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lingDAC - cost effective RBCD multibit DAC design
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Old 17th September 2019, 03:53 AM   #471
abraxalito is offline abraxalito  United Kingdom
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Adapting PhiDAC to work with multiple DACs will be quite tricky. The first AD8017 (whose feedback resistor is R4) can't have lower gain than with 620ohm for R4 as its a CFB amp which will go unstable with R4 much lower in value. So to lose enough gain we'd need to adjust the second AD8017 which has all the filter components around it. That's certainly possible as on the filter stage the feedback resistor's 4k7 so has some flexibility to be reduced for lower gain. It would need fiddling with in LTSpice to work out the new values. My gut feel is its going to need more rather spendy NP0 capacitors than there's room for on the PCB, meaning you'll end up stacking them to reach enough uF.

There is a more thorny issue related to the CLC filter - there's a possibility of exceeding the maximum current of the MLF inductors (5mA) when feeding in the output of eight paralleled DACs (8-10mA). Whilst I doubt there'll be any damage to these inductors, running at current levels above the maximum are going to result in saturation and hence loss of inductance. Meaning the filter's effectiveness won't be guaranteed at all possible signal levels.

The current sink (Q5, Q8) needs to be increased, I'd guess reduce R18 to 110ohm initially, add another paralleled R to tweak the DC level to half the supply. R51 will need reducing too if high PSRR is a desired feature of the CCS, suggest 30k. R14 goes down to 12k.

The current sink devices on both the AD8017s are to keep their output stages from going into classAB. With 8X as much current they also need to be 8X higher current (divide resistor values by 8, similar to the Q5,Q8 pair).

On the new paralleled DAC, I've not decided yet but it will certainly be more than 16. Therefore you can use the board and just populate with as many chips as you want. I've a 36 DAC chip board in manufacture as I write, when that comes back, assuming no errors I'll publish the gerbers.
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Last edited by abraxalito; 17th September 2019 at 03:56 AM.
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Old 17th September 2019, 03:28 PM   #472
obscurus is offline obscurus
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PHILDAC REVIEW

Ok, So I have both PhilDAC and PhilDAC SE from Richard. And I already finished both with chassis and spend quite some time with both of them.

My setup are:

PSU: Salas SSLV 1.3 10V ( normal version ) and 13V ( SE version ).
USB 2 I2S: Taobao XMOS module

I did change some resistors in the i2s input area and some in the gain structure of output of the opamp from thick film to thin film. And replace the output resistor in normal version to serial of two F.B. And output capacitors to my fav kind instead of what came in the kit.

The extra caps and a good PSU is what required to get the most from this DAC.

I listened to them with my AKG K812 and through my stereo speakers.

And I have to say that it is quite a very good DAC, it truly different from DACs I have listened before.

It has:

Very clear human voice ( male + female ) which I always desired. It solved the mudding high problem of my AKG 821.
Very nice separation between sound layer. Plenty of bass, feel very real. Instrument feel real too and cross well with the singer voice.
It sounds close to the real live music, not the same but closer than other Dac I heard before. All my friends agreed about that.
No fatigue. I can listen through it for hours.
Make me dig to my old music library and with any old song, it gives me something new.
Quite an addictive DAC. Made me listen to music more than I usually do.

If I have to compare PhilDAC SE + PhilDAC, I prefered the SE version a bit more than normal version, may be 10 / 9. The SE version seem a have firmer bass + mid compare to the normal one.

I already sold / gave all of my DAC kits to my friends and every one seem happy. For the low price, it is a steal

Now I really want to try the better bother of the PhilDAC, could be Ling DAC or the newest, parallel 208 one
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Old 17th September 2019, 10:34 PM   #473
sajunky is offline sajunky  South Africa
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Quote:
Originally Posted by abraxalito View Post
Adapting PhiDAC to work with multiple DACs will be quite tricky. The first AD8017 (whose feedback resistor is R4) can't have lower gain than with 620ohm for R4 as its a CFB amp which will go unstable with R4 much lower in value. So to lose enough gain we'd need to adjust the second AD8017 which has all the filter components around it. That's certainly possible as on the filter stage the feedback resistor's 4k7 so has some flexibility to be reduced for lower gain. It would need fiddling with in LTSpice to work out the new values. My gut feel is its going to need more rather spendy NP0 capacitors than there's room for on the PCB, meaning you'll end up stacking them to reach enough uF.

There is a more thorny issue related to the CLC filter - there's a possibility of exceeding the maximum current of the MLF inductors (5mA) when feeding in the output of eight paralleled DACs (8-10mA). Whilst I doubt there'll be any damage to these inductors, running at current levels above the maximum are going to result in saturation and hence loss of inductance. Meaning the filter's effectiveness won't be guaranteed at all possible signal levels.

The current sink (Q5, Q8) needs to be increased, I'd guess reduce R18 to 110ohm initially, add another paralleled R to tweak the DC level to half the supply. R51 will need reducing too if high PSRR is a desired feature of the CCS, suggest 30k. R14 goes down to 12k.

The current sink devices on both the AD8017s are to keep their output stages from going into classAB. With 8X as much current they also need to be 8X higher current (divide resistor values by 8, similar to the Q5,Q8 pair).

On the new paralleled DAC, I've not decided yet but it will certainly be more than 16. Therefore you can use the board and just populate with as many chips as you want. I've a 36 DAC chip board in manufacture as I write, when that comes back, assuming no errors I'll publish the gerbers.
Thanks very much for the explanation. This is a solid project with all details well thought. I feel like a fool for not considering that second amplifier would need the same treatment as the first one (Q7&Q10).

All above changes are possible except saturation of LC filter, which I completely missed. For this reason paralleling 8 chip is not practical, so I am giving up. Four chips are rather possible, while still on the border of crossing saturation point.

As for a new parallel DAC, I am concern that failure of a single chip can bring all network down. Probability of a failure is raising proportionally to the number of chips, it is why it may be practical to stop at 16. Beside of fun I want a practical solution. However as you said that I can put less chips if I want and by doing this it won't affect design principles, it is a good news. You can count on me for a one kit. Thank you for your time you spent on the project.
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Old 17th September 2019, 11:52 PM   #474
sajunky is offline sajunky  South Africa
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Originally Posted by obscurus View Post
PHILDAC REVIEW
USB 2 I2S: Taobao XMOS module
What is a link to Taobao, please?
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Old 18th September 2019, 12:25 AM   #475
obscurus is offline obscurus
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Quote:
Originally Posted by sajunky View Post
What is a link to Taobao, please?
XMOS +CPLD U208 USB???? I2S??-???

sure.
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Old 18th September 2019, 01:22 AM   #476
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by sajunky View Post
All above changes are possible except saturation of LC filter, which I completely missed. For this reason paralleling 8 chip is not practical, so I am giving up. Four chips are rather possible, while still on the border of crossing saturation point.
If you were really determined to adapt PhiDAC to multiple DAC chips you could use a transformer to (say) divide the DAC's output current by 8. But really once you're on the slippery slope with transformers you might as well go the whole hog down the rabbit hole and dispense with the opamps too.

Quote:

As for a new parallel DAC, I am concern that failure of a single chip can bring all network down. Probability of a failure is raising proportionally to the number of chips, it is why it may be practical to stop at 16.
Its a very good point and one I've pondered for quite a while. So far I've had zero failures when already running in-circuit, my failures have been right at the start where a chip hasn't worked on first powering up. I've gotten a bit of a handle on one of the failure mechanisms now - solder shorts between the pins of the DAC, from the desoldering process.


Using multiple small boards to build a parallel DAC gives some ability to adapt to DAC faults by swapping out a board. To improve reliability still further it would be fairly easy to run the chips at reduced supply voltage (I run mine very close to the absolute max of 6V) - this will decrease both voltage and thermal stress. On the 36 DAC board at ~6V the chips get quite warm to the touch in the open air - in a case they might reach 60oC. The downside to running at lower voltages is you need more chips to reach the same current level.
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Old 18th September 2019, 02:01 AM   #477
sajunky is offline sajunky  South Africa
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Quote:
Originally Posted by abraxalito View Post
Using multiple small boards to build a parallel DAC gives some ability to adapt to DAC faults by swapping out a board. To improve reliability still further it would be fairly easy to run the chips at reduced supply voltage (I run mine very close to the absolute max of 6V) - this will decrease both voltage and thermal stress. On the 36 DAC board at ~6V the chips get quite warm to the touch in the open air - in a case they might reach 60oC. The downside to running at lower voltages is you need more chips to reach the same current level.
Multiple boards is an excellent idea, I will go for it!
As for the voltage I will depend on your expertise. I feel however that a function of temperature vs voltage is not linear. At a certain voltage temperature may start raising much faster. I would not exceed 45degC. Remember also that some of us would like to increase sample rate to allow oversampling, it must be a room for it.

Would you consider to include in the kit USB to I2S converter module linked by @obscurus in a previous post? I think many members would chose this option. This one seems is very good with XMOS XU208 for a low price and I see three Sitime quality oscilators on the board. I would order two boards.

Last edited by sajunky; 18th September 2019 at 02:14 AM.
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Old 18th September 2019, 02:59 AM   #478
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by sajunky View Post
Multiple boards is an excellent idea, I will go for it!
As for the voltage I will depend on your expertise. I feel however that a function of temperature vs voltage is not linear. At a certain voltage temperature may start raising much faster. I would not exceed 45degC.
I shall design the kit version to be more conservative then, no higher than 5.5V power to the DAC chips.

Quote:
Remember also that some of us would like to increase sample rate to allow oversampling, it must be a room for it.
This is a point I'm curious about. I can see that running 2X OS has certain advantages - it allows the anti-imaging filter to be more effective at removing stop-band images. But is there some interest in going up to 4X or 8X? I can't see any advantage of those rates myself and quite likely they're going to reduce the subjective dynamics. On my first prototype 36 DAC board I have used an LED as shunt regulator to the 74HC86 logic driving the DACs, it turns out to need twice as much current at 88k2 than it does at 44k1. So I would need to design for up to 8X as much current if customers wanted to run at 8X OS. I don't think the current requirement for the TDA1387s themselves rises so much with frequency though.

Quote:
Would you consider to include in the kit USB to I2S converter module linked by @obscurus in a previous post? I think many members would chose this option. This one seems is very good with XMOS XU208 for a low price and I see three Sitime quality oscilators on the board. I would order two boards.
Individual buyers are free to request whatever they need. I'd not include it as standard in a kit though as I wouldn't want to be the one to whom they're returned if any fault develops with them.
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Old 18th September 2019, 09:32 PM   #479
nautibuoy is online now nautibuoy  United Kingdom
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matt_garman is kindly going to get me into the game by providing one of his spare PhiDAC kits - thanks again Matt and thanks also to Abraxalito for sharing his adventure with us.

Just wondering if gerbers files are available for the PhiDAC as soldering the board in a reflow oven might be a good proposition. I have a friend with a suitable oven and he's also working on developing his own pick and place machine - populating the board coul be a really useful test use-case for the pick and place if the files are available.

I did have a look for PhiDAC gerbers but didn't spot anything in the search results.
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Old 19th September 2019, 01:21 AM   #480
abraxalito is offline abraxalito  United Kingdom
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Gerbers for PhiDAC are here : lingDAC - cost effective RBCD multibit DAC design
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