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Old 29th October 2011, 02:19 AM   #1
AWaudio is offline AWaudio  China
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Default PCM1794 SPDIF decoder DAC

After several weeks I get this done. Sounds very nice but I'm still thinking about anything more we can do to improve it.

It's DIR9001 + PCM 1794 + several OPA2134 and Toshiba Transistors for head AMP. Oh a PCM2706 is also there for USB.

Now I am thinking about adding EQ to it? any idea? Personally I'd like to supress the mid and boost the bass and treble as I guess maybe this will sounds better for headphone directly output without any tone tunning circuit?

I'd like to post more information here but looks like direct image upload is not available here, so...forgive me, post the ebay link there PCM1794A 24 Bits HIFI SPDIF+TOSLINK Decoder + USB DAC + Headphone Amplifier | eBay.

We are tying to figure out what's the significant diffrence between the DIYed pieces and the thouands bucks ones from top brand. Appreciate very much if we can explore this. Is it possibel that the DIYed piece sounds better than the famous ones?
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Old 29th October 2011, 02:35 AM   #2
AWaudio is offline AWaudio  China
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I've tested a typical negative feedback tone controlling circuit with opamps opa2604, and thin film capacitors. Indeed it's able to adjust the EQ obvious with lots of dBs. However it's not that sweet.... any more ideas.
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Old 29th October 2011, 02:47 AM   #3
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. Sounds very nice but I'm still thinking about anything more we can do to improve it.
Re-think the grounding. Looks like you've got SPDIF input going in to a daughter board on the rear panel. Do you have a dedicated ground wire for that input or is it isolated via transformer?

I'm playing with a DAC design at the moment and have obtained what seemed to me to be surprising improvements in clarity from re-jigging the grounding of the digital parts even though they're fed from a separate winding of the mains trafo.
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Old 29th October 2011, 03:05 AM   #4
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I am trying to isolate this digital part from the system by connecting it the the common ground by a single point, to be specific, via one of the PINs in the 6PINs I2S connector cable to the DAC board. Doing this to avoid cross talk over the digital and analog modules. However, redundant design to allow the GND plane of the daughter card to connect to the enclouser, test show there is not much diffrence...

It sounds dead quiet if I leave the optical input open and turn the volume to max.

But comparing to the 1.5 thousand dollar Marants CD, I still think it's not "sweet" enough, and the negative feedback EQ circuit doesn not work. Professional makers should have something tricky we will never know ha?

Quote:
Originally Posted by abraxalito View Post
Re-think the grounding. Looks like you've got SPDIF input going in to a daughter board on the rear panel. Do you have a dedicated ground wire for that input or is it isolated via transformer?

I'm playing with a DAC design at the moment and have obtained what seemed to me to be surprising improvements in clarity from re-jigging the grounding of the digital parts even though they're fed from a separate winding of the mains trafo.
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Old 29th October 2011, 03:22 AM   #5
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It sounds dead quiet if I leave the optical input open and turn the volume to max.
I think noise shows up as intermodulation products so in the absence of signal its not a good test. At least I hear no difference when nothing's playing.

Quote:
But comparing to the 1.5 thousand dollar Marants CD, I still think it's not "sweet" enough
Exactly - noise induced intermod takes away from music's sweetness. So maybe those Marantz designers know a thing or two about grounding?

Quote:
Professional makers should have something tricky we will never know ha?
No, there was a time when they didn't know either. Such knowledge comes from experience of modding and listening.
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Old 29th October 2011, 04:00 AM   #6
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I love the idea of re-gidding digital ground brings siginificant improvement. Could you share some idea? principles and practical action. Isolating the digital board from other parts of the circuit, allowing them only to connect at the input of the I2S interface for nex stage(the DAC) is the basic idea in my mind.

So in my idea is separated power supply(or heavily de-coupled and heavy PSRRed dedicated regulator as in my design) for digital board, and single common ground wiring point at the I2S interface.

I'm playing with a DAC design at the moment and have obtained what seemed to me to be surprising improvements in clarity from re-jigging the grounding of the digital parts even though they're fed from a separate winding of the mains trafo.[/QUOTE]
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Old 29th October 2011, 02:28 PM   #7
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Quote:
Originally Posted by AWaudio;2762428

I'd like to post more information here but looks like direct image upload is not available here, so...forgive me, post the ebay link there PCM1794A 24 Bits HIFI SPDIF+TOSLINK Decoder + USB DAC + Headphone Amplifier . [/QUOTE
Absolutely possible to post images here, go to advanced, then "manage attachments." Posting links to commercial products one is promoting on a non vendor forum is actually a violation of forum anti-spamming rules.
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Old 30th October 2011, 01:11 AM   #8
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Could you share some idea? principles and practical action.
Sure - the general observation is that in most designs the 0V does double duty - provides power and is an audio signal reference. The provision of power through a reference means crosstalk occurs. Rigorous star grounding is a way to minimize this digital-analogue crosstalk.

Often a DAC is designed with separate 0V for digital and analog - this is a start but an SPDIF receiver has analog parts in it - the PLL. So should that be powered from the analog supplies? So you maybe can begin to see that separating out the power supplies isn't at all straight forward.

One change I made to my current modding DAC - this one from taobao:

TDA1543 4.0U ͬ USB -Ա

is to separate the digital supply 0V pin from the WM8805 and give it a direct route back to the digital star point. I did this also on the CM102 USB chip on the board. Seems to have made a worthwhile improvement in the sound.

Here's a link to a thread I've contributed to on Head-Fi about another similar DAC - the pics are my modded Muse with lots of yellow wires for the additional grounds:

Mini Dac TDA1543 X 4 NOS
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Old 30th October 2011, 02:49 PM   #9
AWaudio is offline AWaudio  China
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Photos included:
1)front_s, a front view of it.
2)rare1, rare view, three digital inputs, USB Fiber and COAX for SPDIF
3)power, power supply of the suite. Seperated power supplies for Large current amplifier and low noise signal processing
4)output, 8 transistors there for output, plus a relay for protection.

The digital and analog ground of each module is isolated by inductor or bead, seperated groud plane is there for them. This time some optimization is done over the power supply and it sounds great, better than my Marants Pure CD player.
Attached Images
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File Type: jpg rare1.JPG (65.2 KB, 367 views)
File Type: jpg output.JPG (131.7 KB, 368 views)
File Type: jpg power.JPG (107.9 KB, 362 views)
File Type: jpg 2134.JPG (323.4 KB, 362 views)
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Old 31st October 2011, 12:11 AM   #10
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Default Mode select between NON (ZERO)-Oversampling and Oversampling wanted

How can I realize a non oversampling DAC with the PCM1794?
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