was@hp-lsd.COS.HP.COM (Bill Stubblebine) (07/27/90)
Some time ago at a swap meet, I picked up a nifty ICOM (familiar ham radio brand) headset containing a microphone and earpiece. I wanted to use it for hands-free dictation with my VOX controlled microcassette recorder. The earpiece in the headset worked as I expected, but the microphone generated only the faintest sound for recording. Later, I spoke with a ham friend familiar with ICOM equipment, and he said that these headsets use an electret microphone. All of a sudden the light bulb went on: tape recorders expect microphones to transduce acoustic energy directly into electrical energy without an external power source, as crystal, ceramic or dynamic microphones do. Now, I remember reading a long time ago that an electret microphone is basically a capacitor whose value varies in proportion to incident acoustic energy. Therefore, one would expect a circuit to make an electret microphone do its thing would hold the electret capacitor at a relatively fixed voltage and detect the charge shuffling in and out of the capacitor as its capacity varies. Intuition tells me such a circuit might look something like this: |<--audio-->| | | ----- ----- decoupling ----- ----- caps | | __________|__/\/\/\___|_________ microphone | R | DC source (variable-C) ------- ------- + ------- --- - |______________________________| I've never designed a circuit like this, and I'd rather not experiment with my nifty new headset, so can anyone supply some pointers? Voltage level for the DC source? Is the polarity important? What's the right value for the resistor to match the tape recorder mic input? (The recorder manual says the mic jack is designed for a nominal mic impedance of 200-600 ohms. Should the value of R be in that range?) Do I need the decoupling caps? Book recommendations would also be welcome. Thanks. "I groove on 1's and 0's; this analog stuff is mind bending" Bill Stubblebine Hewlett-Packard Logic Systems Div. Colorado Springs, CO 80920 was@hp-lsd.hp.com (719) 590-5568
jws@thumper.mlb.semi.harris.com (James W. Swonger) (07/30/90)
The condenser mic (electret) is usually packaged with a FET gain stage because of the extremely high impedance (oo) of the electret element. These assemblies usually have three wires (+, gnd, sig) and run off +3V. Some will work on 1.5V and some only use 2 wires. Did the headset have a battery compartment?
was@hp-lsd.COS.HP.COM (Bill Stubblebine) (08/01/90)
jws@thumper.mlb.semi.harris.com (James W. Swonger): > The condenser mic (electret) is usually packaged with a FET gain stage > because of the extremely high impedance (oo) of the electret element. Email from several other people also pointed this out. It sounds plausible, but the boom microphone in the headset is difficult to disassemble, so I can't directly verify whether or not there is an FET in there. > These assemblies usually have three wires (+, gnd, sig) and run off +3V. > Some will work on 1.5V and some only use 2 wires. Did the headset have > a battery compartment? The microphone connection is two wires. One suggestion I received was to series the microphone with a 3V DC source through ~3K and decouple the audio across the mike. This is almost like my original figure, except that the audio is developed across the microphone, not the series resistor, with the FET presumably supplying the impedance reduction: _____________/\/\/\__________________||___ 3V DC | R=3K | || source + ------- electret ------- audio - --- + FET ------- |______________________________|_____||___ || Since I posted my original message, I located a schematic for an ICOM VHF transceiver that uses this kind of headset. The circuit it uses seems to employ the principle shown above. Thanks for all the suggestions. I'll report back on the results. Bill Stubblebine Hewlett-Packard Logic Systems Div. was@hp-lsd.hp.com (Internet) (719) 590-5568
was@hp-lsd.COS.HP.COM (Bill Stubblebine) (08/13/90)
A couple of weeks ago I posted a note asking for help coupling the electret microphone in an ICOM ham transceiver headset to a VOX controlled micro- cassette recorder. I received numerous helpful suggestions, mostly via email. Most were similar to the one from Mark Zenier (markz@ssc.uucp): > Most electret microphones (new & cheap ones) have a built in > FET amplifier. All you need is a pullup resistor to a DC > supply (1.5 to 10 V) and a decoupling cap. See the Digi-Key > or Mouser Catalogs for examples. Indeed, the electret microphone listings in the Digi-Key catalog were quite helpful, even showing recommended interface circuits. Anyway, the headset is now working fine. The interface circuit I finally settled on was: 0.1 uF _____________/\/\/\__________________||___ 3V DC | R=3.5K | || source + ------- electret ------- audio - --- + FET ------- |______________________________|_____||___ || 0.1 uF I packaged the circuit inside a little plastic battery holder I found at Radio Shack. The microphone draws ~200 uA DC (as best I can measure) and produces ~20mv peak-to-peak audio for normal volume speech. This is about the same output level produced by a crystal microphone I had lying around, and more output than the small hand-held ceramic microphone I normally use with the microcassette recorder. (The recorder has AGC, so the microphone output level doesn't seem to matter too much.) The ICOM headset boom mike has a distinctively crisp response to voice frequencies, and excellent noise rejection. By the way, the battery polarity did matter. Both polarities produced audio, but one polarity was 6-10 dB louder than the other. I used the louder polarity. The only thing I'd still like to do is connect the 3V microphone supply to the recorder's internal 3V supply so that it switches off when the recorder is off. As it is now, I have to remember to unplug the microphone from its supply to stop the current draw. This should be an easy modification. Thanks for all the suggestions. The net is a truly wonderful thing. Bill Stubblebine Hewlett-Packard Logic Systems Div. was@hp-lsd.hp.com (Internet) (719) 590-5568