The 640A builds upon the impressive features of the 540A resulting in a quality of performance you would expect from amplifiers costing considerably more. This development provides true audiophile levels of sound reproduction and takes the Azur marque to the highest level.

Output is 65 watts per channel into eight ohms and the inclusion of a Class 'A' follower driver stage, together with high quality polypropylene bypass capacitors reflects the attention to detail that sets the 640A apart from its rivals.

CAP5 (Cambridge Audio Protection 5) is an innovative and intelligent protection system. It constantly monitors your amplifier, without degrading signal purity, ensuring ultimate reliability. This incorporates a clipping sensor that reduces volume to keep levels in a safe range. There is also short-circuit detection and a solid-state temperature sensor with over-heat cut out. Combined these technologies ensure your Cambridge Audio Azur amplifier will deliver years of reliable high performance.

A proprietary non-invasive five-way protection circuit CAP5 was the subject of a separate research and development programme within Cambridge Audio. The aim was to develop a highly innovative protection system that would not affect the sound quality of their amplifiers.

Perhaps most innovatively the circuit measures the distortion of the output signal relative to that of the input. This means the CAP5 circuit is able to tell when the unit is being driven to hard and apply the protection. The mechanism triggers a two-stage response. For small excursions into clipping, CAP5 'nudges' down the volume control. For gross clipping the CAP5 protects the unit by going into Standby.

The circuit constantly measures the DC level at the output terminals. It immediately disconnects the speakers if any DC is detected. This protects the attached speaker in the event of an amplifier fault.

The circuit constantly monitors the current in the output stage and the voltage across it. It ensures levels remains within a safe operating area. The CAP5 immediately protects the amplifier should this be exceeded.

The circuit measures the temperature of the internal heatsink. If this exceeds a set temperature reading the unit is shut down.

Each time the amplifier is turned on, CAP5 measures the resistance across the speaker terminals. It checks for a short every time the unit is turned on before allowing use. The most likely time this occurs is when a customer changes his/her speakers.