The Holy Grail
Dec 11, 2006 at 7:38 PM Post #226 of 232
Quote:

Originally Posted by Magsy /img/forum/go_quote.gif
Is there an easy place to tap in and provide external power?


You are entering into uncharted water here. If you look at the botom of the board (http://www.ecp.cc/images/HPDAC_0.1b_bot.GIF) the power pin from the usb connector has 3 spokes. One goes to the reg101 regulator, one goes to the host pin, and the third goes to the dc-dc converter and the led. Presumably, it is this third one that you would want to cut and replace.

As for places to apply power, one option is that there is a via right before the dc-dc converter, but this would not bypass the converter, which I think is a source of more noise than the usb supply itself.

Another option might be to not populate the dc-dc converter section (and not cut any traces) and apply external power to the output pins of VR1 and VR2. This would keep the LED powered by the usb supply. If you only wanted to apply power to one spot only, it could be to the via next to the "2" in "L2". So long as you jumpered D1 this would power both the amp and DAC chip.

All of this stil leaves the PCM powered via USB, but I don't see an easy way around that.

-d
 
Feb 12, 2007 at 8:17 PM Post #227 of 232
I'm using a PCM2706 in internally-regulated mode. I did not populate C7, C8, C21, L6, and L10. Based on a review of the PCM datasheet, it looks like the PSEL pin should be LOW to trigger the internally-regulated mode. Is this correct? Should I remove L9 (or jumper it) to set the pin low?

Also, I'm wondering why the SSPND pin receives voltage; since it's an output, shouldn't it be left to float?
 
Feb 12, 2007 at 8:48 PM Post #228 of 232
Quote:

Originally Posted by Skubasteve /img/forum/go_quote.gif
I'm using a PCM2706 in internally-regulated mode. I did not populate C7, C8, C21, L6, and L10. Based on a review of the PCM datasheet, it looks like the PSEL pin should be LOW to trigger the internally-regulated mode. Is this correct? Should I remove L9 (or jumper it) to set the pin low?


Jumpering it is the same is populating it. L9 presents a high impedance to high frequency noise, but a low resistance to DC.

Anyhow, you need PSEL high to tell it to be in buss powered operation.

Quote:

Originally Posted by Skubasteve /img/forum/go_quote.gif
Also, I'm wondering why the SSPND pin receives voltage; since it's an output, shouldn't it be left to float?


It is a (inconsequential it seems) bug. There is a note about it on the website which says:
Quote:

There appears to be a small bug in the schematic and hence in the board. To date, there have been no reports of problems being caused by it, but if you are concerned, it is an easy fix. Pin 11 (SSPND) on the PCM2707 is powered while it should not be (it actually provides voltage to turn on some DAC chips.) To disable this, you can either cut the small trace that connects pin 10 to pin 11 (it is right between the two pads -- this is most easily done before the board is built) or, once assembled, use an X-acto knife to cut pin 11 off of the chip.


 
Feb 12, 2007 at 9:57 PM Post #229 of 232
Thanks dsavitsk!

Another noob question: I plan on adding VR3 at some point; would I then want to ground PSEL to get the chip to run in self-powered mode? Or does the voltage at VBUS take care of this?
 
Feb 12, 2007 at 10:11 PM Post #230 of 232
Quote:

Originally Posted by Skubasteve /img/forum/go_quote.gif
Thanks dsavitsk!

Another noob question: I plan on adding VR3 at some point; would I then want to ground PSEL to get the chip to run in self-powered mode? Or does the voltage at VBUS take care of this?


TI's terminology here is a little confusing. Self powered is as opposed to bus powered. The hpdac is always bus powered, meaning powered off of the USB supply. Self powered would mean that you built a stand alone power supply (batteries or AC.) So, psel should always be high.
 
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