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

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#### jfarrow

##### New Member
The VHF radio installed in my Italian aircraft is a military model. The mic audio-in is labeled "carbon mic". Don't know much more about the radio except that it says Marconi model number ART 151. I think it is in fact a Collins unit made under license because it looks like a Collins. Anyway, we've been using two noise canceling headsets with carbon mics that run though an audio panel/intercom made by some unknown (to me) Italian company called Gemelli. It's noisy in the cockpit because there is no VOX or squelch.

I just brought a new NAT AA12-200 audio panel/intercom for use with general aviation headsets and made a patch cable to connect it to the aircraft in place of the existing unit. Using a new set of general aviation noise canceling headsets with electret (Lightspeed 3G's) I find the intercom works great (of course - that's only a function of the NAT unit) plus you can hear the radio. However, I cannot transmit or hear a side tone (you can't hear your transmission in the headset).

To make matters more complicated, there are two resistors and a capacitor in the mic audio line from the audio panel to the radio. One resistor and the capacitor are in series and the other resistor connects the mic line to ground. I see then on the aircraft wiring schematic but I have no idea where to find these components in the aircraft! They are buried. The mic line ohms out to 92 ohms to ground. I could run a new line from the radio to the intercom to bypass these components if necessary. My guess is the capacitor is blocking dc voltage getting to the radio. I also think that this has something to do with the need for a DC bias voltage on the Lightspeed electret mic. I tried apply a voltage of around 4volts but still nothing. There is still obviously an interface issue.

Anyone know how to get this to work?

FYI - the aircraft operates on 28Volts DC.

#### MikeMl

##### Well-Known Member
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.

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#### jfarrow

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

http://www.electro-tech-online.com/custompdfs/2009/07/804-0410.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.

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?

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

#### MikeMl

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

http://www.electro-tech-online.com/custompdfs/2009/07/804-0410-1.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.
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.

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.

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#### jfarrow

Per what you said about not know what the capacitor was for, NAT make a product the aa34-300 which is an interface between various radios and headsets. I'm told it would solve my problem; for a mere $500+! Here is the installation manual: http://www.northernairborne.com/pdfs/install_Ops/SM36_AA34/Install_Ops/804-0400.pdf If you look at page 2-4 it says half way down the page under "bypass switch for blocking capacitor in mic out circuit": "....if there are any concerns about dc interaction between the aa34 and the connected radio" So there are apparently radios that may require a capacitor to avoid damage. Thus you see my concern in my installation. However, I think I'm going to try connecting the radio directly to the intercom. Wish me luck! #### marcbarker ##### New Member Would an Electret capsule develop enough AC volts (even shouting into it) to drive a "Carbon mic" input? Last edited: #### Nigel Goodwin ##### Super Moderator Most Helpful Member 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. #### tezaa ##### New Member aviation electret bias voltage Hi all, Would like the 'gurus' among you to clarify a point or two regarding aviation microphones. Thought I would use this thread as it seems to be related. It seems that the 'standard' for aviation microphone bias voltage is 8-16V. I'm guessing that this comes from the RTCA. RTCA, Inc. "DO-170, Audio Systems Characteristics and Minimum Operational Performance Standards for Aircraft Audio Systems and Equipment" 1) So why is this 'specific' voltage range listed? Is required for impedance matching purposes? (ie: raising the output voltage of electret and dynamic microphones so they can be used with the old carbon mic compatible aircraft radios?) OR ... Is this voltage simply required for powering the FET electronics in the electret mics? 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'? ***I really struggle to get my head around impedance matching. Just when I think I've got it sussed ... it always comes back to haunt me**** 5) Are carbon mics actually still used in aviation these days? Looking forward to your replies. tez #### marcbarker ##### New Member 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! Last edited: #### marcbarker ##### New Member ... old UK telephones used a carbon mike, and they later brought out a fully compatible electronic replacement - just connected on the same two wires. Sounds like a solution to me. Now an enterprising person could find one of these phones on eBay... #### marcbarker ##### New Member (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... ... 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. ... plus you can hear the radio.... OK, sound getting from the Radio to the intercom. (during non-PTT mode) 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. [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. 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. Last edited: #### MikeMl ##### Well-Known Member Most Helpful Member Per what you said about not know what the capacitor was for, NAT make a product the aa34-300 which is an interface between various radios and headsets. I'm told it would solve my problem; for a mere$500+! Here is the installation manual:

http://www.northernairborne.com/pdfs/install_Ops/SM36_AA34/Install_Ops/804-0400.pdf

If you look at page 2-4 it says half way down the page under "bypass switch for blocking capacitor in mic out circuit":

"....if there are any concerns about dc interaction between the aa34 and the connected radio"

So there are apparently radios that may require a capacitor to avoid damage. Thus you see my concern in my installation. However, I think I'm going to try connecting the radio directly to the intercom. Wish me luck!
Read section 1.2. Notice that this box is to interface AIRCRAFT MICS to radios that do not accept aircraft mic levels or do not generate mic bias. Yours does.

You DO NOT need THIS BOX!

Where are you. Maybe I could get it working for you and you could send me the \$500

#### MikeMl

##### Well-Known Member
Would an Electret capsule develop enough AC volts (even shouting into it) to drive a "Carbon mic" input?
Most modern aviation headsets use electret mic elements (two of them arranged back-to-back for noise-canceling) with an audio amplifier which boosts the output level to ~1/4V. The source of the bias current for the electret mics and the amplifier (and load resistor across which the audio appears) is inside the transmitter. There is no battery for mic circuit inside the headset. If there is a battery in the headset, it runs the Active Noise Reduction circuitry which feeds the earpieces in the headset; not the mic.

#### MikeMl

##### Well-Known Member
...
This is where I think it's going wrong.
I'm not sure that the radio really needs a DC voltage from the microphones. Does it?
The "LS electret mic" works fine (see earlier) during intercom mode, but does it lose it's 'bias' voltage when the Radio's PTT operated? If so, does the NAT do something like "transfer" the electret mic circuit over entirely to the Radio?

I would expect maybe 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: not enough information. Gather more data!
Marc,

The whole problem all along is that the Marconi radio is internally making the DC bias, but it is not reaching the mic amplifier during the time the PTT is keyed. The mic doesn't make the DC bias, the radio does!!!

SideTone is a separate issue. If there is no bias to the mic during PTT, there is no mic audio. If there is no mic audio; there is no sidetone. Sidetone is generated by sampling transmitter audio inside the transmitter, and then it is fed back to the headphones during PTT. Fix the transmit audio, and the sidetone will take care of itself.

#### marcbarker

##### New Member
I think our postings had crossed, I was editing an earlier posting I made.

Yes, I realise now. The Marconi set's DC bias (from the radio) doesn't seem to be reaching the LS mic. What I didn't realise was that during PTT, the raw mic circuit is transferred away from the intercom, over to the radio.

The missing sidetone of course is a symptom, not a cause.

Red herring I'd inadvertantly created was me assuming that an electret mic (few millivolts) is interfaced into a carbon mic circuit (few hundred mV) without an amplifier.

#### jfarrow

##### New Member
almost there?

What a great response! Mike, I'm in Los Angeles - my cell is 323-428-0559.

So how is it that the headsets with the carbon mics and the old Gemelli audio panel/intercom manage to work with that capacitor and resistors in the way?

Although the Marconi radio is operating on civilian frequencies, it is worth noting that the aircraft (it's an SF260, a 2 seat trainer) was in service with the Italian military back in the 90's. In addition to the Marconi VHF radio it also had installed a military UHF radio. Both radios fed through the Gemelli audio panel to the headsets. Perhaps this is why the headsets required carbon mics?

If I put a 500 ohm resistor across the mic in of the radio, I should see 4 volts, right? If that's the case then I think I could do no damage to radio if I jump the radio's "mic in" directly to pin 12 of the NAT.

#### MikeMl

##### Well-Known Member
...
So how is it that the headsets with the carbon mics and the old Gemelli audio panel/intercom manage to work with that capacitor and resistors in the way?
I'm guessing that for whatever reason, in the old set-up, the resistor network supplied the mic bias current from a regulated voltage source which may have been inside the old audio panel. When that was removed, the resistor network stopped working.

I'm 90% sure that the Marconi (Collins) will source the bias current. Possibly, the military UHF radio didn't, so someone cobbled that network to make it work with both radios. If one did, and the other didn't, then the capacitor would block the dc from the the one that did so as not to change the operation of the carbon mic when that transmitter was selected.

Although the Marconi radio is operating on civilian frequencies, it is worth noting that the aircraft (it's an SF260, a 2 seat trainer) was in service with the Italian military back in the 90's. In addition to the Marconi VHF radio it also had installed a military UHF radio. Both radios fed through the Gemelli audio panel to the headsets. Perhaps this is why the headsets required carbon mics?

If I put a 500 ohm resistor across the mic in of the radio, I should see 4 volts, right? If that's the case then I think I could do no damage to radio if I jump the radio's "mic in" directly to pin 12 of the NAT.
Yes, use a 500Ω resistor as a mic substitute. Key the radio with the PTT, and you should see 2 to 6V. Unkeying will likely make the voltage be zero.
If for some reason, you do not get the mic bias when the Marconi is keyed, it may be an option. Otherwise, write back and we will cobble our own bias network, or trick the NAT into doing it.

btw: I got a 1 hour ride in a Fouga here in the SLC area with Jon Bowman about 12 years ago (his son Tim worked for me), during which I got to try a loop (~4 Gs), aileron rolls, a hammer-head and some other cool maneuvers.

#### jfarrow

##### New Member
results

Mike - I pulled the transmitter from the plane today and fired it up (with no head unit attached so no freq selected). Across a 480 ohm resistor between mic audio in and ground I see:

14 volts with ptt floating
6-7 volts with ptt ground

Looks like we are on the right track.

I couldn't trace the wire or find the capacitor so I stopped for the day. Will borrow a tone generator tomorrow.

#### jfarrow

##### New Member
I think I may officially be quiting this job! With a direct connection to the radio mic audio, there is still no transmit. In fact now that the radio is connected to the head unit, voltage across a 480 ohm resistor from mic audio is 14 volts ppt floating and then rises to 22 volts with ppt to ground. With the NAT audio panel connected, you get a whole different set of results, also depending upon whether pilot isolate is selected or not. I think the radio is doing something very weird. I'm going to try to contact Marconi. Wish me luck with that!

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