- Written by Rick Schmidt
- Published on 07 November 2011
Design of the CEntrance DACMini
The headphone amp is a class-A amplifier design. The unit I reviewed was the standard model with 10 Ohms of output impedance. A modified version is available that lowers the headphone out impedance to 1 Ohm. CEntrance offers this mod to enable the flattest possible frequency response and for best coupling with low impedance headphones. Also available is a modification to provide more gain at the headphone output, for better driving of high impedance headphones (such as electro-static models).
Additional modifications are available - a black anodized case and Variable Output to convert the DACMini into a preamp.
There is no power supply upgrade which is what I first went looking for when I realized the supplied supply was a switching model. My power supply prejudice prompted me to push back on CEntrance about this.. why no PS upgrade mod?? I got this reply from Michael Goodman CEO and chief designer: "The external power supply that ships with DACMini is a specially designed, ultra-quiet, high-stability power supply. It is optimized for best performance in audio products. Inside DACMini, there is a number of additional power supplies with galvanic ally isolated grounds -- they totally decouple the internal circuitry from the external power supply."
Yeah, yeah, but have you tried a linear PS?
"We've had a number of outside parties attempt to improve the audio quality by using an external linear power supply. Nobody reported hearing any audible difference. Technology has moved on from the time when switching power supplies were a total taboo in audio."
And there is this from the design blog for the DACMini (http://www.centrance.com/products/dacmini/blog/)
"DACMini is so insensitive to external power because its internal power station utilizes a high-frequency transformer-isolated circuit to re-generate all the needed voltages, maintaining cleanliness and stability of audio power independently of brownouts and spikes on the line."
And this.." A cheaper USB DAC does not need an isolated power supply, because its own self-noise will mask any noise related to ground differences. On most USB DACs, when you turn the volume control fully "on", you will hear a loud hiss, which covers up the computer switching noise. However, with DACport and DACMini, we are employing breakthrough headphone amp technology, which ensures no noticeable noise floor in the headphones. It's actually "pitch black" in the headphones when you crank the volume knob all the way up. Now, at this level of low-noise performance nothing masks the computer grunge anymore, so additional isolation is needed to fully get rid of the ground-coupled signal impurities. This is why for DACMini we designed a transformer- and opto-coupled power supply that fully disconnects the analog ground from the digital ground. Galvanic isolation ensures that you are hearing the actual music and not the artifacts of computer chips clicking "on" an "off" on the motherboard."
Ok, ok, I give, we'll see when I listen. I had to ask though.. Opto-coupled power supply? This means that the digital signals are sent optically at key junctions, like at the input and output of the DAC, so that the power supplies for the different parts of the circuit are isolated. Simple and utterly effective, so much so that it's hard to believe that no one has done this before. Of course, cost might be a factor. The DACMini is pioneering a price point that gives me a bit of hope for the future of hifi – charge a little bit more for a high end product, in the consumer realm, and maybe some people who didn't know what was possible will give it a try. It's hard to know though what people will spend, to me the $795 price tag for the DACMini seems quite reasonable but I'm into this stuff. The ace in the hole for the DACMini is its design. It's the perfect mate for a MACmini. The two stacked together would look pretty cool.
Getting back to the power supply though – just for one second – bare with me - one of the satisfying aspects of traditional, heavy iron power supplies is heft. When one is paying big (or even small) dollars for an electronics product it's much easier to justify if the thing has some mass to it. Well the DACMini has heft. As I said the case is strikingly beautiful with a sparkly glow, but it is also substantial. By my crude estimations the DACMini weighs 2.5-3 lbs and feels and looks like it was made to last. A paper available on the CEntrance web site goes into detail about the internal design of the case. At what must be considerable expense, the inside is compartmentalized for the different functions. The compartments are separated with tongue and groove construction to provide thorough RF isolation. This is not just a pretty box.
That isolation is all the more necessary because there is not just a DAC in there, there is a processor as well. Gathering the data from the USB input, volume control, switching inputs and the on/off switch are all handled by the processor, not by mechanical switching which is firmware upgradable. The volume control, while handled by the processor is done in the analog realm which is what you want, digital volume control means throwing bits away.
But what about the DAC in there? Which one is it? Wolfson? Cirrus? I had all but given up on finding out. CEntrance doesn't divulge this info even though it is the custom to do so. Well your intrepid reporter poked and prodded to get the story behind the DAC. The DAC chip in the DACMini is the AK4396 from the Japanese company AKM. Michael Goodman explained that this DAC chip is special in that even though it is a delta-sigma design it is a new variety with far less noise. Delta-sigma D/A conversion requires a high clock rate, many multiples of the sampling rate, which brings its own set of noise issues. I'm not sure what exactly but the AK4396 does something radically different (or so it is claimed) in delta-sigma conversion that greatly reduces (ala 60db) the noise issues from this high frequency circuitry. It is a different sort of sound as compared to other DACs I had heard. Liquid and very listen-able (i.e., not fatiguing). The Transporter from Slimdevices/Logitech also employs an AK4396.