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Mic distribution

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edsetsdude

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I have a motorcycle with a factory audio system auxiliary input and use a Garmin GPS that has an audio output (to aux) and also a mic input on it.

You can pair a phone with the GPS and make calls using the helmet headset. Just needed a voice signal for outgoing phone calls.

I've made a mic splitting harness that uses a transformer to have a direct and an isolated mic signal, the isolated one going to the GPS mic input. That transformer blocks the DC from the GPS, which is good since the mic is dynamic... and therein is part of the problem, tho.

The dynamic needs a boost, perhaps to 60mV or so, not sure but testing tells me I need to yell into the mic to get the signal up to usable.

I would like to do this without the transformer, though, since it picks up engine and computer noises too easily.

So...I am going to use a TL074 and try to make one of the units a buffer for the incoming signal (capacitive coupled), then the other three will be a 'through' buffer, a '10x gain', and a 50x gain'. (or... Perhaps tap the first buffer for THRU and do a 10, 20, 50 gain with the other three OPs.

The GPS has an AGC circuit so that any signal higher than it's max is just clipped, but it does not seem to increase gain of any signal lower than the threshold. I've tried to analyze the GPS mic input, but about all I can say is it seems to have an input impedance of 1K ohms.

Any suggestions?
 
Not clear why you need the transformer to block DC. Couldn't you just use a coupling capacitor for that?
 
It is a 2-for-1 situation: the xformer is mostly for ground loop control and it also does the DC blocking. motorcycles are horrendous for bad grounding.
 
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