I just finished my first DAC build, the reference design from the TI PCM2902 data sheet. I had some difficulties on the first PCB version; seems the oscillator being under the chip was a bad idea. Rev 2 is up and working with a REG103ua-a regulator outputting 4V through a diode to Vccci.
I noticed a loss in volume and what appeared to be a loss in dynamic range with my second version which included an external (to the chip) LPF with metal oxide 1k Dale Vishay resistors and Kemet 4n7F Polyester caps. This combo puts the -3db cutoff at 33khz or so. Removing them seems to have helped some, but I may just be hearing a difference in gain from the headphone jack splitter I'm used to. Does the -30deg phase shift at 10khz possibly have an effect?
Anyway, since the PCM2902 includes antialiasing filters and an analog LPF filter, are there any advantages to having an external one on the card? An external LPF is shown on the datasheet schematic but it explicitly states one is included on the chip.
Thanks.
I noticed a loss in volume and what appeared to be a loss in dynamic range with my second version which included an external (to the chip) LPF with metal oxide 1k Dale Vishay resistors and Kemet 4n7F Polyester caps. This combo puts the -3db cutoff at 33khz or so. Removing them seems to have helped some, but I may just be hearing a difference in gain from the headphone jack splitter I'm used to. Does the -30deg phase shift at 10khz possibly have an effect?
Anyway, since the PCM2902 includes antialiasing filters and an analog LPF filter, are there any advantages to having an external one on the card? An external LPF is shown on the datasheet schematic but it explicitly states one is included on the chip.
Thanks.
Still getting enumeration errors in about 1/10 attempts, but for the most part it's working.
Performance via USB->DAC->TOSLINK->AMP is phenomenal. With the same source it sounds better than the boxee box TOSLINK which is our main AV playback source, possibly because the gain is higher on the USB DAC. Analog performance is well, so-so. It doesn't sound bad, but the mids and highs are kind of shadowed under the lower frequencies. The dynamic range of the piece sounds a little compressed too. I was hoping with the $7 TI onboard voltage regulator, this little chip would put out a big sound. That does not appear to be true.
I do have another DAC with the PCM2902(b) chip... a USB phono plus from ART. This one has a volume knob on the analog out which allows me to bring the gain up to that of the digital out. The dynamic range sounds more normal with the higher input gain to the amp, but the highs still aren't there.
It was a good first attempt though. I had big plans to make a car version for phone playback, but obviously i need to learn more first. Perhaps I need a quality op-amp (and +/- boost regulator) on the line out to boost gain? Should I put a scope on the output to find out what Vout RMS I'm getting?
Reading the other posts in this sub forum, you are all way ahead of me on much more complicated projects, but any help you can throw a ME trying his hand at electronics would be appreciated!
Performance via USB->DAC->TOSLINK->AMP is phenomenal. With the same source it sounds better than the boxee box TOSLINK which is our main AV playback source, possibly because the gain is higher on the USB DAC. Analog performance is well, so-so. It doesn't sound bad, but the mids and highs are kind of shadowed under the lower frequencies. The dynamic range of the piece sounds a little compressed too. I was hoping with the $7 TI onboard voltage regulator, this little chip would put out a big sound. That does not appear to be true.
I do have another DAC with the PCM2902(b) chip... a USB phono plus from ART. This one has a volume knob on the analog out which allows me to bring the gain up to that of the digital out. The dynamic range sounds more normal with the higher input gain to the amp, but the highs still aren't there.
It was a good first attempt though. I had big plans to make a car version for phone playback, but obviously i need to learn more first. Perhaps I need a quality op-amp (and +/- boost regulator) on the line out to boost gain? Should I put a scope on the output to find out what Vout RMS I'm getting?
Reading the other posts in this sub forum, you are all way ahead of me on much more complicated projects, but any help you can throw a ME trying his hand at electronics would be appreciated!
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