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tda1387 dac pcb "front end"
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Old 22nd March 2018, 03:30 AM   #121
stellarelephant is offline stellarelephant  United States
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Matt, it sounds like you do not have the noise issue. Mine is very noticeable, especially through headphones. My mystery fix makes it completely silent though!
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Old 22nd March 2018, 05:12 AM   #122
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by matt_garman View Post
Sounds like in general they are a good thing to have. What would the optimal R values be for them?
I'm not sure, I normally use 470R. You could go higher but that increases the rise time at the DAC's input pins, not normally a good outcome as it tends to increase jitter. However I've not yet encountered a jitter problem in my listening (by other means than series resistors, e.g. using disrespected USB->I2S interfaces) so perhaps attention to jitter is overrated?
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Old 22nd March 2018, 11:50 AM   #123
stellarelephant is offline stellarelephant  United States
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This post got left on the previous page, so not sure if it was seen. The picture doesn't show much detail, but maybe the grounding tests are of interest. I want to tinker more, but wifey says I need to get our taxes done pronto!
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Old 22nd March 2018, 02:40 PM   #124
randytsuch is offline randytsuch
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Any idea how much current the pi hat dac version uses?
I'm looking at my PS options, trying to get it fired up soon, but I also have taxes to work on

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Old 23rd March 2018, 01:36 AM   #125
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
Hi Randy,

A quick guess is it should be under 50mA. The tda1387 datasheet says the max current at 5v is 6.5mA. The other active components are the tl431 shut regulator, two bc560 common base transistors, and the power-on LED. Maybe I can try to take a reading this weekend...
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Old 23rd March 2018, 02:42 AM   #126
stellarelephant is offline stellarelephant  United States
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Curiouser and curiouser...

I no longer think the Pi DAC noise is a grounding issue. I don't know what I'm talking about, but I believe it could be EMI. Here's why...

I can affect the noise without physically touching a thing. Matt, if you have a headphone amp, you can put on your headphones and try this yourself (for science!) by just cranking the volume with no music on. Now move your hands around the DAC as if you are playing the theremin. I find the area near the big output caps to be particularly sensitive. Once my hands are less than an inch away, the buzz increases significantly with proximity. Wild.

I'm happy to say that I have tracked down my mystery noise cure. Recall that I accidentally attained complete elimination of noise when I connected the Pi DAC into the same switch box as my Mac's Firewire DAC via RCA plugs. Early on, I tried to see if I could just ground something to get the same results. Nope. I find that the noise actually increases when I run a wire from any part of the Pi stack (or the barrel of the iPower, for that matter) directly to earth. I tried earthing the audio ground wire too. No luck. Bizarrely, touching these same metal areas with my fingers reduces noise, a little.

Eventually I set about trying to narrow down how the noise was disappearing when I connected my Mac's Firewire DAC to the switch box with the Pi DAC. I guess the effect has to do with leakage currents getting absorbed somewhere in my Mac (yikes!) or more likely its PSU. I unplugged the Firewire cable (which supplies the power) from the DAC, and the noise came back. Finally, I realized that I could bypass the Firewire DAC entirely. By leaving the Firewire cable connected only to my Mac, and simply touching the steel outer barrel of the Firewire cable connector to the audio ground wire (center screw terminal on the Pi DAC board) I can get sweet, sweet silence! Weird, especially since the Googling Firewire pinouts suggests that the outer barrel has no function. I guess it must be grounded, though, right?

How in the world does this work? And does anybody have any suggestions for eliminating noise that does not necessitate using my Mac as a dedicated noise scrubber?
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Old 23rd March 2018, 02:52 AM   #127
abraxalito is offline abraxalito  United Kingdom
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Yeah EMI could indeed be it, very hard to track down though. I had a problem when I connected an early prototype of my lingDAC to an SD card player. Its using a much, much slower CPU than a RPi (100MHz or so I think) but I could not place my DAC within the same enclosure without getting nasty clicking noises on the audio. In the end I mounted the lid of the box on 20mm long spacers creating a 'power bulge' and taped my DAC to the underside of the lid. The extra distance was enough to render the clicks inaudible.
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Old 23rd March 2018, 02:42 PM   #128
randytsuch is offline randytsuch
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Quote:
Originally Posted by matt_garman View Post
Hi Randy,

A quick guess is it should be under 50mA. The tda1387 datasheet says the max current at 5v is 6.5mA. The other active components are the tl431 shut regulator, two bc560 common base transistors, and the power-on LED. Maybe I can try to take a reading this weekend...
Thanks, don't worry about measuring, it should be fine.

I'm going to start with a linear walwart I have in my junk drawer, puts out around 7.5v at 2a. Will just use a 7805, or a lm317(if I'm more motivated lol) to generate 5v. Probably also add a couple lifepo4 batteries.

FYI, lifepo4 batteries are pretty easy to implement, relatively cheap, and do a great job, especially for digital circuits, imho. They can be charged with a constant voltage, unlike normal lithiums which need a special charging circuit.

So, for 5V, I take two in series, discharge them with a power resistor until their voltage is a little less than 5V, then connect to regulated 5v, and you have a high quality 5v ps. If you leave power on all the time, you don't have to worry about switching. I plan to add a two pole switch or relay, and only connect battery when dac is on. Keeps battery from getting discharged when dac is off. If no switch, you do have to be REALLY careful when you connect, because it will be hot.

Deciding if I will make a 5v ps for the dac, and another one for the allo. I know this should be done to best sound. For this project, it would just mean two 7805's, two pairs of batteries, and two switches or relays.

I'll power the pi from a different 5v ps, just a little usb charger for now.
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Old 23rd March 2018, 08:55 PM   #129
stellarelephant is offline stellarelephant  United States
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Randy, your battery power scheme sounds rather ideal. I was thinking about trying battery power as well, yet now I don't think my problems are PSU related any more. Many use the iPower with various DAC hats and have reported very low noise. I have to assume that Allo and other commercial manufacturers have some tricks for isolating their products from the noise and EMI of the Pi itself. I don't know how much is coming through the GPIO versus just being radiated upward as EMI. But I guess a long GPIO ribbon cable and a wide enclosure might help the latter, at least. I have worked with muMetal foil before too (all the power bricks for my Mac and peripherals are stuffed in a grounded steel ammo box lined with the stuff!) and the results were good. But man, there must be a much easier (and cheaper) way!
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Old 23rd March 2018, 09:20 PM   #130
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
stellarelephant - I have a few more random thoughts; probably low likelihood of success, but fairly easy to try:
  • A different AC outlet, or preferably a different AC outlet in another room. The goal is to get on a different circuit. How well do you know the wiring in your house? Some homes have strange wiring, e.g. a bedroom circuit is shared with the kitchen, and you get the refrigerator compressor noise. I know we think it's probably not power supply issue, but if it's not to hard to move your stuff around (or maybe run a long extension cord), it's an easy test.
  • Have you noticed if time of day makes any difference? If you can say it always goes away at such and such time or a certain day, that might be a clue.
  • Can you try at a friend's house, preferably one that is a good distance from your own? If indeed it is EMI, is it from the RPI itself, something in your house, or something near your house? If you can fire it up at another house without the noise, then we can say there's something in your environment.
  • If the noise is coming from the RPI itself, you might try some of the hacks I mentioned above, or give DietPi a try. I.e., reduce the amount of power the RPI is drawing. In the next few days, I'll try to do the opposite on mine (deliberately turn off those power-saving features and artificially load the CPU and IO on the RPI); maybe I can induce my RPI to create the noise you're seeing.

One more question: are you using wired or wireless networking on your RPI? Looking at that pic you posted, it looks like the wired network port is empty on your RPI... if you're using wireless, do you have the ability to test with wired networking? I've never enabled the WiFi radio on my PIs, that's something else I could try when I have time.
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