Ramsey Electronics SHA1 - Manual (Page 4)

SHA1 • 4
What is it and how does it work?
Let’s take a look at the schematic diagram, and we will follow
through from input to output to get a general idea how this kit
works, and why. We will look only at the left channel circuitry
starting at J1 since the right channel is identical to the left.
A line level audio signal is placed in J1. Line level means an audio
signal of around 1V peak to peak, and will give a reading of 0dB
on a VU meter. The audio passes through C1, a coupling
capacitor. This capacitor prevents DC from entering the circuit
from external components and interfering with audio quality. The
capacitor lets the audio pass through to R3, the left channel
volume control.
The volume control simply varies the incoming signals level that
goes into U1, the LM386. U1 is a fully integrated audio amplifier,
capable of driving low impedance loads. It requires very few
external components, runs very efficiently, and has great fidelity.
With the sibilance switch open, the audio passes directly into U1,
and the signal is amplified to drive a low impedance speaker like
those in your earphones.
With the sibilance switch closed, the capacitor C12 is switched
into circuit. This capacitor shunts some of the higher frequencies
to ground, but leaves the lower frequencies alone. C12 creates a
low pass filter when combined with R2, thereby reducing the level
of high frequencies that are amplified in U1. This is really handy if
you have a noisy weak station coming in, or the signal source is
really tinny sounding like from a poorly recorded CD.
R4 and C5 on the output side of the LM386 is for preventing
oscillations due to the inductive nature of a speaker coil being
driven by the LM386. This makes the load of the speaker look
more like a resistive one than inductive, which prevents
“motorboating” of the audio signal.