The quest for ultra-low distortion
This month, we present the first article on the Ultra-LD Mk.3 amplifier module. This is the second time we have revisited this class-AB amplifier design and it’s almost getting monotonous, as we announce even lower distortion. Exceedingly low! In these days of iPODs and MP3 players, why do we bother?
The reason is that those sound sources are so mediocre. Why put up with mediocrity? The same comment can be made of the vast majority of home-theatre systems and indeed, virtually any sound source that most people listen to. Compared to where we were with high-fidelity sound systems based on CD players 25 years ago, we have gone backwards. Furthermore, it can safely be said that the vast majority of people under the age of 30 probably have never experienced a good hifi sound system. If they have, it was probably years ago while they were still living with their parents.
We at SILICON CHIP, on the other hand, believe that the best quality sound is still worth striving for. We also have better tools and design methods to help us get there than we did years ago. Our benchmark is the standard of performance available from the best CD players available today, such as the Marantz CD player reviewed in last month’s issue. So in designing a no-holds-barred amplifier, our task is to produce something that does not degrade the sound quality of such a CD player in any way. Or for that matter, the sound quality of a premium Blu-Ray DVD player.
Ideally, this means that its total harmonic distortion across the entire audible spectrum up to 20,000Hz must be less than .0015% and its signal-to-noise ratio should be better than -110dB. If we get more distortion or more low-level noise, we are degrading the signal quality. It turns out to be a very difficult task to achieve such a standard. Indeed, home-theatre equipment, from even the best brands, is hard-pressed to achieve .01% harmonic distortion – about 10 times worse. And the typical home-theatre system with class-D amplifiers is a good deal worse again.
Our 20W Class-A Amplifier has been our best attempt to date and its distortion is far below that of the best CD and Blu-ray DVD players, for frequencies below 10kHz at least. We have always aimed for the same standard with our more powerful class-AB designs and that is a much more difficult task. But with the Ultra-LD Mk3 amplifier module we have come tantalisingly close, as readers who like to closely compare performance graphs will attest (the relevant graphs for our 20W Class-A amplifier were featured in the May 2007 issue). The difference is that the Ultra-LD module is a great deal more powerful than the class-A design.
Significantly, the new module typically has half the harmonic distortion of the module it supersedes. And while we are very pleased to be able to present it, we have not left all those people who built the previous module in the lurch. We intend to bring out a small adaptor PCB which will bring it up to the same standard.
Is this the ultimate in amplifier design? In the true sense of the word, it is but only for now. Who knows what new devices and new techniques may bring in the future. In the meantime, our challenge is to produce a complete integrated amplifier featuring the new module and with facilities for analog and digital sound sources. If the development of the new Ultra-LD amplifier module is anything to go by, that could take some time. Stay tuned.