Gentlemen: I think you muddy the waters when you refer to preamps and electrolet headsets such as the CM500 in the same context. There is no appreciable current flow in the CM-500. That's why you can run the bias for years with a pair of AAA batteries. Let's take discussions of preamps with 10 ma circuits and legacy items like carbon mics to another thread. 73 Fred, AE6QL
-----Original Message----- From: elecraft-boun...@mailman.qth.net [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Mark Goldberg Sent: Thursday, March 14, 2013 1:28 PM To: Joe Subich, W4TV Cc: Elecraft Mailing List Subject: Re: [Elecraft] CM-500 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" <li...@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 >> ______________________________**______________________________**__ >> Elecraft mailing list >> Home: http://mailman.qth.net/**mailman/listinfo/elecraft<http://mailman.qth.net/ma ilman/listinfo/elecraft> >> Help: http://mailman.qth.net/mmfaq.**htm<http://mailman.qth.net/mmfaq.htm> >> Post: mailto:elecr...@mailman.qth.**net <Elecraft@mailman.qth.net> >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> >> ______________________________**______________________________**__ > Elecraft mailing list > Home: http://mailman.qth.net/**mailman/listinfo/elecraft<http://mailman.qth.net/ma ilman/listinfo/elecraft> > Help: http://mailman.qth.net/mmfaq.**htm<http://mailman.qth.net/mmfaq.htm> > Post: mailto:elecr...@mailman.qth.**net <Elecraft@mailman.qth.net> > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
