Core Audio Kratos MKII Fully Digital Amplifier Preview

While some circles in the audiophile community stubbornly refuse to embrace the sonic and convenience benefits of digital music, others are on the front lines of the movement. There are a growing number of high-end companies starting to design digital components to please the next generation of Audiophile. Core Audio Technology is one of those companies. Their products seem to dance the line between what I would call “high-end” and “esoteric”. You won’t find stupid expensive gear like an $80,000 record player or single driver “full-range” speaker with a higher MSRP than a Porsche 911. But the gear isn’t entry level either, ranging from about $600 to $19,000. I might summarize Core Audio Technology as a company who offers many of the design elements, look and feel of ultra-expensive gear with one less digit on the price tag.

Straddling that line between high-end and esoteric is the Kratos MKII fully digital amplifier, the replacement for their original Kratos MKI. It’s rated at 100wpc into 8ohms and 200wpc into 4ohms at .1% THD. We're not sure if the power rating was taken with a 1kHz tone or 20-20kHz full bandwidth test.  Reported dynamic range is 140dB. The Kratos MKII weighs in at 22lbs, which would be poultry for a traditional class A or A/B amplifier, but weight doesn’t tell you much about an amp of this design.

At this point, it’s important to clarify what “amp of this design” actually means. I’d suspect that many readers are unfamiliar with what a fully digital amplifier actually is. A quick glance at the back panel will tell you that this isn’t a typical amp. The back panel sports the expected two sets of speaker binding posts, but it also has a 2 digital optical and 2 digital coax inputs that can handle 24 bit/192kHz native. On the front you will find a volume/Input control and LED display. It also comes with an IR remote. It’s similar to traditional integrated amp in terms of functionality, but with a few very important differences.

Kratos Fully Digital Concept

The Kratos is designed to have no “digital to analog conversion, no analog to digital conversion, no analog to triangle wave conversion, no analog to pwm conversion, no DSP, no sample rate conversion, and no switching power supplies.” It does not accept analog inputs of any kind; instead, everything is done in the digital domain and ONLY digital inputs are accepted. The logic is that no DAC is better than even the best DAC. The amp takes a PCM digital input, amplifies it, then the signal is demodulated to drive the speaker directly. One of the added benefits is also that THD does not change with power output. Ultimately, the goal of this design is to achieve better sound quality by eliminating unnecessary conversions, cables and circuitry.

Core Audio doesn’t provide as detailed of specs about the amplifier measurement as we sometimes find, but they do go into far greater that normal detail about the power supply. It’s a dual rail design with roughly 96,911uF of capacitance generated by 86 capacitors per rail. They claim that, “several LC filters and expensive feed-through filters are utilized in the regulation circuitry to eliminate noise in the bandgap and prevent spikes and ringing through capacitive coupling of higher frequencies”. It also utilizes a Wolfson 8805 Digital Interface Receiver. The Kratos MKII draws less that 1w of power when idle, and about 120w at full power (one channel driven). In comparison, the power supply is actually larger than the amplifier. Core Audio claims that, “We found a strong correlation between power supply performance and sound quality. That’s probably why the power supply is larger than the amplifier.”

So how is the Kratos Digital Amp different than less costly solutions on the market?

A digital amplifier is extremely sensitive to two things. Load impedance and noise. Core Audio designed four exchangeable output filters for the Kratos unit to accommodate 4, 6, 8, and 16ohm nominal loads. The customer can specify the load impedance of their speakers at the time of purchasing so they can ship the properly configured device. This makes the Kratos uniquely optimized for active amplification and mating the amp to the particular drivers being used -- for another day.

The ability of the amplifier to drive a complex load, however, comes from the power supply. Conventionally Class D and FDAs use switching power supplies. Switching power supplies are typically both quite noisy and quite slow. They also have large issues with parasitics where common mode noise is introduced into ground which causes ground voltage mismatches and noise in the rest of the audio system. So two things happen here, you have high frequency noise from the switchmode power supply being folded over into the square wave and causing amplitude distortion, and you have common mode current creating noise on everything else in the system.

"I think one element that people miss is that switching noise isn't entering other components, it creates ground voltage mismatches which means any common mode noise in the wall or being filtered out by the system turns into differential mode noise and differential mode signals. Noise always returns to its source, so EMI filters just make the noise loop through a shorter circuit." says Ryan Mintz, Owner of Core Audio.

The Core Audio solution eliminates the SMPS in favor of a linear power supply.  They also fully decouple the supply for better high frequency noise immunity; a step many amplifier manufacturers either skimp on or don't do properly according to Ryan.

For example, say the amp is switching at 384khz or even 2mhz, any noise above that is folded over into the base band and introduces harmonic content into the square wave that doesn't exist in real life. This is where quantization error and foldover come from. It's important to eliminate high frequency noise or risk amplitude errors on the digital square wave.   Core Audio does several things in their power supplies.  They employ discrete regulation circuits and feedthrough filters for example, to increase the filter bandwidth and improve transient response. The transient response increases regulator bandwidth and the supply's ability to control and filter high frequency transients. The problem is that the band gap becomes wider and more sensitive to noise. So increasing bandwidth and filtering band gap noise are mission critical.  In the PWM controller they have implemented several algorithms to control jitter.

The Kratos MKII has a base price of $2,250 and comes with a 5 year warranty, free shipping, and 30-day trial. Within 30 days, there’s no restocking fee and return shipping free. Outside of 30 days, Core Audio offers and trade in program good for 2 years after the original purchase date that can get you between 75, 50, or 40 percent of the original value, depending on how long ago the product was purchased. However, it’s important to keep in mind that not all of their products, and in particular custom products, qualify for the 30 day trial period and some are nonrefundable. However, all products do still qualify for the trade in program.

Kratos MKII Rear Panel

Although Core Audio offers a plethora of custom build options, there are also a few standard options to choose from when purchasing a Kratos MKII.  You can add a pair of line level subwoofer outputs for an addition $250. And eventually they will offer an option for a 32/384khz DSD-capable USB Input for another $250. We really like both of those options, especially USB. Most computers don’t sport a digital coax or optical outputs unless they have an upgraded sound card. If the MKII came with a USB port by default, it would make the product much more enticing. As of right now, consumers would have to use an external USB to SPDIF converter. The last optional upgrade is the feet. The manufacturer claims that, “High Frequency vibration in audio electronics can wreak havoc on your sound. Microvibrations down in the nanosecond range create stray magnetic fields, which in turn create noise voltages in your component and limit dynamic range, resolution, and harmonic content.” While that statement might throw off many audiophiles not in the esoteric community, luckily Core Audio follows it up with the statement that they don’t believe in tuning equipment through the use of feet. But it’s evident that they believe in the added benefit of anti-vibration feet. The Stillpoints Ultra SS feet will set you back an extra $1000 and the Stillpoints Ultra 5 add a blistering $2,300 to the price.

Conclusion

I think we will start to see more fully digital amps hit the market in the coming years. The product class offers a way for audiophiles to embrace the world of high-end digital audio while holding true to principles like simple, clean signal paths. For those who are particularly enticed by the Kratos, Core Audio Technology also offers package deals where they will pimp out a Mac Mini to make it a lean-mean-music-serving-machine and give you a few hundred bucks off of the combo. For those looking for the fewest pieces and least conversions possible in the pursuit of digital audio nirvana, the Kratos MKII can surely help out.

For more information visit Core Audio Technology

Unless otherwise indicated, this is a preview article for the featured product. A formal review may or may not follow in the future.