Q & A
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QCan you please help with a simple diagram for the FM radio shown in Figure 1 I would like to add an
LM386 amplifier — less than or equal
to one watt — with minimal parts and
an eight ohm speaker to it. I’d like to
eliminate the need for a ‘crystal
earphone,’ using just one power
supply — that of the FM radio or from
a wall wart ( 9 VDC 100 mA).
Since I couldn’t locate an original
365 pF variable capacitor at an
affordable price, I substituted one
rated 0-532 pF. Consequently, do I
need to change the value of C4?
Also, I couldn’t find the two metal
PNP transistors (25A220). Can you
please recommend other substitutes,
without modifying the entire
■ FIGURE 1.
— Don Franklin
AThis is not the usual FM radio; it is an AM radio that is able to receive FM by “slope detection.” When
the radio is tuned off center
frequency, the slope of the response
curve converts the FM to AM, which
is then detected.
The LM386 datasheet has several
circuits that you could use. The
simplest (minimum parts) has a gain
of 20 which is not enough to drive
the speaker. The circuit with a gain
of 200 is the one to use. In Figure 2,
I show how it should be used in this
C4 — called a padder — is used
to reduce the capacitance of C3 and
thereby reduce the tuning range. It is
not necessary to change C4, but if
you want the same tuning range, the
capacitance of series capacitors is
calculated similar to parallel resistors:
C=C3*C4/(C3 + C4) = 365pF*50pF
/(365 + 50) = 44pF
■ FIGURE 2.
Here’s a little algebra to find the
value of C4 with 532 pF: