[Elecraft] CM-500

[Joe Subich, W4TV]

> Active Noise Cancelling headsets have other power options, maybe
> running off 28V. I am not aware of 48V systems in Commercial
> aviation, but I don't know everything that is out there. Carbon mics
> were different but not common any more.

Maybe it was 28 instead of 48 volts ... it has been a while since I
ran into the problem of making the aviation headsets run in an "AV"
environment.  I do know that the headsets in question were spec'd as
"carbon mic" equivalent and required a high voltage bias until the
resistors were reduced by a factor of about 20.

73,

    ... Joe, W4TV


On 3/14/2013 4:27 PM, Mark Goldberg wrote:
> A standard aviation microphone approved to TSO C58 is supposed to meet
> DO-214 "Audio Systems Characteristics And Minimum Operational Performance
> Standards For Aircraft Audio Systems And Equipment" although not all do.
> Unless you belong to the RTCA, it will cost money to see that document. The
> standard is bias excitation is 8 to 16V with a 470 ohm current limiting
> resistor, providing about 10 ma to what usually is an amplifier circuit in
> the mic. The ac output level is supposed to be between .02 and .08 V/Pa,
> where  1 Pa is 94 dB SPL referenced to 20 uPa. Normal close speech is in
> the range of about 1 to 10 Pa, so this is pretty high compared to a lot of
> other mics, up to .8 V. I have not tried connecting one up, but I would
> think if you had a resistor that provides about 10 ma it should work, if
> the rig can stand that high an input and if the bias can supply 10 ma.
> Active Noise Cancelling headsets have other power options, maybe running
> off 28V. I am not aware of 48V systems in Commercial aviation, but I don't
> know everything that is out there. Carbon mics were different but not
> common any more.
>
> One issue is to keep the load mic preamp from disturbing the bias. I had to
> filter the bias end of the series resistor on one rig with a big cap. Also,
> connections vary.
>
> Mark
>   On Mar 14, 2013 12:27 PM, "Joe Subich, W4TV" <lists at subich.com> wrote:
>
>>
>>   I use my headset with my K2, but would like to try it with another
>>> rig that supplies 10v. To get my other rig voltage down to the 3-4v
>>> region require about 12 meg ohms in series with the 10volts
>>> supplied.
>>>
>>
>> An electret does not draw current in the classic sense.  5.6K seems
>> to be a good value of resistor for almost anything from 5 to 8V but
>> 12K might be safer for your 10-12V.
>>
>> The "big name" aviation headsets use 33 - 48K resistors for the typical
>> 48V aircraft system.  They tend to not work well with the 3-5V ham and
>> computer supplies but if they are opened and the resistor removed (or
>> replaced with a 2K2 depending on whether the equipment already contains
>> a resistor), they work perfectly well at the lower voltage.
>>
>> I've always used a rule of thumb of 1K/V for electret mics.
>>
>> 73,
>>
>>     ... Joe, W4TV
>>
>>
>> On 3/14/2013 2:45 PM, Dr Ira J Saber wrote:
>>
>>> GA  Everyone
>>>
>>> I hope this has not been answered before, but I could not find my answer
>>> in the archives, so here goes.
>>>
>>> Can anyone tell me how much bias the microphone is able to tolerate?
>>> Since the battery pack supplies 3v
>>> dc, I am sure that 4-5 volts probable won't hurt.  But how about 10-12v
>>> DC?
>>>
>>> I use my headset with my K2, but would like to try it with another rig
>>> that supplies 10v.  To get my other rig voltage down to the 3-4v region
>>> require about 12 meg ohms in series with the 10volts supplied.
>>>
>>> All of you guys and gals are a great source for info regardless of topic
>>> and I wish to thank all and any of you who choose to answer my "novice"
>>> question.  I have learned something in my years on this earth, that if
>>> you don't ask you do not get any answers.
>>>
>>> Thanks again in advance.
>>>
>>> 73
>>>
>>> Ira    N2IS
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