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tda1387 dac pcb "front end"
tda1387 dac pcb "front end"
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Old 18th March 2018, 12:44 PM   #111
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
Hey, I used to have one that looked just liked that!

More good news: I have my balanced RPI HAT DACs working now. Somehow the pin designations for the TL431 got messed up between schematic and board layout. It's a lousy pic, but eagle-eyed viewers will see the TL431s (through-hole TO-92 devices) are now "rotated" with respect to the footprint silkscreen.

Now the only thing that doesn't work is the power-on indicator LED! Not sure why, that's the simplest part of the circuit.
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Old 21st March 2018, 12:59 AM   #112
stellarelephant is offline stellarelephant  United States
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I'm in love with this thing. I have set up an A/B switch box but I hardly need it to hear the difference. The delicacy is addictive...imaging is insane. Its an incredible match for my Fostex planars but the real magic happens with speakers...I finally got a taste of that "holographic" thing that the 1543 lovers rave about. Real listening impressions to come.
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Old 21st March 2018, 02:54 AM   #113
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
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Originally Posted by stellarelephant View Post
I'm in love with this thing. I have set up an A/B switch box but I hardly need it to hear the difference. The delicacy is addictive...imaging is insane. Its an incredible match for my Fostex planars but the real magic happens with speakers...I finally got a taste of that "holographic" thing that the 1543 lovers rave about. Real listening impressions to come.
Great news! I'm thrilled you like it!

The radio silence had me worried that maybe my "baby bias" had finally gone off the deep end. (And I fully acknowledge that I suffer baby bias; it's a huge part of the fun and appeal of DIY for me. But I'd like to think I retain a semblance of objectivity!)

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Old 21st March 2018, 05:15 PM   #114
stellarelephant is offline stellarelephant  United States
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Nah, it's legitimately killer sound.

I prefer it to my AKM and both of my Cirrus DACs, and the Dragonfly Black (Sabre) I had for a bit. I can see why people compare the sound of these chips to analog. I am especially thrilled at how this DAC allows me to enjoy music without the usual listening fatigue.

Only caveat is that I am still trying to troubleshoot a substantial level of background noise from this Pi hat. I did not expect to have any noise problems at all, with the super clean iPower PSU. The hum/switching noise is almost certainly the fault of the Pi, and not the DAC, because when I crank the volume with music paused, I can actually hear the noise fluctuate in sync with the flashing red LED on the Pi board! The iPower is not grounded, but I found somebody recommend grounding the barrel of the iPower to reduce noise. I have yet to try this. But unexpectedly, I solved the issue last night (on accident) by connecting the new DAC to my A/B switch box. I had the same noise at first, with just the Pi stack connected. But as soon as I connected my Mac's Firewire DAC to the switch box, the Pi noise dropped to zilch!!! This gives me hope that it can be fixed pretty easily. Both DACs are hooked up via RCA. Matt, you have the same exact setup as me (iPower->Pi->Kali->1387). Do you have any noise issues or a recommendation about this? Before my accidental remedy, the noise got worse when I touched the audio connector cable, and better when I touched any metal part on the Pi.

Last edited by stellarelephant; 21st March 2018 at 05:23 PM.
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Old 21st March 2018, 07:14 PM   #115
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
stellarelephant - My gut instinct to what you describe is a ground loop or a similar grounding problem. That the A/B box fixes it suggests to me that the box is breaking the ground loop or providing the "missing" piece of the grounding scheme.

Yes, I am using the same setup. In fact I have two setups, my main one with the Kali, and the basement setup for testing and burn-in (which does not have a Kali). In both cases I'm using the iFi iPower PSU.

I don't have any noise, but I'm also using software volume control. On my main system, I have to keep the volume very low due to small room, semi-sensitive speakers, and an over-powered amp. However, if there is noise, I think that by definition, software-based digital volume control should keep the noise proportional to the signal. Meaning, if there was noise I think I should be able to here it, even at my low volume setting. On the basement system, I've cranked it to roughly 3/4 max volume, and still nothing like what you describe (there I can't go louder because the speakers can't handle it).

What are you using for volume control?

Do your preamp, amp and other downstream components have a safety earth connection on the mains plug (i.e. 3-pin plug)?

Lastly, I'd be surprised if this made a big difference, but I'm using DietPi as my RPi distro. It has a setting "PSU Noise Reduction", which sets the CPU governer to Powersave and disables HDMI output. Though I don't do this, I've heard of people disabling the status LEDs on their RPI to further reduce power consumption (and implicitly help with PSU noise). Even better than using the powersave governer is to set the CPU to a fixed frequency (the lowest frequency you can get away with). This would also present the power supply with a more fixed load. But still my gut says all this should be "fine tuning" rather than the core of the problem, which I still suspect is a grounding issue.

For what it's worth: one of my highly modded x8 DACs has a ton of noise. For a long time, I didn't know, because I was using a stepped attenuator between it and the amp for volume control. This was a nearfield setup, so volume was again super low. I never heard any noise because the volume was so low (analog volume control, which should lower noise along with the signal). I never took the time to debug it, though.

Do you have a way to provide the RPI stack with 5v power via battery? That breaks the connection with your mains on the source (RPI) end. 5v is an uncommon battery voltage, but you could whip something up quickly with a couple 18650 cells or a car battery and a heatsinked LDO. E.g., I have a couple adjustable LT1083-based regulators like this that come in handy from time to time. You can use a transformer + AC to get a target output voltage, or just use a battery in place of AC input and adjust down to what you need.
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Old 21st March 2018, 09:11 PM   #116
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
Another thing that might be relevant, and hoping Abraxalito can jump in and offer some thoughts: if you look at the DAC PCB, you'll see I have resistors in series with DAC pins 1, 2 and 3, which are the I2S input pins. I've seen other people put those resistors in place, but not always. I kinda-sorta have a vague notion of their function, but not enough to set their value appropriately. So those are actually zero-ohm resistors currently. I was hoping eventually someone would come along and improve my understanding of those resistors, and enable me to set them to a more appropriate value (or omit them entirely).
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Old 21st March 2018, 11:43 PM   #117
stellarelephant is offline stellarelephant  United States
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So far I have fed the hat DAC into my battery-powered DIY Pocket Class A headphone amp (it is silent with every other source I have tried) as well as two different Tripath amps with pots in front of them, each powered by a two prong (floating ground) SMPS. These are also normally quiet. I got the noise with all three scenarios. Have you tried pausing playback and then putting software volume to max, to listen for noise? I might have a crude way of testing the Pi battery power with some wimpy boost converters I have lying around, if I get creative...but they aren't totally silent either to my knowledge.
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Old 22nd March 2018, 01:52 AM   #118
abraxalito is offline abraxalito  United Kingdom
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@matt - the resistors on the I2S do a couple of things. They work to reduce ringing on the wires (they're a kind of 'series termination') and they low pass filter any noise on them so hopefully the noise is less likely to affect the DAC.
On my schematics with the 74HC86 before the DAC they have a third function - current limiting any voltage swings above the HC86's supply voltage (2.5V).

@se - I agree with matt that your noise problems are grounding related. Without a full picture of what's connected to your RPi I wouldn't be able to diagnose further. I'd guess when you inserted the Mac Firewire DAC that (or the switch box) provided a path to earth for the leakage currents. The give-away is when you touch a 0V connected part, the noise reduces - in that situation you're providing a not-very-effective earth connection via your body.
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Old 22nd March 2018, 03:18 AM   #119
matt_garman is offline matt_garman  United States
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tda1387 dac pcb "front end"
Quote:
Originally Posted by stellarelephant View Post
Have you tried pausing playback and then putting software volume to max, to listen for noise?
I did that just now - only the slightest of hissing if I put my ear right up to the tweeter. I also did another test: I picked a song I know that starts out extremely quietly (Ravel Bolero), and then I could safely crank the volume all the way up with music playing. Again, a bit of hiss with my ear close to the speaker, but nothing like you describe (and that could very well be tape hiss from the recording itself).


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Originally Posted by abraxalito View Post
@matt - the resistors on the I2S do a couple of things. They work to reduce ringing on the wires (they're a kind of 'series termination') and they low pass filter any noise on them so hopefully the noise is less likely to affect the DAC.
Sounds like in general they are a good thing to have. What would the optimal R values be for them?
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Old 22nd March 2018, 03:22 AM   #120
stellarelephant is offline stellarelephant  United States
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The give-away is when you touch a 0V connected part, the noise reduces - in that situation you're providing a not-very-effective earth connection via your body.
This was my initial thought as well. Curiously, a little experiment I tried seems to contradict this. I have an anti-static wrist strap that plugs into an electrical outlet ground hole. When I touch it to the same metal parts (the Pi's steel Ethernet jack housing, for example), the noise gets louder!!! My finger still makes it quieter, so long as I am not grounded.

This pic shows my cheap Radio Shack switch box. I have no idea what the circuit inside is like...could all the RCA barrels share a ground? I need more time to experiment but I can confirm that all noise disappears completely when the second DAC gets connected. When I took the pic, I was running through the Pocket Class A as a preamp, but the noise is the same when going direct.
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