Elenco Projects 102-305 - Manual (Page 50)

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Project #240
Power Amplifier
OBJECTIVE: To check stability of power amplifier with open
input.
When you turn on the slide switch (S1), the power amplifier IC (U4)
should not oscillate. You should be able to touch point X with your finger
and hear static. If you do not hear anything, listen closely and wet your
finger that touches point X. High frequency clicks or static should be
coming from speaker (SP) indicating that the amplifier is powered on and
ready to amplify signals.
The power amplifier may oscillate on its own. Do not worry, this is normal
with high gain high-powered amplifiers.
Project #241
Feedback Kazoo
OBJECTIVE: To show how electronic feedback can be used to
make a musical instrument.
Use the circuit from project #240 shown on the left.
When you place one finger on point X and a finger from your other hand
on the speaker (SP) snap that is not connected to the battery (B1), what
happens? If the amplifier starts to oscillate it is due to the fact that you
just provided a feedback path to make the amplifier into an oscillator. You
may even be able to change the pitch of the oscillation by pressing harder
on the snaps.
This is the principle used to make an electronic kazoo. If you practice and
learn the amount of pressure required to make each note, you may even
be able to play a few songs.