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00940 21st February 2004 09:47 AM

Pcm2702 Usb-dac
Here's an attempt to make a compact pcm2702 DAC pcb. I'm aware that KECES has a kit but it's too big for my taste. I also don't want it to be wall powered. I also know than Ukram has a pcb but it's a pcm2902 and it doesn't have a low pass filter.

So, here we are, grid is half an inch :

Schematic :

Now, I'd need some advice, I've 2 ideas for the supply :

1- One option would be to build a 5 AA battery pack. As recharging 5 AA would be painful (not really standard), we could just integrate a small charger and add a DC jack. Battery life could reach near 40 hours. No need to change batteries, just connect the device to a small wallwart (9V should be ok).

2. Another options would be too go for less batteries. With 2 AA, we get 2.4VDC. Add to this a DC-DC converter and we're quickly back to 5.5V. It would have an estimated battery life of 15 hours. It's not more boring than having a pcdp.

What do you think ?

hifi 21st February 2004 10:49 AM

If you do the battry approach, why not make it charge from the 5v (100mA) availbe at the USB port?


00940 21st February 2004 11:10 AM

charging 5 AA 1.2V/2000mA batteries from a 5V-100mA source would take forever and nimh should be charged in serie. Which means 5V isn't enough to begin with.

Furthermore, i don't know for you, but i shut down my laptop when I don't use it. When would it charge ?

hifi 21st February 2004 12:01 PM

I was thinking more in the lines of maintanace charging the battery

/ Mikael

00940 21st February 2004 12:51 PM

oh, i see. Yes, it's an idea to explore.

I thought about using the MAX1722; here what's Maxim claims :


Built-in synchronous rectification significantly improves efficiency and reduces size and cost by eliminating the need for an external Schottky diode. All three devices feature a 0.5 N-channel power switch. The MAX1722/ MAX1724 also feature proprietary noise-reduction circuitry, which suppresses electromagnetic interference (EMI) caused by the inductor in many step-up applications. The family offers different combinations of fixed or adjustable outputs, shutdown, and EMI reduction (see Selector Guide).
The max1722 can output 150mA @ 5.5V, regulated, enough to have a regulation to 5V with a low-dropout regulator. It's compact too and cheap : less than 2$ a piece !

Neteagle 21st February 2004 02:14 PM


Do you have the schematic and board origianle files ( it seems to be made by eagle ...) ?



00940 21st February 2004 03:50 PM

yes, it's done under eagle. I don't have a .schm since i only work on the board.

I'll upload the .brd later this evening or tomorrow, there are some changes to be done.

Gyula 25th February 2004 10:20 PM

Hi 00940!

I suggest you to remove the ground-loop from the PCB :yikes:!


Rookie 26th February 2004 01:10 PM

Hi all!

I recently build a USB DAC with PCM2702. It doesn't use analog filter in the output. The output is taken directly from the PCM2702 through a 4,7uF WIMA MKS4 DC blocking capacitor. The power supply is separated for the analog and digital section and double regulated (LM317+ADP3303-3.3 for the digital section and LM317+ADP3303-5 for the analog section). The transformer has two secondary windings for the analog and digital section. Total capacitance is ~10000uF.

The sound is noticably better compared to the integrated sound card on the motherboard. More highs, more natural, better channel separation, better soundstage. Unfortunatelly, I don't have a camera to take some pictures. I put it in old PC supply case and made a aluminium front panel with a blue LED in the middle. It looks nice.:)

Later I will upload the image of the PCB made in Eagle.

Best regards,

00940 26th February 2004 01:27 PM

sorry, I almost forgot this thread. (there's a more alive thread on headfi about this).

Thx for the suggestions. In the latest version, we scrapped the LPF. I'll just a small blackgate NX.

The new layout will be double sided and use a groundplane.

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