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My first DAC on PCM1794A - help needed.
My first DAC on PCM1794A - help needed.
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Old 18th April 2018, 04:04 AM   #21
tjaekel is offline tjaekel  United States
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Thumbs up SCK and BCK, audio caps

Very good points, thank you. And all correct you have mentioned!

1) SCK and BCK:
Yes, true, both are connected to each other (RPi does not provide SCK, just a three signal I2S). But IT WORKS! For sure: I tried, and also these guys did: DDDAC 1794 NOS DAC - Non Oversampling DAC with PCM1794 - no digital filter - modular design DIY DAC for high resolution audio 192/24 192kHz 24bit
What really happens inside - no idea, sorry. Potentially (based on datasheet) it is a bit wrong (and 'against' datasheet):
with 48 KHz and 24bit - and RPi will play as 32bit (per channel, use RPiDAC driver) - we have just 64 fs (48 KHz, 32bit stereo results in 3.072 MHz SCK instead of 6.144 MHz for 128 fs). If it results in 1/2 effective fs (48 KHz is played like 24 KHz) or if the DAC would do "averaging" (or "dithering") between two consecutive PCM words, the filter characteristics do not match with datasheet ... no idea, sorry. It works, for sure, it sounds great and is the only way to use in a simple way on a Raspberry Pi (otherwise use a FPGA/CPLD and generate SCK from BCK, so you could add also a PLL ;-) ).

2) audio caps:
Sorry, I am not an expert. There is a big discussion which caps to use in audio signal path. I am potentially wrong, sorry. The message I tried to provide: check a bit which caps (which dielectricum) when fighting for best audio.
Thank you for the correction, I appreciate.
(I go often with high voltage ratings on such caps, but true: some types might be worse due to voltage dependencies, I agree. And my thought: SMD caps are even better as large through hole MKT: less parasitic inductance).
Anyway a valid point: check which type of caps are best for audio signal path.

Thank you for correction and pointing out.

Last edited by tjaekel; 18th April 2018 at 04:07 AM.
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Old 18th April 2018, 12:21 PM   #22
Chulia is offline Chulia  Lithuania
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You shouldn't have posted that link ((

I need to finish my current project before starting a new one :]

And thanks so much for the NP0 recommnedation - I would have definitely missed it and went the wrong way. Just ordered these.
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Old 18th April 2018, 05:24 PM   #23
00940 is online now 00940  Belgium
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Quote:
Originally Posted by tjaekel View Post
1) SCK and BCK:
Yes, true, both are connected to each other (RPi does not provide SCK, just a three signal I2S). But IT WORKS! For sure: I tried, and also these guys did: DDDAC 1794 NOS DAC - Non Oversampling DAC with PCM1794 - no digital filter - modular design DIY DAC for high resolution audio 192/24 192kHz 24bit
What really happens inside - no idea, sorry. Potentially (based on datasheet) it is a bit wrong (and 'against' datasheet):
with 48 KHz and 24bit - and RPi will play as 32bit (per channel, use RPiDAC driver) - we have just 64 fs (48 KHz, 32bit stereo results in 3.072 MHz SCK instead of 6.144 MHz for 128 fs). If it results in 1/2 effective fs (48 KHz is played like 24 KHz) or if the DAC would do "averaging" (or "dithering") between two consecutive PCM words, the filter characteristics do not match with datasheet ... no idea, sorry. It works, for sure, it sounds great and is the only way to use in a simple way on a Raspberry Pi (otherwise use a FPGA/CPLD and generate SCK from BCK, so you could add also a PLL ;-) ).
Or simply drop in a src4192/ad1896 with a local clock. That would also clean up the pretty bad clock from the Pi. A resampler has also the advantage of defining once for all the operation of the DAC and thus of the analog filter.

The DDAC is a bit of a different beast as it doesn't use the digital filter section of the pcm1794. I'd rather not comment further on it as I think it does almost everything wrong.
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Old 18th April 2018, 08:12 PM   #24
Chulia is offline Chulia  Lithuania
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Could you expand more on the src4192/ad1896 solution? Maybe some example circuits would be perfect, didn't managed to really grasp how it would work just from datasheets.
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Old 18th April 2018, 09:33 PM   #25
00940 is online now 00940  Belgium
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http://www.audioxpress.com/assets/up.../negro3051.pdf
The Metronome ASRC Module (check the schematic and manual)
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Old 19th April 2018, 02:46 PM   #26
andrewlebon is offline andrewlebon  India
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Default Replacement Transistors

Hello
greetings can BC546/BC556 work in place of BC550/BC560

warm regards
Andrew
Attached Images
File Type: png circuits_powersupply_large.png (106.3 KB, 196 views)
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Old 12th June 2018, 03:26 PM   #27
Chulia is offline Chulia  Lithuania
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IT WORKS!


Try#1 missed one via and one op-amp section was not getting -15V, though getting +15V rail. A puff of smoke and a run around the city to get replacements (of course they had to discontinue LME49990 right during these days :] ). Managed to find them, replaced, got 3 extra spares just in case.
Try#2 Just a random noise from loudspeakers. Believe it or not, failed to connect GND between RPI and PCBs :]
Try#3 Voila!


I took balanced output right after I/V section and fed them into my Hegel amp balanced input. It does sound fantastic, I don't have many in my list, but this project clearly outplays every DAC I've heard and tried in my system. Whatever the intensity of the track - it always feels there is still so much headroom left. So excited with so many music to go through :]

Some points:
1) Will have to put everything in a nice enclosure and arrange all the cabling, shielding, etc - will keep it posted here. Together with questions, I guess :]
2) I plan to integrate WaveIO board as an additional USB input option. How do I best switch the signal? Mechanical switch with 3 contacts? That would make quite long runs of I2S signal wires (rpi/waveIO -> switch in panel -> back to PCBs), is it ok? Maybe some kind of electronic solution? Mosfets/CD4053 IC? Won't it influence the signal?
3) LME49990 seem to heat up quite a lot, is it normal? So far managed to get away with several DC fans around the boards, but as is - they definitely need a heatsink. Haven't found anything about that in a datasheet, so trying to make sure that they really do need it and it's not a fault in assembly of sorts (which is doubtful, since everything works great).
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Old 13th June 2018, 05:38 PM   #28
weissi is offline weissi  Austria
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Old 13th June 2018, 11:44 PM   #29
cozycactus is offline cozycactus  Israel
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My first DAC on PCM1794A - help needed.
I don't understand why to use different supply for left and right channel...
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Old 14th June 2018, 10:53 AM   #30
Chulia is offline Chulia  Lithuania
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This is a classic and often used (though expensive) concept of channel separation. In general, you don't want anything happening in one channel affecting the other. If they share same power source, voltage drops due to to the loads in one channel affects the voltage another one is receiving.
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