Integrated Amp Reviews

Unless you are a human FFT analyzer, figures 4 and 5 can't be seen as spectra.

Sorry about the mislabeled graphs. I am traveling today but I'll fix this later today.

Update: Okay, got to where I was going today (flying from Seattle to Ashland Oregon) and have fixed the caption problem.

John Atkinson

Editor, Stereophile

Is the nova125 review available online? 

From the specifications of the amp section itself, it appears that the Decco65 is using the Texas Instruments TAS5613 150WPC class-D amplifier IC. This IC can deliver 65WPC into 8 ohms and 125W at 4 ohms with 1% THD, and claims both >90% efficiency and flat THD content across the entire audio bandwidth, with extended bandwidth to 80kHz. Pretty darn awesome for something the size of a postage stamp. The key to this, according to the TI data sheet for the IC, is their "PurePath HD" feedback scheme, which enables wide bandwidth and extremely low distortion (compared with other class-D ICs). A larger 300WPC amplifier IC is also available, the TAS5630. For those that are new to all this, an IC-based class-D amplifier builds everything but the input filtering caps and the output filter into the chip itself - which means the output devices are also onboard. A benefit to this is the extremely short signal paths on the amplifier IC's silicon die. By contrast, the popular ICEpower and Hypex UcD/nCore amplifiers use discrete components.

What this review says to me is that affordable class D amps (and particularly IC-based class-D amps) are finally "getting there" in terms of audio quality to where they can be used in a credible high-end audio product. I lost a channel in my class-AB amplifier recently and am using a Chinese stereo amp (Sinewave Genius200) based on the TAS5613 while the big iron is being repaired. Overall, my impressions mirror the author's - while not the last word in transparency, there is a wealth of detail being unveiled. Also, there are no sweeteners in the amp IC itself - tonally, it is on the neutral to cool side (like a class-AB amp based on bipolar FETs instead of MOSFETs). But the amp is remarkably free from the phasey 'hash' and elevated treble THD that I've heard from other class-D IC amps, mainly the low-cost Tripath variety or later ST Micro ICs (though the Tripaths were at least listenable), so imaging is much improved over what I've heard before from affordable class-D amps.

Still, the work is not done for engineers of class-D ICs, or those amp manufacturers that use them. Even the TI PurePath ICs are power-rated at THD levels  (1% THD and 10% THD) that would be considered unacceptable in a Class-AB amplifier at full power, so there is room for improvement. (The TAS5613 is rated for 0.03% THD at 1W - the foregoing statement is only meant to highlight that at full power, class-D amps still have distortion issues.) Also, this indicates that class-D amps should actually be de-rated slightly when comparing them to similarly rated class-AB units to put them on equal footing where THD is concerned. Still, I am glad to see these improved class-D ICs making their way into higher-end products as their performance rises to the necessary level.

There is still some room for designers to affect the sound of these amps, even though the 'guts' are hidden inside the IC. What amp designers can do is make careful choices in the selection of input filtering caps and also in the design of a properly-damped output filter that provides maximal bandwidth and uses components designed for high voltage and high current applications, such as metalized film capacitors, potted large-gauge inductors, and high power resistors of good quality. A well-designed output filter will be more than a simple second-order lowpass LC network - it will include a parallel leg containing a shunting capacitor in series with a damping resistor to prevent additional HF distortion from voltage spikes that overwhelm the amp IC's feedback network. The math for output filter design is very simple, so when amp manufacturers leave this stuff out, I wonder what they are thinking other than trying to shave off every penny of cost - not a mindset that will succeed in the high end.

Lastly, to stay competitive, I'd like to see ICEpower A/S and Hypex Electronics BV create their own IC designs incorporating their patented technologies, such as a second feedback loop that monitors the output of the output filter and attempts to exert greater control via the amp stage. The very short signal paths on an IC would be of clear benefit to these already class-leading technologies.