| 00940 |
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 ? |
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| hifi |
If you do the battry approach, why not make it charge from the 5v (100mA) availbe at the USB port?
/Mikael |
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| 00940 |
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 ? |
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| hifi |
I was thinking more in the lines of maintanace charging the battery
/ Mikael |
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| 00940 |
oh, i see. Yes, it's an idea to explore.
I thought about using the MAX1722; here what's Maxim claims :
| quote: | | 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 ! |
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| Neteagle |
Hi,
Do you have the schematic and board origianle files ( it seems to be made by eagle ...) ?
Thanks
Neteagle |
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| 00940 |
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. |
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| Gyula |
Hi 00940!
I suggest you to remove the ground-loop from the PCB :yikes:!
Gyula |
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| Rookie |
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 |
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| 00940 |
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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| 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. |
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| 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 |
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| 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. |
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| 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 |
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| 00940 |
nice to know those devices are so good :clown:
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 ? |
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| 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 |
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| Rob M |
| quote: | 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? |
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| 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 |
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| 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 :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 :) . |
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| Rookie |
| quote: | Originally posted by jmar
Is this a filter that is intended to filter high freq digital noise because of the mixed signal situation? |
Yes.
| quote: | Originally posted by jmar
Low pass refers to passing audio and placing the cutoff above the audio range? |
Yes.
| quote: | Originally posted by jmar
If so, at what freq is it usually set? |
In the PCM2702 evaluation board the cuttof frequency is ~33kHz.
| quote: | 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 :) |
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| 00940 |
I've done my homework.
Here's a single sided pcb, analog and digital grounds connected. Power supply traces are 40mil, signal is 16 mil. Clearance is 8mil. Grid is 1 inch. I hadn't a correct layout for the inductor, so I used a cap layout. The computer ground is not directly connected to the groundplane but to the chip only.
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| Gyula |
Hi Rookie!
In this way, everything is coupled capacitively to your ground plane, and it's Full of ground loops!
Good luck,
Gyula |
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| Gyula |
Oops!
I have to apologize from Rookie, I wrote the reply above to 00940. |
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| Adam Reed |
Gyula--I would have to say that with this single sided board the capacitive coupling from a trace to the ground plane is next to nil. Capacitance between two plates (trace and ground plane for example) is proportional to the area of the plates in parallel, and traces side by side with a ground plane on the same layer are coupled only by the thickness of the 1 ounce copper, NOT the width of the traces.
We do this all the time on low noise photoamps here at work. it works fine, no ground loops. And it's not really a problem with 2 or 4 sided boards either. Calculate the capacitance with 0.062 FR-4 separating two plates, for example (2 sided standard board). It's pretty small. I have in production 4 layer boards (0.015 inch insulating layers between 1 ounce copper traces, 2 signal layers with a ground plane and power plane) with 10 megohm feedback resistors in current-to-voltage converters where 1 picofarad is a big deal. These boards work fine, and the typical parasitic capacitance is on the order of 1/4 picofarad. Not enough to create much of a ground loop....
cheers
Adam |
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| Rookie |
| Here is my PCB. According to Gyula, it is fool of ground loops too. :) |
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| Adam Reed |
That looks pretty nice, Rookie!
One other comment I forgot to add to the previous post: Ground loops are created when various parts of a circuit have different return paths to ground with significant resistances to those paths. In an extreme case, where a circuit drawing significant current causes a measurable voltage drop across a return trace and shares that ground return with, say, a low level analog signal, real havoc can result--oscillation, noise, etc. As far as the analog circuit is concerned, its ground reference is bouncing up and down as the load varies.
The main reason a fill, or ground plane, is used is so that all the ground return paths are through a large area and thus low impedance conductor, thus insuring that all return paths have a low impedance to ground. And still, if you have a significant current draw (hundreds of milliamps or more), you should have a separate ground plane for the high current circuit only, tied (as I described in the previous post) to the other ground planes where power comes in to the board.
There is a more elegant way to do this--a star ground, where **every** ground path has its own trace back to a single grounding point. I have a picture of a preamp designed by Dan Schmalle (Doc Bottlehead) where every signal or power path has a separate wire, and all the return wires lead to one single point on the chassis. It's definitely a 3-D rats nest and nearly impossible to do properly on a PC board...but technically speaking that is the BEST way to design a circuit. But then again that is why PC boards have ground planes instead...much easier to implement and nearly as effective.
cheers
Adam |
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| 00940 |
| If the first wasn't good, would this be better ? |
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| Gyula |
Hi Adam!
Had wrote that observation because I think the tracks and the ground plane are too close to each other.
The capacitance depends on the Area and the Distance of the conductors at a given permittivity.
Gyula |
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| Gyula |
Hi Adam!
The ground loop is coming from the parasitic impedance and capacitive coupling. These are the two terms of oscillation. This is not coming from the resistive voltage drop as you wrote, the current division is an another phenomeon. Every path has her own LC, and begin to oscillate with each other.
Gyula |
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| 00940 |
| no comment on the new layout ? |
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| Adam Reed |
Been out of town for the weekend, sorry! Ive been off with a group of fellow Porsche 912 owners visiting the Spruce Goose in McMinnville Oregon...
I'll try and look at your design at lunchtime if I have time.
Adam |
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| 00940 |
sorry, what for ? :bigeyes: I just noticed that I forgot to post the schematic :xeye: So stupid..
Anyway, I hope you had fun :) |
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| 00940 |
Here we are, what do you think about it ?
Schematic :
Silkscreen :
Top/pads (+ parts outline) :
Bottom (+ parts outline) :
A few comments on the parts.
Chips on board : max1722, dc-dc booster from Maxim, REG102 (5V and 3.3V), low dropout regulator from TI, PCM2702, USB DAC from TI.
Capacitors : C1, C2 are 10uF tantalum. C3, C9 are Pana FC, 1000uF/6.3V. C6, C12 are Pana FC 100uF/6.3V. C15, C16 are ceramic, 32pF. All the other caps are wima mks-02 100nF. 220nF would fit.
Grid is 1cm. The board is about 3cm/6cm (1.2"/2.4"). |
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| Adam Reed |
Hi again
Took a look at your setup and compared it to the TI eval kit. Interesting, they don't even regulate the voltage input and get their 3.3V with 2 diode drops! pretty crude.....
One thing I noticed on your schematic--you have no pull downs on the audio out. The risk here is that you will get a POP when you plug in the DAC to your preamp and it discharges the output caps. It does not take much load to fix this--I used 330K to ground on each output on my prototype.
I like your power supply scheme. I have several products where I have done just what you propose--a small switcher followed by a linear for good noise performance. Should work fine. Thorsten Loesch has suggested that the TI TL431 is one of the better sounding regulators, might be an alternative to try although I do not know how much difference it will make overall.
On the layout, I would strongly suggest using surface mount components everywhere you can. Especially for all decoupling caps--0805 ceramic caps perform very well and can be put right next to the IC for the best possible decoupling of high frequency noise. Makes the board smaller too!
Nice work!
cheers
Adam |
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| 00940 |
| quote: | Originally posted by aj912
Took a look at your setup and compared it to the TI eval kit. Interesting, they don't even regulate the voltage input and get their 3.3V with 2 diode drops! pretty crude..... |
Indeed, their version is quite stripped down.
| quote: | | One thing I noticed on your schematic--you have no pull downs on the audio out. The risk here is that you will get a POP when you plug in the DAC to your preamp and it discharges the output caps. It does not take much load to fix this--I used 330K to ground on each output on my prototype. |
True. Well, I could mount the resistors right on the RCA jacks.
Thanks for the comments :) |
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| wwwdcn |
I want to make my own usb dac,but I don't well up to Protel
can you sent the Protel file to my email box
my email "www.d.cn@163.com"
thank you |
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| tifosi_f1 |
Hi, I'm going to build one of these pcm2902-based dacs after reading other people's experiences with it.
I was wondering, does anyone know of software/drivers available that can be used (along with, say, 3 dacs plugged into separate usb ports) for surround sound - e.g route bass/centre, rears and fronts to each dac.
If this could be done it would be nice for movies, games etc (when not listening to stereo audio) :D .....
Thanks, Pete. |
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| iampivot |
| It is at least possible with eg Xine under Linux with ALSA drivers. Xine can decode AC3 in software and route to different USB soundcard/DAC drivers. |
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| guzzler |
I've sort of taken over development of this board from 00940. Here's a 3D render of the the board I'm going to send off as a beta sometime next week:
I've included pull down resistors (the 0805s near the output), replaced the resistor for the oscillator with an 0805. Also, there's a pad to bypass the switcher or the 5V regulator. All the red caps are WIMA MKS-02 100nF for size reference. The output caps can be Black Gates
If all is well with this board, I'll put a smallish order in for other people
g |
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| tifosi_f1 |
Thanks iampivot, I dont use Linux too much but I'll keep that in mind for Xine. :)
I'm still searching for some kind of way to do this in windows (like a 'generic' surround sound driver where the individual outputs can be mapped to different devices) |
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| Wizard |
| Where is the circuit? |
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| tifosi_f1 |
| quote: | Originally posted by Wizard
Where is the circuit? |
Try the previous page? There is a diagram there, or did you mean something else? |
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| Wizard |
| Yes ,I know,but the photograph can't display...... |
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| iampivot |
| quote: | Originally posted by tifosi_f1
Thanks iampivot, I dont use Linux too much but I'll keep that in mind for Xine. :)
I'm still searching for some kind of way to do this in windows (like a 'generic' surround sound driver where the individual outputs can be mapped to different devices) |
It's possible using an AC3 DirectSound filter. See posting on AVSForum or sourceforge for more information. |
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| ITZBITZ |
FYI: I'm handling US distribution of this kit. Guzzler has sent me a good portion of the boards and related components. Estimated cost is $30, but we won't know for sure until I get it all together. The ICs are not included (PCM2702E, voltage regulators, and the Maxim DC-DC convertor) but can be obtained on their own.
There is a web site for the project as well:
USB DAC Project Site
E-mail or PM me if you are interested in one of the kits. |
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| Pierre |
Hello.
I would like to know some software aspects of the USB DAC based on TI PCM2702:
Does it run under Win95 OSR2.1 or Win98?
For example, once installed under Windows 2K, does it appear as a soundcard in the device manager?
If so, what controls do you have available? Volume and mute, perhaps? Does it implement balance?
Another question: how were the RMAA results shown in that page obtained? Using the USB DAC as output and selecting another soundcard line in as input?
Thanks! |
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| Nisbeth |
Pierre, there's a long thread on head-fi.org describing the usb-dac. I think that thread answers your questions :)
/U. |
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