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my DAC design - pup1 DAC - Page 2

post #16 of 674
Thread Starter 

Well as well as it can live considering the charge pump is only putting out -1V and the -2.5V reg is then regulating that to -0.25V. As a result to the output op-amps rails are 2.5V and -0.25V.

Setting Foobar's volume way low, I can listen on my capactively coupled cmoy and it sounds pretty decent. No further comments on the sound until I can get the output rails straightened out and listen to it in my normal setup.

Things learned in debugging so far:
1. Reversing Data+ and Data- from the USB won't get you anywhere
2. If you want I2S out of the DAC, you need to tell it that.
post #17 of 674
Thread Starter 
I've now determined that that the -2.5v regs that Digikey sent me are not the right part. Removing them and the charge pump gives me a perfect -5v negative rail.
post #18 of 674
Thread Starter 

Finally complete

The last post of mine is cow-patties. Turns out that the issue was mine - I assumed that the pins on the negative regulator were the same as the positive ones which is not the case. On the negative regulator the Vin and GND pins are reversed compared to the positive regulators so the issue is on the PCB itself.

However, with some creative bridging and placement, I was able to place the regulator on the board and get a nice -2.5V rail. As a result, everything on the board is now working as intended.

The result (in need of a good cleaning):

To my ears, this board sounds very good with the LMH6643 as the output op-amp. I'm not going to try to compare to the other USB DACS (gamma1 and AlienDAC) I have since as much as I try I'm going to be biased. Someone else will need to build one to be the judge of the outcome.

My next build of this is going to try unregulated rails with a THS4222 as op-amp
post #19 of 674
Thread Starter 
I've had a chance to slowly complete my second board now. My original intentions for this second board was to use the THS4222 as it seemed like to would be a good fit. Aside from the op-amp, the only other change from my BOM were that the C31 and C33 were replace with larger 120uF Panasonic FM caps.

The THS4222 is a fairly high speed opamp (950v/uSec slew rate) and it turned out to be somewhat unstable on my board as laid out. While there was no audible distortion, the left channel was experiencing 80mV offest voltage while the right channel only had 1mV offset. Not feeling like fighting with it to get the opamp stable, I replaced the THS4222 with a LME49721.

So I have two functional boards at this point in time. Proto 1 use a LMH6643 and 47uF UCC PSA caps as C31 and C33. Proto 2 uses a LME49721 and 120uF FM caps. Both boards, to my ears sound really good. Proto 1 has a bit more of an intimate sound - vocals can really sound like you are right there. Proto 2 seems to lose a bit of the intimacy for a bit more bottom end. To me they sound as good as the gamma1, but I am biased.

The biggest difference that I have qualitatively tested between the two boards is noise. On both boards, any noise is not noticeable to me at listening levels. If I stop playing music and then crank my Pimeta to the end stop, proto 1 is noisier than proto 2 by a fair margin. I would say that proto 2 is as quiet as my gamma1. I do not have any good tools to quantify this. I am however quite happy that the charge pump does not seem to make any noise into the audio spectrum.

I'm slowly working to r3 fixing the obvious mistakes and tweaking the board based on what I'm learning and reading. That being said, if anyone wants to try building one of these, PM me. A PCB will be yours for cost + shipping. I can easily describe the three fixes that need to be done to get the board working.
post #20 of 674
Looks good. If I didn't have a handful of USB-dacs already I'd definitely want one

post #21 of 674

Well, I built one over the last few days and it sounds glorious!


Look to see more of cobaltmute's excellent DAC soon!! wink.gif

post #22 of 674

Wow, looks like a nice, little DAC. Any impression between it and the Grub?

post #23 of 674
Originally Posted by Lil' Knight View Post

Wow, looks like a nice, little DAC. Any impression between it and the Grub?

Well, you know I don't like make comparisons in the DIY threads.wink.gif  Let's just say that it's not bigger and a bit more expensive for nothing.  Cobaltmute has designed in the same feature on the Pup that exists in the Grub: a charge pump that needs no output coupling caps.  Further, the TI DAC chip is one of the best out there ~130dB SN ratio, among other things.  Almost every voltage source is regulated on the PCB.  The result, IMHO, is an outstandingly clean and detailed sounding DAC. That's really an understatement, but it's probably because I'm very excited about this and am trying not to gush over everyone.smily_headphones1.gif 

post #24 of 674

Nice build.  I take it to mean you are going to pick up distribution of this?


FWIW, lift pin 2, put a switch on it for high/low selection.  It's 5V tolerant and allows selection of the digital filter rolloff on the PCM1794/PCM1798.

post #25 of 674

While I'm not a fan of the passive I/V section (in my experience, active I/V stages sound better with the pcm179*), I must admit that it makes a lot of sense for such a compact DAC.


The whole implementation is very interesting and I've no trouble to imagine it sounding absolutly wonderful. Are you planning to sell boards ?

post #26 of 674
Originally Posted by 00940 View Post

While I'm not a fan of the passive I/V section (in my experience, active I/V stages sound better with the pcm179*), I must admit that it makes a lot of sense for such a compact DAC.


The whole implementation is very interesting and I've no trouble to imagine it sounding absolutly wonderful. Are you planning to sell boards ?

Yes - kits, too, we hope.  I'll have to talk to cobaltmute, but we may be interested in a few prototype builds from volunteers.  I understand he has a few PCB's left and I know I'm building a second one.  In its present form, it's a perfect fit for the Hammond 1455C801 with no wiring required.

post #27 of 674
Thread Starter 

I do have a few boards left over and it would be nice to get a couple of other opinions as I rework the board.


That being said, if you're considering putting your hand up, I want to be up front:

  1. The board has two part layout defects.  Very easy to overcome, but want you to be aware.
  2. The layout around the op-amp is "marginal".  The feedback loop is too long and you may experience issues.  With the high speed, best sounding op-amp, I have seen offset issues by doing something as simple as mounting the resistors on the bottom of the board.  That can be fixed by swapping out for a slower op-amp, but it is definitely a trade-off in sound.


That being said, follow the BOM, don't do anything crazy and you'll have something very nice sounding.


If you can't accept those points, wait for the rework.  Either way, its worth building, in my utterly biased opinion.

post #28 of 674

#1. is a piece of cake, IMHO - not a problem.  Yes, it's not perfect, but it's a prototype.  The fixes (2 of them) are easy with only moderate SMD soldering skill.  One of them you can easily see in my pics above and requires only a through-hole-skill simple fix.  There are two resistors on the left USB side of the board that are vertical with their leads looped over a resistor crossing underneath.  The other is a small TPS-style regulator at the bottom right (toward the 3.5mm output jack).  It needs to be angled slightly in order to be offset on the pads.

post #29 of 674

I would like to have one board. Do I have to pay something for it?

post #30 of 674
Thread Starter 

tomb will think me nuts for this as he has heard the DAC, but given that a) I don't remember what I paid for the boards, and b) the issues with the board, I will give the extras away for cost of shipping.


So $5 Canadian gets you a board in an envelope mailed to your address anywhere in the world.  To ensure fairness, I'm only putting one board per envelope.  You want more than one, you pay extra.


All I ask in return is that you give your opinion of the DAC here in the thread afterwords.


PM me for a board.  First come, first served.  Spacehead is first on the list.


And of course, it is rather silly to post this right now, as I'm packing to go away for the weekend.  So I won't be replying until Monday.

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