Impedance matching headphones and carbon microphone adapter

Question

I'm trying to convert a regular pair of headphones (audio technica anc-9) with a electret microphone (headset buddy) to a converter box so I can use that setup as a aviation headset. All the information I have found isn't very straight forward and I'm hoping to get some help with a adapter box. I know the headphones have 100 ohm impedance and when I wire them together I get 50 ohms (the aircraft uses mono output). The aircraft needs a 150 ohm impedance. My question is: I figure I need a audio transformer of about 1:3 but I have no idea how to search digi key for the one I need, all I know is I want it to be small as possible.

The next part is that I'm stuck on is the microphone. I know the aircraft is expecting a carbon style microphone so the electret microphone needs to be adapted to work. From what I understand this requires around a 1volt output and I can get the power I need to amplify the voltage from the bias voltage provided from the microphone leads. I have found a diagram posted by Andy aka from this post:

Electret microphone amplifer for general aviation

Will this produce the required voltage? And do I need to match the impedance for the microphone too? Also what would I have to change be able to change the gain of the microphone?

to summarize:

1. how do I go about choosing a suitable transformer?

2. Does the microphone also require the impedance to match?

3. what would I do if the bias/phantom voltage is not 5-7 volts?

Thanks for any help

Answer 1

Giving the headphone amplifier a 100 Ω load rather than 150 Ω is unlikely to cause any trouble. If you're really concerned then connect them in series for 200 Ω but make sure that they're in phase so you don't get the weird "hole-in-the-middle" effect when out of phase.

Figure 1. Image from Electret microphone amplifier for general aviation.

What's not shown in the schematic is the source resistance in the AUDIO+ input. If you want to generate more volume then you need to generate more voltage and current in that resistor. To do that you would need to decrease the value of the 100 R.

Try experimenting.

Note that most aviation headsets use a pair of microphones in anti-phase - one facing the mouth and one facing away. This helps to cancel out cabin noise without cancelling out speech.

Answer 2

If you can only wire your headphones in parallel (for 50 ohms total impedance), then simply throw a 100 ohm resistor in series to bring the total impedance up to 150 ohms. You will need to experiment to see whether that provides adequate sound levels for the typical noise environment in the cockpit of a general aviation craft. Because we don't know the output levels provided by the intercom system and we don't know what is the acoustic sensitivity of the headphones. Your chances of finding an appropriate transformer fall somewhere between slim to none.

A carbon microphone doesn't actually produce a voltage. It varies its resistance to cause a variation in the applied voltage from the intercom system. There is nothing in the Andy Aka circuit that would suggest it is not perfectly appropriate for your application.

1. You probably do NOT need to impedance "match" the headphone circuit. Furthermere, your chances of finding an appropriate transformer are virtually nil.
2. No, the microphone circuit does NOT need "impedance match". The carbon (or equivalent) circuit merely produces a varying resistance which causes a varying voltage from the intercom system current.
3. It probably won't may any significant difference if the microphone voltage from the intercom system is not exact. You could change the values of the microphone circuit if you find the sensitivity is too great or too small. As discussed in the cited Q/A.