Electret Mic "Splitting"

Question

I've got a Baofeng UV-5R . I've done some research, and I can see how its mic/headphone plug is wired . It seems to me like it expects an electret mic as the element. I've got a a Rexon RHP-530 airband radio. I am pretty sure it has the same wiring as the Baofeng, and wants an electret mic. I'v got a Lexin LX-B2 bluetooth intercom . It has a completely different wiring, but I took apart the boom mic that came with it, and it appears to have just an electret element in it. It has its own plug configuration.Unlike the radios above, the mic terminates separately from the headphones, with the mic wired to a 3.5mm trs. Only the tip and sleeve are wired. the ring seems unterminated. So... the point of all of this... I would love to cobble together some interconnect that allows me to attach the electret to all 3 devices. They each obviously have their own PTT. Can I just connect all three devices to the electret in parallel? Is there some small circuit (opamp? transistor?) I can build to "fan out / share" this one mic element? Any thoughts would be appreciated. Thanks, Buzz

Gough Lui · Answer

As mentioned by dougw , electret microphones are biased with a DC voltage and produce a very small signal in response to vibrations in air. You shouldn't just connect them together for a number of reasons: There may be too much bias as suggested by Doug. The audio signal when "split" amongst them all may not be sufficient to reach full deviation - this may be masked by the AGC in the radio (if one exists - the Baofeng are notorious for being poor in this regard), but would result in poor signal to noise ratio. The bias voltage sources may be different (not all radios/HT will have the same source voltage - this may depend on the battery). This could result in a continuous circulating current which could induce hum in the audio, damage biasing resistors, drain power from one or more of the radios. Faults or interference from one radio may propagate to another - especially when it comes to transmitting, the connected cable may allow for unintentional RF radiation into an adjacent radio which could cause it to malfunction. Other times, poor design of a radio that may generate internal spurious emissions may travel to another radio via direct connections that could lead to desensitisation of the receiver. Ideally, I think a bit of careful circuit design is necessary - in essence we are sort of building a mixer-in-reverse. Perhaps you could: Make a standalone circuit that provides the bias to the microphone from another battery. Use separate op-amps as voltage followers or amplifiers to produce the drive into each radio. Pass the output from these amplifiers through a potentiometer so you can adjust the drive into each radio to its optimal point. Add a DC-blocking capacitor in the path to prevent the bias current from going up into the opamps. Add ferrite beads to suppress RF interference from travelling up the cables. Add the necessary PTT buttons for each of the outputs. Optionally, add some sort of level meter if you want to see that it's all working well. Things such as the drive voltage required for full deviation will vary from radio to radio, so it's really about making the design as flexible as possible to accommodate the needs of each. For really annoying issues (e.g. where radios cannot share a common ground), then you may also need to add in isolating audio transformers. Just my 2c. - Gough

dougw · Answer

Simple Electrets are usually biased(powered) by a pull-up resistor (about 2.3K for 5V power supplies). If each target input has one of these bias resistors, the electret would have too much bias current (which would reduce volume). Many electrets have a built-in FET (still having just 2 leads to the mic) which may be able to handle the extra current.

A single transistor buffer should work, or you could try to disconnect the bias resistor in 2 of the input stages.

Electret Mic "Splitting"

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