SHA2
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5
C12(C24) is another coupling capacitor, and it serves the same purpose as
C9 at the start of the circuit. This prevents the DC portion of the signal on the
output of U2(U4) from being sent to the headphones.
The clipping detector circuit
The clipping detector circuit watches the headphone output signal level and
flashes an LED when the output signal comes close to the maximum level
available from the LM386. This level is determined by the voltage divider
formed by R15 and R17.
U1C and U1D form comparator circuits for the left and right channels. The
output signals from U2(U4) pin 5 is fed to U1C(U1D) pin 10(12), the ’+’ or non-
inverting input, of the comparator. If this signal level exceeds the level on pin
9(13) the output, pin 8(14), will go to a high state about equal to the supply
voltage of 9 volts. This forward biases diode D2(D5), causing it to conduct
and apply a positive voltage through R16 and light the clipping LED D3. R16
is simply a current limiting resistor to prevent D3 from conducting too much
current and ’burning out’. Because the outputs of the two comparators are
basically at ground under non-clipping conditions D2 and D5 are needed so
that if one comparator output goes high and the other is still low the current to
light D3 is not grounded through the low output.
The looping buffer
The purpose of this circuit is to provide a signal which is identical to, but
isolated from, the input signal to feed other devices such as additional SHA2
units, your tape recorder, etc. The isolation feature prevents the additional
devices from interfering with the original input signal. For example if the other
the device were to develop a short it will not affect the unit that it is being fed
from.
U1A(U1B) are used as a unity gain amplifier or buffer. The input signal is feed
through C1(C2), a coupling capacitor, to the buffer amplifier input pin 3(5).
The output of the buffer then goes through C31(C32) to the buffered loop
output. R1(R4) and R2(R3) form a voltage divider which sets a bias level on
the ‘+’ input of the amplifier which is half of the supply voltage. This allows the
input signal to swing around this point and produce an output signal which is
between ground and the power supply voltage. This is basically the same
way the clipping detector works except the output of U1A(U1B) is made to
swing between 0 and the supply voltage as long as the input level does not
exceed a peak-to-peak level greater than the supply voltage. Capacitor C31
(C32) removes the DC level of the output and presents a signal which is
centered around 0 volts to the loop output jack. R19(R18) Provide a path for
C31(C32) to discharge and also sets the impedance of the output. This low
