Description of Four Microphone Infrasonic Summing Amplifier

This infrasonic amplifier accepts the signal from up to four electret condenser microphones and sums them. It provides the correct microphone operating voltage and load resistance for microphones using the Panasonic WM-52BM electret cartridge. The summed signal passes through a variable gain amplifier. Differential Gains of between 6 and 48 dB can be selected in 6 dB steps by means of internal switches. The frequency response of the amplifier is 3 dB down at 0.1 Hz and 36 Hz. The differential output amplifier is capable of driving the capacitive loads presented by long cable runs. See the block diagram below.

Block Diagram of Four Channel Microphone Amplifier

Internal Adjustments

There are two adjustments that can be made by opening the amplifier case. There is a trim-pot at one end of the printed circuit board which can be used to remove DC offset from the output amplifier. Set this adjustment in the same mode as the output connection. It is made with respect to ground for single-ended operation or differentially for differential operation. The second adjustment is gain and is set with the DIP switches located in the center of the printed circuit board. The gain DIP switches are numbered from 1 to 8. The lowest numbered closed switch sets gain. Where n is the lowest switch, the gain is set by the following:

n GdB = 20 log 2 .

Microphone Response

Operation of the Panasonic WM-52BM electret microphone cartridge in the infrasonic band (below 20 Hz) is outside the manufacturer’s specified range of operation. Because of this, the sensitivity and response of 25 units were tested in an attempt to characterize the response below 20 Hz. The test showed that the low end response of each unit was dominated by a single high pass pole. The frequency of the pole was between 1.1 and 4.8 Hz. Half the units had poles between 2 and 3 Hz.

Specifications

Amplifier Response: -3 dB at 0.1 Hz and 36 Hz Amplifier Gain: switch selectable 6 to 48 dB differential, 0 to 42 dB single-ended. Output Impedance: 300 Ù differential, 150 Ù single-ended. Open-circuit Output Signal Swing: 12 volts peak differential, 6 volts peak single-ended. Current Consumption at 12 volts: 8 mA quiescent, plus 3 mA per microphone. In Band Microphone Sensitivity: -44 dB ± 2 dB, 0 dB = 1 V / Pa, nominally 6.3 mv / Pa.

Connection Notes

1) The electret microphones must be connected with the correct polarity. The mics with clear plastic cable (zip cord) have polarity marked with tinned copper strands on the negative side. The positive side has plain copper strands. The mics using black cable have their polarity marked by the color of the cable wires. Red is positive. Black is negative.

2) The output of the summing amplifier (OP-, OP+) is differential and should be connected to the Reftek differential channel inputs without grounding either side and with the correct polarity. If a single ended connection is desired either output maybe used with respect to ground but the output level will be halved and the “OP-” signal is phase inverted. Never ground “OP-” or “OP+”. Long cable runs between the Reftek and the summing amp are permitted. Connection Diagram

P. J. McChesney [email protected] 1.5 V RP1A 3.3K C1 C5 10uF RP2A 6.8nF 330K C7 +M 1 2.2nF RP2E 1.5 V 330K R2 1M RP1B C2 3.3K 10uF RP2B C18 330K C6 +7 V 2 R1 0.1uF +M 2 - 3.3uF 1 1M 13 U1A - 4 14 1.5 V 3 + U1D 12 U1 LT1014 + RP1C C3 R3 3.3K RP2C 10uF 1M 330K +M 3 R4 RP3 5 x 5.1K 20.5K 1.5 V pin 1 RP1D 16 15 14 13 C4 3.3K RP2D 1 2 DIP SW1 3 4 10uF 330K +M 4 8 7 6 5

9 R5 10 11 12 324K

D1 C24 RP4 5 x 82K pin 1 +12 V 1N5819 0.1uF C8 R10 C10 9 3.3uF TVS3 C23 10nF - 10K R11 10nF 20V 8 10K U1C C20 10 R19 22K + 11 C19 +7 V 0.1uF 0.1uF R7 +7 V R6 2 - R17 1.5 V out in 1M 8 150 OP- 4.99K R12 1 U2A sen fb -7 V 510K C13 C9 P1 3 shut 5V + 10uF R20 2.2nF 100K TVS1 7.5V gnd err 100K R13 U3 LP2951 5.1K R14 R8 1M -7 V 6 10K - C11 C22 C12 7 10nF U1B 10nF 10uF 5 + R21 47K R9 1M R15 10K R18 out in 6 - OP+ R16 7 150 sen fb U2B C14 5.1K shut 5V 5 10uF R22 + 4 TVS2 gnd err 10K U2 7.5V U4 LP2951 LT1355 NOTES C21 -7 V 0.1uF +7 V 1) Use with Panasonic WM-52B bst V+ Electret Condenser Microphones C17 C+ osc 0.1uF 2) All unmarked IC pins are Four Input Infrasonic Microphone C15 gnd lv drawn in pin order top view. 10uF C- out -7 V Summing Amp U5 7660 C16 10uF Pat McChesney 8/99