Download 6.0 SD II Final Documentation

microphone or electric guitar pickups are in the range of 1.5-50 mV. This is to low
for regular audio amplifiers. If a transducer with high output impedance is
connected to an audio amplifier with also a high input impedance the connection
will “load down” the circuit and would act as a voltage divider. This may reduce
the signal level by up to 6 dB. To avoid this, the low level microphone or
instrument signal is connected to a preamplifier circuit that would raise the signal
level to the 100ʼs of mV range. As discussed in the impedance matching
subsection the input of this pre-amplifier must be at least 10 times greater than
the output impedance of the source feeding the signal. Line level signals in the
other hand do not require any pre-amplification. They can be fed directly into the
audio frequency amplifier. This implementation is done in the Unplugged by
running the Microphone/Instrument input through a preamplifier with a voltage
gain of 10 and running the mp3 player input directly into a 2 band equalizer that
is then fed into a unity gain mixer that combines both the mp3 player and the
microphone/instrument signals.
A more convenient way of differentiating line level signals from microphone level
signals is the dBu. It is a decibel unit where the reference voltage for the ratio is
0.775 Vrms and the output load is 600 Ω. The 0dBu point will happen when the
load dissipates 1mWatt of power. Line level signals are referred to be at +4 dBu
and Microphone and Instrument level signals at -10 dBu.
2.11 Audio Equalizing
2.11.1 Background
The main goal of equalization in the product is to compensate the signal
deficiencies of each input among the audio frequency spectrum. This allows the
user to shape the sound within reasonable parameters by increasing the audio
gain of different frequency bands. This audio equalizing chain is implemented as
an active filter.
Active and passive implementations offer distinct advantages and disadvantages.
The main advantages of active filters are that they can easily be implemented
with operational amplifiers and few resistors and capacitors; they can deliver high
quality factors with high accuracy. The disadvantages of using active filters are
that most op-amps that can be used for them such as the NE5534 require a dual
sided Voltage source of at least 12 Volts (Datasheet 1) that would draw
anywhere from 1.7 mA to 2.8 mA depending on the ambient temperature. This
amount of current is given per each individual op-amp. In the case of the
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