Gamma-2 (γ2) DAC Thread

I was poking around on MisterX's photobucket page and saw this:



I usually go to Headwize for DIY news, but that has been down. Does anyone know anything about this?

[updated September 15, 2009]

News

- The γ2 website is now live.

Introduction

γ2 (gamma2) is an extension to the γ1 full++ configuration that MisterX and I have been working on for the past few months. It builds on the "modular" idea of the γ1, and pushes the concept further into the high-end. Basically, if you take the γ1 USB and DAC boards in its original, un-broken form and then plug in the γ2 board, the whole assembly now fits into the nice and compact Box Enclosures B2-080 case.

Circuit description

γ2 leverages γ1's USB and S/PDIF receiver sections and taps the I²S datastream. Then, an (optional, jumper bypass-able) SRC4192 or AD1892 ASRC chip upsamples to 96KHz. The resultant data stream is then sent to the top-of-the-line Wolfson WM8741 or WM8742 DAC chip (WM8740 also supported).

The SRC4192 and WM874x DAC chips are clocked by an onboard ultra-low jitter 24.576MHz CMOS oscillator when the ASRC is installed.

If the WM8741 or WM8742 is used, γ2 allows you to select between three different digital filter responses (minimum phase, linear phase slow rolloff or linear phase brickwall), as well as enable/disable the anti-clipping mode via front panel switches. WM8740 does not have these features.

The differential analog outputs from the DAC chip is then buffered by a rail-to-rail opamp (OPA2365 or AD8656) which also serves as an analog low-pass filter (fc ~= 100KHz) and balanced-to-unbalanced converter. The output is then capacitively-coupled to the output jacks (both RCA and 3.5mm jacks are provided, and could be used concurrently to drive two amps).

Separate onboard LDO voltage regulators are used for the analog and digital sections of the circuit, with extensive use of ferrite beads.

The γ1 board's WM8501 DAC section can be populated and used in conjunction with the γ2 board. The result is a compact case with two different DACs in it. You can use them both concurrently. If either of these are equipped with 470uF output capacitors (Nichicon FW or KW) then it could be used to directly drive headphones down to 32Ω impedance (You'll need to use a software volume control). On the γ2, the AD8656 is recommended for headphone drive due to its high output current capability.

The γ1 board's USB-to-S/PDIF converter section can also be fully populated, Providing front panel coax and optical S/PDIF outputs of the data stream from USB. Note that this is only possible if the γ1 is set up as a full config. Otherwise, γ2 could be plugged into a either a full or lite config γ1 board.

Schematic diagram

The following is the current schematic diagram:



Board layout

And here is the current PCB layout. The board has 3 copper layers, soldermask on both sides, and silkscreen on the top side. The top and bottom layers each also have a ground plane. The representative colors are: top=red, bottom=blue, inner=grey. Where the top and bottom layers overlap the color shows as lavender.



3D rendering

An updated 3D rendering of the board (partially populated):



Parts list

Please see the γ2 website for the most up-to-date BOM.

Reminder to self: "Keep an eye on what I have in my Photobucket public folders"

Looks great Ti!

Those R+,R-L+,L- are the balanced outs of the DAC right? Do they also need caps on the output?

You have sharp eyes...

Yes, those "test points" are the differential analog outputs straight from the DAC chip. There is not enough panel space for additional jacks (let alone large, XLR ones) on the Box B2-080 case, so we did not design for balanced outputs. You could air-wire to those points if you use a larger case, and use external coupling caps. You don't get the opamp's buffering or low-pass analog filter that way.

Oh yes yes yes

Well, I've been looking forward to this. I can hardly think of any reason or detail to change to improve this DAC.

The only thing I'd like to ask about is the 96KHz upsampling. What's the deal with that? I mean does the DAC chips have better measured performance at 24/96 compared to 16/44.1?

I expect the main reason to use the ASRC is for jitter removal? I understand why you choose 96 instead of 44.1 reclocking... I was just wondering if it would be possible (perhaps with a 2nd off board clock) to have a switch between 44.1 and 96? In other words to compare reclocking to upsampling.

*watches with great interest*

The ASRC's main purpose is indeed jitter removal, but the DAC chip also benefits at 96KHz with optimized digital filter performance.

Ok great. Since it's going to be asked anyway, let me be the first... ETA? :P

Aww I know the answer is not going to be any exact date etc but I can't wait lol.

No ETA yet, sorry. It will take as long as it needs, but no more.

An evolutionary step towards the full featured desktop dac?

Darn, this pops up! This should be a good competitor to the Opus, as they are pretty similar hardware-wise (WM8741, asynchronous ASRC, balanced, etc), in one compact enclosure! Very exciting! I finished populating the Opus (four modules total), it's my second time with SMD, but man... eight hours of staring at less than 1mm components. At least I got the 28-pins perfectly . If the γ2 stays through-hole, with the exception of the crystal, which is SMD, then it's a great next step from the γ1.

Actually the Opus focuses on balanced output having SE as option using a Balsie.

THe y2 focuses on SE outputs having the possibility to "hack" balanced outputs and wire the output caps and connectors.

Amb, any chance of having a servo to offset output voltage instead of caps?
Is thi even possible?

No, it's not possible. We're also tapping the power supply from the γ1, which is 5V single-rail. In order to have any kind of DC servo you'll need a dual-rail supply. We're also out of board space. Despite the seemingly expanses of "unused" areas on the γ2, we can't actually put anything there because they would run into parts on the γ1 when plugged together.

No problem, we'll wait for that fully featured desktop DAC you've been talking about for a while...