Poor quality audio from Phat DAC


#1

I am having issues with the audio quality of the pHAT DAC. I have it hooked and working with shairport-sync to an Xtrememac Tango TRX speaker. The vocals sound really thin and faint while the instrumentation sounds reasonable.

If I hook the tango up to my MacBook Pro with the same cable everything sounds very good. I’m not sure where to begin with troubleshooting this.

I have shairport-sync running on a regular raspberry pi 3 connected to the TRX and it sounds fine. Not super high quality, but everything sounds normal, no problems with vocals or anything and perfectly suitable for some background music.

I was hoping to get some better quality out of the pHAT DAC, but something is definitely not right somewhere. Any ideas where to begin on troubleshooting this.

I’m using the latest raspbian with the auto installer.


#2

After playing around with this some more it sounds more like the top end and bottom end is cut out. If I play titanium through it the thumping bass is missing and the vocals sound kind of faint and distant, but the mids of the instrumentation and vocals in another part of the song sound pretty good.

Anyone know what might chop off the highs and lows? Is there some kind of config setting I’m missing somewhere? I haven’t really done much with the config other than install shairport sync and run the auto installer.

I have another Pi with the built in audio working fine on the same peripherals so I know it’s either something with the board itself or the software setup.


#3

Reloaded the card with Jessie lite and I get a constant hum or buzz. I’m not sure where to go from here. It seems this card might be bad.


#4

Played around some more with this and it turns out that just plugging in the power adapter will instantly start the hum/buzz over the speakers. After realizing that it has nothing to do with software I disconnected the phat dac and just touching the header pins with it unplugged causes the hum/buzz. It seems it’s acting almost like an antenna and sending the noise straight through the audio jack.

When it’s hooked up to the zero, touching the metal of the usb jack will start the buzz.

Is this normal? I don’t see how anyone could use this board like this. There doesn’t seem to be any way to keep it from being noisy?


#5

The problem turned out to be no ground on the 3.5mm jack. I rigged a wire between one of the RCA jack ground points and the 3.5mm ground pin on the side of the jack facing the RCA location and it’s working nicely now. No more hum/buzz and no more odd sounding music.

Also, for shairport-sync it helps to uncomment volume_range_db = 60. I also set the output format to S24.

Sounds really good on my second hand desktop speaker.


#6

Ended up buying two more Phat DAC boards from digi-key. These boards have solder pads on the 3.5mm jack that look a little different than the first board I bought, and the grounds check out fine when tested with a meter. So either I had an earlier revision that didn’t have a ground or it was just a bad board. Either way seems to be fine on the newer ones I bought.