Aviation Headset with Flex-5000
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Aviation Headset with Flex-5000 Expand / Collapse
Posted Wednesday, December 1, 2010 4:01 PM
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I want to use my Bose Aviation headset with my FLEX-5000.

The headset emulates a standard aircraft carbon microphone.

I am looking to buy an adapter to convert the aviation mic output to XLR balanced-differential format that I will input to the F5000 via the rear panel 1/4 inch tip/ring/sleave jack.

I think the circuit is could be as simple as a 9 volt, battery, a 1K resistor and a small audio transformer, but I would rather buy it than design and build one.

Any leads on an existing product or a known working solution.

Thanks, K6THH - Tom



Post #3495
Posted Thursday, December 16, 2010 8:21 PM
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This website shows a circuit



Post #3571
Posted Friday, July 27, 2012 8:20 PM
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Like K6THH, wanted to use my aviation headset with the Flex5000A.
An aviation microphone is designed to electrically 'look like' a carbon microphone (an interesting relic of WW2!) when it's plugged in to an aircraft microphone jack.
This means that (a) it draws a few mA of DC current (b) it generates a few hundred mV of audio onto the input line and (c) it's average DC potential is about 'half way up' to the +12V energising supply rail.
Modern headsets of course don't use carbon microphones and as far as I can tell (having investigated just a few headsets) they have a combination amplifier and DC current adjuster which makes all the above happen. For instance, the one I had intended to use (a Beyerdynamic) ends up with a DC sit point of about +5V over quite a wide range of currents.
The line input port on the F5k accepts audio signals of this magnitude and so it's just necessary to feed the amplifier and condition the signal a little.

I also wanted a single location where I could plug in the headset phones and the microphone side by side (like in an airplane) and so I built a combo box which housed both the headphone jack and the mic jack as well as a T-R switch.
On an aviation microphone, the microphone connection is on the ring terminal, not the tip. The tip is used for a handheld or headset-mounted PTT button if you happen to use one.

I first used a 2.2k resistor from the mic jack ring terminal to +12V, thus giving me about 4mA through the amplifier. This method of course puts any noise or hum present on the +12V line straight into the line-in connection on the F5k. The audio signal must have its ~+5V DC bias removed by use of a suitable coupling capacitor.
I also found I got terrible RFI with the RF in the shack getting back into the line input or wherever and causing bad 'motor-boating', especially if I was using a linear amp.
I suspect that the F5k Line-in port doesn't have as much in the way of RF filtering on the inside plus my shack is pretty 'live' at the moment.

After a lot of experimentation I came up with the approach shown in the attached .pdf schematic. It features liberal use of RF chokes and RF bypass capacitors and I'm not sure which ones are absolutely essential. It kind of evolved as a 'try this, try that' operation.
I took the +12V power from the Flexwire connector and used a +9V 3-terminal voltage regulator to remove any noise or hum on the +12V line. Because of the lesser supply voltage of +9V I reduced the energising resistor to 1k which still gives about 4mA of energising current.
I fed the audio into the F5k via the Line-in pin on the Flexwire connector. Don’t forget to select the Flexwire Line input in the F5k Mixer control or you won't get any audio!

An interesting feature of the SDR is that there is a finite fixed time delay between the audio going in and the RF coming out. So, if you have an RFI problem it manifests itself as a distorted echo on the audio.
I exploited this feature by installing a Test Port which connects to the Line-in line via a feedthru capacitor for RFI filtering.
I connected this port to a pulse generator which puts out ~5msec wide pulses at about a 2Hz rate with an amplitude of ~0.5V. This causes the F5k to generate brief burst of RF a short time after the pulse. The delay time is determined by the buffer size and sample rates set up.

I then set the F5k display to 'Scope' mode and I saw the 2Hz pulses each followed by an RFI 'echo'.
This was the only way I could find to somehow objectively evaluate RFI susceptibility and the countermeasures I tried. The amplitude of the echo indicates the RFI susceptibility of the setup.
I guess theoretically one gets a string of exponentially decreasing echoes although if the RFI suppression is any good you can really only see the first one.
If the RFI is bad, the echoes can end up being bigger than the stimulus. Then they build up and up and you end up with continuous ring-around oscillation or 'motorboating'.

I also discovered that some aviation microphone/amplifiers are themselves susceptible to RFI so no amount of filtering in the headset box is going to solve the resulting RFI problem.
Some microphones worked OK with my box (e.g. a David Clarke H10-30 and two Avcomm models), one was a bit marginal (a Beyerdynamic unit with a separate cell-phone interface module) and one was not good at all (a Telex lightweight/moulded earpiece unit).

Overall though,I like the hands-free operation and noise-canceling you can get by using an aviation headset/microphone but, as in a lot of things, it’s never simple!
I hope my experiences may be of value to others trying to use aviation headsets.

GL es 73

De Mike, VE3EBR

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