[Ham-Mac] Griffin iMic

[Chris Smolinski]

>Bill and others,
>
>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.
>
>I don't know about other Kenwood rigs; however,  the fine print in the
>TS-570 user manual says Pin 3 ANO (from the ACC2 port) has an output
>impedance of 4.7 ohm and God only knows the input impedance of the Griffin
>iMic.  Griffin's tech staff talks in music audio standards and were not
>prepared to address data.  The bottom line is the iMic acted like a signal
>attenuator and little on no audio made it from the TS-570 to the G4.  Yes, I
>know the iMic is an A-to-D converter but it acted like an attenuator due to
>the impedance mismatch.  I finally just ran a standard stereo jumper from
>the SignaLink SL-1+ interface box to the G4 sound input and it works FB.
>Sorry to hear the new Mini does not have sound input . BTW, I still have the
>iMic in line and use it for the transmit side. It works great with the
>SignaLink SL-1's auto PTT feature. The iMic uses the USB port and this lets
>me retain the internal Mac speaker for standard system beeps and Mac stuff.
>If you plug something into the G4's sound output jack it mutes the internal
>speaker. I tried running external powered speakers via a Y-connector but
>found the Mac system beeps to be QRM generators.  The iMic solved all kinds
>of issues on the transmit side. I wish it would work on the input side.

Well, we're all amateur radio operators, so it should be fairly 
trivial for us to take some quick measurements, and come up with some 
back of the envelope calculations to see what is happening.

I have an iMic, so I did some quick tests, all at 1750 Hz, which I 
assumed would be a good freq since it is in the middle of the SSTV 
audio band. First, there is a small switch on the iMic which sets the 
input level range. In the normal setting, a 1V p-p signal produces a 
full response (just at the clipping level) with the iMic. This makes 
sense, 1V p-p is a common audio level. Flipping the switch the other 
way changes the input range to 0.1 V p-p, that is, ten times more 
sensitive.

I then connected my function generator to the scope input, and 
connected/disconnected the iMic. I could see no change in the audio 
level on the scope. This would mean that that iMic input impedance is 
much higher than the generator output impedance, which in my case is 
50 ohms, which is an order of magnitude more than your quoted 4.7 
ohms for your radio. I than ran some quick tests using a resistor in 
series with the function generator and the iMic, so that the resistor 
and iMic became a voltage divider. With a little hand-waving (since 
we're ignoring any capacitance effects here) I came up with a 
ballpark value of 10K input impedance for the iMic. This was with the 
input range switch in either position.

So, what can we conclude? The iMic is a high impedance device, which 
makes sense, as most audio input devices are. So it is operating in a 
voltage mode, that is, the A/D is looking at the voltage in the 
input. We're not concerned with power transfer here. So "impedance 
mismatch" is completely irrelevant in  our situation. That's like 
saying you have an impedance mismatch when you use your voltmeter 
with a 1M ohm input impedance to look at the output of a circuit with 
a output impedance, say a power supply.

What is happening?  I see three possibilities, there could be more:
1. The 570 is not capable of producing a sufficient output voltage to 
use the full range of the iMic. This seems unlikely as only a 0.1V 
p-p range is required.
2. The 570 does not like it when a high impedance load is connected. 
This seems unlikely, but if you want to test this theory, connect a 
suitable resistor across the iMic. The 570 will see that as the load 
impedance. The iMic will not care, remember, it is operating in 
voltage mode.
3. There is a wiring error.


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