The Panasonic WM61A amplifier outlined on the Using the Panasonic WM61A as a Measurement Microphone page can be used with the Dayton Audio iMM-6 Calibrated Measurement Microphone. The iMM-6 has very low distortion, (1% at 127 dB SPL,) and very reasonable bandwidth, (18 to 20 kHz.,) and noise, (70 dBA,) characteristics. The amplifier can be modified to accommodate these characteristics:

Figure I is the schematic of the iMM-6 amplifier, (which is available in a larger size png file,) designed on the gEDA Project system, and simulated with NGSPICE, (both available from the Linux repositories.) The design files (including NGSPICE models for the WM61A and CMA-4544 electret microphones, including 2SK3372 and 2SK596B J-FET transistors,) are available in a tape archive, imm-6.tar.gz, (about 33 KB,) to replicate the entire design, (including a single layer DIY PCB):

Quick Start:

1. tar xvfz imm-6.tar.gz
2. cd imm-6
3. make co
4. make
5. emacs plotit.gp
6. make realclean
7. cd ..

The amplifier is designed to run on 4 AA batteries to provide 4.8 V DC, which will last several hundred hours-this decision was made to avoid stray noise pickup on the microphone element's ground, (which is single ended,) and an eternal issue with electret microphones, (specifically, an environment with "wall wort," switching power supplies.) The LT1677 operational amplifier was chosen for a single, (minimum,) 3 V DC power supply operation, and rail-to-rail input and output capability, allowing +4dBU 1.228 V RMS studio level outputs, with reasonable noise and distortion specifications.

Note that the signal is applied to the inverting input of the operational amplifier, which is a virtual ground to AC-meaning the drain of the J-FET is at constant Voltage within the amplifier's bandwidth, omitting channel length modulation, and lower noise performance. The gain is product of gm of the J-FET and R3, (110K.) The low frequency F3 is determined by R1, (4.3K,) and C3, (4.7uF,) and, the high frequency F3 is determined by R3, (110K,) and C5, (30pF.)

With the above parametric values, the low frequency F3 of the amplifier is 8.6 Hz., and the high frequency F3 is 41 kHz. The output Voltage with the amplifier with the WM61A electret capsule is 1 V RMS @ 100 dB SPL, 0.5 V RMS @ 100 dB SPL for the CMA-4544, and, 0.7 V RMS for the iMM-6 microphone. The openloop Ft, (0 dB gain,) of the amplifier is 2.2 MHz., with a phase of -127 degrees.

The LM431 provides a 2.5 V reference for the positive terminal of the operational amplifier, although a resistor divider off of the power supply is workable, also.

License

A license is hereby granted to reproduce this design for personal, non-commercial use.

THIS DESIGN IS PROVIDED "AS IS". THE AUTHOR PROVIDES NO WARRANTIES WHATSOEVER, EXPRESSED OR IMPLIED, INCLUDING WARRANTIES OF MERCHANTABILITY, TITLE, OR FITNESS FOR ANY PARTICULAR PURPOSE. THE AUTHOR DOES NOT WARRANT THAT USE OF THIS DESIGN DOES NOT INFRINGE THE INTELLECTUAL PROPERTY RIGHTS OF ANY THIRD PARTY IN ANY COUNTRY.

So there.

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