Someone sent this question in via email, and I thought it would be smart to answer it publicly (with his permission of course). It will eventually find its way into my FAQ. Any comments on whether this makes sense, confuses or confounds you would be appreciated.
Kevin I been following the post of your amp with great interest.
I am not very knowledgeable about audio. So I wonder how is it possible for your amp which has an A/D converter in it to sound analog(better than your SET amp), as I am used to thinking that digital sounds inferior to analog... (This is a sincere questions )
This is a good question. The answer is that digital amps and digital sources share little in common. Yes, they encode waveforms in a digital format, but that's about it. A digital source uses a low sampling frequency (44.1kHz) compared to digital amps, which run around 500 kHz. So Class D amps have a much higher resolution representation of an analog signal. SACD samples at 2.8 Mhz, and this is one of the primary reasons why SACD sounds so much more "analog". The LC Audio modules I am using can be clocked up to 1 MHz.
Class D amps and digital sources have even more differences... A big one is that the Redbook digital format is designed for transmitting signals over distance (e.g. a digital cable). As a consequence, there is error correction information in the signal, and the signal is processed in a way to make it more fault tolerant. Decoding this requires a very sophisticated device, and it requires very precise clock matching. Errors in the clock (e.g. jitter) cause huge problems in sound quality, and this is why installing an ultra-low jitter clock is one of the best tweaks you can do to a CD player.
Class D amps do something entirely different with the digital signal. They amplify it in place, and then simply pass the signal through a lowpass filter. This filter acts as the "decoder" and usually consists of one inductor and one capacitor. There's no clock involved there! So Class D amps are essentially immune to jitter.
Regarding sound quality, Class D amplification takes the best characteristics of both SS amps and Tube amps, while leaving out the worst problems. The key concepts here are
harmonics and
linearity.
Harmonics are the spice of life as far as music is concerned. The complex interplay of harmonic frequencies and levels is what makes music sound so beautiful (or awful). The ear is much better at understand harmonic information than any instrument, or any spec sheet which horribly condenses all harmonics information down to the total average harmonic distortion. This is probably the second most misleading spec in audio (after total RMS power ratings).
Tubes have a strength in harmonics, because the harmonic distortion waveforms from a single note tend to die out quickly. Solid State has a big weakness in that the harmonics from a note tend to "ring" and persist much longer after the note has passed. The amplitude of these rings are very very small, but they still can intermodulate with successive notes and cause a harsh, piercing anharmonic sound. This is why no-negative feedback amps are preferable, because feedback increases the "intermodulation distortion". Tubes also avoid discordant and unnatural odd-order harmonics that are inherent in SS devices. So even though the Total Harmonic Distortion of SS is much lower than tubes, it's not the total amount that matters, but rather what kind and when.
Single Ended Triodes, in particular, have extreme amounts of 2nd order harmonic distortion, up to 5%, but yet people absolutely love the sound. Why?? Well, the answer is that 2nd order harmonics are especially pleasing, almost like seasoning on food. If food is bland, add spice! The right amount of seasoning can bring the taste to a zenith, but too much can obscure some of the inner detail of the taste. Everyone has different opinions on what tastes or sounds best to them.
Linearity is the big problem that analog amplification devices have. A 20 Hz signal should have exactly the same amount of amplification as a 20,000 Hz signal, and everything in between should be the same too. This is not as easy as it sounds. Tubes are notoriously non-linear, especially the classic SET tubes like the 300B and 2A3, and a really good designer knows how to build circuits that utilize only the linear operating points of tubes. SS transistors are usually a lot more linear than tubes, but finding high power varieties is not that easy. It is in fact entirely possible that they are no longer commercially produced at all in the near future, because of the prevalence of digital. To sum up, SS devices are typically more linear across the entire frequency band.
Class D AdvantagesSince Class D amplifies at only one frequency (the carrier freq ~500kHz), it has NO problems with linearity! Also, harmonics of the carrier frequency (mulitples of 500kHz) are so far out of the audio band that there is no effect whatsoever on the music. A Class D amplifier is capable of reproducing music without any harmonic intermodulation distortion, and zero non-linearity problems. This is why it can sound so good. The resulting accuracy, detail, imaging, and emotional involvement is stunning.
The problem with Class D is that sometimes it can be TOO accurate, and sound dry or analytical. We figured out a trick that completely solved this problem for the LC Audio modules, and now there is no discordant information in the music. It is only the purest representation of the original analog signal.
Energy Usage of Class DOne more huge advantage of Class D over analog is the method in which an amplified waveform is created. With Class D, the output transistors are switch from full-off to full-on to essentially "reassemble" the analog signal in a high-voltage state. Transistors (and tubes)
"like"[/] that. In contrast, analog amplifiers have to be on all the time, and perfectly respond to the subtle variations in level and frequency of the musical signal. This uses a lot more energy, and puts more stress on the amplifier. In contrast, Class D amps are incredibly efficient, and should last a long time because of the low stress on the components.
