CH and CL Series Power Amplifiers
5 Principles of Operation
The signal next enters the main amplifier error amp where it is mixed with a small portion of the output voltage and current in such a way as to control the amplifier’s overall output perfor- mance.
Following the error amp is the modulator stage where the audio signal is compared to an extremely accurate 250-kHz triangle waveform. Comparators output a Pulse Width Modulated (PWM) string of pulses at 250 kHz that vary in width depending on the level of the input sig- nal. These strings of pulses, one for the posi- tive side and one for the negative side, are connected to the output stage via optocouplers.
The signals from the optos are then passed to gate drivers that amplify the pulses to the level required to drive output devices. The driven
output devices are now able to produce PWM pulses that have an output voltage from the negative high-voltage rail (–Vcc) to the positive high-voltage rail (+Vcc). This output voltage is always the same (2 * Vcc) but the width of the pulses is still dependent on the level of the input signal. The positive and negative output PWM pulses then pass through inductors and are summed together. Summing the output sig- nals through inductors reconstructs the audio signal, amplified to the desired level. There is a small amount of ripple on the output that is at double the switching frequency (500 kHz).
The amplified audio signal is then passed through an output filter that removes the resid- ual ripple voltage.
Protection for the output devices is performed by a very precise pulse-by-pulse current limiter circuit that operates each time the output devices switch. The current limiting is “flat” meaning that, regardless of the output voltage, the output current always limits at a certain value.
The turn-on delay circuitry functions to keep the modulators turned off (which keeps the out- puts from switching) until all supplies are up and stable.
Thermal probes monitor Heatsink tempera- tures and power transformer temperature. As the temperatures rise, the probes send a pro- portional voltage to the fan control circuit and the Thermal Limit Control (TLC) circuit. The fan
Figure 5.4 Model CL4 Circuit Block Diagram
