Class D FAQ

Why move to class D?

Is a class D amplifier a digital amplifier? When does it qualify as one?

Are class-D amplifiers as reliable as conventional amplifiers?

Are Class-D amplifiers a potential EMI source?

How can I infer sound quality from measurement data?

What is behind the power rating?

Class D FAQ
 

Why move to class D?

Class D amplifiers are chiefly chosen for their power efficiency, small physical volume, low weight, low airflow requirements, and less demanding power-supply requirements. All these properties add up to more design freedom.

Is a class D amplifier a digital amplifier? When does it qualify as one?

It is common to attribute the word "digital" to any class D amplifier because the output devices are principally operated in the ON and OFF states. However, since they spend between 1% and 10% of their time transitioning between those two states and the amplitude of the switched voltage is not fixed they are not ‘digital’ in the true sense. A strict definition of a digital class D power amplifier would be a power amplifier where the switching pattern is a bit pattern, generated entirely using digital means, and independently of the output stage. An amplifier is not digital if an analog processing is present, the input signal is analog, feedback or other error correction is employed, or large timing and/or amplitude errors are present.

Are class-D amplifiers as reliable as conventional amplifiers?

There is no reason why class D amplifiers should be any less reliable than linear amplifiers. Some people wonder whether they ever mis-switch; they don't. The three most important factors dictating reliability of electronic products are temperature, thermal behaviour and heat management. Class D amplifiers’ high efficiency leads to good thermal characteristics, which in turn, can lead to higher reliability.

Are Class D amplifiers a potential EMI source?

EMI (electro-magnetic interference) is caused by rapid changes in voltages and currents. When the power stage makes a transition, the switch outputs change across the entire power supply voltage and the entire loudspeaker current is re-routed through the output stage. This is the root cause of EMI in class-D amplifiers. It can be satisfactorily addressed by relaxing switching speeds (if distortion is controlled by feedback), and meticulously planning the board layout.

How can I infer sound quality from measurement data?

The best way to determine the sound quality of an amplifier is connect a source and good speakers and listen.

What is behind the power rating?

The power rating of finished amplifiers is already an issue. One amplifier could have the following power ratings:

• 50W 1%, 60Hz-20kHz (IHF – a mandatory rating system for products sold)
• 65W 10% THD (EIAJ - the output power at 1kHz for 10% THD)
• 80W Music Power (DIN - RMS’ power on a single-cycle 1ms tone burst)
  130W Peak Power (‘Sales talk’ – twice the average power, assuming a non-clipped sine wave)