Portable Microphone Preamplifier

High headroom input circuitry, 9V Battery operation

This circuit is mainly intended to provide common home stereo amplifiers with a microphone input. The battery supply is a good compromise: in this manner the input circuit is free from mains low frequency hum pick-up and connection to the amplifier is more simple, due to the absence of mains cable and power supply. Using a stereo microphone the circuit must be doubled. In this case, two separate level controls are better than a dual-ganged stereo potentiometer. Low current drawing (about 2mA) ensures a long battery life.

Circuit Operation:

The circuit is based on a low noise, high gain two stage PNP and NPN transistor amplifier, using DC negative feedback through R6 to stabilize the working conditions quite precisely. Output level is attenuated by P1 but, at the same time, the stage gain is lowered due to the increased value of R5. This unusual connection of P1, helps in obtaining a high headroom input, allowing to cope with a wide range of input sources (0.2 to 200mV RMS for 1V RMS output).

Portable Microphone Preamplifier Circuit diagram:

Parts:
P1 = 2.2K
R1 = 100K
R2 = 100K
R3 = 100K
R4 = 8.2K
R5 = 68R
R6 = 6.8K
R7 = 1K
R8 = 1K
R9 = 150R
C1 = 1uF-63V
C2 = 100uF-25V
C3 = 100uF-25V
C4 = 100uF-25V
C5 = 22uF-25V
Q1 = BC560
Q2 = BC550

Notes:

• Harmonic distortion is about 0.1% @ 1V RMS output (all frequencies).
• Maximum input voltage (level control cursor set at maximum) = 25mV RMS
• Maximum input voltage (level control cursor set at center position) = 200mV RMS
• Enclosing the circuit in a metal case is highly recommended.
• Simply connect the output of this device to the Aux input of your amplifier through screened cable and suitable connectors.

Source :  http://www.ecircuitslab.com/2011/06/portable-microphone-preamplifier.html

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Portable Detector Comparator Temperature

This circuit called the Portable Detector Comparator Temperature, to ascertain actual baby changes in temperature in affiliation to the Ambient temperature.

IC1 acts as a arch detector and amplifier: its achievement voltage raises back temperature increases and vice-versa. This happens because the n.t.c. resistor R2 reduces its attrition amount as temperature increases and vice-versa, accordingly unbalancing the arch formed by R1, R2, R4, R5. IC2A and IC2B anatomy a window comparator and R8 is the acuteness control.

Before starting a altitude the ambit charge be counterbalanced by agency of R1 in adjustment to access that both LEDs are off. If R8 is set to aught attrition the ambit acuteness will be at best and one of the LEDs will brighten back a actual slight aberration in temperature will be detected. As R8 amount is added the ambit acuteness will decrease.

Portable Detector Comparator Temperature Part List

R1 = 22K  Linear Potentiometer
R2 = 15K  @ 20°C n.t.c. Thermistor
R3 = 10K
R4,R5,R7,R9 = 22K
R6 = 220K
R8 = 5K
R10 = 680R
C1 = 47µF
D1 = Red LED
D2 = Green LED
IC1 = TL061 Low current BIFET Op-Amp
IC2 = LM393 Dual Voltage Comparator IC
P1 = SPST Pushbutton
B1 = 9V

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