Project: integrated amp/DAC: The Hotbox

[j4cbo]

I've been struck by the elegance of an integrated amp/DAC for some time, and finally decided to sit down and design one. In exchange for giving up some flexibility (and cost), there are a number of advantages: a vastly shorter overall signal path, no line-level interconnects and connectors, direct control over potential DC offset issues, and of course smaller size and lower cost.

So, after hunting around a bit, I've picked the various components:
- PCM2704 for USB-to-S/PDIF (onboard DAC not used)
- ESS Sabre DAC (using the Twisted Pear Buffalo design)
- THS4131 I/V conversion (taken from Twisted Pear IVY)
- LME49720 and LME49600 buffer (from LME49600 datasheet, without DC servo)
- AVR microcontroller

Both the DAC and amp are fully balanced, all the way through; it'll require balanced headphones. I'll be recabling my HD650s at some point with a 4-pin XLR. Two three-pin XLRs is the standard, but ugly and bulky, so I'm just going to use one four-pin connector for both left and right.

I'm going a little overboard with the power supply, as well. A separate box provides switched preregulated +/- 18V DC and always-on +5V; the DAC/amp unit itself then has separate onboard regulators for:
- Left amp +15v and -15v
- Right amp +15v and -15v
- Sabre 3.3v digital, left analog, right analog, 1.2v core
- PCM2704 and S/PDIF receiver 3.3v

The high bipolar supplies are switched on and off in the PSU, controlled by a signal sent back from the microcontroller; the logic supplies are switched in the DAC itself. There will also be a logic-level trigger input, so the whole thing can be switched on and off from remotely. I'll be using a Slim Devices Transporter, which has a 3.3v output that can be set to indicate power status.

I'm thinking of simplifying the design a bit more, removing the LME49720 opamps in the output stage and simply connecting the LME49600 directly within the feedback loop of the THS4131. I'm not sure whether that would be advantageous or not; advice would be particularly appreciated there.

The amplifier and power supply each fit in a Hammond 14455K1201 case (3" wide, 1.7" high, 4.7" long). I don't have the power supply schematic uploaded, but it's quite straightforward, with Amveco PCB-mount toroids (one for the bipolar supply, one for +5v), LM317 regulators, and some filter caps.

This is my first attempt at designing an amplifier end-to-end. Including the Twisted Pear designs for the DAC and I/V was a huge help; big thanks to Brian and Russ for releasing their designs. I'd love any comments, tweaks, or suggestions.

Here's the schematic. This isn't quite complete; the power control input isn't there, and I suspect I've forgotten a thing or two. I also don't have a full datasheet for the Sabre yet, so that portion may change once I get better information. The board layout is mostly done, and I'll post that once any more schematic tweaks are finished. If all goes well, I'll be able to build the thing at some point this summer.

 

[FallenAngel]

Looks like a very nice project, but may I recommend a few changes.

1) Take I2S from the PCM2707 instead of S/PDIF
2) Allow for Sabre DAC Outputs (RCA/XLR)
3) Allow for Sabre DAC Input (S/PDIF Coax/Optical)
4) Allow for Amp input (RCA/XLR)

With those in place, it'll be one nice little monster

 

[akione]

Hi J4CBO,

You might be interested to see my efforts in creating a similar pre-amp/DAC, using TPA modules (Darwin, J/Tree, Opus, IVY)

You can find details in the gallery section on Twisted Pear Audio's support forum.

Your ideas look very similar to my own, although I haven't bothered with a micro-controller and remote control.

I can definitely recommend this approach, the sound is awesome. It should also be a bit easier using Buffalo.

Have Fun.

 

[j4cbo]

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif Looks like a very nice project, but may I recommend a few changes. 1) Take I2S from the PCM2707 instead of S/PDIF 2) Allow for Sabre DAC Outputs (RCA/XLR) 3) Allow for Sabre DAC Input (S/PDIF Coax/Optical) 4) Allow for Amp input (RCA/XLR) With those in place, it'll be one nice little monster

I'd considered all of those, but:
- My understanding is that the Sabre's jitter rejection is such that there's no advantage to using I2S over S/PDIF, and using S/PDIF makes layout and such significantly easier. I'd rather go with what's simple.
- I do have a separate S/PDIF coax input; the unit will switch to it automatically if USB is not connected.
- There's no room for separate analog inputs or outputs.

Four XLRs would be about as big as the rest of the board, and I don't want to have differential-to-single-ended or vice versa.

I'm doing a bit of a redesign to split the main unit into two boards; one will have the amp, DAC, and local regulators for those two, while the S/PDIF and USB input circuitry and microcontroller will be on a separate input board mounted above.

Good grief, the Sabre is expensive.

$250 for the minimum order of five samples... this may be put off for a few more months while I save up

 

[fierce_freak]

I was considering doing something very similar to this (using TPA's uber controller when they have it ready), but I hadn't gotten everything completely fleshed out. Looks like a nice project.

 

[labmat]

This sounds like an awesome project; I'll be keeping an eye on this one.

 

[BrianDonegan]

Weird. The only thing in your list we don't already have (or about to have) is the LME49720 and LME49600 buffer , which is actually in the works at the moment. Have you been monitoring our conversations and dev notes?

I am actually planning a similar project to keep my wife happy (less boxes in the living room). The only additional piece mine will have is a balanced 100W/ch amp for speakers. What we want is what we build.

EDIT: Actually, mine will also have multiple S/PDIF inputs, probably 2 optical and 2 coax. We are also designing an S/PDIF mux using the CS8416 receiver to handle this. It will also have an AES input, but I don't think I will be using it as I don't have any sources for it.

 

[fierce_freak]

Any updates?

 

[jonnywolfet]

check this out
amazing dac/headamp at diyaudio

 

[TheRobbStory]

Did this ever take off? I'm mainly intersted in the microcontroller aspect of the design.

 

[sid_]

I'm also interested in this project, as I've been looking for a fairly high-end DAC/Pre-Amp/Headphone Amp combined for a while.

Multiple sources, both analogue and digital (bypassing the DAC as necessary) with a simple micro-controlled source selector a la Darwin - would be awesome. Similarly, I'd like to see an onboard attenuator a la Joshua Tree.

I'm not an electronics expert, but a single board solution using SMD components where possible would be a dream come true in terms of space saving and elegance. I understand the importance of modular design and understand why the TP guys offer their designs as such, however it'd be really nice to have an 'all the bells and whistles' integrated solution.

Fingers crossed someone continues work on this project.

 

[bidoux]

I was thinking to do something similar.
I would do a millet "starving student" with an "Alien DAC" on the top of it.
It would looks like a big wood cube with a medium wood cube on the top of the big one.

If I do it properly, the shape will actually look like this: