HPDAC2 and HPDAC3

I wasn't really intending to do these, but I got started and couldn't stop. If you are unfamiliar with the original HPDAC, you can read about it at: http://www.ecp.cc/HPDAC.html

The HPDAC2 is a 8x oversampling design using the pcm1794 or pcm1798. It has the same foot print as the original (in fact it will slide in the same case), is still usb powered, etc. It is a very challenging build, I think.

The HPDAC3 is a little larger 100mmX70mm (the 2 is 80mmX50mm). It also uses the pcm1794/8 as the 8x oversampling dac, but it also uses an asynchronous reclocker to upsample to 96kHz (or wherever you set it actually) and to reject jitter, and is wall powered.

The amp section in both is essentially unchanged except opamps have been changed to DIP for rolling purposes (and because the NJR4556 is only available in DIP anymore). Also, while both use a passive (resistor) I/V*, they use an opamp for differential to single ended conversion. The 2 has some coupling caps between the dac and the amp due to the usb power.

Below are some preliminary board and schematic images. There are still a few changes and tweeks to be made, and any comments are appreciated. I am looking to do some prototyping in the next few weeks ...

* It should also be noted that the I/V sections in both are based on the work by ezkcdude (Evan) on his ezdac. The "2" adds some extra coupling caps and changes the resistor values such that the 3db point is not too high, but the "3" is pretty much stock. Much appreciation goes to Evan for his earlier work.

What he said.

The original HPDAC used a dc-dc converter to increase the 5V for the usb jack to ~12V. This was reregulated down to 5V which ran the dac chip. The DAC chip drew 60mA or so, which means that it was drawing 60mA at 12V, and since the boost converter is about 75% efficient, it is more like drawing
180mA or so from the usb jack. This is a lot.

The new one (the "2") uses the the 1794 right off the usb line. It also only draws, about 55mA (at 44.1kHz) which frees up a lot of current for the opamps, and especially for the extra opamp. It is still a balancing act, and low current opamps are probably best.

Well, it remains to be seen if it will work at all, but in theory, it should be the better performer. The cost is the need for a wall wart.* The ezdac, which as I've said was very influential in the design, is said to be quite good.

Whether boards will ever be produced for either of these depends on whether they work , how well they work, and whether I get my act together. But, I also need to go through the powers that be at Head-Fi before anything can be said here about selling anything, so for now lets discuss technical things.

* By the way, a neat project for someone would be a rechargeable battery based power supply that fit in a hammond case. It would be basically a PCB with a some 9V batteries on it, a charger circuit, and maybe a regulator -- use it power your amp/dac, and plug it in and use it as a regulator when the batteries die.

Contrary to a lot of people, I actually like passive I/V solutions. To my ear they sound better. My dac at home uses a passive I/V directly connected to the grid of a tube. Sounds good enough that my cd player has not been on in months and is now on the market.

As for the opamps, I'm no great fan, but people like to be able to roll opamps to tune the sound, and this seems like a plus here. Someone could always leave out the I/V resistors and build a discrete I/V that could plug into the opamp socket if they like -- perhaps I'll add an extra ground connection to facilitate this. But, I'll take the suggestion under advisement

The target chip for the '2' is actually the pcm1798. It is pin compatible with the pcm1794, but is even cheaper than the cirrus.

But, I like the suggestion. My fear is that that are 100 chips out there that would be just fine to use, so picking only one is tricky. But, I'll look into the suggestion and see what I think. Might be worth the variety if nothing else.

Looking at the datasheet, I don't see anything about Vout, though?

Update

The first HPDAC3 prototype is up and playing music, and sounding very good if I do say so myself There are still some issues to work out, of course, but so far so good.

My website may be down, but the first HPDAC2 is up and running. Compared to the original, first impression is that I think it sounds a little more full. However, it is a total pain to build.



I have a very small number of prototype boards left (5) that I'd like to sell off at my cost to get other people's impressions. I am undecided whether this project is worth continuing with, and whether other people can build it will be an important bit of information. Slight priority will be given to owners of the original as they can do a comparison, and they should be up to the soldering challenge. Oh, and the footprint is the same as the original, so it should fit in your case if you have one.

Boards are $7.50 shipped. Send me a PM or an email (hpdac @ ecp.cc). Total cost to put it together is in the area of $50 to $70 (+ a case) I think.