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Old 5th December 2007, 03:15 PM   #1
sangram is offline sangram  India
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Default Planning first DAC build, help please.

Hi,

Planning to start building a small USB-DAC on PCM2702 (driven by combination of simple circuitry, good specs and re-clocking possibilities) for my office laptop, to be combined with a compact power amp, maybe LM4766. Have some noobish questions, kinda long. Thanks for looking and your help.

Need to know:

1. Can I use universal SMD proto boards to construct the DAC? I'm referring to: http://protoboards.theshoppe.com/pcb_index.html These have groundplanes on one side which will hopefully help with decoupling, and are flexible enough to accommodate the DC supply regulation and the output filter. Any drawbacks of such a method? If it is a huge problem I could copy the Eval board layout, but then it'll depend on the shop to deliver quality, I'm not sure I want to take that risk.

2. How important is output-filtering? I was wondering if a passive RC filter is enough. The application is small desktop speakers so quality is probably not as much an issue as size and flexibility. I can also look at a R+C combination in the power amp feedback network, if that approach is feasible to implement post filtering.

3. What should the cutoff frequency for the post filter be? Is 40Khz a decent enough -3dB point, or do I need to go higher? Does this also depend on whether my filter is passive or active (higher order, high frequency and low order, low frequency)? I remember there was a link to some app notes on post filters somewhere on this board, can't find where that is.

4. Power supply: I will probably be sharing the power transformer of the amp with the DAC. The other option is to use bus power, which to me seems a worse option as it's coming off the laptop's USB. I want to use Alu tubing to house everything (including PS) so a second transformer is kind of out of the question. Is this a huge problem? I was planning on 27 volt rails for the amp, and regulate it using resistors and zeners (which I suppose will work better than 78xx, which is my other choice) plus some HF decoupling. If there are nice low-noise options in monolithic packages I would love to hear suggestions. The DAC requires 5V analog and 3.3V digital supplies (3.3V can be supplied from the 5V rail with two diode drops, in the eval board that's the way it's connected).

5. Formats and data streams: will my DAC support all formats supported by software players, or will I need decoders (FLAC, APE, Vorbis and MP3). Basically, does the USB audio driver stream PCM streams to audio devices, or native data? Also, does the USB driver support multiple sample rates or does it resample to 48KHz like the Dsound drivers/kmixer? The PCM2702 supports 32, 44.1 and 48 KHz. Will the USB driver detect and stream 44.1 data to the DAC, or should I resample to 48KHz using ASRC?

6. Tools: I also have very little test equipment. I have a PC with a soundcard which I can use for basic analog measurements to about 20Khz. Is it worthwhile to attempt this with only a soldering iron and a multimeter? I could buy a little controller for 10 dollars (the sme price as only the TI chip) and hook up the analog output to a small amp. I do have a little SMD soldering experience, and was hoping this would be a good first SMD project while my eyes can still handle it.

TI for all your help!

Sangram
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Old 5th December 2007, 04:47 PM   #2
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1. These boards will do just fine.

2.3.4. Are probably trivial and covered by the datasheet. This chip will at best provide good mid-fi sound so getting overboard with PS and filtering is pointless. I would even go for bus power.

5. Bypassing the k-mixer with ASIO is a good idea in any case. There will be no resampling by Windows unless you choose the software player to do it. All file formats are completely transparent to the dac as long as the player supports them and they don't exceed 44/16.

6. You'll be just fine with a voltmeter provided the circuit is built without errors. Having a scope eliminates some head scratching but is not a must.
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Old 5th December 2007, 06:07 PM   #3
00940 is offline 00940  Belgium
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If you want it easy, there's always that option, it works : http://glassjaraudio.com/category.sc?categoryId=4

Otherwise:

2-3. not very important. You already have an analog lpf onboard, its curve can be found in the datasheet under the title "analog filter performance".

4. Take care to somehow keep seperate the analog and digital supplies. Use ferrites on each line before the final decoupling. That will make more difference than using a zener rather than a lm7805.

5. The connection in between the pc and the usb dac doesn't know the difference in between flac or wav. Just keep the bitrate under 48khz. Have a look at ASIO4ALL too.
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Old 6th December 2007, 05:47 AM   #4
sangram is offline sangram  India
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Thanks!

I had no idea anyone was building a kit for this that chip, maybe that is the way to really go.

The datasheet showed a separate analog filter after the onboard LPF, so was wondering how necessary it was.

Great to know that the DAC will support all formats. I have used ASIO4All before so hopefully I can get it working here. I don't use kmixer as all my pro cards use ASIO natively, and my SBLive is hacked up using Kxdrivers, hence my apprehension.

Thanks again, and maybe the kit is really the way to go. I'll have a look.
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Old 23rd January 2008, 05:07 AM   #5
sangram is offline sangram  India
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Thanks for all the help. I ordered a bunch of Aliens and protoboards would have been a nightmare. It took me a few hours to get everything soldered in and sorted out. Man, can those SMD resistors fly!

The PCM2702 is smoother and gives up a bit of detail to the TDA1334 and the CS4398, but is leagues ahead of onboard sound of any nature. The first version I built last night uses bus power (the 'regulated USB' configuration on the website) and I'm planning on a stripped down version for the second build, to fit my all-in-one plans.

The Alien boards will accept external supplies and can be tweaked towards enhanced or basic versions far as I can see, and Jeff's service is top-notch. I got my kits in ten days from the US, all parts intact and well-labeled.

You were right about not needing to bypass kmixer. The USB audio generic driver does not support kernel streaming, either through ASIO4all or Foobar's built-in KS plugin. It natively supports 44.1 KHz though, so I'm a happy camper.

Pretty kicked about having built something so small by myself.
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