aviation headsets and tecpro beltpacks

[aidso]

Hi we have a customer who has requested cheap noise cancelling headsets for use with canford beltpacks. Our first idea was £60 aviation headsets from CPC. When we got the headsets we releised that they are condenser mics. The brain storm we had was to wire a 12V car keyfob battery on the ground side of the mic. This does work but gives us very low volume when compared to the standard techpro headsets. The head sets we got were CPC product code AV12745. Any ideas?

[boatman]

... The brain storm we had was to wire a 12V car keyfob battery on the ground side of the mic. ...

 

You must have made two connections to the battery for it to make any difference. Where was the second one? A diagram would help.

[cedd]

There should be a voltage supplied to the beltpack by one of the pins on the XLR, another should be ground. You could use this to provide a more useful power supply. A little research may be required as to what voltage the headset is looking for. If it's 48v phantom power then I doubt you'll manage it, however the 12 or thereabouts voltage present may mean you can loose the battery.

 

You need to wire the battery across the mic capsule, not to one leg of it.

[aidso]

here you go

 

http://i277.photobucket.com/albums/kk42/aidanhughes1/mic.jpg

[cedd]

And here we have your problem

 

Your voltage needs to be applied across the capsule, ie. in parallel with the capsule, not in series.

 

Provided your capsule is looking for 12v this should then work, and should account for your lack of signal.

[KevinE]

I believe most aviation comms stuff uses 8V DC Bias which has to be supplied via an appropriate resistor (usually via a 12V bus), the comms receiver will have a DC blocking capacitor to prevent the bias fron upsetting the pre-amp, which your beltpack may not have..it might be worth adding one.

[aidso]

I know the beltpack uses 14-18v. we did have the idea of tappig into the 24v line in the beltpack but the problem being that they might want to use them on 2 way radios as well.

[SteveL]

Aviation uses 28v as its dc standard voltage. This may explain the low volume with 12v.

[boatman]

I know the beltpack uses 14-18v. we did have the idea of tappig into the 24v line in the beltpack but the problem being that they might want to use them on 2 way radios as well.

 

As you'll need to break into the comms loop to get 24V, it sounds as though you should fix the correct plug to the headsets for the radios you are using; then build an interface to the beltpack in a small plastic box with jack sockets to match the headsets. You'll need three cable mounted XLR's (1 off 3-pin male + 1 off 3-pin female for the comms loop and a 4-pin female for the beltpack headset connection) on the beltpack side. The radios will probably need either two mono jacks (3.5mm speaker + 2.5mm mic) or a stereo jack (2.5mm or 3.5mm); although some Motorola radios use a 4-pole 3.5mm jack. Most PMR radios are designed to use electret mic and have the appropriate bias voltage on the mic input.

[aidso]

ok so spent an hour on it today. put the batterys in parallel and it did make a bit of a difference not a huge one mind you. There is white noise overlaid with the mic. could this be caused by the 24v entering the beltpack if so how can I get rid of this? I then added a 10uf cap in series with the hot leg of the mic and wham, the signal jumped up big time. If I increase the cap the signal increases. Which is grat but it also increases the white noise.

[boatman]

ok so spent an hour on it today. put the batterys in parallel and it did make a bit of a difference not a huge one mind you. There is white noise overlaid with the mic. could this be caused by the 24v entering the beltpack if so how can I get rid of this? I then added a 10uf cap in series with the hot leg of the mic and wham, the signal jumped up big time. If I increase the cap the signal increases. Which is grat but it also increases the white noise.

 

Connecting 24V across the mic input may have damaged the beltpack, causing it to generate the white noise. You need to try your beltpack with the original headset to check that it is OK. Don't forget that the Techpro system is designed to work with low impedence mics (200 - 600 ohms) and may not like a high impedence electret capsule. You may also have to put a small buffer in the project box I suggested earlier.

[aidso]

... capsule. You may also have to put a small buffer in the project box I suggested earlier.

 

what sort of valuse and wire it which way? sorry for being a pain. I can fix most things but cant make them. ** laughs out loud **

[KevinE]

Aviation uses 28v as its dc standard voltage

 

 

We're talking the bias voltage here for the noise-cancelling mics used on cockpit headsets.

It's 8V on the unit in question, I checked on the CPC datasheet.

 

A quick google check shows 8V is pretty standard for airborne comms mic bias though there are exceptions.

 

The improvement with a capacitor suggests you've been feeding the beltpack preamp with the bias voltage which needs to be removed to prevent it offsetting the signal or upsetting the agc/dc conditions etc. You dont need a massive capacitor, low leakage is best, maybe 1uF or so, tantalum or similar. It doesn't need to be bipolar so long as its connected the right way round with reference to the bias voltage.

 

Bear in mind the mics contain a FET preamp of their own which can be damaged by even momentary overvoltage through wrong connection.

 

Some Martin lighting with the sound to light function (mainly disco/bar stuff) use condenser mics with DC bias and occasionally the blocking cap goes leaky and the mic goes very low volume. If the cap goes short it takes out the mic completely which has to be replaced.

[aidso]

right ok sorted. voltage is 28v tried building it up and didnt get any meaningful level until 24v. So I have 2 x 23a 12v car keyfob batteries wired in series the wired parrell accross the mic. then on the hot leg I have it feeding into a 22uf 63v cap(the only one I had laying about. then into a 100ohm 1/4 watt resister then into the pack and it works fantastic. same level as the tech pros but a lot lot lot clearer. Thanks for all your help. if anyone wants a copy of the circut please let me know. thesere a good cheap comftable change from the tech pros.

[boatman]

Excellent news. Unfortunately, it sounds as though those headsets may not work with hand-held radios as the bias voltage there will only be around 1.5V to 2.5V. You may have to use the same trick with two batteries and a capacitor.