Pcm2702 Usb-dac

[jmar]

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.

[Adam Reed]

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

[jmar]

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.

[Adam Reed]

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

[00940]

nice to know those devices are so good

Just one question to those who build their own PCB : how do you deal with grounding ? I mean, how and where do you connect analog and digital ground ?

[Adam Reed]

I built mine on a piece of perf board with groundplane on one side and plated through holes on 0.100" spacing. I did not try to separate analog and digital grounds, though if I did a PC board I would do separate planes (even on a 2 sided design) and tie them together at the power input with a ferrite.

cheers
Adam

[Rob M]

Originally posted by aj912
I did not try to separate analog and digital grounds

Did you do anything to isolate things from the USB ground?

[Adam Reed]

**Did you do anything to isolate things from the USB ground?**

Nope, all grounds went directly to the single ground plane. I do this with low-noise photo amp prototypes at work all the time and it seems to work quite well as long as you have a continuous ground plane...

cheers
Adam

[Rookie]

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 . 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 .

[Rookie]

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

Yes.

Originally posted by jmar
Low pass refers to passing audio and placing the cutoff above the audio range?

Yes.

Originally posted by jmar
If so, at what freq is it usually set?

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

Originally posted by jmar
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