Crown Audio S Series operation manual Principles of Operation, S-Series Power Amplifiers, page

S-SeriesPower Amplifiers

5 Principles of Operation

For the sake of simplicity, only channel one of the amplifier is described.

Signal is presented to the S amplifier through one of three connectors when using the stan- dard input module. Each channel is outfitted with a balanced XLR/phone jack and a barrier strip. These connectors are wired in parallel, which allows daisy chaining when needed. The signal is then converted from balanced to unbalanced in the Balanced Input Stage where it also receives RFI protection. Signal then flows into the Variable Gain Stage where the front panel level controls are allowed to affect the gain.

Following this stage, the signal is put under the control of a full-time compressor circuit com- prised of a symmetrical window detector, a buffer amplifier, and the gating op amp which uses several small components to set the com- pressor’s attack and decay characteristics. The actual compressing is accomplished by an opto-isolator which affects the gain in the sig- nal path.

Next the signal enters the error amplifier circuit, where it is mixed with a small portion of the output signal in such a way as to control the amplifier’s overall output perfomance.

Following the error amp is the LVA stage, which is where the low-voltage referenced signal gets translated to the output high-voltage rails. The last voltage amplifier, in conjunction with a bootstrap current source, drives both predriv- ers and the bias servo. The bias servo is mounted in such a way as to translate the out- put heatsink temperature into a controlled bias current to prevent thermal runaway and hold the amplifier’s notch distortion to a minimum.

The predrivers provide enough signal to acti- vate the drivers, which together operate in the class AB range. For the major output current requirements, the drivers feed the various num- bers of paralleled output transistors which operate in a class B mode. This we call the Tri- ple-Deep Darlington Output Stages.

The output transistors are protected by the Time Dependent Voltage & Current circuit. This cir- cuit protects the devices from extending beyond their safe area of operation, but allows the devices to provide high bursts of peak power with music, allowing your amplifier to deliver more punch. This amplifier output topology offers a good combination of low qui- escent amplifier heating, great distortion per- formance at high powers, and relative simplicity, with impressive reliability and value.

All output power is delivered through 4-circuit Speakon® connectors. These connectors have been wired in such a way as to allow you the most versatility. The Channel 1 and 2 connec- tors are cross wired so you can cover all options, from dual cable stereo (typical), to Bridge-Mono in one connector, to running a bi- amp speaker with one amplifier and one cable per speaker cabinet.

The output relay, in conjunction with input sig- nal mute circuit, assures the amplifier will be well-behaved during turn on and off. In the event of an amplifier output failure, a triac will activate to turn off the offending channel and protect your speakers.

The turn on delay circuit functions to keep the output relay open until all the voltages are up and stable, both in the amplifier, and in all the components in the system ahead of the amplifier.

Heatsink temperature is monitored by a thermal probe attached to the heatsink. As the tempera- ture rises, the probe sends a proportional cur- rent to the proportional speed fan circuit which starts the fan. Should the power transformer

reach its maximum safe temperature, an inter- nal thermal switch opens and the fan circuit turns on full speed to quickly cool down the amplifier. It also disconnects the load via the output relay, removing any output current and further speeding a cool-down cycle. Extra care was taken during the design stage to set this point both to protect your investment and to guard against nuisance tripping.

Whenever the heatsinks or the transformer reach a maximum temperature, or during the normal turn-on delay window, the front panel Fault LEDs will blink to get your attention.

A modular jack is mounted on the back panel (same type as used on telephones). Pins 2 and 5 are connected to an opto-isolator which is always in a low-resistance state whenever the unit is on and functional. Should a fault be detected or should the amplifier lose AC power, the opto-isolator will change to a high resis- tance, allowing the user to remotely detect the status of the amplifier.

The Signal Presence Indicators tap the signal chain just before the level controls and prior to the power amplifier chain. They are not ampli- fier output indicators and should only be used to indicate the presence of signal to the ampli- fier front end.

The Clip light is driven from the output of the compressor circuitry and lights to indicate the onset of audible distortion.

The Power LED is driven from the low-voltage supply.

A positive and negative regulator form the ±15- volt power supplies. Add to that the main trans- former, a full-wave bridge rectifier, and high energy electrolytic to form the main power sup- ply. They are protected by the front-panel line circuit breaker and controlled by the front-panel power switch.

Operation Manual page 19