- Dec 22, 2008: The official γ1 website is now live.
- Dec 12, 2008: Production PCBs (v1.0) and related parts are now available.
- Nov 24, 2008: This post updated to v1.0, which incorporates what we learned from the prototyping phase. Farnell and RS Components part numbers are added to the BOM.
- Oct 22, 2008: v0.6 boards have been sent to prototype team.
- Sep 30, 2008: We are moving forward to the prototype phase.

Introduction

I am pleased to announce the γ1 ("gamma 1") project, a miniature but full-featured stereo DAC offering a high level of performance, flexibility and configurability.

I had expressed an interest in a DAC project for quite some time. MisterX and I have been working on it for several months and the circuit evolved into something that we both feel is ready for prime time. We have also solicited the feedback from a few other people offline. I think at this stage the schematic is fairly firm, although not cast in concrete. Your feedback will be, as always, appreciated.

We believe that this little DAC with the big heart will be a superior companion for high-quality portable amps like the Mini³ v2 and others, and is good enough to serve as a quality home/desktop DAC. There has been no shortage of DIY DACs of late, but we hope this one should be compelling enough to make you take notice.

Basic feature set

- Designed for the Hammond 1455C80x case (same as Mini³ v2).
- Supports USB, S/PDIF coax and S/PDIF optical inputs (switchable)
- 24-bit 192KHz high-performance Wolfson DAC chip.
- 192KHz S/PDIF receiver chip with low-jitter clock recovery.
- Modular: A separate DAC board and a USB interface board that could each be used standalone, or mated to each other for the full feature set.
- The maximum S/PDIF resolution and sampling rate is limited to 24-bit, 96KHz for best digital filter response.
- The maximum USB resolution and sampling rate is 16-bit, 48KHz.
- When used standalone, the USB interface board can be configured as a bus-powered USB-to-S/PDIF or USB-to-I²S converter.
- When used standalone, the DAC board supports only S/PDIF (coax or optical) inputs and must be powered by a wallwart.
- When mated to each other, the two boards support all inputs and can be USB-powered or wallwart-powered.
- Auto-sensing of wallwart-power (no external switch).
- Onboard power supply voltage regulation (separate LDO voltage regulators for the USB board, DAC digital and analog sections).
- Unbalanced analog line output (3.5mm stereo mini jack), with high output current and low output impedance.
- Illuminated mini-toggle input selector switch (USB/coax/optical) with power-on indication and operation status (green = normal, red = error or no audio).
- 1.3mm miniature barrel-type DC power jack, RCA phono S/PDIF coax jacks, standard Toslink-style S/PDIF optical jacks, 3.5mm stereo mini jack for audio line output, mini-USB connector.
- Through-hole resistors, capacitors, ferrites and other parts to reduce build difficulty. The only SMD parts are the various ICs and voltage regulators.
- In default configuration all parts (including jacks) are PCB-mounted. No internal wiring necessary.
- USB host interface supports Linux, MacOS and Windows (no additional software driver necessary).

Modularity

Due to γ1's modular nature, there are five main possible configurations:

1. USB+DAC "full" (USB board configured for S/PDIF format and the DAC board fully populated with coax and optical S/PDIF inputs and front panel selector switch).
   
2. USB+DAC "lite" (USB board configured for I²S format, no coax or optical S/PDIF inputs, no input selector switch).
   
3. USB board only, as a USB-to-S/PDIF converter.
   
4. USB board only, as a USB-to-I²S converter.
   
5. DAC board only, with coax and optical S/PDIF inputs only (no USB).

γ1 boards

The boards will be supplied as "sets" only. A board set includes one DAC board, one USB board, five ferrite beads, NS 9P 0.318" header (which should be separated into 3P and 5P segments for J2U and J3U). I will also stock the WM8501 DAC chip, TPS2115A power mux chip, PCM2707 and several other related parts that might be difficult to source in certain parts of the world.

Main components

Some of these are optional depending on the desired build configuration (see circuit description below).

- Texas Intruments PCM2707 - USB interface and conversion to either S/PDIF or I²S
- Cirrus Logic CS8416 - 24bit/192KHz S/PDIF input multiplexer and receiver, clock recovery and other functions
- Wolfson WM8501 (or WM8759) - 24bit/192KHz high performance DAC
- Toshiba TORX147PL - optical S/PDIF input module
- Toshiba TOTX147PL - optical S/PDIF output module
- Pulse transformer (Schott, Newava, Pulse or Murata) - for coax S/PDIF input and output coupling and galvanic isolation
- Texas Instruments TPS2115A - auto-switching power multiplexer for auto-sensing/switching between USB and wallwart power
- Microchip MCP100, MCP101 or Maxim DS1818, DS1817 - reset manager chips for power-up sequencing
- National Semiconductor LP2985A-3.3, LP2985A-4.5 or Texas Intruments TPS79333, TPS793475 - LDO voltage regulators for independent USB section, DAC digital and DAC analog supplies.

Schematic diagram, Circuit description, PCB layout, Parts list

Since the official γ1 website is now up, rather than duplicating the information here, please refer to the website.

[Edited by amb on 12-25-2008 at 06:17 AM.]

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[Quoting amb]

- 24-bit 192KHz high-performance Wolfson DAC chip.

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[Quote]

- In default configuration all parts (including jacks) are PCB-mounted. No internal wiring necessary.

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Ooh, sounds interesting, perhaps some crossover with the TwistedPear Opus? The neat, small trouble free form factor is cool.

Mindyou, it's got to be said I for one was looking forward to the first fully discreet DAC from amb?

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[Quoting mwofsi]

Mindyou, it's got to be said I for one was looking forward to the first fully discreet DAC from amb?

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The only place for a discrete solution might be in an I/V or analog output stage, but in this case, our small form factor precludes it. But I'll drop a hint: there are no opamps at the output of this DAC either.

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[Quoting cobaltmute]

So are you planning on the WM8716 or the WM874041? I had been looking at the datasheets and was looking at the recommended dual DAC setup listed on the datasheet for the WM8716 as a project.

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Wait until I post the schematic. I'll describe the chip choices and the reasons behind them.

I have just finished an AlienDAC and in its inital setup found it a little underwhelming so I very interested in seeing what comes of this. Especially since you say most of it is through hole.

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[Quoting cobaltmute]

Patience is a virtue I guess then.

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I second that. I was just thinking of a DAC project to do and was looking at the Bantam DAC but this sounds more interesting. Go for it! But don't make us too virtuous by making us wait too long.

Btw - I hope the parts won't be too difficult to get.

[Edited by heady on 08-14-2008 at 07:20 AM.]

I wonder how good a WM8727 could sound with a good design around it. The DAC is so small it makes a lot of room for supporting chips.

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[Quoting heady]

I hope the parts won't be too difficult to get.

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[Quoting yuujin]

Will you be making an I/V board

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[Edited by amb on 08-14-2008 at 11:46 AM.]

I would hope that you can have the USB converter go from USB to I2S to drive the DAC's input, so we can avoid the troubles of SPDIF altogether.

I would also hope that a battery-power option can be considered for the DAC side of this unit. Power supply isolation issues are going to be a design challenge here. A lot of new laptops manage power by scaling the clock speed of the CPU dramatically, switching it from very slow to very fast when loads appear; this creates very audible and hard-to-filter nastiness on the USB power buses (as well as the harder-to-manage problem of varying USB clock speeds on many systems).

Another design challenge is giving the Wolfson chip a DC-coupled output. Personally I think these things sound excellent with an electrolytic cap on the output, but in keeping with the principles of your other designs, you might want to find a way to DC couple it (which probably sounds marginally better). I'm not sure whether this could be done easily by applying a countervailing DC bias (probably before a simple buffer). I actually removed the output capacitors from my Wolfson DAC and ran it through a CK²III, using its DC servo to zero the output... But that really is not the intended use of this design.

I hope you can leave space for a good-quality transformer input on the SPDIF. This makes a significant difference.

Finally, I hope this is a trial project before you try to tackle a really no-compromise, non-portable, no-expenses spared DAC with differential outputs... a y24, maybe.

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[Quote]

I would hope that you can have the USB converter go from USB to I2S to drive the DAC's input, so we can avoid the troubles of SPDIF altogether.

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There currently is an option for "simplified" USB to I2S to DAC configuration but it remains to be seen if that will survive the prototype process.

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[Quote]

I would also hope that a battery-power option can be considered for the DAC side of this unit.

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You could always a plug an external battery pack into the DC power jack but given the size of the default enclosure I think that is far as a battery powered option will go.

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[Quote]

you might want to find a way to DC couple it

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The feasibility of that is questionable with the DAC chip we have selected.
(even if the coupling capacitors are omited there is still an internal RC filter on the DAC chip to deal with)

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[Quote]

I hope you can leave space for a good-quality transformer input on the SPDIF.

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That is the plan.

[Edited by Mister X on 08-15-2008 at 12:19 AM.]

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[Quoting cobaltmute]

Is it going to be built with a standard USB or mini-USB socket? Mini-USB would be nice to keep travel cable weight and size down.

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[Quoting amb]

We're using Mini-USB. Not for the reason you state, but because it uses less space, both in terms of board area and panel area. Of course a slimmer cable is a bonus.

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I thought the smaller board space after I posted and thought about an edit to comment on that but I was just too lazy.

Is the USB to I2S/SPDIF the "standard" PCM2704/7 approach or do you have some magic up your sleeve?

(the PCM2704 cannot be configured to output I2S data, did you mean the PCM2706?)

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[Quoting Mister X]

Nope, no magic.
The standard approach is adequate.

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A nice touch might be a spot for a SRC4192 -- would allow reclocking of the I2S as well as some jitter reduction. It's easy to jumper over as well.