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The preamp is usually the voltage amplifier part of the
audio amp.
Since the maximum output voltage from the CD player is about 0.5V,
we will require our preamp to have a maximum voltage gain of
approximately 20 for all frequencies over the audio range.
The Common Emitter amps from Lab 2 are probably not good choices
for achieving this gain because they are capacitively or AC coupled,
and thus amplify lower frequencies less than higher frequencies
in the audio range. (Typically sized coupling capacitors block signals at low
range audio frequencies.)
From Lab 1, we know that such a gain is attainable using
an op-amp circuit for two main reasons:
(1) Op-amp
circuits can be DC coupled thereby allowing us to amplify
low frequency signals as well as high frequency signals.
(2) Op-amp circuits are connected with feedback that ensures
a flat frequency response over the audio range.
Also the use of feedback also reduces distortion.
The preamp stage is also where you would probably
want to include volume and tone controls.
Volume control can be achieved either by varying the gain
of the preamp stage, or by following the preamp
with an adjustable voltage divider which will
attenuate the signal before it reaches the power stage.
741 op-amps are a reasonable choice for designing the
preamp.
Next: Power Amplifier Stage
Up: Design Considerations
Previous: Complementary Symmetry Power Amplifiers
Neil Goldsman
10/23/1998