Pcm2702 Usb-dac

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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 :

An externally hosted image should be here but it was not working when we last tested it.


Schematic :

An externally hosted image should be here but it was not working when we last tested it.


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 ?
 
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 ?
 
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§Ù 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 !
 
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,
Dejan
 
I've built a PCM2902e DAC (only now to realize the 2702 has better performance) and wanted to ask about the purpose of the "low pass" filter on the audio outputs.

I'm old school so when I think of low pass....it means LOW.

Is this a filter that is intended to filter high freq digital noise because of the mixed signal situation?

Low pass refers to passing audio and placing the cutoff above the audio range?

If so, at what freq is it usually set?

You mentioned "analog" filter.....so now I'm really confused.
 
USB DACs

I went a little different route since I wanted to compare the sound of the TI oversampled DAC with a non-OS DAC run off the same USB bus. I used a PCM2707 with a well filtered 3.3V power supply. I put a switch on the FSEL line so I could switch between internal and external use of the I2S bus and stuck a Philips TDA1387 (very similar to a TDA1545) on the I2s spigot.

I decided to go balls-out on extreme simplicity so the output of either DAC goes straight out **with not even a blocking cap** to my passive preamp and from there to a home-brew 6C33C Class A tube amp. The amp input is cap coupled, so I was not afraid of DC problems...

I started out listening to the 2707's internal DAC. I never got to the Non-OS design that first day!!!!! I have never heard such a natural and gutsy sound from my system, plain and simple. I spent the entire day trying every piece of music I could get my hands on. The frequency extremes are fast and deep, the midranges make voice sound as natural as I have ever heard.....

Only got to spend a little time the next day listening to the TDA1387. First impression is that it is close to the PCM internal DAC, but I need a bit more time for comparison.

FWIW: I ripped to the hard drive using EAC and did playback with Foobar. Both highly recommended!

More to come

Adam
 
I've been out of audio for a long time...nothing more than a Sony CD player and my beloved Sansui AU717 with EPI speakers and after breadboarding this 2902 DAC, I could not believe what I was hearing. (I've been putting my cd's on the hard drive with exact audio copy and listening to the wav files).

And all I did was put some (hefty) 47uf caps on the 2902 outputs straight into the Sansui.

I have a hard time believing it can get better than what I'm hearing because I'm going through my music and actually hearing instruments in songs I'VE NEVER HEARD BEFORE.

I am getting a bit of obvious digital noise though, right before I click on media players "play" button and right after I "stop" the track.....like a 2 second "ticking" sound. (The noise is not there during the song--just before and after)

My PCB is kinda sloppy but I'd like to know....are the op amps primarily used to get rid of this noise I'm hearing. (I know they're used as "buffers" as well.

My only problem is this noise.....if it wasn't there I wouldn't see the (audible) need for ANY opamps.

Damn thing sounds great to me.
 
I doubt that output op amps will help with the clicking noise. I suspect what you are hearing is coming in on the power leads and you need better filtering/decoupling. Are you running the DAC off the USB bus? if so, the noise is probably coming in on the USB power line. Try powering the chip off an independent 3.3V supply. Also decouple the Vcc line really well. I used 100uF electrolytic and 0.01uF film caps in parallel on both input and output of a low noise Linear Tech 3.3V regulator. Dead silent!

Adam
 
My greatest problem is that my DAC is connected to a cheap Aiwa mini HI-FI line so I can't hear it in its full shine :bawling: . But even now the difference is noticable compared to the integrated sound card. My next project is a Gainclone and some ML TL boxes with Monacor full range units. My budget is very tight, but this should be good enough to me. When I finish that, I'll be able to comment more on sound qualities :) . I can hardly wait.

Regarding the PCB, I used a single ground plane. In some datasheets it is recommended to separate the analog and digital ground, but some people claim that this is not a good solution. I will upload my PCB tonight, cause now I'm at work. It is very small and compact. Maybe it has some flaws, but I like it :) .
 
jmar said:
Is this a filter that is intended to filter high freq digital noise because of the mixed signal situation?

Yes.

jmar said:
Low pass refers to passing audio and placing the cutoff above the audio range?

Yes.

jmar said:
If so, at what freq is it usually set?

In the PCM2702 evaluation board the cuttof frequency is ~33kHz.

jmar said:
You mentioned "analog" filter.....so now I'm really confused.

I really meant to say low pass filter, but yes, it's analog, not digital :)
 
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