The predecessor to this miniature micropower microphone preamp was the Mitey-Mike (Speaker Builder / 6/90) designed by Joe D'Appolito and placed on the market by Old Colony Sound in 1990. This version can still be purchased as a kit or completely assembled, with an optional calibration certificate from Joe D'Appolito.
The MM1 housing is quite large and even though the microphone capsule is at the end of a long thin tube, the reflections from the end of the housing could seriously affect the high frequency performance of the microphone system. The electronics, however, are excellent and achieve superb performance considering the miniscule battery drain.
The MM2 circuit board is a re-designed MM1 executed in two-sided surface mount (and with newly available integrated circuits) to make it small enough to fit into a 50mm (2 inch) length of 17mm (5/8 inch) diameter tube which can be mated with a conical adapter to a long probe carrying the microphone capsule. The circuit is modernized but the electrical characteristics are essentially the same.
Now the answer to the first question --
The MM2 uses the MAX874 low-dropout micropower precision voltage reference to excite the capsule. This means that the capsule d-c excitation voltage is extremely stable over the design battery discharge range from 9 volts to 4.5 volts. Typically, the output voltage of this chip will not change more than 0.5 millivolts from 4.5 to 20 volts input, with an initial accuracy on 0.2% and a drift of 40 ppm/C. Filtering of this supply assures that electrical noise does not contribute to the inherent capsule noise.
The latest circuit board design optionally allows a modified capsule to be run in 3-wire mode as a source follower. In addition, beginning in the fall of 2003, the Analog Devices AD8671 ultra-low-noise precision opamp will be used.
The current output amplifier is the micropower OP121 with 6 dB of gain to mtch the MM1. The overall performance of the MM2 is impressive: 180 microamperes nominal current drain (including capsule) and a response from 7 Hz to 62 KHz +0/-1 dB (amplifier only). With no load, the output clipping level is 2 VRMS and the wide-band noise is below 60 microvolts. The amplifier will successfully drive a 10K load with 0.002µF shunt capacitance (with higher battery current, of course). That capacitance represents 12 meters (40 feet) of typical coaxial cable.
Although almost any electret will work, the capsule for which the MM2 is designed is the Panasonic WM-60A available from Digikey. This capsule has an advertised sensitivity of -44dB re: 1V/Pascal (10 µbar) and a signal-to-noise ratio in excess of 58 dB. It is advertised as essentially flat from 20 Hz to 20 KHz.
MM2-5354 Statistical Performance Data (dummy capsule)
DC current at 1000 Hz, no load except instrument cables
assembled by mbc, measured by rhc, parts lots 280C/286G
(Standard Deviation, SD as a percentage, Mean, Max, Min)
Count: Sigma--> 3.77 4.82 0.24 1.47
23 Sigma%-> 2.1 2.5 3.3 2.3
Mean---> 178.7 195.9 7.2 62.5
MAX----> 181.0 200.0 7.6 66.5
MIN----> 165.0 179.0 6.7 60.7
I_idle I_0dbm fcoLF fcoHF
Sample Code Date microamperes Hz KHz
1 6/1/96 180 194 6.8 66.5
2 6/1/96 165 179 6.7 63.9
3 6/1/96 180 196 6.8 65.2
4 6/1/96 179 195 6.9 61.6
5 6/1/96 180 197 6.9 61.8
6 6/1/96 180 196 7.0 61.7
7 6/1/96 179 197 6.9 61.3
8 6/1/96 169 184 6.9 61.9
9 6/7/96 178 194 7.3 64.9
10 6/7/96 180 200 7.6 62.3
11 6/7/96 179 196 7.4 62.2
12 6/7/96 181 199 7.3 60.8
13 6/7/96 178 196 7.2 62.3
14 6/7/96 179 197 7.2 62.5
15 6/7/96 181 199 7.5 61.7
16 6/7/96 181 199 7.4 63.2
17 6/7/96 181 199 7.3 62.9
18 6/7/96 181 200 7.3 60.7
19 6/7/96 179 197 7.2 61.0
20 6/7/96 179 197 7.3 60.9
21 6/7/96 181 199 7.2 62.6
22 6/7/96 180 199 7.3 61.3
23 6/7/96 180 197 7.2 63.4
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