Microphone Current?

[JLM]

My airplane's radio transmits on the AM band and outputs about 10 watts.  I am experimenting using Bose in-ear noise cancelling earphones to receive audio.  It works great but there is an overwhelmingly loud, indeed painful, "sidetone" of my own voice in my ears when I transmit (I am using the microphone from an aviation headset).  Although there is a setting in the radio (a Garmin GNS430W) to reduce or turn off the sidetone, it does not seem to have an effect.

My idea for a workaround is to use a normally-closed relay in series with the earphone circuit which will sense when the microphone is keyed and then open the earphone circuit.  I would like to build a box which sits between my earphone/microphone combo and the panel, in essence an interface that I can choose not to use when I want to plug in a regular aviation headset into the panel (which does not have the sidetone).  My question is whether a current flows through the microphone circuit when the mike is keyed but there is no voice modulation.  And if so, what is the typical amount of current?  Is it enough to trigger a relay, or would I have to amplify the current to a usable level?

Thanks! 

[Rerouter]

another way to reduce microphone volume would be to strap a resistor across the 2 pins before its amplified, as piezo mics dont put out much current start in the 3.3K range

[JLM]

I don't follow this. The idea is to have the same power transmitting, while
 not hearing myself while I transmit

[Whales]

I'm assuming this feedback is not an intentional sidetone made by your equipment, but instead leakage from headphones to microphone.  If it is intentionally put there by your equipment, the best solution will be to modify your equipment's settings.

Better muffed/isolated headphones are going to be the least irritating solution to getting rid of feedback.  Noise-cancelling only cares about reducing noise to your ears, but what the rest of the world (ie your microphone) does.  If there is not enough padding, sound will escape and get to your mic easily, mostly unaffected by the active noise-cancellation. 

• Can you improve the padding/isolation of your current headphones?
• Would you consider obtaining and trying different headphones?

Without knowing exactly what headphones you are currently using, I'm going to lean in the direction of "they are made and marketed by BOSE" and hence recommend that you consider other options.  BOSE is a company known for its brand name and advertising.  In practice their products are renowned for being absolutely sucktastic and poorly-engineered, for the price you pay for them.  None the less anything with better sound isolation will help.


If you want to pursue your original plan:

• The power microphones provide is definitely not enough to open/close a relay.
• A passive-approach may be possibly using a single FET and a resistor (variable-muting of your headphones) but this will cut out a portion of the signal constantly due to ambient noise
• And active approach with a FET/relay, a voltage-following op-amp, a demodulator, a comparator and a tuning pot would provide a variable 'cut-out' level for your heaphones based on microphone amplitude.

Keep in mind however that this will:

• Need tuning to determine the threshold between "I'm talking" and "I'm not talking", which will vary depending on ambient noise/how loud you think you need to shout at the present moment for the mic to notice.
• Add another failure point to your radio.  I'm assuming this is a primary communications method on your craft?

[ve7xen]

Noise "cancelling" = active, which means they have electronics inside. Given Bose's awesome reputation, it wouldn't surprise me at all if the nearby 10W transmitter was being "received" directly by the noise cancellation electronics. If it were somehow coupled into the cancellation microphone preamp, even slightly, it would have a lot of gain and would be applied (out of phase) directly to the headphone audio output (since this is how these things work). Maybe try some ferrites on the headphone cable?

It seems passive headphones would be preferable in an aircraft.

[Whales]

Given Bose's awesome reputation, it wouldn't surprise me at all if the nearby 10W transmitter was being "received" directly by the noise cancellation electronics.

Reminds me of the time I made an AM radio simply by plugging a pair of ear-phones into the microphone jack of a laptop.  The sound-card electronics must have provided enough enveloping and capacitance to make a basic crystal radio.

I'm leaning more towards the fact his headphones will be blasting in order to be heard over the in-air noises.  That and possibly the 'sound steering' from the cancellation which could possibly make the sound source 'appear' to be more in-front of his microphone than to the side (shot in the dark: I'm a practical audio guy, not grounded deeply in theory). 

[Richard Crowley]

My airplane's radio transmits on the AM band and outputs about 10 watts.  I am experimenting using Bose in-ear noise cancelling earphones to receive audio.  It works great but there is an overwhelmingly loud, indeed painful, "sidetone" of my own voice in my ears when I transmit (I am using the microphone from an aviation headset).  Although there is a setting in the radio (a Garmin GNS430W) to reduce or turn off the sidetone, it does not seem to have an effect.

Then you have something connected or deployed incorrectly. There should be every expectation that the sidetone adjustment should work with your noise-cancelling earphones just as it would with any other earphones.  Perhaps what you think is "sidetone" is actually some wierd artifact of using your noise-cancelling headphones in a manner they were not designed.

My idea for a workaround is to use a normally-closed relay in series with the earphone circuit which will sense when the microphone is keyed and then open the earphone circuit.  I would like to build a box which sits between my earphone/microphone combo and the panel, in essence an interface that I can choose not to use when I want to plug in a regular aviation headset into the panel (which does not have the sidetone).  My question is whether a current flows through the microphone circuit when the mike is keyed but there is no voice modulation.  And if so, what is the typical amount of current?  Is it enough to trigger a relay, or would I have to amplify the current to a usable level?

Thanks!

No, trying to sense the audio level, especially at microphone-level is a terribly unreliable way of doing this. Absolutely NOT recommended, especially for a life/safety function like aircraft communications.  If you have access to the microphone audio signal, then you must also have access to the transmit keying signal from the button, don't you?  That would seem like the perfect place to derive the TX/RX status signal.

[JLM]

Thanks for all the thoughts.  I think the best course would be to replace the spst momentary contact switch used for keying the mic with a dpst. One pole would be used to transmit, the other to trip the relay which takes the earphones out. Good idea?

[Whales]

Thanks for all the thoughts.  I think the best course would be to replace the spst momentary contact switch used for keying the mic with a dpst. One pole would be used to transmit, the other to trip the relay which takes the earphones out. Good idea?

If you go for a double-throw switch you could get it to kill the headphones directly without a relay.