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1.20 Pre-Amp Section
The pre-amp section also uses the Signetics 5532A op amp. The circuits of the pre-amp
are of opposing signal polarity to provide push pull characteristics and are connected via the
stereo/bridge switch. Each of the two pre-amp channels has 15.7 dB of gain but channel A is
inverting and channel B is not.
Since the main amplifier is inverting, channel A will not invert the signal and channel B
will. Doing this facilitates two things. When the stereo/bridge switch is in the bridge mode
channel B will already be out of phase with A so no signal inversion will have to be done to
achieve bridge operation. Also by operating B out of phase all the time, (even in stereo
operation) getting the phase of the signal flipped back at the speaker terminals, the amplifiers
low frequency power bandwidth will be increased. This allows the power supply to be utilized
more efficiently.
This section also shapes the bandwidth of the amplifier further via another low pass pole
being added to each stage. First and second stage high pass filtering occurs via the 100uf
coupling capacitors preceding the volume pots and after the pre-amp stages. The pre-amp
stages drive the main amplifier directly.
ALINK connections at the junction between R8, C2 and R13, C5 are for sending the
output signal of the input stage at a low impedance for use with auxiliary equipment. The
connection between R9, R10 and R14, R17 are to be used to for defeating the level controls by
supplying an input signal from a very low source impedance to swamp out the signal from the
pots.
1.30 Main Amplifier Section
The main amplifier is composed of four sections:
à
The monolithic front end.
à
A complimentary voltage gain stage or transconductance stage
à
The output driver
à
The output stage
The amplifier is configured in the inverting mode. This allows for the inputs of the op amp
to remain at a 0 voltage potential and ease operation on a +/- 15 volt supply. The large voltage
swing is accomplished with a discrete, complimentary darlington connected transconductance
stage Q1, Q2, Q3, and Q4. The current in the transconductance stage is set by the voltage
divider network made up of R19, R20, R21, and R22. The first transistor in the darlington is a
TO-92 packaged device with the current set by R27, and R28. The collector of this transistor is
not tied to the second collector in the darlington connection but rather to the +/-15 volt supply to
help increase the bandwidth of the stage and reduce the dissipation in the TO-92 devices. The
closed loop gain of the amplifier has been set for 10X or 20dB. The DC output offset is a
function of the input offset voltage error of the op amp times the gain of the system. With the
offset error of the op amp at about +/- 1mv the amplifier will have less than +/- 10mv of output
offset. The feedback network is composed of R25, R18, and C11. C11 reduces the bandwidth of
the amplifier. The large low frequency gain of the op amp helps to reduce the supply ripple that
is introduced into the system by the lack of supply rejection in the voltage gain stage. Under
driven conditions at mild loads of 4 or 8
will appear not to have any signal or very little signal on it. When the amplifier is clipped, or
driven to its rail, the output of the op amp will be driven to its rail, or about +/- 14 volts. It is trying
to correct the loop error or non linearity of clipping in the amplifier. The only other time the output
of the op amp will become a large signal is at a 2
the loss of loop gain when the transconductance stage is required to deliver large amounts of
current to the output stage drivers. CR1 and CR2 are connected as clamps to the discrete
Alesis A4/A8 Amplifiers Service Manual------
at mid and high frequencies, the output of the op amp
condition at lower frequencies. This is due to
2
03/07/03
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