Monday, March 24, 2008

New HTPC amp

There's recently been some discussions on AVSForum about a new amplifier board that was developed by D2Audio. Actually, most of the comments have been about a new MSI 7411 motherboard that supports the D2Audio technology. Instead of using the typical on-board 7.1 HD-Audio chips like those from Realtek, they have included D2Audio's DAE-3 class-D PWM controller with this motherboard. This controller includes the standard HD-Audio interface and works with one of two add-on PCIe cards, either a 7.1 preamplifier card or a 5.1 amplifier card.

This isn't the first time someone has integrated an audio amplifier with a PC motherboard. A little over 5 years ago, Acer introduced their AOpen AX4B-533 Tube motherboard with built-in tube amps. I don't know how well it sold, but it got some attention, probably because of the novelty of the combination. Obviously, it didn't revolutionize the way other manufacturers designed motherboards because we haven't seen anyone else release a similar product until now. However, it does look like more PC manufacturers are working toward making convergent products and taking the HTPC market more seriously. This time, instead of using tubes, the D2Audio amps use class-D circuitry.

These products are exciting to AVSForum readers because they may finally have an HTPC that can replace their A/V receiver and many of the source devices. With something like this, you wouldn't have to deal with the current hassle of trying to pass HD encoded audio, like Dolby TrueHD and DTS-HD MA, via HDMI to your AVR. You can just connect your HTPC directly to your hifi speakers. I guess the next question is, how will it sound compared to a good AVR? Based on D2Audio's literature it is designed to compete head-to-head with current AVR gear. Chris Morley, President of Omaura North America has listen to it and says it sounds fantastic.

I don't doubt that it would sound as good as a low to moderate priced AVR, but after researching class-D amps for several years, I'm skeptical that this technology can sound as good as dedicated or even integrated amps. For those of you that are interested in high end sound quality, I created this chart to compare common specs of the D2Audio amp card to other class-D amp modules:

ProductPower Rating
SNR / Dynamic Range
Power Efficiency
Peak Output Current
Frequency Response
D2Audio 5-Channel x 100W (AAIC100-5) Card
100 W
at 8Ω
less than 0.1%,
f = 1KHz, P = 1W
more than 105 dB
±0.5 dB (20Hz to 20KHz)
125 W
at 8Ω,
210 W at 4Ω
f = 1KHz,
P = 1W
112 dB
93 %
> 25 A
±0.3 dB (20Hz to 20KHz, all loads)
Hypex UcD180
HG Version
105 W
at 8Ω,
180 W at 4Ω
f = 20Hz to 20KHz,
P = 1W
120 dB
92 %
10 A
±0.3 dB (10Hz to 50KHz, all loads)
100 W at 8Ω,
200 W
at 4Ω
f = 1KHz,
P = 10W
123 dB
91 %
26 A
+0/-3 dB
(DC to 150KHz, at 8Ω)
CAD Audio
250 W
at 4Ω
110 dB
20 A
+0/-1 dB
(20Hz to 20KHz)
CL3 Gemincore
250 W
at 8Ω
f = 1KHz,
P = 1W
115 dB
97 %
19 A
0 to 70KHz
PowerPhysics A-108
100 W at 8Ω
less than 0.05%
P = 0.1W
115 dB
more than 90 %
20Hz to 20KHz

It's hard to make valid comparisons because all manufacturers don't use the same measures. My first concern with the D2Audio amp board is its power rating. To be fair, I limited the comparison to modules that were close to the AAIC100-5's power rating of 100W. D2Audio has calculated the power rating based on an 8Ω speaker load. A lot of the amp manufactures will quote power ratings based on 4Ω loads, but 200W at 4Ω is similar to 100W at 8Ω. The specs from D2Audio's website list their power rating as 100W peak at 8Ω. Most of the other manufactures will list the power rating using an rms (root-mean-squared) calculation. D2Audio may mean you can get 5 x 100W peak power for a short period of time before the PC's power supply cuts out. That's probably why they state in their literature, "the DAE-3 engine provides real-time power management to protect against power supply overload and potential Media PC shutdown from loud music or explosive sounds that occur during movies or games." So maybe if you have a large enough power supply it can provide a continuous 100W rms power. If not, if they really mean peak, then the rms power is actually 50W per channel.

The distortion figures for the AAIC100-5 are not that impressive when compared to most of the others in the chart. Not too bad when you compare it to some tube amps, but not as good as most solid state AVRs on the market. It also depends on the type of distortion. A tube amp may have a lot more distortion, but some people perceive the relatively high 2nd order harmonic distortion to be pleasing and adding "warmth" to the sound. Even though solid state amps have much lower 2nd order harmonic distortion, the other higher order harmonic distortion is not too pleasing. With most inexpensive solid state amps, the distortion increases as frequency increases making them sound bright or harsh. The THD (total harmonic distortion) for class-D amps are primarily second harmonic in nature, but there are also some higher order harmonics present. The best class-D designs use a feedback loop to compare the output to the input so they can minimize any errors in the output. With a pure digital PCM-PWM amp, you have nothing in the input to compare to, so without some pretty expensive and complicated circuitry, it is nearly impossible to get good distortion ratings. That's probably why D2Audio's digital amps don't match the THD specs of these other analog class-D designs. It's also probably why most respected class-D designers abandoned the pursuit of pure digital amps a few years ago in favor of analog class-D. I've been very impressed with Hypex's UcD amp modules and if you look at their THD figures you can see why. They are able to get very low THD over the entire frequency range (well at least from 20Hz to 20KHz). Older class-D amps without feedback circuitry had pretty poor THD at higher freqencies and this is why they were used mostly in active subwoofers and got a bad reputation for true hi-fi. I hope the high frequency performance of the D2Audio PCIe amps don't help to reinforce that reputation.

You can also see that the dynamic range of the AAIC100-5 doesn't compete with the others in the table. Considering the fact that many of the DACs in popular soundcards have SNR specs around 120 dB, the D2Audio amps might become the weakest link in the audio electronics chain of your system.

The other specs compare favorably to the others in the chart. Power efficiency and frequency response are in the same range with the other amp modules.

There are a few other audio specs that are not listed in D2Audio's literature that would be worth knowing. Since the AAIC100-5 is installed inside your HTPC and is dependent on the PC's power supply, I'd like to know the spec for power supply rejection ratio (PSRR). PSRR indicates how good a device is at rejecting noise from the power supply. The PSRR of the ICEpower and Hypex amps are about 60 and 65 dB, respectfully.

In conclusion, I think the new MSI motherboards with integrated D2Audio technology is a pretty cool thing. I'm happy to see any product that makes HTPCs more useful and popular. I'm not a big fan of digital amps, mainly because their specs, especially THD, don't measure up to good quality class-D analog amps. I also don't like the fact that these amps are totally dependent on the PC's power supply. If you've read some of my previous blog posts about HTPCs, you also know that I like my HTPCs to be pretty small. Ideally, I'd like a fully functional HTPC about the size of a Mac Mini. This would be impossible if you throw in the amps. So, unless this sort of technology makes some dramatic improvements in the future, you won't see anything like this available from Amplio.

update 3/28/08: I received a more detailed datasheet for Huygen's MHzpower-2 amp module and noticed some differences with the specs in my chart. So I updated the chart above to match their datasheet. I also noticed that the PSRR of the MHzpower-2 is 71 dB. These are pretty impressive specs. Unfortunately, I haven't had a chance to test them out. Fumac, if you read this post, please remember to email me your price sheet. I also haven't read or heard any feedback from any of Huygen's customers, but I know there are a few European amplifier manufactures that are using the Huygen modules in their expensive amps.

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