- 6,960 Posts. Joined 9/2007
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my DAC design - pup1 DAC - Page 3
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If you look at the picture of the board, point 1A is the 5V USB taken after the ferrite and right next to it is a ground.
So I suspect that yes it can. I haven't read all the latest on the Carrie, but I know that there was some start-up issues that were happening if you didn't take the power off the bottom of one of the caps on the grubDAC, so I can't say 100% that it would work.
And also the pup1 is the full length of the 1455C801 case. You'd have to look at something like the 1455C1201
- 203 Posts. Joined 7/2008
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Does the IV opamp put out enough power to drive headphones directly do you think? Is the power supply robust enough to handle such a load? This would make a really cool single board solution for portable listening.
You'll have to wait for cobaltmute to answer that one.
Meanwhile, for those of you who commit to building one of these, I can perhaps supply some of the TI chips. Since it's for prototyping, the TI sample program should apply. Just be aware that if I supply you with the chips, you can't eventually sell the device except for a price that will only recoup your miscellaneous parts investment (not the chips). Those are Head-Fi rules, anyway.
I'll coordinate with cobaltmute on those of you who have requested a PCB and send you what I can. I'm pretty sure I can supply the DAC chip and perhaps some of the TPS's. I'll take stock tomorrow and we'll see.
As I've said, I'd deal with the PMs on Monday, but everyone that has PM'ed me will get a board.
So that you can have this before you commit, this is an up to date Digikey BOM:
I only have a BOM for Digikey for this project. It is complete, short the Hammond 1455C801 case.
Hmm ... looks like they'll have the 10K resistor on 3/23. In the meantime, this one should work: P10.0KCACT-ND
As for the PCM2707, I think DigiKey has stopped stocking them altogether. Let's just assume that I will provide you with those, along with the DAC chip. I don't have the opamps, but may be able to supply a lot of the TPS chips, too. Once cobaltmute gets me a count of the PCB's he's going to distribute, I'll get a better idea what I can supply and announce it here.
If your tastes involve listening at quiet to moderate levels, you could likely get away with it. Higher impedance cans would also be a better choice.
The charge pump for the negative output rail only outputs 60mA and this is used for both channels. Low impedance cans will likely draw more than this design can output.
That being said, I'm listening to "Buena Vista Social Club" at -20dB in foobar with my HD650 and it sounds great.
As an update for the prototype team, tomb is going to send me the PCM chips to me to include with the boards when I mail them out. I will ask for the money for shipping and the addresses of those that are getting protos when I am ready to ship.
I've been asked about op-amp rolling, and here are my thoughts on that.
So far, I've attempted to use three DACs on the board
- THS4222 - extremely high speed, but was not stable at all
- LMH6643 - this is the current choice for me. Personally, I think the board sounds so good with this, I haven't seen the need to try others
- LME49721 - this is a "slow" opamp and seems to be stable when the other start to be unstable and show offset issue.
There is two choices for the rails and this can affect your choice of op-amp
- Regulated - using the BOM parts, you get +-2.5V on the rails.
- Unregulated - don't install the regulators, and jumper 1A to 1B and 2A to 2B. You have to understand what you're dealing with though. USB input power can be 4.75 - 5.25V. The charge pump will invert that to equivalent negative voltage. The issue comes that the charge pump droops as it gets more loaded. The rail will droop up to 0.5V, so the negative rail could end up at about -4.25V. When considering op-amps, for safety, I would consider +-4V as working, but they need to be able to handle +-5.25V.
If you are looking at the regulated option, consider rail-to-rail output amps to ensure that you don't have issues.
As I mentioned in the initial post for prototypers, the layout of the board, may be prone to offset issues. My board is stable with the LMH6643, but I have built one (where I put the output resistors on the bottom side) and it had offset issues on one channel (150mV IIRC). You will need to check your build for offset, and if it is excessive, the easy fix is to replace the LMH6643 with a LME49721.
Minor offset, depending on your setup, you can live with. If there is any stage that is AC coupled, you are good, If you have a DC servo in your signal chain, you are also likely good depending upon its range. If you are full DC coupled with no servo, you'll have to look at the offset and your gain and decide upon your risk tolerance.