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1.00 Theory of Operation
The A4/A8 amplifiers are basic stereo amplifiers. They have 0dBm input sensitivities for
rated output at 4 , with the ability to drive any load impedance from 2
have balanced inputs via Neutrik combination connectors providing XLR and 1/4" TRS
connections and also a barrier strip. The output of the amplifier is obtained by way of four five
way binding posts. Reference designations in the text refer specifically to the A4 amplifier except
where otherwise noted. Though reference designations between the two units are different, the
designs are virtually identical. The only significant differences occur in the output section (since
the A8 is required to handle much more power than the A4).
Here are some of the major features and building blocks of the Matica:
à
An input balanced to unbalanced converter.
à
A second stage pre-amp and an amplifier gain stage utilizing a monolithic front end with
discrete complimentary transconductance stage and a complimentary output stage in a
common collector configuration.
à
Output device protection is accomplished with a conventional volt-amp current limiter circuit.
The output devices use a new perforated emitter technology unique to MOTOROLA. The
output devices are driven by similar technology devices, but they have been optimized for
extremely linear current gain with a unity gain bandwidth (F t ) of 50 MHz.
à
The speakers are protected by output relays. They are activated during the first 3 to 5
seconds the amplifier is turned on. Also, if a DC condition exists at the output of the amplifier
or the amplifier is driven to full output below 5Hz the relays will be activated.
à
Thermal management and protection are accomplished with a large heavy aluminum
extruded heat sink the is fan cooled. If the sink gets warmer than 55 deg. C, the fan is
automatically stepped up in speed and if the heat sink gets warmer than 80 deg. C the fan is
run at high speed and the output relays are activated to disconnect the load until the unit has
cooled to 65 deg. C. There is also a unique feature of the amplifier fan circuit in that when the
amplifier is providing an output signal of a little more that a watt the fan speed is modulated
or increased by the signal. This will help get longer run times with the amplifier under
extreme load conditions.
1.10 Input Section
The input stage is made up of a dual Signetics 5532A op amp. This is a low noise
selected version of the 5532. One half of the dual is used for each channel. The circuit is a basic
balanced to unbalanced converter. It can be driven unbalanced but 6dB differences in gain may
result for various hookups. If the (+) input is driven with the (-) input grounded the gain will be
unity. If the (+) input is driven with the (-) input left open there will be a gain reduction of 6dB.
This is not recommended as a noisier condition may result. If the (-) input is driven, gain will be
unity and will not change with the grounding or ungrounding the (+) input The maximum input
level before clipping in about +21dBm. Since there is no level control in front of the converter this
is the maximum input level for the amplifier. Connections to the Alink connector are between R1,
R2 and R3, R4. These are provided so an outboard impedance may be connected to modify the
gain and overload characteristics as well as its frequency response. Following the amplifier is a
passive low pass (R7, C1) filter that begins to limit the high frequency gain of the amplifier.
Alesis A4/A8 Amplifiers Service Manual------
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to an open circuit. They
03/07/03
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