 ohm impedance.


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Chris Smolinski
Black Cat Systems
http://www.blackcatsystems.com