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lingDAC - cost effective RBCD multibit DAC design
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Old 10th February 2020, 06:50 PM   #641
pelopidas is offline pelopidas  United States
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Never mind the above. The ground I2s cable has a defect and after replacing it the low level hiss went away.
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Old 11th February 2020, 11:21 AM   #642
abraxalito is offline abraxalito  United Kingdom
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Today I'm officially released from the quarantine period which means I'm allowed to go outdoors

I've spent the past few quarantined days not on DACs, rather on a digital project (which has been in my in-tray for a while) to see what's possible with ST's cheapest ARM CPU. I blogged about the STM32F030 (see here : Cheapest 32bit CPU to date... - diyAudio ) and planned to interface it to a DAC. Well its only taken 5+ years to get around to but this afternoon I got the result I was hoping for and its implementing a very rudimentary I2S pass-through.

The reason for the doubt and pondering is this device is so basic it doesn't support I2S, I had to coax its SPI interface to work in I2S mode. Perhaps I'll start another thread to talk about that though - I'll put the link up when I've opened it.

@pelopidas - glad you got your hiss sorted. I was going to suggest a test with an Audacity file which contains only dither, but as its solved now I won't describe that.
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Old 13th February 2020, 03:23 PM   #643
pelopidas is offline pelopidas  United States
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So what do you want that chip to do?
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Old 14th February 2020, 01:26 AM   #644
abraxalito is offline abraxalito  United Kingdom
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Several things I have in mind for it. First is to slow down the I2S feed to my DACs, from the usual 64fs to 32fs. I made a circuit that did this a few years ago and I got the impression that the SQ improved with the slower clock. But that was on a much earlier DAC design with passive I/V, so I wonder if the apparent SQ lift will be anywhere noticeable on a PhiDAC Quad.

Second target is to create a digital volume control so that a future PhiDAC can be more 'stand-alone' in that it could work in conjunction with a CD player or SD card player.

Other ideas are to create a dual-mono and balanced DAC where one stack of chips is used to handle both phases of a single channel (either L or R) and to implement some kind of digital filtering - either to create an active XO or just to give better stop-band rejection of image frequencies.

On this last notion, I am still planning to create a much lower power implementation of the venerable old SAA7220. That chip is well known for being a major contributor to lousy SQ - not because of its filtering but because its a noisy power hog. The latest Cortex M0+ chip out of ST holds out the promise of being able to replicate the filtering of the 7220 but at perhaps just 5% of its power draw. The idea is to create a plug-in PCB which can replace the original chip for those upgrading old style CD players.

Beyond that I think the STM32F030 (and the later generation G030) could be made into digital audio 'lego' bricks to build more complex digital filters and also potentially be used to enhance the distortion performance of multibit DACs. They could also be the core of a DAC which uses not-for-audio DAC chips - those chips don't have I2S interfaces so the CPU can do the interface translation.


Any of those thoughts interesting to anyone else besides me?
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Last edited by abraxalito; 14th February 2020 at 01:29 AM.
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Old 14th February 2020, 11:43 AM   #645
GORDMUS68 is offline GORDMUS68  South Africa
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Richard judging by the fact that you are sticking your tongue out, and you're being stupendously productive, as usual, bodes all is well there.

Loving the 8 chip LingDac variant I have, pity that development reached a conclusion so soon, although I don't have time to tinker at all, and am a complete electronics pleb, I read as much as possible here, in the advent that one day i will have the time to at least try.

Bring it on Sir!
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Old 14th February 2020, 12:58 PM   #646
Markw4 is offline Markw4  United States
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Quote:
Originally Posted by abraxalito View Post

Any of those thoughts interesting to anyone else besides me?
Sure. Its all interesting. Be interesting to see what you find out.
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Old 14th February 2020, 02:04 PM   #647
pelopidas is offline pelopidas  United States
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Ok, just to show how ignorant I am of how all this works, I will ask a question that I am not even sure how to ask correctly.
If you can do dual mono, would you not also be able to do 128 chips per channel to get it to 24bit resolution? So, a modest 256 chip NOS DAC to play high bit rate files?
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Old 15th February 2020, 01:58 AM   #648
abraxalito is offline abraxalito  United Kingdom
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Its an interesting question, one I've thought about myself on and off for some time. I think the answer is likely 'yes' but first I'd want to reduce the target number of bits. Even though plenty of recordings touted as '24bit' exist, they aren't in reality 24bits (in the sense they don't have 144dB dynamic range) as the bottom 4 bits (at least) will be noise. I can't see any point in working hard to get the noise level down when the DAC's internal noise will be swamped by noise on the recording. Therefore I think a more realistic target for a parallel array of 16 bit DACs is 20bits.

The typical noise spec of TDA1387 is 98dB which is 22dB away from 120dB. Noise will in general reduce 3dB for each doubling of number of chips, therefore we will need 7 doublings to reach 119dB, your modest suggestion of 256chips should get us to 122dB. Simply paralleling chips doesn't get us more resolution though, just lower noise. To improve the resolution we'll need some DSP to send the various DACs slightly different data, the same DSP could also randomize the low-order bits to effect distortion reduction. Looks like it could be possible with an array of cheap STM32 chips, not sure how many we'd need though. Definitely a worthwhile project - anyone want to contribute to it?
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Old 15th February 2020, 02:22 AM   #649
sumotan is offline sumotan  Indonesia
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I like your idea of slowly down the I2S speed Richard. Very clever of you.
With regards to noise, do we really need 122db reduction ? I have experimented with my AyA dac for many years & my findings are to to minimize PS noise as much as possible first.
Here transformer types, caps values etc matters alot. The last experiment that I tested was suggested by Max on Ultimate 1541 thread, a powder coated ferrite ring surprisingly it work very very well. Caveat though is the wire used must be at least 1mm in dia. Give this a try cost peanuts. Yes Im aware that the engineer part of you will just look at component specs but I can assure you that there's really a place for so call boutique caps in audio. In fact no need boutique caps to test, every brand of caps of same value will sound different. On my dac there's more then 6 types/brand of caps that I use on different locations.

Cheers
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Old 15th February 2020, 02:35 AM   #650
abraxalito is offline abraxalito  United Kingdom
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Hi @sumotan, thanks for chipping in! I've gotten the slower I2S working and while I can't put my finger on any particular changes, its more addictive to listen to. Perhaps the low frequency bloom is more pronounced, can't say yet - I need my wife's ears to examine it. Yes, agree with PS noise being the focus, but I have addressed that already on PhiDAC (even made some measurements which showed decreased LF rail noise with more caps, hence PhiDAC SE).

As for 122dB - well its a noise level which makes sense for 20bit input material. I don't think I have any as pretty much all I have is RBCD. But I like the challenge of getting there. The idea can go the other way too - I have some 'industrial' 12bit DACs sitting in my intray, I'd like to build an array of them to give 16bit performance.

For those who love optimizing their sound with fancy caps, I say great. I don't have the inclination to do that, I prefer to focus on things where I can understand the whys. If I tweak a cap to a boutique one I haven't learned anything about why it raised the SQ, to me that's not giving any satisfaction, I want to tie improvements to circuit details so I understand what's happening to some degree.
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