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My first DAC on PCM1794A - help needed.
My first DAC on PCM1794A - help needed.
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Old 15th April 2018, 02:05 PM   #11
Chulia is offline Chulia  Lithuania
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I'm back to this project - and I have some questions at my hands :] PCM1794a has a de-emphasis filter (pin3). By default in original project of the author it's enabled (pull-up resistor), but defeatable with jumper to ground.

Do I need to keep it enabled at all? I wasn't able to find enough info what this filter does, just that it's somehow related to a very rare use cases. Do I need to worry about it at all. Maybe simple short it to gnd and disable?
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Old 16th April 2018, 05:26 AM   #12
PaulFrost is offline PaulFrost
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You would only care about demphasis if your source has pre-emphasis (like some CDs). As you are using an RPI as a source, I doubt you will ever need demphasis.

You can read more about CD emphasis filters here:Pre-emphasis - Hydrogenaudio Knowledgebase
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Old 16th April 2018, 05:44 PM   #13
Chulia is offline Chulia  Lithuania
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Thank you so much, that's all I needed. Will keep progress posted :]
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Old 16th April 2018, 11:09 PM   #14
Chulia is offline Chulia  Lithuania
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Hm, ok, one more. The original designer of this project put 2.7n caps in I/V stage opamps. What he also did - is added a note about using 6.8n. This led me to some confusion which ones am I supposed to use.
1) What's the influence of this choice? I see some notes with resulting frequencies, but no idea how to interpret them.
2) If I go with 6.8n, does it apply to all 8 of them, or only the 4 ones with note "6.8n/16V" next to them (in the right-side part of the I/V schematics)?

If this is influenced somehow by 1K...1.47K resistor choice, I already bought some metal film 1.2K ones and would like to use them. As for 360R/715R ones, they're in the basket but will be ordered together with caps.

Click the image to open in full size.
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Old 17th April 2018, 03:06 AM   #15
24century is offline 24century
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Send a message via ICQ to 24century My first DAC on PCM1794A - help needed.
Did you actually make it work as the drawing?

From pi/volumio to dac via i2s without USB is not going to work since 1794@ doesn’t has internal pll
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Old 17th April 2018, 10:51 AM   #16
Chulia is offline Chulia  Lithuania
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Well, they guy made it (even in some variations), tested the sound and made the design public, so I guess there is a good chance it will work :]

I'm still in the process of soldering the pcb, so can't confirm that yet.
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Old 17th April 2018, 07:15 PM   #17
tjaekel is offline tjaekel  United States
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Quote:
Originally Posted by 24century View Post
Did you actually make it work as the drawing?

From pi/volumio to dac via i2s without USB is not going to work since 1794@ doesn’t has internal pll
Sorry, I2S and USB? Two different topics. The PCM1794A is connected via I2S directly to the RPi digital audio interface. The PCM1794A is clocked by the I2S SCK signal, with an automatic clock detection. A PLL is not really needed as long as the I2S SCK clock is stable and not with so much jitter.
Even with the "jittery" signal out from Raspberry Pi - the DAC follows well the I2S clock. The jitter might just create a bit of distortions but the DAC works for sure as shown in the schematics. I have not realized an audible effect based on the (acceptable) jitter generated by the RPi.
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Old 17th April 2018, 07:56 PM   #18
tjaekel is offline tjaekel  United States
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Quote:
Originally Posted by Chulia View Post
Hm, ok, one more. The original designer of this project put 2.7n caps in I/V stage opamps. What he also did - is added a note about using 6.8n. This led me to some confusion which ones am I supposed to use.
1) What's the influence of this choice? I see some notes with resulting frequencies, but no idea how to interpret them.
2) If I go with 6.8n, does it apply to all 8 of them, or only the 4 ones with note "6.8n/16V" next to them (in the right-side part of the I/V schematics)?

If this is influenced somehow by 1K...1.47K resistor choice, I already bought some metal film 1.2K ones and would like to use them. As for 360R/715R ones, they're in the basket but will be ordered together with caps.

Click the image to open in full size.
All these RC circuits on the OpAmps create low-pass filters. They are "needed" (actually suggested) in order to get rid of the high frequency noise generated by this Delta-Sigma DAC (due to the "noise shaping"). Otherwise, noise which is not audible (way above 24 KHz) could confuse a following power amp.
For the filter calculation - you can check here: Op Amp Low Pass Filter | Active Filter Circuit | Radio-Electronics.Com

So, the filter corner frequency is given by: fc = 1 / (2*Pi*R*C) .
You can vary R and C and set a corner frequency as you like, as long it is above approx. 24..30 KHz.The resulting corner frequencies for the R and C used are given in the schematics.
If you double C (or R) the corner frequency is lowered by one-half. So, with 1.2K and 6.8n you would get 19.5 KHz which might be a bit too low!: corner frequency has already a -3dB attenuation, so you would cut a bit from the audible audio spectrum (if your ears are able to realize 18 KHz or higher, quite unlikely if you are not so young anymore).
BTW: do not make the R too small: it sets the input impedance of the OpAmp circuit and acts as a load on the DAC output.
If you make R way too large you have to consider that it will act as a thermal noise generator. (a too large R needs a very small capacitance but think about tolerances which will result in a "wrong" corner frequency).

If you change - change all in in the same 'stage' in the same way: see that we have two low-pass filters in series (gives you a second-order filter). Both filters have different corner frequencies (the second one filters very high frequencies left by the first one again).

How do you set the corner frequencies is up to you. Just bear in mind they should just remove the not-audible part of noise which might still come out from the DAC (above 30..50 KHz). It is just to 'protect' the following power-amp: often such one has a low-lass filter also on its input, but in case not and the power-amp is very wide-band (e.g. able to amplify also frequencies above 30 KHz) the amp could run into saturation and will burn power by amplifying 'just' high-frequency noise (which will not come out to the speaker as audible signal but the amp could 'run away' by such noise).

Metal-film resistors are a good choice (lower thermal noise). Also to use good caps, e.g. as MKT (but not available as SMD), e.g. as NP0 (great temperature coefficient). I prefer often X7R, X6R, X5R. Sometimes also a need to optimize for low ESR but on these tiny caps it does not matter so much.
More details to find here: Basics of Ceramic Chip Capacitors

Nothing wrong with the schematics, room for changing R and C, just to cross-check the resulting corner frequencies - just cut outside of your audio spectrum (e.g. assuming 22..28 KHz).
BTW: actually, it depends a bit which sample frequency do you want to run: 44.1/48 KHz gives you a corner frequency as 1/2, so 22..24 KHz. With 192 KHz sample frequency on play via DAC - you could think to set 96 KHz, but you cannot hear anyway. So, the given values here in schematics are a good fit for not so drastic filtering somewhere in between.

My suggestion: go with nominal values, e.g. 1.2K and 2.7n first: if you need more drastic noise filtering (which depends on the following amp) - you can just solder a parallel cap on top of the SMD cap (make C a bit larger by adding a parallel C, with parallel R you could just make it smaller which increases frequency).

Last edited by tjaekel; 17th April 2018 at 08:10 PM.
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Old 17th April 2018, 08:10 PM   #19
Chulia is offline Chulia  Lithuania
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My project just got so much more satisfying in an educational way :]
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Old 17th April 2018, 09:05 PM   #20
00940 is offline 00940  Belgium
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Quote:
Originally Posted by tjaekel View Post
Sorry, I2S and USB? Two different topics. The PCM1794A is connected via I2S directly to the RPi digital audio interface. The PCM1794A is clocked by the I2S SCK signal, with an automatic clock detection. A PLL is not really needed as long as the I2S SCK clock is stable and not with so much jitter.
I guess the problem raised by 24century is that the pcm1794 needs 4 I2S lines, including a master clock, and that the PI only provide 3 I2S lines, missing the master clock. DAC ICs with an internal pll, such as the pcm5102, will run fine on 3 lines, not the pcm1794.

It seems that you deal with the problem by tying bck and sck. Since the pcm1794 works at least with a 128fs system clock, you need a 5.6448MHz clock at least. A 44.1KHz/16bit signal will have a BCK of 1.4112MHz so I'm not sure how the digital filter can properly work...

BTW, X7R (or even worse X5R) caps should never be used in filters as their capacitance is voltage dependant. Among ceramic caps, NP0 only should be used in the signal path. X7R is good for PS decoupling.
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