Thread: trying to get an electret mic to work with a radio looking for a carbon style

All standard aircraft radios (since WWII) are designed for a carbon mic. The Lightspeed and all modern electret mic headsets are designed to operate correctly on the (carbon mic) bias current that the aircraft radio feeds to the mic. The headset has an amplifier which operates on the aircraft radio's bias current, and produces an output level equivalent to what a carbon mic would put out, but with much less background noise and better fidelity than a carbon mic.

If you connect an aircraft intercom (portable or panel-mounted) or an audio-panel between the headset and the aircraft radio, during the time that the transmitter is not keyed, the intercom supplies the mic bias current to the headset microphone amplifier. While the transmitter is keyed, the headset mic bias comes from the transmitter; not the intercom. The mic bias switching is done automatically inside the intercom using the PTT line.

A normal test if using a portable intercom is to unplug it, and plug the headset mic directly into the transmitter's mic jack. The transmit audio level should not change with or without the intercom. Some intercoms optionally (internal jumper?, rear connector wiring option?) deliver bias current (instead of the transmitter doing it) while the PTT is keyed, but this is not commonly done. A lot of aircraft have a "failsafe" mic jack which is wired so as to bypass the intercom/audio panel in the event of a failure. I carry an old hand-mic in my glove box for that possibility in my 182.

If someone wired a series capacitor between the intercom mic output and the mic input to the transmitter, that would block the transmitter's bias current from reaching the headset mic during transmit.

btw- I have wired dozens of intercoms/audio panels/radios for aircraft. I have a pretty good collection of pinouts for most common ones. What audio panel do you have?

ps- I have Lightspeed 20Gs in my 182.

Originally Posted by jfarrow

Interesting.... as I said, I am trying to install the Northern Airborne Technologies (NAT) aa12-022. The install diagram is here:

http://www.northernairborne.com/pdfs...s/804-0410.pdf

As in most audio panels there is a pilot isolate switch on the NAT intercom/audio panel that, as you said, connects the mike and headset of the pilot directly to the radio. I notice when you flip that switch the mic bias voltage drops to zero from 4 volts. My question is how does the old system manage to pass voice to the radio and why is the NAT unable to do the same? When not in isolate mode there is a voltage on the mic when transmit is pressed and still nothing.

If it were installed the way most audio panels are, while either Pilots or Copilot's PTT is keyed, the bias applied to the respective mic would be coming from the radio; not the NAT! The fact that the mic bias disappears when the PTT is pushed means that either the Marconi transmitter is bad, or there is missing connection between the NAT and the COMM, or there is a dc blocking capacitor in the way! Pin 12 of the NAT should go directly to MIC INPUT of the Marconi transmitter.

Originally Posted by jfarrow

Is the solution as simple as taking out the capacitor and resistors? I have to wonder why they are in the circuit? It seems to me the capacitor is either protection for the radio or to isolate the radio from dc voltage while the resistor is to match impedance? The last thing I want to do is blow up the radio! Should I connect the audio panel mic out directly to the radio mic in?

Yes, the NAT install manual shows the direct connection. It is possible that the resistors and capacitor was someone's attempt at reducing the transmitted noise level because you have a high ambient noise level aircraft (Jet?)

All aircraft radios I have ever worked with are designed so that the carbon mic (or amplified dynamic or amplified electret) are intended to be connected directly between the MIC HI connection on the radio and ground. No intervening resistors/capacitors are required or normally used.

Obviously, your mic element is not getting any bias current when the transmitter is keyed. I do not understand what the extra resistors and capacitor is intended to do.

Originally Posted by jfarrow

BTW, I notice you say current not voltage. Is that an important distinction?

The aircraft radio transmitter back-feeds the carbon mic element from an 8V internal regulated power supply though an internal 510Ω resistor. If the mic element was shorted, the current would be ~15mA. If the mic element is open, the voltage across it would be 8V. If the mic element is nominally 500Ω, then you would see 4V and 8mA though it.

The effective resistance of carbon mics varies all over the map, so the voltage across the element can practically be as low as 1V or as high as 7V.

Would an Electret capsule develop enough AC volts (even shouting into it) to drive a "Carbon mic" input?

Originally Posted by marcbarker

Would an Electret capsule develop enough AC volts (even shouting into it) to drive a "Carbon mic" input?

You use an amplifier that boosts the output, and drives the supply rail - old UK telephones used a carbon mike, and they later brought out a fully compatible electronic replacement - just connected on the same two wires.

Originally Posted by tezaa

1) So why is this 'specific' voltage range listed?

2) Dynamic mics don't require bias voltages to 'operate' - do they?

3) Am I able to bias an electret microphone with a lower voltage without damaging it? (eg: +5Vdc). (NB: This is for personal use - I'm not modifying the aircraft radio).

4) I assume changing the bias voltage will change the microphones impedance, but does this really matter, if I can just 'turn up input gain'?

5) Are carbon mics actually still used in aviation these days?

1. supplies a buffer transistor inside an electret Mic. Transistor just provides electrical isolation between diaphragm and external world. A plug-in pc soundcard mic is an electret. (It's based on a voltage that wobbles inside a capacitor). Without the transistor it's a condenser mic instead, a recording studio CM uses as much as 48 V. I've seen 200 V used before

2. nope. A DM is just a coil that wobbles within a magnet. A DM is even lower sensitivity than a electret, you'd need sound level of an air raid siren to match a carbon mic.

3. I find electret still works on lower V, just seems to gives a lower mV output. But compared to a carbon mic, you'll need a megaphone to develop enough voltage to match a carbon mike. Carbon mikes invented before amplifiers, a party trick was you wired a telephone earpiece and carbon mike and battery together, they screech feedback without any amplifier.

4. ha ha impedance matching....

5. Yes, in air radio sets apparently! I think they used to be 'throat-mikes', they are strapped to the throat with a elasticated strap. I had one of these about 40 years ago.

As for "Impedance Matching", you probably don't want take it too seriously. When the Beatles were recording the bass guitar of "Paperback Writer" at Abbey Road, the engineers had tried every microphone in the studio and the beatles still couldn't get the fat sound they heard 'somewhere', but what they really wanted somehow. A young engineer (was it Neil Emerick?) had the idea of using a speaker still in it's cabinet as a 'microphone', pressed up against the bass guitar amp's speaker. He was handy with a soldering iron and wired a mic cable to the speaker socket. The fab four loved the sound, and so did the public. While the song was soaring high in the charts, this engineer was summoned to visit none other than the chief engineer of EMI, so jubilantly he went up there expecting to be rewarded for his inginuity. But instead he was reprimanded for failing to observe correct impedance matching and for misuse of company equipment!

Here's a link that was posted on these forums a while back, you could strip the mike part out of it.

http://www.electronic-circuits-diagr...simages/13.gif

(HumbleMode=On) Of course, I should of read the previous postings better before I posted. MikeMI had covered it well. Oops.

Oh well here goes I'll try and sum up and get back on topic...

Originally Posted by jfarrow

... a new NAT AA12-200 audio panel/intercom for ..... and a patch cable to connect it to the aircraft in place of the existing unit....
... I find the intercom works great...

OK, new mikes making signal loud enough for earphones, amplifier in the NAT working too. Intercoms talk to eachother, good. Almost there.

Originally Posted by jfarrow

... plus you can hear the radio....

OK, sound getting from the Radio to the intercom. (during non-PTT mode)

Originally Posted by jfarrow

However, I cannot transmit, or hear a side tone...

I presume that during Radio PTT, the side tone comes from the radio itself, through the same audio output. (Rather than the side tone locally within the NAT, during intercom) I'd think it'd be the latter, so the radio would sound 'dead' if it wasn't receiving voice OK during PTT.

Originally Posted by jfarrow

[paraphrased]
...two resistors and a capacitor in the mic audio line, from the audio panel to the radio.....an R & C in series... and an R to ground. .... on the aircraft 'somewhere', buried.

Sounds like a pot-down attenuator, with DC blocking. I can see why you considered running a bypass between the NAT and the Radio. Again, MikeMI is on the ball here.

Originally Posted by jfarrow

My guess is the capacitor is blocking dc voltage getting to <...> the Lightspeed electret mic. I tried apply a voltage of around 4volts but still nothing.

This is where I think it's going wrong.
Does the NAT do something like "transfer" the electret mic circuit over entirely to the Radio?

I would expect any locally-generated side tone (for the intercom) to be deactivated during radio PTT, so that the sidetone from the radio is heard instead of from the intercom.

Bottom line is: MikeMI above knows what he's talking about.