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Improving a cheap chinese PCM1794 DAC
Improving a cheap chinese PCM1794 DAC
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Old 11th January 2019, 01:35 PM   #1
pachy is offline pachy  Bulgaria
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Default Improving a cheap chinese PCM1794 DAC

Sorry if this board was already discussed but I didn't find anything related to it.

I few years ago I paused my diy audio hobby and sold/gifted everything I had built but an year ago I felt the need to start enjoying music once again. In no more than 1 month time I bought the ATC SCM11 curved speakers which I fell in love with at the first note, built a Raspberry Pi 3 + Volumio streamer, built the PeeCeeBee V4H known to most of you and now I was in need of a new DAC which will be used as my main audio source. The requirements were simple:
– Digital input switching. I wanted to use the same DAC whether I am listening from my DIY Volumio streaming device or watching a movie/playing Xbox.
– Good to exceptional sound quality.

After reading an article by a russian author, the PCM1794 datasheet and talking to a friend of mine who works in a professional audio equipment manufacturing company, I decided to try the PCM1794 from Burr Brown for this project. I did a quick search on Aliexpress for readily available PCBs based on PCM1794 and I came across one that looked well made. The PSU part is well separated utilising:
– Four separate transformer windings with 3 bridge rectifiers (MCU+LCD; digital section PSU; analog section PSU)
– Five on board regulators (LM7805 for the uC/LCD, LT1968-3.3V for the AK4118 and PCM1794 digital supply, LM317(5V) for the PCM1794 analog VCC, LM317/337(+-15V) for the opamp supply.
– Utilized ground planes


The board costed only 47$ so I expected cheap capacitors and probably fake AD827 opamps. After 20 days I received the board. I was right, they used cheap caps(except the big filter Nichicons, which seem good) and the opamps are probably fake at this price so I started planning mods to the board.

The first thing I did was to test the board before doing anything so I know if it works as expected. I plugged in all required windings, plugged my set-top box as digital source with an optical cable and flipped the switch…

F**k… The thing is not even working…

Click the image to open in full size.

Chinese PCM1794 AK4118 board

The first thing I did was to measure all the supplies. I found out that the LT1968-3.3 had 2.0V at its output so the AK4118/PCM1794 didn’t get enough voltage to work. I desoldered the LT1968 and soldered a LD1117V33. It finally worked. Good thing is I don’t have to return it to the seller and I can start with the mods.

After inspecting the board and following some traces I have annotated the photo of the board of all planned mods for convenience.

Click the image to open in full size.

I have done the following modifications:

1. Change all big diodes in the rectifier bridge to SB5A0 fast recovery diodes because the old ones were standard Chinese diodes with shady letters on them.

2. I checked all resistor values according to the following OPA1611(single version of OPA1612) schematic and found out that the 8200pF capacitors were actually 220pF(marked on board as 820pF) and the 2700pF were actually 270pF(marked on board as 270pF). Resistor values were right according to this schematic.

Click the image to open in full size.

OPA1612 as I/V converter
Changed those to Wima FKS2 capacitors with the right values and also changed the 2200pF metallized film capacitors in the feedback of the I/V stage with same value Wima FKS2.

3. Changed all electrolytic capacitors to Nichicon UPS which are low impedance, high temp range capacitors suitable for PSU usage. Some of the capacitors were 47uF instead of the 10uF according to the PCM1794 datasheet.

4. The final thing to do was change two of the AD827(probably fake) to two OPA1612 opamps in the I/V stage and change the third AD827 in the differential to single convertor stage to OPA2132.

Click the image to open in full size.

I will now let the DAC burn in for a couple dozen hours and I will start listening! The next planned mod is to change all regulators to discrete ones based on a new schematic I am now evaluating. I will combine this DAC in one enclosure with my Volumio streamer and the JLSounds XMOS in the next few days. Any other ideas for possible mods/swaps?
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Old 12th January 2019, 01:27 PM   #2
abraxalito is offline abraxalito  United Kingdom
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Don't connect the I/V opamps direct to the PCM1794, rather insert a passive (CLC) filter between the DAC current out and the -ve input of the I/V opamp. It provides bandlimiting of the fast edges out of the DAC meaning that the opamp has a much easier task. This has been a significant upgrade on my own DACs which use active I/V, making the sound holographic.
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Old 12th January 2019, 06:32 PM   #3
phase is offline phase  United States
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While most of the output configurations that I have seen for the pcm1792/4 are fairly simple, I am surprised that there aren’t any resistors at the least before the op amps.
The output on this board looks like the suggested/data sheet implementation, at least after adjustments to it as it had been delivered.
I would try the opa1612 op amps or some of the latest Burson modules if you decide to keep it.
Those filter caps were a great move, and would keep playing with the power supplies after the diodes. Snubber circuits on the diodes would help also, along with an X cap .1uf on the incoming ac power.
I would try just some standard 100 - 220uf after the lm317 regs and see if they work better than the FCs. Have yet to hear an FC that sounded good in whatever position. It appears you’re using some decent, low loss/low noise parts around the chip, that will go a long ways.

I have found that fast recovery diodes may sound different and affect the spectrum that way, but the smoothest are from those that are also soft recovery. For small 3a applications the nte571 works very well, are relabeled Philips parts.
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Old 12th January 2019, 06:46 PM   #4
dreamth is online now dreamth  United Kingdom
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Quote:
Originally Posted by abraxalito View Post
Don't connect the I/V opamps direct to the PCM1794, rather insert a passive (CLC) filter between the DAC current out and the -ve input of the I/V opamp. It provides bandlimiting of the fast edges out of the DAC meaning that the opamp has a much easier task. This has been a significant upgrade on my own DACs which use active I/V, making the sound holographic.
Would you be a bit more specific about that, please?I'd be interested to apply that to a very old unbalanced Iout dac, tda1543 if it makes sense as it's using a lm833 for i/v stage, not a modern fast op-amp.
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Old 12th January 2019, 07:21 PM   #5
Koifarm is offline Koifarm  Netherlands
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pachy, reading your start thread i do not get the reason for the mods.

You asume that the parts give not the right sound. Why did you not discribed what was wrong with the sound of the origenal parts and make some measurements like ThD, noise harmonics footprint. So that you after the mods can see what made the difference.
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Old 12th January 2019, 07:38 PM   #6
phase is offline phase  United States
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Also, if you can use some polystyrene capacitors in the filters, along with the polypropylene that you have, will help the sound. The polystyrene give more crisp treble details, while the polypropylene tend to give more punchy dynamics, bass details.

I’m sure it sounds much better now than when you received it; not working!
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Old 13th January 2019, 01:04 AM   #7
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by dreamth View Post
Would you be a bit more specific about that, please?
Here is an example filter I recently posted to another 'Digital Line' thread : IV Opamp converter after DAC: which of the two circuits?

Quote:
I'd be interested to apply that to a very old unbalanced Iout dac, tda1543 if it makes sense as it's using a lm833 for i/v stage, not a modern fast op-amp.
Certainly looks to make sense to me. Do remove the cap across the feedback resistor when you add in the CLC, as that's no longer needed.
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Last edited by abraxalito; 13th January 2019 at 01:14 AM.
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Old 13th January 2019, 02:54 AM   #8
dreamth is online now dreamth  United Kingdom
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I just hope that i won't kill the iout output if i load it with the filter.At least the opamp is made for this thing, i have no idea how would the dac react to a passive filter .
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Old 13th January 2019, 05:20 AM   #9
dreamth is online now dreamth  United Kingdom
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Now i see what you really did...you have a noise penalty if you put any resistor in series with iout, i don't know if i can afford more noise with my last toy added to my cd player, but the only thing that worries me is a 200nf to ground before anything...
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Old 13th January 2019, 05:36 AM   #10
Markw4 is online now Markw4  United States
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Why don't you try it with and without the filter and see how it sounds? That's the only way find out if its a good idea or not. Some kind of filter between the dac and I/V input can help improve sound quality for many dac chips. It depends how much clock and or other HF noise is coming out of the dac outputs. The HF noise can cause slew-induced distortion in the I/V amp, which if that happens tends to worsen sound quality.

However, clock noise tends to be a bigger problem with older dac chips as compared to at least some of the newer ones. Probably not a good idea to do that with an ESS Sabre dac because HF noise is relatively small and any resistance between the dac outputs and I/V will increase HD. However, a single cap across the outputs may still be an option even for Sabre's.
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