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HPDAC2 and HPDAC3

post #1 of 21
Thread Starter 
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.







post #2 of 21
Doug, how much current can the USB port supply?
post #3 of 21
Both USB 1.1 and 2.0 specifications state that maximum current starts with 100mA but device can request the subsystem to supply up to 500mA in 100mA intervals. A lot of modern PCs will go straight to 500mA by default. Still, my old Toshiba laptop with USB 1.1 was not able to run the original HPDAC without adding noise whenever I moved my USB mouse. After testing with a number of different computers, the 2 older ones with USB 1.1 just had noise issues, newer computers with USB 2.0 had none.
post #4 of 21
Thread Starter 
Quote:
Originally Posted by ezkcdude View Post
Doug, how much current can the USB port supply?
Quote:
Originally Posted by FallenAngel View Post
Both USB 1.1 and 2.0 specifications state that maximum current starts with 100mA but device can request the subsystem to supply up to 500mA in 100mA intervals.
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.
post #5 of 21
How can I get boards for both?
post #6 of 21
Sign me up too! From what I can gather, would you consider the HPDAC3 to be the best of the bunch in terms of performance?

Aditya
post #7 of 21
If there are going to be boards produced and sold I'd be interested in a couple of each.
post #8 of 21
Thread Starter 
Quote:
Originally Posted by a1rocketpilot View Post
From what I can gather, would you consider the HPDAC3 to be the best of the bunch in terms of performance?
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.
post #9 of 21
looks good but it will probably perform very well just because it has had some expensive parts thrown at it like pcm1794. Maybe HPDAC3 can do better with the I/V. Loose the passive I/V and opamps and make a discrete solution for I/V and buffer to drive headphones. Something simple, a while ago I read about a simple I/V with a couple of transistors and no feedback design can sound good (much better than opamp for I/V according to some).

Still I do not know much about designing DACs i'm still learning. Just some ideas there worth thinking about i hope.
post #10 of 21
Also for HPDAC2, if the aim is to keep it small and portable (USB powered), and sound quality is not most important, then you could use a DAC like CS4398; the flagship DAC from cirrus. It is much cheaper and has voltage output so no need to worry about I/V. Specs are good, I think it doesn't sound too bad as well. availability for the CS4398 should be ok.
post #11 of 21
Thread Starter 
Quote:
Originally Posted by Dave_M View Post
Maybe HPDAC3 can do better with the I/V. Loose the passive I/V and opamps and make a discrete solution for I/V and buffer to drive headphones.
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

Quote:
Originally Posted by Dave_M View Post
you could use a DAC like CS4398; the flagship DAC from cirrus. It is much cheaper and has voltage output so no need to worry about I/V. Specs are good, I think it doesn't sound too bad as well. availability for the CS4398 should be ok.
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?
post #12 of 21
I think I made a mistake about the V out. not sure where I got the idea that it outputs voltage from

But there is a very nice DAC chip from Wolfson, the WM8740.

That definitely has voltage output, and is even cheaper than the cirrus (about $8 compared to $12 for the cirrus - UK prices). It is used in some nice CD players that cost around $800. I have heard some good things about its sound quality. It should be excellent for Hi-Fi.

Wolfson is UK based so to order WM8740 I think you will have to have them shipped overseas. Not much of a problem, just takes a little longer to arrive.

Would also be good to try and make use of the volume control on the WM8740. That DAC can be programed in hardware, but to use the volume control you might need a micro controller so might not be easy, but it will be something different at least.
post #13 of 21
It's not really fair to ask Doug to change the D/A chip. Do you realize how much effort is involved in laying out the pcb? That's what takes about 90% of the time in doing these projects. PCM1794 is a top-of-the-line chip, and Doug is obviously trying to make an "audiophile-quality" DAC. (Of course, I say this having made the same decision as Doug.) However, if you are really interested in a WM8740 -based DAC, just go to the diyaudio forum. Russ White (twisted pear audio) has already developed one.
post #14 of 21
Thread Starter 

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.

post #15 of 21
Thread Starter 
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.
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