Competitive class D amp suggestions
I have been Class D fun since a few years ago when i bought my first class D amp. I like the concept, in general, and all the attractive features of this class of amplifiers. I tried 4 different ones, currently i stayed with one of them that i consider to be the best among all four amps. I do enjoy and like it. At the same time, my 5 watts SET amplifier (with more than 100 times higher distortion according to the specs) gives more natural and (surprisingly) notably cleaner sound (THD of the class D amp is 0.001). The soundstage of the class D amp is not so bad but that of the tube one is still better.
I remain attracted by class D amps though.
Any fresh suggestions on reasonably priced class D amps (i mean excluding non-reasonably priced class D amps, e.g., Merrill amplifiers)?
Any comments on non-reasonably priced class D amps are also welcome (so far i was not able to audition many class D amps and am curious if there are some which could really compete with Class A).
the numbers might get you in the right theater but you will only find the perfect seat by trial and error because the amp has to mate with the source or preamp and speakers in a way that sounds great to you.
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In a nutshell, if there's enough lower ordered harmonic distortions, they can mask the higher orders. The higher orders otherwise cause harshness and brightness. Either the 2nd or 3rd can predominate, but if the 3rd dominates then it has to be at a lower level to begin with- about 1/10th what you might see if the 2nd were dominant in the design. For example almost any SET makes 10% THD at full output- most of which is the 2nd harmonic. If you have an amplifier design that makes the 3rd instead of the 2nd (which can happen if the amp is fully differential and balanced, since even orders are cancelled throughout the design) then the THD will and must be much lower. This isn't that hard to achieve since a design that exhibits a dominant 3rd harmonic will tend to have less distortion overall anyway. On top of that the distortion cannot rise as frequency is increased, which is a problem with most solid state amps made (and with tube amps employing feedback). Class D offers a solution for this- you can have the distortion vs frequency look like a straight line, and you can run a lot of feedback at the same time. In this way you can achieve distortion levels that are several orders of magnitude lower than you might see in a tube amp ye the actual distortion signature (2nd and 3rd vs the higher ordered harmonics) can look quite similar to a tube amp. This allows greater transparency (since distortion obscures detail) yet the same kind of smooth mids and highs you associate with a really good tube amplifier- IMO, the best of both worlds. In case you don't get my drift, I'm drawing a very straight causal line between the distortion measurements and how the amplifier actually sounds; IOW its predictable based on sufficient measurement! The tech to do this really didn't exist 40 years ago but it does now. A lot of audiophiles don't know that yet; like so many industries tradition plays an enormous role in this. |
In unrelated news, Topping has recently released a new version of its D90SE DAC, called the D90LE, and that is unremarkable in the current context. What may be of interest, perhaps for educational purposes only, is that included in the new firmware is the option to simulate either tube harmonics or transistor harmonics (or neither). Specifically, just 2nd and 3rd harmonics, fixed for either option. Scroll over three quarters way down here to "Sound simulation", and there are images of the fundamental and its two prominent harmonics spikes (sadly the images are not very clear).
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Ralph, your distortion discussion is very helpful. What can you say, generally speaking about the distortion make up for class A and highly biased Class a/b solid state amplifiers? Are they generally the higher distortion 2nd harmonic type or the lower distortion 3rd harmonic type.. or have I missed the mark completely??? |
Most of them tend to have unmasked higher ordered harmonics at a low level, and because the ear uses the higher harmonics to sense sound pressure, its keenly sensitive to their presence! Also because the ear assigns a tonality to all forms of distortion, this causes most of said amps to sound harsh and bright even though they might have less higher ordered harmonics than a tube amp! This is how important the masking that the 2nd and 3rd provide can actually be. The reason so many class A and AB solid state amps have this issue is they lack the required Gain Bandwidth Product to support their feedback at all frequencies. This is why they can play good bass, since at those frequencies the feedback is enough to do the job. But at some higher frequency the amp runs out of GBP and so the feedback decreases: and with it the distortion at those frequencies rises. That is why distortion vs frequency is an important measurement. The distortion simply should not rise at all if brightness and harshness is to be avoided. There are a few A and AB amps that appear to have enough GBP to get around this problem and I think most of you know of one of them- the Benchmark- but you still have the issue of the distortion spectra itself. Most of the measurement guys that poo-poo subjective listening will tell you that the distortion is inaudible if about 100dB down. I personally doubt that; if it were true there would be no controversy around the ’sound’ of these amps. Since getting the distortion that low (< 0.005%) is pretty hard you can see that the distortion spectra still plays a role. One thing to keep in mind is that the distortion spectra at 1 Watt doesn’t tell the whole story! A classic example is SETs, which at lower power really do quite well. But when you get over -6dB of full power the higher ordered harmonics become more prominent. This causes the amp to sound ’dynamic’ at first (as power is increased) since the power needed is on the transients. So the ear reacts to the loudness cues on the transients, converting the result to ’dynamic’. I’m convinced that when audiophiles use the word ’dynamics’ to describe what they hear in an amplifier or stereo, that you can safely replace that word with ’distortion’ and not change the meaning of the conversation at all. In many class A and AB amps what happens is not quite the same- as power is increased the distortion spectra changes but the result is more harshness and brightness, since that’s a problem with them even at lower volumes.
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