Speech Amplifier


This circuit is intended to be placed in the
same box containing the loudspeaker, forming a compact microphone
amplifier primarily intended for speech reinforcement. A device of this
kind is particularly suited to teachers, lecturers, tourists’ guides,
hostesses and anyone speaking in crowded, noisy environment. The
circuit’s heart is formed by the TDA7052 Audio power amplifier IC,
delivering a maximum output of 1.2W @ 6V supply. An external microphone
must be plugged into J1, its signal being amplified by Q1 and fed to
IC1. R1 acts as a volume control and C3 tailors the upper audio
frequency band, mainly to reduce the microphone possibility of
picking-up the loudspeaker output, causing a very undesirable and loud
“howl”, i.e. the well known Larsen effect. Therefore, C3 value can be
varied in the 4n7 – 22nF range to ensure the best compromise from speech
tone quality and minimum Larsen effect occurrence. Dynamic or electrets
microphone is warmly recommended. It has a useful feature that can be
used to momentarily mute the microphone by connecting SW1 shown in
diagram.

Parts:

P1 = 22K
R1 = 1M
R2 = 15K
R3 = 470R
R4 = 47K
R5 = 4.7K
R6 = 4.7K
C1 = 100nF-63V
C2 = 100nF-63V
C3 = 10nF-63V
C4 = 100nF-63V
C5 = 220uF-25V
C6 = 10uF-25V
Q1 = BC547
IC1 = TDA7052 B1
J1 = Mono Jack Socket
B1 = 6V Battery
SW1 = SPST Slider Switch
SW2 = SPST Toggle Switch

Notes:
Please note that hands-free, uni-directional headset or ear clip
microphone types are very well suited for this device, as also are
Clip-on Lavaliere or Lapel microphones.
If a small electrets capsule is used for the microphone, R5, R6 and C6 must be added to the circuit to provide power supply.
Choose a loudspeaker as large as possible, in order to increase circuit performance.
You can use also two 4 Ohm loudspeakers wired in series or two 8 Ohm types wired in parallel in order to obtain better results.
The box containing the amplifier and loudspeaker(s) can be fitted out
with a belt and carried like a shoulder-bag or, if you build a smaller
unit, it can be used as a Pick & Go Belt Clip Speaker.


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