Mensaje originado el: 07/08/10 a las: 23:08 z From: G8MNY@GB7CIP.#32.GBR.EU To : TECH@WW By G8MNY This homebrew design has the advantage that it has clear audio, as the treble has been lifted. Most /M mics have a flat responce & when placed near the mouth for background noise reduction (as needed for /M work), the LF bass is far too loud & very WOOLY audio is the result. Indeed with some preamps the LF engine & rumble noise from the car can cause problems if no filtering is used. Here a 3 terminal electret mic is used, it is mounted (soldered) on to a thin wire (flower arranging type) stalk inside a thin AF lead for about 20cm. The end of the wire is placed in plastic insulation tube & shaped into a loop and tied off. I wear the loop around my ear. As the mic is mounted right in front of my mouth, there is next to no background noise. /----\ __ ! ! / \ / / Mic \__/ ! / Ear \________________/ / loop ---/ / / cable Electrically the treble has been lifted by a 0.2uF tantelum cap on the electret mic's internal FET transistor, source to ground. This simple treble lift is only possable with a 3 terminal mic capsule. The normal +ve rail drain input now becomes O/P as in a 2 terminal mic type. A shunt R of about 3.3K (select on test) across the mic sets the AF O/P level, & a 10nF across the mic in the gear lever switch to roll off the AF lift above 3kHz. RIG MIC<------ PTT Switch -------------------------- screened lead ! ! screened lead ! ! !10n _ 3 term ~3K3 === + / mic ! ! 0.2u=== ! ! ! ! ! RIG GND<-------------------------------------------- The result is a clear punchy mic that has very little background noise. A small sponge covers the face of the mic to reduce poping/blow etc. It is comfortable to ware, and I have to make sure I remove it before getting out of the vehicle. The only problem I have found is while eating Sandwiches!! If your rig does not supply power for a 2 terminal type mic & the PTT operation you will need a C & R as well. +5-12V >-----------------------! smooth DC R - C + ! RIG MIC<--------!!-------------------------- RIG PTT<---------! after PTT / PTT SWITCH as above ! RIG GND<------------------------------------ The C needs to be about 1uF (10n-100n if not treble lifting), & the R between 1K-33K select for the gain you need. ------------------------------------------------------------------------------ Doepie, ZS1HK @ ZS0HER says these electret mics can turned into noise canciling mics by drilling 3-4, 0.8mm holes through the insulation between the terminals of your mic. This will influence the gain but makes it a high quality noise canceling mic. This will take away all back ground noise. I suggest you only try this if you have a spare insert! 73 De John, G8MNY @ GB7CIP