Ramsey Electronics SHA2 - Manual (Page 4)

SHA2 • 4
CIRCUIT DESCRIPTION
Let’s take a look at the schematic diagram, we will follow through from input
to output to get a general idea how this kit works, and why. We will look
mainly at the left channel circuitry starting at J1. Items in parentheses ,
‘(xx)’, refer to the right channel signal path. Standard configuration settings
will be assumed. See the optional configurations section for additional
operation information.
The nitty-gritty of it all
A line level audio signal is connected to J1. Line level means an audio signal
of around 1V peak to peak, will give a reading of 0dB on a VU meter. The
audio passes through C9 (C10), 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 pin 19(2) of U3, an
LM1036, which is the tone/volume/balance control IC.
This IC can be thought of as the heart of the SHA2. It provides control of all
the major functions of the unit including volume, tone, balance and loudness.
Without this IC there would be a large quantity of resistors, capacitors and
amplifiers required to achieve the same functionality it provides. By the way,
remember that it is actually two sets of these controls since this is a stereo
system. R9, R10, R11 and R12 are the volume, tone and balance controls.
These functions are controlled by U3 with a DC voltage applied to the
respective control pins. This voltage is supplied by a built in reference source
on pin 17 and each control varies the voltage on the control pins which is
applied to its control pin. The loudness control, S1A, is used to provide a
slight amount of bass boost when the volume is set to lower settings.
Because your ear is less sensitive to low tones at low levels this provides a
more robust sound at these levels. Capacitors C15 and C21 determine the
way the treble control operates. Capacitors C19, and C20 are for the bass
control while C16 and C18 are used both treble and bass.
After being processed the signal is sent from U3 pin 8(13) to the input of the
output amplifier, U2(U4) pin 3 through C10(C17), another coupling capacitor
like C9. U2(U4) 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. U2(U4) amplifies the signal to drive a low
impedance speaker like those in your headphones.
R7(R14) and C14(C27) on the output side of the LM386, U2(U4) 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 resistive
rather than inductive which prevents “motorboating” of the audio signal.