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
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