The γ3 high resolution DAC

History & acknowledgements

After releasing the γ1 and γ2, I began thinking about a bigger, more ambitious DAC project (the γ3). While the γ1 and γ2 are both excellent, certain design constraints were imposed due to the small size. There was no balanced output, and the power source had to be either from a single external 5V power supply, or from a computer's USB port. Also, at the time they were designed, the Texas Instruments PCM2707 was the only viable USB audio solution, limiting it to no more than 16 bit, 48KHz sample rate, despite the fact that γ1 supports up to 24 bit, 96KHz and γ2 up to 192KHz via S/PDIF.

I mentioned a desire to develop the γ3 as a high-end DAC a long time ago, but began work in May 2014 with a wish-list of features. Meanwhile I acquired the Audio Precision System Two SYS2322 Dual Domain audio analyzer and the Altor Audio JK-GEN 384 digital signal generator to help with the development and testing. These were costly but became indispensable.

As the overall γ3 architecture began to take shape, I knew I needed a high performance fully-differential analog line amplifier to serve as the analog output stage. A separate project, the α24 was spawned. It resulted in a versatile line stage that could be used in many different applications. The α24 performed extraordinarily well on the test bench, and sounds excellent.

An effort was also taken to find a 24 bit, 192KHz high resolution USB audio solution. After evaluating a number of possibilities the final choice was easy to make -- the Audio Widget, an open source project that is DIY-friendly, free of licensing or royalty, fits with the AMB way of conducting its projects, and last but not least, has great performance. I decided to offer an AMB implementation of the Audio Widget, the ζ1, as an independent module, and took a slight detour from the γ3 design effort to develop the γ1.5, a miniature high resolution USB DAC and headphone amplifier that utilizes the ζ1. It turned out to be a little gem with excellent sound, and served as an excellent test bed for the ζ1.

The main components of γ3 were also chosen early in the project, consisting of the Texas Instruments SRC4392 and dual Wolfson WM8741s operating in differential-mono mode. I also decided to further modularize the concept into a backplane supporting the γ24 high performance DAC core module, which contains much of the heart of the DAC.

Software control is a necessity for the SRC4392, and enables many more features that are otherwise unavailable from the WM8741 such as software volume control and up to five selectable filter responses. This, of course, means that there must be a microcontroller in the system. Naturally, the LCDuino-1 was brought into the mix, which added remote control and many other features while keeping the front panel very simple and elegant.

I developed special firmware for the γ3 to run on LCDuino-1, which is heavily modified from the Volu-Master code base (used for the δ1 relay stepped attenuator and δ2 relay input/output selector).

Galvanic isolation of all inputs, including USB, was an important feature that I wanted the γ3 to have, along with the use of three high performance power supplies. Configurable digital input types is also a feature that I feel is a great differentiator, no other DAC, as far as I know lets you do this.

With input from forum members, many design details were honed to perfection, including the selection of ultra low-jitter, low phase noise oscillators, a low jitter clock buffer, and premium S/PDIF transformers from Scientific Conversion. Forum members also contributed comments about the PCB layout that led to improvements.

The overall product is far more than just which DAC chips are used. The devil is in the details, and none of it is left to chance in the γ3 development process. You can read the γ3 development forum thread for the entire process.

A prototype team of 8 people was created to build the first version of γ3. Several minor issues were identified and corrected for the final release in late November 2015.

I can say without reservation that this has been the most complex project I have worked on. It involved both hardware and software development, on a system that is quite advanced, especially in the world of DIY audio. The result is immensely satisfying, it performed like a champ on the test bench, besting many high-end commercial DACs. I was astounded by the sound, it is without a doubt a big step up from the already excellent γ2. Others who have auditioned the γ3 also confirmed what I heard.

I want to thank everyone who helped make this project a reality. Including everyone who contributed in the forums, members of the prototype team, MisterX for working with me on past DAC projects which became valuable learning experiences, and LinuxWorks who co-developed the LCDuino-1 system with me.

Look forward to more exciting projects from AMB!

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