Crown Audio MA-36X12 Theory of Operation, Macro-Tech MA-24X6 & 36X12 Power Amplifiers, page

Macro-Tech MA-24X6 & 36X12 Power Amplifiers

6 Theory of Operation

Each channel is powered by its own power transformer, T100 or T200. Both channels share TF-1, a low voltage transformer. The sec- ondary outputs of each transformer are full- wave rectified by heavy duty bridge rectifiers and are filtered by large computer grade capac- itors. A thermal switch embedded in each transformer protects them from overheating.

The low voltage transformer TF-1 uses a sepa- rate fan motor winding. The TF-1 output is rec- tified by diodes D1-4, generates an unregulated

24volts. Monolithic regulators U1-2 provide a regulated ±15 volts.

For simplicity, the following discussion of the circuitry and operation will refer to one channel only. Please refer to the block diagram in Figure 6.1

The input signal at the phone jack passes directly into the balanced gain stage (U104- C,D). Use of a PIP module for input signal causes the input signal to pass through the PIP and then to the balanced gain stage.

The balanced gain stage (U104-C,D) causes balanced to single-ended conversion to take place using a difference amplifier. From there, gain can be controlled with a potentiometer. The error amp (U104-A) amplifies the differ- ence between the output signal and the input signal from the gain pot, and drives the voltage translator stage.

The voltage translator stage channels the signal to the Last Voltage Amplifiers (LVAs), depend- ing on the signal polarity, from the error amp U104-A. The +LVA (Q105,Q125) and the –LVA (Q110,Q126), with their push-pull effect through the bias servo Q318, drive the fully complementary output stage.

The bias servo Q318 is thermally coupled to the heat sink, and sets the quiescent bias cur- rent in the output stage to lower the distortion in the crossover region of the output signal. D301, D302, D303, and D304 are used to remove the charge on the unused portion of the output stage, depending on the polarity of the output signal.

With the voltage swing provided by the LVAs, the signal then gains current amplification through the Darlington emitter-follower output stage.

The bridge-balanced circuit (U104-B) receives a signal from the output of the amplifier, and differences it with the signal at the Vcc supply. The bridge-balanced circuit then develops a voltage to drive the bridge-balanced output stage. This results in the Vcc supply having exactly one half of the output voltage added to their quiescent voltage. D309, D310, D311 and a trimmer resistor set the quiescent current point for the bridge-balanced output stage.

The protection mechanisms that affect the sig- nal path are implemented to protect the ampli- fier under real-world conditions. These conditions are high instantaneous current, excessive temperature, and operation of the output devices outside safe conditions.

Q107 and Q108 act as a conventional current limiter, sensing current in the output stage. The allowable current level is also adjusted as a function of voltage. When current at any one instant exceeds the design criteria, the limiters remove the drive from the LVAs, thus limiting current in the output stage to a safe level.

To further protect the output stages, a specially developed ODEP (Output Device Emulation Protection) circuit is used. It produces an ana- log output proportional to the always changing safe operating area of the output transistors. This output controls the translator stage by removing any drive that exceeds the safe oper- ating area of the output devices. Thermal sen- sor S100 gives the ODEP circuits vital information on the operating temperature of the heatsink on which the output devices are mounted.

Should the amplifier fail in such a way that would cause DC across the output lead, the DC protection circuit senses this on the negative feedback loop and shuts down the power sup- ply until the DC is removed.

Operation Manual page 19