dynamic mic to electret input, with splitter - diyAudio
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Old 2nd May 2015, 08:13 AM   #1
clavin is offline clavin  United States
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Default dynamic mic to electret input, with splitter

Total NOOB here! Be gentle.

I want to build a passive circuit to split a dynamic mic source to 2 inputs: 1 is a handheld (battery operated) radio, that expects a dynamic mic, and 2 is a recorder, that expects and is tuned for an electret mic input. The sound quality does not have to be all that great and it is voice only. This is for a racecar application.

I simply wired the mic to both inputs, which made the radio work just fine but you have to yell into the mic to get anything on the recorder. After that disappointment I did some research on what the heck a dynamic and electret mic are and how they work and I'm a little surprised that it works at all. I guess the small voltage supplied by the recorder (to power an electret mic) isn't affecting the radio negatively. The recorder has some gain controls but I have it up at max and it is at that point where yelling is only somewhat audible.

So I found a circuit online that has a transistor which boosts the voltage of the dynamic mic to something usable by an "electret input". Dynamic to electret mic . It seems reasonable to me at a glance and meets my need to have a passive circuit.

Will I be able to safely split the mic signal at the source, before the booster circuit, and have both radio and recorder work correctly?

The more expensive car harnesses (radio audio only, not considering an additional recorder) have an isolation transformer on the headphone side of the circuit, to cut the noise from the car's electrical system when the radio is powered by the car. In my case the radio is self-powered but the recorder will be powered by the car. Will it help me at all to add a transformer on the mic side?

Thanks so much for taking a look. It is actually more complicated than that even, but let's start here so I can understand the basics.

Last edited by clavin; 2nd May 2015 at 07:16 PM.
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Old 5th May 2015, 09:11 AM   #2
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Mic inputs are usually AC coupled, plus the few hundred ohms of the mic would reduce bias voltage considerably (voltage divider), that's why the radio isn't bothered.

The way you describe the setup, you should not need an isolation transformer (the radio is not connected to the car's electrical system anywhere else, just mic and headphones or internal speaker, right?). I don't see much of a problem with splitting the mic either; estimated base current is so low that input impedance of the mic-to-electret booster should still be well in the kOhms. Just make sure everything is properly shielded, including the booster.

You may want to optimize the booster circuit a bit. For this, first measure unloaded electret bias voltage at the input, and later note what sort of base-emitter voltage you get at the transistor. If it's more than half the unloaded voltage, you'll want to reduce the bias resistor value, while if you get close to 0.3-ish V, you'll want to increase it. Using a high-beta transistor like BC549C/550C, 2N5088/89 or KSC945 may allow inserting a degeneration resistor between emitter and ground for improved linearity (but don't overdo it, maybe 100-150 ohms or so). Do this particularly if you find that you get ample volume with the booster in.

Note that the single-resistor bias is quite basic and means that your operating point will vary a fair bit depending on transistor beta (which in turn is hardly a constant and also depends on temperature, for example).
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Old 5th May 2015, 12:50 PM   #3
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Pretty much just guessing, I wonder if something vaguely like the below might work for you. It's from Why Not Wye?
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Last edited by bentsnake; 5th May 2015 at 12:52 PM.
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Old 5th May 2015, 03:27 PM   #4
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Hold it. Or much more like this. The below is again adapted from Rane's "Why Not Wye?" I think it might actually work. Female jacks are shown, but the mechanics can be whatever's convenient.
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Old 2nd August 2015, 11:19 PM   #5
clavin is offline clavin  United States
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Sorry it's taken me so long to get back to this. All your tips have been well regarded!
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