Class D Amps
Types of Amplifiers – Class D
Technology is a wonderful thing; new developments all the time make our audio world a wonderful place to be in. One of the latest to come out are these so-called digital amplifiers. Technically the name is incorrect, although they do have a digital aspect about them. Still fairly thin on the ground, you will surely hear a lot more about them over the coming years.
In the beginning we had valve amplifiers, these evolved to great heights and even now continue to be produced. When the transistor came out, it was rapidly put to use in audio amplifiers once enough power could be mustered to drive speakers! Bipolar transistors evolved into MOSFETs (metal oxide semiconductor field-effect transistors since you asked!), a name commonly heard in describing higher end amplifiers, generally they have better stability and performance characteristics than the older bipolar type.
The latest type of amplifier is often called Class-D; “D” being again incorrectly assumed to be a reference to digital, rather the name is a logical follow-on to Class-A, B, A-B, and C type amplifiers in describing a different technology used to produce the sound. Class D amps are not new, they have been around since the 1960’s but not often used in hifi applications until recently. One unusual use was by Mercedes-Benz in their top-of-the-range S-Class car, it was fitted with a 7 channel, 13-speaker sound system by Becker and Bose. The Bose amplifier had Class D technology driving the subwoofers and the front speakers. The system was widely regarded as one of the best factory fit sound systems of its time. Infinity also had Class D amplifiers at one stage but they were discontinued in the 1990’s. All these amplifiers were small and used relatively little power.
These last two characteristics are still valid for today’s crop of Class D amplifiers, often achieving 90% to >95% efficiency. That means that of the power coming into the unit, most of it is converted into power to drive the speakers; so the amplifier itself will tend to run a lot cooler than more traditional designs. As an example, Class A amps are around 30-40% efficient and A-B Class amplifiers about 55-65%. The means smaller power supplies and smaller cabinets for the Class D versions. Ultimately this should also result in cost savings once mass production starts and development costs are recovered.
Class D amplifiers are currently available in limited number; a few mainstream brands are now coming out with versions in the surround sound and car audio area. Some use hybrid chips, most of the circuitry is embedded in an IC making it easy to build the final product, these often have extra signal processing in the digital domain. These are akin to the hybrid amplifier ICs that were popular many years ago in midi systems and lower-end amplifiers. Their performance is petty good for what they are but still not at real hifi levels.
Bang and Olufsen of Denmark have developed the ICE module, a dedicated amplifier module working in the Class D domain. Several manufacturers have been using it in their own equipment with great results. Several other companies have also developed Class D amplifier modules, LC Audio, Hypex and NewClassD are a few, all from Europe and mainly cater for the do-it-yourself market.
Professional audio is another area that would really benefit from Class D amplifiers, especially due to their high efficiency. Many 1000’s of watts can come from these amps but you can easily carry them making the roadies’ job much easier! So what about the gear we all buy at the local hifi store? As mentioned earlier a few brands are starting to make some Class D models such as Sharp and Yamaha. The performance levels of digital amps can be very high if designed correctly and some of these DIY versions mentioned earlier are extremely well regarded. This will surely filter through to mainstream products within a few years. Early Class D amps tended to be harsh, this is no longer the case as lessons are learnt and designs are improved. The development of new componentry is also helping as faster switching rates are achieved with lower noise levels.
So how do these things work? Most work by using PWM or Pulse Width Modulation. The audio signal is compared to a high frequency signal of say 500,000 bits per second and this then creates a signal of pulses which drives the output stage. These are usually MOSFETS but instead of acting in a variable fashion such as in traditional amps, they are switched either on or off, no in-between. The output signal goes to the speakers, but since the pulsing is so fast, the speaker simply “sees” it as a continuous waveform. Of course it is much more complex than that, output filters are required, load constraints and more need to be taken into account.